Andrade, M. C. R., G. O. Ludwig and S. J. Camargo, 2000: Self-consistent equilibrium calculation through a direct variational technique in tokamak plasmas. Plasma Physics and Controlled Fusion, 42(12): 1269-1289.
A self-consistent equilibrium calculation, valid for arbitrary aspect ratio tokamaks, is obtained through a direct variational technique that reduces the equilibrium solution, in general obtained from the 2D Grad-Shafranov equation, to a 1D problem in the radial flux coordinate rho. The plasma current profile is supposed to have contributions of the diamagnetic, Pfirsch-Schluter and the neoclassical ohmic and bootstrap currents. An iterative procedure is introduced into our code until the flux surface averaged toroidal current density (J(T)), converges to within a specified tolerance for a given pressure profile and prescribed boundary conditions. The convergence criterion is applied between the (J(T)) profile used to calculate the equilibrium through the variational procedure and the one that results from the equilibrium and given by the sum of all current components. The ohmic contribution is calculated from the neoclassical conductivity and from the self-consistently determined loop voltage in order to give the prescribed value of the total plasma current. The bootstrap current is estimated through the full matrix Hirshman-Sigmar model with the viscosity coefficients as proposed by Shaing, which are valid in all plasma collisionality regimes and arbitrary aspect ratios. The results of the self-consistent calculation are presented for the low aspect ratio tokamak Experimento Tokamak Esferico. A comparison among different models for the bootstrap current estimate is also performed and their possible Limitations to the self-consistent calculation is analysed.
Arfeuille, G., L. A. Mysak and L. B. Tremblay, 2000: Simulation of the interannual variability of the wind-driven Arctic sea-ice cover during 1958-1998. Climate Dynamics, 16(2-3): 107-121.
A thermodynamic-dynamic sea-ice model based on a granular material rheology developed by Tremblay and Mysak is used to study the interannual variability of the Arctic sea-ice cover during the 41-year period 1958-98. Monthly wind stress forcing derived from the National Centers for Environmental Prediction (NCEP) Reanalysis data is used to produce the year-to-year variations in the sea-ice circulation and thickness. We focus on analyzing the variability of the sea-ice volume in the Arctic Basin and the subsequent changes in sea-ice export into the Greenland Sea via Fram Strait. The relative contributions of the Fram Strait sea-ice thickness and velocity anomalies to the sea-ice export anomalies are first investigated, and the former is shown to be particularly important during several large export events. The sea-ice export anomalies for these events are next linked to prior sea-ice volume anomalies in the Arctic Basin. The origin and evolution of the sea-ice volume anomalies are then related to the sea-ice circulation and atmospheric forcing patterns in the Arctic. Large sea-ice export anomalies are generally preceded by large volume anomalies formed along the East Siberian coast due to anomalous winds which occur when the Arctic High is centered closer than usual to this coastal area. When the center of this High relocates over the Beaufort Sea and the Icelandic Low extends far into the Arctic Basin, the ice volume anomalies are transported to the Fram Strait region via the Transpolar Drift Stream. Finally, the link between the sea-ice export and the North Atlantic Oscillation (NAO) index is briefly discussed. The overall results from this study show that the Arctic Basin and its ice volume anomalies must be considered in order to fully understand the export through Fram Strait.
Beal, L. M., A. Ffield and A. L. Gordon, 2000: Spreading of Red Sea overflow waters in the Indian Ocean. Journal of Geophysical Research-Oceans, 105(C4): 8549-8564.
As a result of its remarkably high salinity and despite its small volume input, remnants of Red Sea Water (RSW) have been identified in the Agulhas Current, over 6000 km distant of their source. This provided the motivation to investigate the long-term mean spreading of RSW throughout the Indian Ocean, using a comprehensive set of observations, taken from the National Oceanographic Data Center archives and from the World Ocean Circulation Experiment Hydrographic Program for the Indian Ocean. After emerging from the Gulf of Aden into the Arabian Sea, RSW spreads predominantly southwestward along the African continental slope, as indicated by strongly inclined isohalines across the Arabian Sea. There is some monsoon variability, so that during the winter monsoon there is more RSW present in the Gulf of Aden and an intensification of southward spreading along the western boundary, between 12 degrees and 5 degrees N. Elsewhere the intermediate depth salinity field of the Indian Ocean appears relatively stationary Between 5 degrees and 10 degrees S, in the region of the South Equatorial Current, isohalines of the RSW layer become quasi-zonal across the width of the Indian Ocean, only dipping southward toward the western boundary west of 50 degrees E. South of here, there is a strong tongue of RSW spreading southward through the Mozambique Channel and into the Agulhas Current. These conclusions concur with previous localized investigations of intermediate water properties. Using a simple mixing model, the percentage of RSW throughout the Indian Ocean was quantified. It was found that the flux of salt into the Gulf of Aden from the Red Sea is similar to that estimated to cross 32 degrees S in the Agulhas Current. This result implies that all the RSW which is mixed into the interior of the Indian Ocean may eventually be exported at the western boundary. Furthermore, it implies that RSW is the dominant component of the salt budget for the intermediate layer and that input from the Indonesian Seas and via diapycnic processes are small.
Bromwich, D. H., A. N. Rogers, P. Kallberg, R. I. Cullather, J. W. C. White and K. J. Kreutz, 2000: ECMWF analyses and reanalyses depiction of ENSO signal in Antarctic precipitation. Journal of Climate, 13(8): 1406-1420.
The El Niño-Southern Oscillation (ENSO) signal in Antarctic precipitation is evaluated using European Centre for Medium-Range Weather Forecasts (ECMWF) operational analyses and ECMWF 15-yr (1979-93) reanalyses. Operational and reanalysis datasets indicate that the ENSO teleconnection with Antarctic precipitation is manifested through a close positive correlation between the Southern Oscillation index and West Antaretic sector (75 degrees-90 degrees S. 120 degrees W-180 degrees) precipitation from the early 1980s to 1990, and a close negative correlation after 1990. However, a comparison between the operational analyses and reanalyses shows significant differences in net precipitation (P - E) due to contrasts in the mean component of moisture flux convergence into the West Antarctic sector. These contrasts are primarily due to the mean winds, which differ significantly between the operational analyses and the reanalyses for the most reliable period of overlap (1985-93). Some of the differences in Row pattern are attributed to an error in the reanalysis assimilation of Vostok station data that suppresses the geopotential heights over Fast Antarctica. Reanalysis geopotential heights are also suppressed over the Southern Ocean, where there is a known cold bias below 300 hPa. Deficiencies in ECMWF reanalyses result in a weaker ENSO signal in Antarctic precipitation and cause them to miss the significant upward trend in precipitation found in recent operational analyses. Ice-core analyses reflect both an upward trend in ice accumulation and the ENSO teleconnection correlation pattern seen in the operational analyses. This study confirms the results of a previous study using ECMWF operational analyses that was the first to find a strong correlation pattern between the moisture budget over the West Antarctic sector and the Southern Oscillation index.
Burnett, W. H., V. M. Kamenkovich, D. A. Jaffe, A. L. Gordon and G. L. Mellor, 2000: Dynamical balance in the Indonesian Seas circulation. Geophysical Research Letters, 27(17): 2705-2708.
