Biasutti, M., D. S. Battisti and E. S. Sarachik, 2005: Terrestrial influence on the annual cycle of the Atlantic ITCZ in an AGCM coupled to a slab ocean model. Journal of Climate, 18(1): 211-228.
An atmospheric GCM coupled to a slab ocean model is used to investigate how temperature and precipitation over South America and Africa affect the annual cycle of the Atlantic ITCZ. The main conclusion of this study is that variations in precipitation and temperature forced by the annual cycle of insolation over the continents are as important as variations in insolation over the ocean and in ocean heat transport convergence in forcing the annual march of the Atlantic ITCZ observed in the control simulation. The processes involved are as follows.
Buckley, B. M., B. I. Cook, A. Bhattacharyya, D. Dukpa and V. Chaudhary, 2005: Global surface temperature signals in pine ring-width chronologies from southern monsoon Asia. Geophysical Research Letters, 32(20): doi:10.1029/2005GL023745.
We analyze Pinus ring width chronologies from three locations across monsoon Asia (Bhutan, India, and Thailand) where climate is dominated by the southwest monsoon in the boreal summer. We compare these records to global surface temperatures for the past 150 years, shifting the correlations through three seasonal averages: two seasons preceding the monsoon (Dec-Feb and Mar May), and the monsoon season itself (Jun-Sep). Clear patterns emerge for each of the chronologies that highlight links to areas of known influence on the Asian monsoon: the Indian Ocean, the tropical eastern Pacific Ocean, and the high-latitude Asian landmass. The Thai and Indian chronologies are from the same species (P. merkusii), and show a strong correlation with tropical Indian and Pacific Ocean bands. The Bhutan chronology (P. Wallichiana) is most strongly linked to climate over the north Pacific and Asian landmass. All of these correlations are strongest in seasons preceding the summer monsoon.
Camargo, S. J. and A. H. Sobel, 2005: Western North Pacific tropical cyclone intensity and ENSO. Journal of Climate, 18(15): 2996-3006.
The influence of the El Niño-Southern Oscillation (ENSO) on tropical cyclone intensity in the western North Pacific basin is examined. Accumulated cyclone energy (ACE), constructed from the best-track dataset for the region for the period 1950-2002, and other related variables are analyzed. ACE is positively correlated with ENSO indices. This and other statistics of the interannually varying tropical cyclone distribution are used to show that there is a tendency in El Niño years toward tropical cyclones that are both more intense and longer-lived than in La Niña years. ACE leads ENSO indices: during the peak season (northern summer and fall), ACE is correlated approximately as strongly with ENSO indices up to six months later (northern winter), as well as simultaneously. It appears that not all of this lead-lag relationship is easily explained by the autocorrelation of the ENSO indices, though much of it is. Interannual variations in the annual mean lifetime, intensity, and number of tropical cyclones all contribute to the ENSO signal in ACE, though the lifetime effect appears to be the most important of the three.
Camargo, S. J., A. G. Barnston and S. E. Zebiak, 2005: A statistical assessment of tropical cyclone activity in atmospheric general circulation models. Tellus Series a-Dynamic Meteorology and Oceanography, 57(4): 589-604.
The properties of tropical cyclones in three low-resolution atmospheric general circulation models (AGCMs) in seven ocean basins are discussed. The models are forced by prescribed, observed sea surface temperatures over a period of 40 yr, and their simulations of tropical cyclone activity are compared with observations. The model cyclone characteristics considered include genesis position, number of cyclones per year, seasonality, accumulated cyclone energy, track locations, and number of storm days. Correlations between model and observed interannual variations of these characteristics are evaluated. The models are found able to reproduce the basic features of observed tropical cyclone behavior such as seasonality, general location and interannual variability, but with identifiable biases. A bias correction is applied to the tropical cyclone variables of the three models. The three AGCMs have different levels of realism in simulating different aspects of tropical cyclone activity in different ocean basins. Some strengths and weaknesses in simulating certain tropical cyclone activity variables are common to the three models, while others are unique to each model and/or basin. Although the overall skill of the models in reproducing observed interannual variability of tropical cyclone variables has not surpassed or often even equalled that of statistical models, there exists potential for higher future skills using improved versions of dynamical approaches.
Cane, M. A., 2005: The evolution of El Niño, past and future. Earth and Planetary Science Letters, 230(3-4): 227-240.