A high resolution, four-open port, non-linear, barotropic ocean model (2D POM) is used to analyze the Indonesian Seas circulation. Both local and overall momentum balances are studied. It is shown that geostrophy holds over most of the area and that the Pacific-Indian Ocean pressure difference is essentially balanced by the resultant of pressure forces acting on the bottom.
Burnett, W. H., V. M. Kamenkovich, G. L. Mellor and A. L. Gordon, 2000: The influence of the pressure head on the Indonesian Seas circulation. Geophysical Research Letters, 27(15): 2273-2276.
A high resolution, regional, non-linear, barotropic ocean model (2D POM) was used to show that a pressure difference between the Pacific and Indian Ocean does not significantly influence the total transport of the Indonesian throughflow.
Camargo, S. J., M. K. Tippett and I. L. Caldas, 2000: Nonmodal energetics of electromagnetic drift waves. Physics of Plasmas, 7(7): 2849-2855.
The linear properties of an electromagnetic drift-wave model are examined. The linear system is non-normal in that its eigenvectors are not orthogonal with respect to the energy inner product. The non-normality of the linear evolution operator can lead to enhanced finite-time growth rates compared to modal growth rates. Previous work with an electrostatic drift-wave model found that nonmodal behavior is important in the hydrodynamic limit. Here, similar behavior is seen in the hydrodynamic regime even with the addition of magnetic fluctuations. However, unlike the results for the electrostatic drift-wave model, nonmodal behavior is also important in the adiabatic regime with moderate to strong magnetic fluctuations. (C) 2000 American Institute of Physics. [S1070-664X(00)02707-5].
Carbotte, S. M., A. Solomon and G. Ponce-Correa, 2000: Evaluation of morphological indicators of magma supply and segmentation from a seismic reflection study of the East Pacific Rise 15 degrees 30 '-17 degrees N. Journal of Geophysical Research-Solid Earth, 105(B2): 2737-2759.
Hydrosweep bathymetry and multichannel seismic reflection data from two contrasting: segments of the East Pacific Rise 15 degrees 30'-17 degrees N are used to assess the relationship between crustal structure, morphological indicators of magma supply, and ridge segmentation. From stacked and migrated seismic profiles we evaluate the width and depth of the axial ma,nma lens and the geometry of the seismically inferred extrusive crust, layer 2A. In contrast to other studies, correlation between axial morphology and magma lens characteristics is found with a lens that, on average, is twice as wide and slightly shallower (100-200 m) beneath the unusually shallow and::broad southern segment and that shoals and broadens beneath the shallowest part of the northern segment. However, large local-scale variations in lens depth and width are also observed that are not related to morphology. We conclude that magma supply:contributes to the regional characteristics of the magma lens beneath a ridge segment, with:large variations within a segment due to local processes of magma eruption and delivery. A negative correlation between zero-age extrusive layer thickness and morphology is found with a thinner extrusive layer along the southern segment. in this innermost axial region above the magma lens,:factors such as magma pressure, not magma volume in the lens, may govern the thickness of extrusives that accumulate, The width of the zone over which the extrusive layer is built is positively correlated with morphology with a wide zone where extrusives approximately triple in thickness characterizing the southern segment (5-8 km), and a narrower zone (2-5 km) of minor thickening (less than double) along the northern segment and toward ridge segment ends. This relationship could reflect changes in lava flow characteristics in these areas of contrasting magma supply. We find a close correspondence between discontinuities in the narrow axial summit trough and:changes in magma lens presence and geometry, indicating a genetic link between the finest-scale tectonic segmentation of the ridge and segmentation of the magma lens. In several locations, discrete magma lenses at different levels within the crust (offset by several hundred meters) are imaged beneath adjacent fourth-order ridge segments.
Chen, D., M. A. Cane, S. E. Zebiak, R. Canizares and A. Kaplan, 2000: Bias correction of an ocean-atmosphere coupled model. Geophysical Research Letters, 27(16).
A serious problem in the initialization of a climate forecast model is the model-data incompatibility caused by systematic model biases. Here we use the Lament model to demonstrate that these biases can be effectively reduced with a simple statistical correction, and the bias-corrected model can have a more realistic internal variability as well as an improved forecast performance. The results reported here should be of practical use to other ocean-atmosphere coupled models for climate prediction.
Chiang, J. C. H. and S. E. Zebiak, 2000: Surface wind over tropical oceans: Diagnosis of the momentum balance, and modeling the linear friction coefficient. Journal of Climate, 13(10): 1733-1747.
Previous diagnostic studies of surface wind momentum balances over tropical oceans showed that, under a linear friction assumption, the meridional friction coefficient is 2-3 times larger than the zonal friction coefficient, and that both friction coefficients exhibit a pronounced meridional dependence. Our diagnosis of a global marine surface dataset confirms these results. Furthermore, we show that to first approximation the friction coefficients are independent of longitude and season in the tropical band between ~20S and ~20N. Polewards of 20N and 20S, the coefficients are no longer solely a function of latitude. To explain these empirical results, we formulate a simple analytical model of the friction coefficient based on the simplest K-theory mixed layer parameterization, assuming constant viscosity. The model does a good job of reproducing the observed zonal friction coefficient, but does poorly for meridional friction. The poor result is thought to be from model sensitivity to the specified PBL thickness. By reversing the calculation, using observed meridional friction coefficients and assuming no meridional winds at PBL top, we derived model PBL heights that compared favorably with zonally averaged inversion heights for June-August over the tropical Atlantic. Our model suggests that both coefficients increase away from the equator because of the decrease in PBL thickness. Furthermore, the zonal friction coefficient is smaller than the meridional coefficient because strong zonal winds at the top of the boundary layer mixes down, reducing the retarding influence of surface zonal momentum fluxes. Our results also suggest that the boundary layer top winds and height are important components in modeling surface winds over the tropical oceans.
Chiang, J. C. H., Y. Kushnir and S. E. Zebiak, 2000: Interdecadal changes in eastern Pacific ITCZ variability and its influence on the Atlantic ITCZ. Geophysical Research Letters, 27(22): 3687-3690.
The eastern Pacific and Atlantic Intertropical Convergence Zones (ITCZ) exhibit the largest year-to-year variations in boreal spring. We show evidence suggesting that Atlantic ITCZ April-May variability is linked to that for the eastern Pacific through the Walker circulation as it respond to changes in equatorial Pacific convection. Analysis of ITCZ proxy indices shows the link appears to be strong in the 1080-90's and 1920-30's but virtually nonexistent in the 1950-60's. We argue that this apparent nonstationarity results from the nonlinear relationship between sea surface temperature (SST) and convection in the eastern equatorial Pacific and its consequent effect on the Walker circulation and the Atlantic ITCZ. This mechanism was modulated over 1856-1998 by interdecadal changes in peak SST attained in the eastern equatorial Pacific during boreal spring.
Chou, C. B. and H. P. Huang, 2000: Application of AVHRR data to a one-dimensional variational retrieval scheme for cloudy TOVS data. Monthly Weather Review, 128(11): 3867-3878.
The use of the Advanced Very High Resolution Radiometer (AVHRR) data in a one-dimensional variational scheme is examined to retrieve cloud parameters and atmospheric profiles. The variational scheme used TIROS Operational Vertical Sounder radiance data for retrieval. The AVHRR data were used in the partly cloudy and cloudy cases to provide initial guesses for cloud parameters in the iterative scheme, to detect the presence of cirrus clouds, and to determine the sea surface temperature used in retrieval. Sensitivity tests showed that the error in the initial guesses of cloud parameters has substantial impact on the accuracy of the retrieved fields; this sensitivity increases with increased cloudiness. Cloud parameters deduced from AVHRR data are nearly optimal, in terms of maximizing the efficiency of convergence, as the initial guesses for the retrieval scheme. In the absence of cirrus cloud, a retrieval procedure incorporating AVHRR initial guesses produced temperature and humidity profiles for partly cloudy cases that are about as accurate as those for clear cases. In both cases the maximum improvement made in the retrieval procedure over background error was about 0.2 K in the temperature profile, and 0.05 tin logarithm of mixing ratio) in the humidity profile. For partly cloudy cases, best retrieval results were obtained for a low cloud top, or a middle cloud top bur with small cloud fraction. Cirrus cloud remains a problem, as its presence generally degrades the quality of retrieval.
Clement, A. C., R. Seager and M. A. Cane, 2000: Suppression of El Niño during the mid-Holocene by changes in the Earth's orbit. Paleoceanography, 15(6): 731-737.
A number of recent reports have interpreted paleoproxy data to describe the state of the tropical Pacific, especially changes in the behavior of the El Niño-Southern Oscillation (ENSO), over the Holocene. These interpretations are often contradictory, especially for the eastern tropical Pacific and adjacent areas of South America. Here we suggest a picture of the mid-Holocene tropical Pacific region which reconciles the data. ENSO variability was present throughout the Holocene but underwent a steady increase from the mid-Holocene to the present. In the mid-Holocene, extreme warm El Niño events were smaller in amplitude and occurred less frequently about a mean climate state with a cold eastern equatorial Pacific and largely arid coastal regions as in the present climate. This picture emerges from an experiment in which a simple numerical model of the coupled ocean-atmosphere system in the tropical Pacific was driven by orbital forcing. We suggest that the observed behavior of the tropical Pacific climate over the mid- to late Holocene is largely the response to orbitally driven changes in the seasonal cycle of solar radiation in the tropics, which dominates extratropical influences.
Cullather, R. I., D. H. Bromwich and M. C. Serreze, 2000: The atmospheric hydrologic cycle over the Arctic basin from reanalyses. Part I: Comparison with observations and previous studies. Journal of Climate, 13(5): 923-937.
The atmospheric moisture budget is evaluated for the region 70 degrees N to the North Pole using reanalysis datasets of the European Centre for Medium-Range Weather Forecasts (ECMWF; ERA: ECMWF Re-Analysis) and the collaborative effort of the National Centers for Environmental Prediction (NCEP) and the National Center for Atmospheric Research (NCAR). For the forecast fields of the reanalyses, the ERA annually averaged P - E (precipitation minus evaporation/sublimation) field reproduces the major features of the basin perimeter as they are known, while the NCEP-NCAR reanalysis forecast fields contain a spurious wave pattern in both P and E. Comparisons between gauge data from Soviet drift camp stations and forecast P values of the reanalyses show reasonable agreement given the difficulties (i.e., gauge accuracy, translating location). When averaged for 70 degrees-90 degrees N, the ERA and NCEP-NCAR forecast P - E are similar in the annual cycle. Average reanalysis forecast values of E for the north polar cap are found to be 40% or more too large based on comparisons using surface latent heat flux climatologies.
Ducharne, A., R. D. Koster, M. J. Suarez, M. Stieglitz and P. Kumar, 2000: A catchment-based approach to modeling land surface processes in a general circulation model 2. Parameter estimation and model demonstration. Journal of Geophysical Research-Atmospheres, 105(D20): 24823-24838.
The viability of a new catchment-based land surface model (LSM) developed for use with general circulation models is demonstrated. First, simple empirical functions, tractable enough for operational use in the LSM, are established that faithfully capture the control of topography on the subgrid variability of soil moisture and the surface water budget, as predicted by theory. Next, the full LSM is evaluated off-line. Using forcing and validation data sets developed for the Project for Intercomparison of Land-Surface Parameterization Schemes Phase 2c, the minimally calibrated model is shown to reproduce observed evaporation and runoff fluxes successfully in the Red-Arkansas River Basin. A complementary idealized study that employs the range of topographic variability seen over North America demonstrates that the simulated surface water budget does vary strongly with topography, which can, by itself, induce variations in annual runoff as high as 20%.
Eshel, G., M. A. Cane and B. F. Farrell, 2000: Forecasting eastern Mediterranean droughts. Monthly Weather Review, 128(10): 3618-3630.
A dynamically motivated statistical forecasting scheme for eastern Mediterranean winter rainfall is presented. The scheme is based on North Atlantic sea level pressure precursors. The resulting forecasts are robust and statistically significant at similar to 13 months lead time, and improve at similar to7 months lead. It is suggested that these Forecasts form a foundation for an operational early-warning system for eastern Mediterranean droughts.
Evans, M. N., A. Kaplan and M. A. Cane, 2000: Intercomparison of coral oxygen isotope data and historical sea surface temperature (SST): Potential for coral-based SST field reconstructions. Paleoceanography, 15(5): 551-563.
We examine the extent to which the large-scale features of the sea surface temperature (SST) anomaly field are represented by a sparse observational network of coral oxygen isotope (delta(18)O) time series. Regression of annually averaged delta(18)O data against gridded estimates of local SST anomaly within the period 1856-1990 confirm the literature regression of delta(18)O anomaly on SST anomaly for Indo-Pacific corals. However, while interannual SST variability is generally well represented by individual coral time series, observed decadal and secular variability does not always display a linear relationship to local SST anomaly. Instead, many records appear to better recover nonlocal, large-scale phenomena, which in turn are related to either the coral local SST or SST covariant changes in seawater delta(18)O. We employ empirical orthogonal function (EOF) analysis to identify common patterns of variability in the coral data. We find two significant patterns which are interpretable as the oceanographic signature of the El Niño-Southern Oscillation (ENSO) and as a near-global warming in which the eastern equatorial Pacific cools. A third pattern weakly resembles the Pacific Decadal Oscillation. These modes are seen more clearly in a singular Vector decomposition (SVD) of the covariance between the coral data and the dominant patterns of large-scale historical SST variability. The results are consistent with those found in EOF and SVD analyses of SST data from the coral locations. As additional coral-based proxy estimates become available, they will improve the resolution of the patterns recovered. These results suggest that a sparse network of coral data may be used to reconstruct interannual, secular, and decadal SST variability for preinstrumental periods, albeit with large uncertainty.
Ffield, A., K. Vranes, A. L. Gordon, R. D. Susanto and S. L. Garzoli, 2000: Temperature variability within Makassar Strait. Geophysical Research Letters, 27(2): 237-240.
Recent mooring observations of ocean temperature provide the first high resolution, long term record of temperature variability in the Makassar Strait of the Indonesian Seas. The mooring observations span the entire cycle of the strong 1997/1998 El Niño. A high correlation (r = 0.67) is found between variability in the average thermocline temperature, to variability in the southward Makassar volume transport: during high (low) volume transport, the average temperature of the thermocline is also high(low). In addition, from nearly 15 years of XBT data, the Makassar thermocline temperature is shown to be highly correlated (r = 0.77) to SOI. This reveals that the Makassar temperature field - when coupled with the throughflow transmits the equatorial Pacific El Niño and La Niña temperature fluctuations into the Indian Ocean. The ENSO variability in the internal energy transport is calculated: 0.63 PW during the La Niña months of December 1996 through February 1997, and 0.39 PW during the El Niño months of December 1997 through February 1998.