We review forecasts of the future of El Niño and the Southern Oscillation (ENSO), a coupled instability of the ocean-atmosphere system in the tropical Pacific with global impacts. ENSO in the modem world is briefly described, and the physics of the ENSO cycle is discussed. Particular attention is given to the Bjerknes feedback, the instability mechanism which figures prominently in ENSO past and future. Our knowledge of ENSO in the paleoclimate record has expanded rapidly within the last 5 yr. The ENSO cycle is present in all relevant records, going back 130 kyr. It was systematically weaker during the early and middle Holocene, and model studies indicate that this results from reduced amplification in the late summer and early fall, a consequence of the altered mean climate in response to boreal summer perihelion. Data from corals shows substantial decadal and longer variations in the strength of the ENSO cycle within the past 1000 yr; it is suggested that this may be due to solar and volcanic variations in solar insolation, amplified by the Bjerknes feedback. There is some evidence that this feedback has operated in the 20th century and some model results indicate that it will hold sway in the greenhouse future, but it is very far from conclusive. The comprehensive general circulation models used for future climate projections leave us with an indeterminate picture of ENSO's future. Some predict more ENSO activity, some less, with the highly uncertain consensus forecast indicating little change. (C) 2004 Elsevier B.V. All rights reserved.
Cherry, J. E., L. B. Tremblay, S. J. Dery and M. Stieglitz, 2005: Reconstructing solid precipitation from snow depth measurements and a land surface model. Water Resources Research, 41(9): doi:10.1029/2005WR003965.
[ 1] The amount and distribution of snowfall in the Arctic has significant effects on global climate. However, measurements of snowfall from gauges are strongly biased. A new method is described for reconstructing snowfall from observed snow depth records, meteorological observations, and running the NASA Seasonal-to-Interannual Prediction Project Catchment Land Surface Model ( NSIPP CLSM) in an inverse mode. This method is developed and tested with observations from Reynolds Creek Experimental Watershed. Results show snowfall can be accurately reconstructed on the basis of how much snow must have fallen to produce the observed snow depth. The mean cumulative error ( bias) of the reconstructed precipitation for 11 snow seasons is 29 mm snow water equivalent ( SWE) for the corrected gauge measurement compared to -77 mm SWE for the precipitation from the corrected snow gauges. This means the root-mean-square error of reconstructed solid precipitation is 30% less than that of gauge corrections. The intended application of this method is the pan-Arctic landmass, where estimates of snowfall are highly uncertain but where more than 60 years of historical snow depth and air temperature records exist.
Clement, A. C., R. Seager and R. Murtugudde, 2005: Why are there tropical warm pools? Journal of Climate, 18(24): 5294-5311.
Tropical warm pools appear as the primary mode in the distribution of tropical sea surface temperature (SST). Most previous studies have focused on the role of atmospheric processes in homogenizing temperatures in the warm pool and establishing the observed statistical SST distribution. In this paper, a hierarchy of models is used to illustrate both oceanic and atmospheric mechanisms that contribute to the establishment of tropical warm pools. It is found that individual atmospheric processes have competing effects on the SST distribution: atmospheric heat transport tends to homogenize SST, while the spatial structure of atmospheric humidity and surface wind speeds tends to remove homogeneity. The latter effects dominate, and under atmosphere- only processes there is no warm pool. Ocean dynamics counter this effect by homogenizing SST, and it is argued that ocean dynamics is fundamental to the existence of the warm pool. Under easterly wind stress, the thermocline is deep in the west and shallow in the east. Because of this, poleward Ekman transport of water at the surface, compensated by equatorward geostrophic flow below and linked by equatorial upwelling, creates a cold tongue in the east but homogenizes SST in the west, creating a warm pool. High clouds may also homogenize the SST by reducing the surface solar radiation over the warmest water, but the strength of this feedback is quite uncertain. Implications for the role of these processes in climate change are discussed.
Cook, B. I., T. M. Smith and M. E. Mann, 2005: The North Atlantic Oscillation and regional phenology prediction over Europe. Global Change Biology, 11(6): 919-926.
We present an integrated modeling study designed to investigate changes in ecosystem level phenology over Europe associated with changes in climate pattern, by the North Atlantic Oscillation (NAO). We derived onset dates from processed NDVI data sets and used growing degree day (GDD) summations from the NCEP re-analysis to calibrate and validate a phenology model to predict the onset of the growing season over Europe. In a cross-validation hindcast, the model (PHENOM) is able to explain 63% of the variance in onset date for grid cells containing at least 50% mixed and boreal forest. Using a model developed from previous work we performed climate change scenarios, generating synthetic temperature and GDD distributions under a hypothetically increasing NAO. These new distributions were used to drive PHENOM and project changes in the timing of onset for forested cells over Europe. Results from the climate change scenarios indicate that, if the current trend in the NAO continues, there is the potential for a continued advance to the start of the growing season by as much as 13 days in some areas.
Curchitser, E. N., D. B. Haidvogel, A. J. Hermann, E. L. Dobbins, T. M. Powell and A. Kaplan, 2005: Multi-scale modeling of the North Pacific Ocean: Assessment and analysis of simulated basin-scale variability (1996-2003). Journal of Geophysical Research-Oceans, 110(C11): doi:10.1029/2005JC002902.