Giannini, A., Y. Kushnir and M. A. Cane, 2000: Interannual variability of Caribbean rainfall, ENSO, and the Atlantic Ocean. Journal of Climate, 13(2): 297-311.
The large-scale ocean-atmosphere patterns that influence the interannual variability of Caribbean-Central American rainfall are examined. The atmospheric circulation over this region is shaped by the competition between the North Atlantic subtropical high sea level pressure system and the eastern Pacific ITCZ, which influence the convergence patterns on seasonal and interannual timescales.
Gordon, A. L., M. Mensch, Z. Q. Dong, W. M. Smethie and J. de Bettencourt, 2000: Deep and bottom water of the Bransfield Strait eastern and central basins. Journal of Geophysical Research-Oceans, 105(C5): 11337-11346.
Temperature, salinity, oxygen, and chlorofluorocarbon (CFC) data obtained in September 1997 define the water types ventilating the deep eastern and central basins of the Bransfield Strait. These water types are observed adjacent to Joinville Island and are clearly derived from the Weddell Sea. The eastern basin bottom water characteristics closely match that of the bottom water at 310 dbar depth similar to 50 km east of Joinville Island. The eastern basin bottom water is a simple mixture of low-salinity freezing point shelf water (65%) and relatively warm saline Weddell Deep Water (35%), with a CFC-113: CFC-11 ratio age of 8.5 years. The eastern basin bottom water shares a common origin with a weak salinity minimum observed at 1000 dbar within the central basin, though overflow from the central basin to the eastern basin floor may also occur. The bottom water within the central basin is colder and saltier with higher concentrations of oxygen and CFC-11 than that of the eastern basin. The central basin bottom water with a CFC ratio age of 7.5 years is composed of same Weddell water types that form the eastern basin bottom water plus a large contribution of high-salinity freezing point shelf water of the characteristics of water observed 10 km northeast of joinville Island and a small amount of Pacific pycnocline water. The components (and their percentage) are Weddell Deep Water (11%), low-salinity shelf (24%), high-salinity shelf (60%), to Pacific pycnocline (5%).
Huang, H. P. and P. D. Sardeshmukh, 2000: Another look at the annual and semiannual cycles of atmospheric angular momentum. Journal of Climate, 13(18): 3221-3238.
The annual variation of global atmospheric angular momentum (AAM) is dominated by its first and second harmonic components. The first, harmonic is associated with maximum global AAM in winter (December-January-February) and minimum in summer, but the second harmonic is important enough to produce a distinct secondary midwinter minimum. Locally, the second harmonic has largest amplitude in the Tropics and subtropics of the upper troposphere. At present, little is known concerning the fundamental cause of this semiannual variation. The problem is investigated here by focusing on the upper-tropospheric winds, whose angular momentum is an excellent proxy of global AAM. The annual variation of the rotational part of these winds (the part that contributes to the global AAM) is diagnosed in a nonlinear upper-tropospheric vorticity-equation model with specified horizontal wind divergence and transient-eddy forcing. The divergence forcing is the more important of the two, especially in the Tropics and subtropics, where it is associated with tropical hearing and cooling. Given the harmonics of the forcing, the model predicts the harmonics of the response, that is, the vorticity, from which the harmonics of angular momentum can then be calculated. The surprising but clear conclusion from this diagnosis is that the second harmonic of AAM arises more as a nonlinear response to the first harmonic of the divergence forcing than as a linear response to the second harmonic of the divergence forcing. This result has implications for general circulation model simulations of semiannual variations, not only of global AAM but also of other quantities.
Huang, R. X., M. A. Cane, N. H. Naik(Henderson) and P. Goodman, 2000: Global adjustment of the thermocline in response to deepwater formation. Geophysical Research Letters, 27(6): 759-762.
The global adjustment of the thermocline in response to deepwater formation is studied in a single-mode model on a beta-plane. The signal is carried from ocean to ocean by Kelvin waves, which travel equatorward along western boundaries, eastward across the equator, poleward at the eastern boundaries, and then eastward around the southern tip of continents into the next ocean basin. The interior is filled by Rossby waves emanating from eastern boundaries. Stronger (weaker) deepwater formation induces an upward (downward) motion of the main thermocline in the world oceans. The adjustment is completed on centennial time scales.
Israeli, M., N. H. Naik(Henderson) and M. A. Cane, 2000: An unconditionally stable scheme for the shallow water equations. Monthly Weather Review, 128(3): 810-823.
A finite-difference scheme for solving the linear shallow water equations in a bounded domain is described. Its time step is not restricted by a Courant-Friedrichs-Levy (CFL) condition. The scheme, known as Israeli-Naik-Cane (INC), is the offspring of semi-Lagrangian (SL) schemes and the Cane-Patton (Cr) algorithm. In common with the latter ii treats the shallow water equations implicitly in y and with attention to wave propagation in x. Unlike CP, it uses an SL-like approach to the zonal variations, which allows the scheme to apply to the full primitive equations. The great advantage, even in problems where quasigcostrophic dynamics are appropriate in the interior, is that the INC scheme accommodates complete boundary conditions.
Joseph, R., M. F. Ting and P. Kumar, 2000: Multiple-scale spatio-temporal variability of precipitation over the coterminous United States. Journal of Hydrometeorology, 1(5): 373-392.
The spatio-temporal variability of precipitation over the United States using a 30-yr, gridded hourly precipitation dataset is studied. Orthogonal wavelet transform is applied to the time series at each grid box to capture the temporal scales of fluctuation at 17 different timescales ranging from 2 h to 15 yr. Rotated principal component analysis is then applied to the transformed series to identify spatial coherence of the temporal scales of fluctuations. The results indicate that the energy of the fluctuations shows an approximate power-law relationship with respect to scale in most regions. The spatial organization of the temporal variability shows coherence at distinct scales identified as the subdiurnal (2-16 h), synoptic (16 h-22 days), seasonal (42 days-1 yr), and climatic mode (15 yr). The synoptic scale explains the largest spatial variance of the fluctuations in precipitation and is spatially coherent; the subdiurnal mode is spatially less coherent. The seasonal mode is dominant over the Pacific Northwest, whereas the climatic mode has large amplitude only over California. When examining the winter and summer seasons separately, it is found that the winter precipitation fluctuation is more associated with synoptic scale; the summer fluctuation is associated with shorter timescales or the subdiurnal scale. Studies of extreme summer drought and hood events over the Midwest indicate that anomalously wet or dry years are manifestations of persistent anomalous wet or dry conditions across all temporal scales, with the maximum contribution for the wet events being affected by the synoptic-scale activities.
Kaplan, A., Y. Kushnir and M. A. Cane, 2000: Reduced space optimal interpolation of historical marine sea level pressure: 1854-1992. Journal of Climate, 13(16): 2987-3002.
Near-global 4 degrees x 4 degrees gridded analysis of marine sea level pressure (SLP) from the Comprehensive Ocean-Atmosphere Data Set for monthly averages from 1854 to 1992 was produced along with its estimated error using a reduced space optimal interpolation method. A novel procedure of covariance adjustment brought the results of the analysis to the consistency with the a priori assumptions on the signal covariance structure. Comparisons with the National Centers for Environmental Prediction-National Center for Atmospheric Research global atmosphere reanalysis, with the National Center for Atmospheric Research historical analysis of the Northern Hemisphere SLP, and with the global historical analysis of the U.K. Meteorological Office show encouraging skill of the present product and identifies noninclusion of the land data as its main limitation. Marine SLP pressure proxies are produced for the land stations used in the definitions of the Southern Oscillation and North Atlantic Oscillation (NAO) indices. Surprisingly, they prove to be competitive in quality with the land station records. Global singular value decomposition analysis of the SLP fields versus sea surface temperature identified three major patterns of their joint large-scale and long-term variability as "trend," Pacific decadal oscillation, and NAO.
Khatiwala, S. and M. Visbeck, 2000: An estimate of the eddy-induced circulation in the Labrador Sea. Geophysical Research Letters, 27(15): 2277-2280.
Regions of oceanic deep convection such as the Labrador Sea are prone to baroclinic instability. The resulting geostrophic eddies play a crucial role in the post-convection adjustment process which involves both rearrangement of mass so as to release available potential energy and exchange of heat and salt with the boundaries. In this study it is proposed that the slumping of isopycnals associated with baroclinic instability drives an eddy-induced "overturning circulation" consisting of a surface intensified flow transporting low salinity water from the boundary currents into the interior; sinking motion in the interior; and an "outflow" at depth transporting newly ventilated Labrador Sea Water towards the boundaries. Typical eddy-induced velocities estimated from hydrographic data are roughly 0.5 cm/s for the surface inflow, 1 m/day for the vertical motion, and 0.1 cm/s for the deeper outflow, in close agreement with those calculated in a numerical model.
Kleeman, R., N. H. Naik(Henderson) and M. A. Cane, 2000: Meridional location of the Pacific ocean subtropical gyre. Journal of Physical Oceanography, 30(8): 1988-2000.
The observed subtropical gyre in the North Pacific shows a shift in meridional location with depth. At shallow levels the density deviation peaks at around 15 degrees N while at deep levels the peak is more like 30 degrees N. It is argued here using analytical solutions to the beta-plane shallow-water equations that such a shift can be explained by the effects of oceanic dissipation processes, These solutions show that the highly damped solution is approximately proportional to Ekman pumping whereas the lightly damped case tends toward the classical Sverdrup solution. In the North Pacific, Ekman pumping peaks near 15 degrees N while the Sverdrup solution peaks at 30 degrees N. It is further demonstrated that 1) density deviations in the upper ocean are more highly influenced by higher order baroclinic modes than those in the deep, which are influenced by the lower modes, and 2) constant dissipation effectively acts much more strongly on the higher order baroclinic modes because of their slower speeds and smaller Rossby radii. These two factors thus explain the observed shift in the gyre with depth.
Koster, R. D., M. J. Suarez, A. Ducharne, M. Stieglitz and P. Kumar, 2000: A catchment-based approach to modeling land surface processes in a general circulation model 1. Model structure. Journal of Geophysical Research-Atmospheres, 105(D20): 24809-24822.
A new strategy for modeling the land surface component of the climate system is described. The strategy is motivated by an arguable deficiency in most state-of-the-art land surface models, namely, the disproportionately higher emphasis given to the formulation of one-dimensional, vertical physics relative to the treatment of horizontal heterogeneity in surface properties, particularly subgrid soil moisture variability and its effects on runoff generation. The new strategy calls for the partitioning of the continental surface into a mosaic of hydrologic catchments, delineated through analysis of high-resolution surface elevation data. The effective "grid" used for the land surface is therefore not specified by the overlying atmospheric grid. Within each catchment, the variability of soil moisture is related to characteristics of the topography and to three bulk soil moisture variables through a well-established model of catchment processes. This modeled variability allows the partitioning of the catchment into several areas representing distinct hydrological regimes, wherein distinct (regime specific) evaporation and runoff parameterizations are applied. Care is taken to ensure that the deficiencies of the catchment model in regions of little to moderate topography are minimized.
Krahmann, G., F. Schott and U. Send, 2000: Seasonal heat content changes in the western Mediterranean Sea as a means for evaluating surface heat flux formulations. Journal of Geophysical Research-Oceans, 105(C7): 16941-16950.
The semienclosed western Mediterranean Sea has proven to be a useful location to evaluate surface heat flux estimates. In the past the directly measured average oceanic heat transport from the Atlantic into the Mediterranean Sea through the Strait of Gibraltar of similar to 5.2 +/- 1.3 W m(-2) has been compared to estimates of the average heat flux across the surface of the Mediterranean Sea. On long timescales both should closely balance each other. By using a monthly temperature climatology of the western Mediterranean Sea we offer the possibility to extend the comparison to the seasonal timescale. This gives additional information with which different surface heat flux data sets can be evaluated. The seasonal heat content changes of the western Mediterranean and the advective exchange of heat through the Straits of Gibraltar and Sicily are estimated on the basis of a new extensive hydrographic data set and of published values for the volume transports. To demonstrate the method, a limited number of surface heat flux data sets are compared with the oceanographically calculated counterpart. The comparison reveals that some heat fluxes do not only agree well for the long-term averages but also for the seasonal timescale, whereas others show larger deviations. The remaining rms discrepancies of +/-10.2 W m(-2) for the best heat flux data set are smaller than the uncertainty of the oceanographic estimate and of a reasonable magnitude compared to the uncertainty of the long-term average of similar to 5 W m(-2).
Lynch, A. H. and R. I. Cullather, 2000: An investigation of boundary-forcing sensitivities in a regional climate model. Journal of Geophysical Research-Atmospheres, 105(D21): 26603-26617.
The sensitivity of a regional climate model to lateral, upper, and lower dynamical forcing is evaluated for a pan-Arctic domain. The experiments focus on a month-long case study of May 1990, but the results are shown to be extendable to January and July 1990. Although the control simulation is found to adequately represent major tropospheric circulation features, a large negative geopotential height bias of up to 700 m is found at the upper levels of the model. Experiments find these biases are remedied through the replacement of a reflective rigid lid upper boundary condition with a local wave-absorbing layer. Geopotential height biases are reduced to a maximum underestimation of only 50 m at the upper boundary. Substantial improvement is also found in the simulated temperature and zonal wind profiles, transient heat and momentum fluxes, and stationary eddies as seen through the Eliassen-Palm flux divergence. With this wave-absorbing upper boundary condition in place, additional improvement is found using a gravity wave drag parameterization appropriate for this domain's 100 km horizontal resolution. Additional sensitivity to lateral forcing is examined, and the relative role of the upper boundary condition to skillful simulations within the Arctic basin domain is discussed.
Maslowski, W., B. Newton, P. Schlosser, A. Semtner and D. Martinson, 2000: Modeling recent climate variability in the Arctic Ocean. Geophysical Research Letters, 27(22): 3743-3746.
Dramatic changes in the circulation of sea ice and the upper layers of the Arctic Ocean have been reported during the last decade. Similar variability is modeled using a regional, coupled ice-ocean model. Realistic atmospheric forcing fields for 1979-93 are the only interannual signal prescribed in the model. Our results show large-scale changes in sea, ice and oceanic conditions when comparing results for the late 1970s / early 1980s and the 1990s. We hypothesize that these changes are in response to even larger scale atmospheric variability in the Northern Hemisphere that can be defined as either the Arctic Oscillation or the North Atlantic Oscillation. Agreement between the direction and scale of change in the model and observations, in the absence of interannual forcing from the global ocean thermohaline circulation, suggests that the atmospheric variability by itself is sufficient to produce basin-scale changes in the Arctic Ocean and sea ice system.
Moritz, R. E. and J. Ukita, 2000: Geometry and the deformation of pack ice: I. A simple kinematic model. Annals of Glaciology, 31: 313-322.
The deformation of pack ice is modeled as the discrete motion of rigid plates. A continuous and differentiable field of large-scale velocity is sampled at the center point of each plate to determine its uniform translation. Discontinuities in the ice velocity occur at the cracks separating pairs of adjacent plates. Ice deformation that is characterized by opening, ridging and sliding coefficients is computed directly by integrating the velocity jumps over the length of each crack, and summing over all cracks in a representative area. These coefficients depend on the large-scale strain rate and the geometry of the cracks. The relevant geometric parameters are the orientations of (a) the cracks with respect to the principal axis of the strain rate, and (b) the cracks with respect to the relative position vectors between the center points of adjacent plates on either side of the crack. For all tilings of uniform, equilateral diamonds (including squares) the opening and ridging are minimized, and the sliding is maximized, when an axis of symmetry of the plate coincides with the principal axis of the strain rate.
Navarra, A., M. Biasutti, S. Gualdi, E. Roeckner, U. Schlese and U. Shulzweida, 2000: Sensitivity experiments to mountain representations in spectral models. Annali Di Geofisica, 43(3): 559-584.
This paper describes a set of sensitivity experiments: to several formulations of orography. Three: sets are considered: a "Standard" orography consisting of an envelope orography produced originally for the ECMWF model, a "Navy" orography directly from the US Navy data and a "Scripps" orography based on the data set originally compiled several years ago at Scripps. The last two are mean orographies which do not use the envelope enhancement. A new filtering technique for handling the problem of Gibbs oscillations in spectral models has been used to produce the "Navy" and "Scripps" orographies, resulting in smoother fields than the "Standard" orography. The sensitivity experiments show that urography is still an important factor in controlling the model performance even in this class of models that use a semi-lagrangian formulation for water vapour, that in principle should be less sensitive to Gibbs oscillations than the Eulerian formulation. The largest impact can be seen in the stationary waves (asymmetric part of the geopotential at 500 mb) where the differences in total height and spatial pattern generate up to 60 In differences, and in the surface fields where the Gibbs removal procedure is successful in alleviating the appearance of unrealistic oscillations over the ocean. These results indicate that Gibbs oscillations also need to be treated in this class of models. The best overall result is obtained using the "Navy" data set, that achieves a good compromise between amplitude of the stationary waves and smoothness of the surface fields.
Orlove, B. S., J. C. H. Chiang and M. A. Cane, 2000: Forecasting Andean rainfall and crop yield from the influence of El Niño on Pleiades visibility. Nature, 403(6765): 68-71.
Farmers in drought-prone regions of Andean South America have historically made observations of changes in the apparent brightness of stars in the Pleiades around the time of the southern winter solstice in order to forecast interannual variations in summer rainfall and in autumn harvests. They moderate the effect of reduced rainfall by adjusting the planting dates of potatoes, their most important crop(1). Here we use data on cloud cover and water vapour from satellite imagery, agronomic data from the Andean altiplano and an index of El Nino variability to analyse this forecasting method. We find that poor visibility of the Pleiades in June-caused by an increase in subvisual high cirrus clouds-is indicative of an El Nino year, which is usually linked to reduced rainfall during the growing season several months later. Our results suggest that this centuries-old method? of seasonal rainfall forecasting may be based on a simple indicator of El Nino variability.
Ou, H. W., 2000: A model of tidal rectification by potential vorticity mixing. Part II: Frontal regime. Journal of Physical Oceanography, 30(3): 564-571.
A model of tidal rectification in a homogeneous ocean (Part I) is extended here to include a front that separates shelf and slope waters. The front is approximated as a density discontinuity, the stratification and anchoring position of which are given, but which otherwise is coupled to the flow field. The dynamical closure is formulated through vorticity balance of the two layers and a parameterization of potential vorticity (PV) flux in terms of local tidal amplitude and the mean field.
Ou, H. W., C. M. Dong and D. Chen, 2000: On the tide-induced property flux: Can it be locally countergradient? Journal of Physical Oceanography, 30(6): 1472-1477.
The horizontal property flux induced by tides is examined by both analytical and numerical models. It is found that this flux is highly heterogeneous in the vertical and may be directed up the mean gradient near the bottom. This countergradient tidal flux is a consequence of differing boundary conditions satisfied by velocity and property fields, and hence a robust feature. The corresponding tidal diffusivity is substantial where tides are strong and hence potentially important in the mean property balance.
Rodgers, K. B., D. P. Schrag, M. A. Cane and N. H. Naik(Henderson), 2000: The bomb C-14 transient in the Pacific Ocean. Journal of Geophysical Research-Oceans, 105(C4): 8489-8512.
A modeling study of the bomb C-14 transient is presented for the Pacific Ocean. A primitive equation ocean circulation model has been configured for a high resolution domain that accounts for the Indonesian Throughflow (ITF). Four separate runs were performed: (1) seasonal forcing with 20 Sv of ITF transport, (2) seasonal forcing with 10 Sv of ITF transport, (3) seasonal forcing with no ITF transport, and (4) interannual forcing with 15 Sv of ITF transport. This study has two main objectives. First, it is intended to describe the time evolution of the bomb C-14 transient. This serves as a tool with which one can identify the physical processes controlling the evolving bomb C-14 distribution in the Pacific thermocline and thus provides an interpretive framework for the database of Delta(14)C measurements in the Pacific. Second, transient tracers are applied to the physical oceanographic problem of intergyre exchange. This is of importance in furthering our understanding of the potential role of the upper Pacific Ocean in climate variability. We use bomb C-14 as a dye tracer of intergyre exchange between the subtropical gyres and the equatorial upwelling regions of the equatorial Pacific. Observations show that while the atmospheric Delta(14)C signal peaked in the early to mid-1960s, the Delta(14)C levels in the surface water waters of the subtropical gyres peaked near 1970, and the Delta(14)C of surface waters in the equatorial Pacific continued to rise through the 1980s. It is shown that the model exhibits skill in representing the large-scale observed features observed for the bomb C-14 transient in the Pacific Ocean. The model successfully captures the basin-scale inventories of bomb C-14 in the tropics as well as in the extratropics of the North Pacific. For the equatorial Pacific this is attributed to the model's high meridional resolution. The discrepancies in the three-dimensional distribution of bomb C-14 between the model and data are discussed within the context of the dynamical controls on the Delta(14)C distribution of bomb C-14 in the Pacific.
Seager, R., A. C. Clement and M. A. Cane, 2000: Glacial cooling in the tropics: Exploring the roles of tropospheric water vapor, surface wind speed, and boundary layer processes. Journal of the Atmospheric Sciences, 57(13): 2144-2157.
This paper is a modeling study of possible roles for tropospheric water vapor, surface wind speed, and boundary layer processes in glacial cooling in the Tropics. The authors divide the Tropics into a region of persistent deep convection and a subtropical region with no deep convection. The regions are coupled via a radiatively driven Hadley cell and a wind-driven meridional overturning cell in the ocean. Radiation and the convective boundary layer (CBL) are treated in some detail.
Seager, R., Y. Kushnir, M. Visbeck, N. H. Naik(Henderson), J. Miller(Nakamura), G. Krahmann and H. Cullen, 2000: Causes of Atlantic Ocean climate variability between 1958 and 1998. Journal of Climate, 13(16): 2845-2862.
Numerical experiments are performed to examine the causes of variability of Atlantic Ocean SST during the period covered by the National Centers for Environmental Prediction-National Center for Atmospheric Research (NCEP-NCAR) reanalysis (1958-98). Three ocean models are used. Two are mixed layer models: one with a 75-m-deep mixed layer and the other with a variable depth mixed layer. For both mixed layer models the ocean heat transports are assumed to remain at their diagnosed climatological values. The third model is a full dynamical ocean general circulation model (GCM). All models are coupled to a model of the subcloud atmospheric mixed layer (AML). The AML model computes the air temperature and humidity by balancing surface fluxes, radiative cooling, entrainment at cloud base, advection and eddy heat, and moisture transports. The models are forced with NCEP-NCAR monthly mean winds from 1958 to 1998.
Sprintall, J., A. L. Gordon, R. Murtugudde and R. D. Susanto, 2000: A semiannual Indian Ocean forced Kelvin wave observed in the Indonesian seas in May 1997. Journal of Geophysical Research-Oceans, 105(C7): 17217-17230.
Recent observations within the Indonesian exit passages and internal seas highly resolve the arrival and passage of a semiannual Kelvin wave. In mid-May 1997, surface and subsurface currents were to the southeast at a mooring located south of Java in the South Java Current, while local wind forcing was northwestward. Subsequent northward fluctuations in the geostrophic current through Lombok Strait and in observed currents from two moorings located in Makassar Strait are commensurate with the speed and passage of a Kelvin wave through the region. The Kelvin wave was due to westerly wind forcing in the remote equatorial Indian Ocean during the semiannual April/May monsoon transition period. This was confirmed through a simple remote wind-forced analytical Kelvin wave model of velocity at the South Java Current mooring location and sea level in Lombok Strait and also in the numerical general circulation model of Murtugudde et al. . Warm temperature anomalies measured at the south Java mooring and within Makassar Strait an associated with the passage of the Kelvin wave. Salinity anomalies measured at the south Java mooring are consistent with an Indian Ocean source. The observed passage of the Kelvin wave during May 1997 unambiguously demonstrates for the first time that equatorial Indian Ocean remote wind forcing may on occasions influence the internal Indonesian seas.
Stieglitz, M., A. Giblin, J. Hobbie, M. Williams and G. Kling, 2000: Simulating the effects of climate change and climate variability on carbon dynamics in Arctic tundra. Global Biogeochemical Cycles, 14(4): 1123-1136.
Through a simple modeling exercise, three mechanisms have been identified, each operating at a different timescale, that may govern carbon dynamics in Arctic tundra regions and partially explain observed CO2 flux variability. At short timescales the biosphere reacts to meteorological forcing. Drier conditions are associated with aerobic soil decomposition, a large CO2 efflux, and a net ecosystem loss of carbon. Cooler and moister conditions favor slower anaerobic decomposition in soils, good growing conditions, and terrestrial carbon sequestration. At intermediate timescales, periods of terrestrial carbon loss are directly linked to periods of carbon sequestration by the ability of the ecosystem to retain labile nitrogen. Labile nitrogen released to the soil during periods when the tundra is a source of carbon (soil respiration > net primary productivity) is retained within the ecosystem and accessed during periods when carbon sequestration is favored (net primary productivity > soil respiration). Finally, the ability of Vegetation to respond to longterm changes in soil nutrient status via changes in leaf nitrogen and leaf area index modulates this dynamic at intermediate to long timescales.
Susanto, R. D., A. L. Gordon, J. Sprintall and B. Herunadi, 2000: Intraseasonal variability and tides in Makassar Strait. Geophysical Research Letters, 27(10): 1499-1502.
Intraseasonal variability and tides along the Makassar Strait, the major route of Indonesian throughflow, are investigated using spectral and time-frequency analyses which are applied to sea level, wind and mooring data. Semidiurnal and diurnal tides are dominant features, with higher (lower) semidiurnal (diurnal) energy in the north compared to the south. Sea levels and mooring data display intraseasonal variability which are probably a response to remotely forced Kelvin waves from the Indian Ocean through Lombok Strait and to Rossby waves from the Pacific Ocean. Sea levels in Tarakan and Balikpapan and Makassar mooring velocities reveal intraseasonal features with periods of 48-62 days associated with Rossby waves from the Sulawesi Sea. Kelvin wave features with periods of 67-100 days are seen in Pall (Lombok Strait), at the mooring sites and in Balikpapan, however, they are not seen in Tarakan, which implies that these waves diminish after passing through the Makassar Strait.
Ukita, J. and R. E. Moritz, 2000: Geometry and the deformation of pack ice: II. Simulation with a random isotropic model and implication in sea-ice rheology. Annals of Glaciology, 31: 323-326.
In this paper, we extend the analysis of geometry and deformation of pack ice initiated in part I by considering random isotropic geometry using the Poisson line process. The model is used to estimate opening; ridging and sliding coefficients for more realistic geometry than the idealized simple and reg;lar geometry considered in part I. We then derive the shape of yield curves by applying minimization of the maximum shear stress to a linear combination of the estimated ridging and sliding coefficients. It is found that isotropic crack geometry results in a sine-lens shape for the yield curve if sliding makes no contribution to the energy dissipation. By contrast, when sliding contributes, the shape of the yield curve becomes teardropped. These results suggest the presence of a consistent relationship between large-scale characterization of inter-flee interactions and small-scale (crack and lead) ridging processes.
Ukita, J., T. Kawamura, N. Tanaka, T. Toyota and M. Wakatsuchi, 2000: Physical and stable isotopic properties and growth processes of sea ice collected in the southern Sea of Okhotsk. Journal of Geophysical Research-Oceans, 105(C9): 22083-22093.
This study presents physical and stable isotopic properties of sea ice in the southernmost section of the Sea of Okhotsk on the basis of observations and measurements made in 1995 and 1996 midwinter cruises. Structural analysis revealed that ice was predominantly of the granular type. Saucer-shaped cores and wedges were seen in the cross sections of the pancakes, which indicate a rafting event in the early stage of the pancake formation. The fraction of snow with respect to the total ice mass was estimated at similar to 8% from independent delta(18)O and density measurements. From these findings we conclude that the initial growth mechanisms of the sea ice over this region are by the frazil pancake cycle with rafting and by the snow ice formation. Analysis also shows that the formation of the snow ice induces similar to 10% more salt flux through the additional freezing of the seawater. In a short (synoptic) timescale this implies that snow becomes a negative (salting) rather than a positive (freshening) source for the buoyancy flux. These results suggest critical influence of the rapid formation of the frazil pancakes and snow ice under stormy conditions on the salt and buoyancy fluxes over the region.
Visbeck, M. and M. Rhein, 2000: Is bottom boundary layer mixing slowly ventilating Greenland sea deep water? Journal of Physical Oceanography, 30(1): 215-224.
Bottom water temperatures in the central Greenland Sea have been increasing for the last two decades. The warming is most likely related to the absence of deep convective mixing, which cools and freshens the deep water. However, recent observations confirm a slow and steady increase of anthropogenic tracers such as chlorofluorocarbons (CFCs). This points to some amount of bottom water "ventilation" in the absence of deep convective mixing and poses a challenge to our understanding of deep water renewal. One explanation for the observed trends in both temperature and CFCs is significant vertical mixing. The basin-averaged diapycnal diffusivity, required to explain both trends, k(v,av) similar to 2-3 (x10(-3) m(2) s(-1)), is very unlikely to occur in the interior of the ocean. However, a diffusivity of k(v,bbl) similar to 10(-2) m(2) s(-1) within a 150-m thick bottom boundary layer would be sufficient to explain the deep tracer increase. The implications of a secondary circulation driven by such large boundary layer mixing are discussed.
Wang, H. and M. F. Ting, 2000: Covariabilities of winter US precipitation and Pacific Sea surface temperatures. Journal of Climate, 13(20): 3711-3719.
The variability of winter average U.S. precipitation displays strong geographical dependence with large variability in the southeastern and northwestern United States. The covariance of the U.S. winter mean precipitation with Pacific sea surface temperature (SST) is examined in this study using the singular value decomposition (SVD) method. The first SVD mode indicates the U.S. precipitation pattern that is associated with the tropical El Niño/La Niña SST variation, while the second and third SVD modes relate the precipitation variability in the Pacific Northwest and southeast that is associated with the North Pacific SST variation. About 45% of the U.S. precipitation variabilities is related to the Pacific SST anomalies, among which, 35% is related to the North Pacific SST and 10% is related to the tropical Pacific SST. Each SVD precipitation pattern is associated with well-organized 500-mb height and zonal mean zonal wind anomalies. It is shown that the North Pacific SST anomalies associated with the U.S. precipitation are primarily driven by extratropical atmospheric circulation anomalies.
Wang, Z. R., Z. Yu and J. Song, 2000: Concentrative function and its primary application. Acta Oceanologica Sinica, Vol.19(No.3): 73-80.
In this paper, a concentrative function used in statistics is designed to present the extent in which an elementary field concentrates in the spatial distribution. Its characteristics are discussed and its availability proved. From its application in the atmospheric and oceanic sciences, we can find some interesting phenomena. For example, there is a clear seasonal concentricity in the occurrence of storms in every oceanic area in a year. The storms-dense months correspond to such an geographical place that goes around anticlockwise in the tropical oceanic areas. The spatial concentricity of SST and that of cloud change with time in a year in a similar way, that is, the higher occurrence of storms, the higher concentricity of SST and cloud, maybe the higher possibility of floods, which shows possibly that SST and its distribution play important role in the formation of cloud and that the formation of storms contributes to the concentration of the cloud.
Waworuntu, J. M., R. A. Fine, D. B. Olson and A. L. Gordon, 2000: Recipe for Banda Sea Water. Journal of Marine Research, 58(4): 547-569.
Water from the western Pacific flows through the Indonesian Seas following different pathways and is modified by various processes to form the uniquely characterized isohaline Banda Sea Water. The processes contributing to the isohaline structure are studied using data from three ARLINDO cruises in 1993, 1994, and 1996. An inverse-model analysis using salinity and CFC-11 data is applied to a vertical section along the main path of flow, from the Makassar Strait to the Flores Sea and Banda Sea. The model reproduces the seasonal and interannual variability of the throughflow and shows reversals of flow in the vertical structure. The model solutions suggest strong baroclinic flows during the southeast monsoon of 1993 and 1996 and a small, more barotropic flow during the northwest monsoon of 1994. The isohaline structure can be accounted for by isopycnal mixing of different source waters and by vertical exchanges, which are significant in this region. A downward flux equivalent to a downwelling velocity of 5 X 10(-7) m/s is estimated for the section. The total balance also suggests that seasonally and possibly interannually variable backflushing of water from the Banda Sea into the Straits contribute to the isohaline structure of Banda Sea water.
Xiao, J. B., R. Bintanja, S. J. Dery, G. W. Mann and P. A. Taylor, 2000: An intercomparison among four models of blowing snow. Boundary-Layer Meteorology, 97(1): 109-135.
Four one-dimensional, time-dependent blowing snow models are intercompared. These include three spectral models, PIEKTUK-T, WINDBLAST, SNOWSTORM, and the bulk version of PIEKTUK-T, PIEKTUK-B. Although the four models are based on common physical concepts, they have been developed by different research groups. The structure of the models, numerical methods, meteorological field treatment and the parameterization schemes may be different. Under an agreed standard condition, the four models generally give similar results for the thermodynamic effects of blowing snow sublimation on the atmospheric boundary layer, including an increase of relative humidity and a decrease of the ambient temperature due to blowing snow sublimation. Relative humidity predicted by SNOWSTORM is lower than the predictions of the other models, which leads to a larger sublimation rate in SNOWSTORM. All four models demonstrate that sublimation rates in a column of blowing snow have a single maximum in time, illustrating self-limitation of the sublimation process of blowing snow. However, estimation of the eddy diffusion coefficient for momentum (K-m), and thereby the diffusion coefficients for moisture (K-w) and for heat (K-h), has a significant influence on the process. Sensitivity tests with PIEKTUK-T show that the sublimation rate can be approximately constant with time after an initial phase, if K-m is a linear function with height. In order to match the model results with blowing snow observations, some parameters in the standard run, such as settling velocity of blowing snow particles in these models, may need to be changed to more practical values.
Yuan, X. J. and D. G. Martinson, 2000: Antarctic sea ice extent variability and its global connectivity. Journal of Climate, 13(10): 1697-1717.
This study statistically evaluates the relationship between Antarctic sea ice extent and global climate variability. Temporal cross correlations between detrended Antarctic sea ice edge (SIF) anomaly and Various climate indices are calculated. For the sea surface temperature (SST) in the eastern equatorial Pacific and tropical Indian Ocean, as well as the tropical Pacific precipitation, a coherent propagating pattern is clearly evident in all correlations with the spatially averaged (over 12 degrees longitude) detrended SIE anomalies ((SIE*)). Correlations with ENSO indices imply that up to 34% of the variance in (SIE*) is linearly related to ENSO. The (SIE*) has even higher correlations with the tropical Pacific precipitation and SST in the tropical Indian Ocean. In addition, correlation of (SIE*) with global surface temperature produces four characteristic correlation patterns: 1) an ENSO-like pattern in the Tropics with strong correlations in the Indian Ocean and North America (r > 0.6); 2) a teleconnection pattern between the eastern Pacific region of the Antarctic and western-central tropical Pacific; 3) an Antarctic dipole across the Drake Passage; and 4) meridional banding structures in the central Pacific and Atlantic expending from polar regions to the Tropics, even to the Northern Hemisphere.
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