A primitive equation ocean general circulation model is used to investigate climate impacts in the North Pacific Ocean in the 1996 to 2003 period. The objective is to assess the model ability to reproduce observed modes of variability and study their impact in the northeast Pacific. This work is done within the framework of the U. S. Global Ecosystem ( GLOBEC) Northeast Pacific Program studying the links between climate variability and ecosystem dynamics. Three large- scale events are considered: The 1997/ 1998 El Niño, the 1999 " regime shift,'' and the 2002 cold/ fresh subsurface anomalous water mass that was observed in the Gulf of Alaska and off the coast of Oregon. The circulation model is shown to generate the correct seasonal to interannual large- scale variability and is able to represent the climatic signals of interest in the eastern Pacific. We show that the influence of the 1997/ 1998 El Niño reached the coastal Gulf of Alaska and induced an increase in the upper ocean heat content along the coast of North America. An analysis of the sea surface temperature for the model years shows agreement between model and data in the representation of the 1999 shift to a cold phase in the eastern and northern North Pacific. Finally, using the model results, we speculate that the origin of the 2002 cold/ fresh anomaly in the northeast Pacific was due to enhanced mixing during the preceding winter in the center of the Alaska gyre. Owing to anomalous changes in the density structure of the upper ocean, this water was able to move geostrophically toward the coast and it persisted in the northeast Pacific below the mixed layer the following year.
Fairbanks, R. G., R. A. Mortlock, T. C. Chiu, L. Cao, A. Kaplan, T. P. Guilderson, T. W. Fairbanks, A. L. Bloom, P. M. Grootes and M. J. Nadeau, 2005: Radiocarbon calibration curve spanning 0 to 50,000 years BP based on paired Th-230/U-234/U-238 and C-14 dates on pristine corals. Quaternary Science Reviews, 24(16-17): 1781-1796.
Radiocarbon dating is the most widely used dating technique in the world. Recent advances in Accelerator Mass Spectrometry (AMS) and sample preparation techniques have reduced the sample-size requirements by a factor of 1000 and decreased the measurement time from weeks to minutes. Today, it is estimated that more than 90 percent of all measurements made on accelerator mass spectrometers are for radiocarbon age dates. The production of C-14 in the atmosphere varies through time due to changes in the Earth's geomagnetic field intensity and in its concentration, which is regulated by the carbon cycle. As a result of these two variables, a radiocarbon age is not equivalent to a calendar age. Four decades of joint research by the dendrochronology and radiocarbon communities have produced a radiocarbon calibration data set of remarkable precision and accuracy extending from the present to approximately 12,000 calendar years before present. This paper presents high precision paired Th-230/U-234/U-238 and C-14 age determinations on pristine coral samples that enable us to extend the radiocarbon calibration curve from 12,000 to 50,000 years before present. We developed a statistical model to properly estimate sample age conversion from radiocarbon years to calendar years, taking full account of combined errors in input ages and calibration uncertainties. Our radiocarbon calibration program is publicly accessible at: http://www.radiocarbon.LDEO.columbia.edu/ along with full documentation of the samples, data, and our statistical calibration model. (c) 2005 Elsevier Ltd. All rights reserved.
Gildor, H. and N. H. Naik(Henderson), 2005: Evaluating the effect of interannual variations of surface chlorophyll on upper ocean temperature. Journal of Geophysical Research-Oceans, 110(C07012): doi:10.1029/2004JC002779.
 An important issue in modeling and predicting upper ocean variability is the nature of the interactions between ocean biology, ocean dynamics, and irradiance penetration. Numerous studies using in situ observations and model simulations to investigate the effects of biota on light penetration have demonstrated that this biological- physical feedback may be significant over a wide range of spatial and temporal scales. Using a general circulation model which takes into account interannual variations in surface chlorophyll for the period September 1997 to May 2003, we investigate the effect of varying chlorophyll concentration on surface temperature. We conclude that, by using climatological monthly mean chlorophyll values, we capture the first-order effect of chlorophyll on light penetration.
Grieco, L., L. B. Tremblay and E. Zambianchi, 2005: A hybrid approach to transport processes in the Gulf of Naples: an application to phytoplankton and zooplankton population dynamics. Continental Shelf Research, 25(5-6): 711-728.
A hybrid numerical approach was developed to study the dispersion of passive/reactive tracers in the Gulf of Naples (GON). To this end, an Eulerian and a Lagrangian scheme were implemented in the barotropic form of the Princeton Ocean Model (POM) and applied to the dispersion of zoo- and phytoplankton in the GON. The hybrid technique was first validated by comparing the tracer concentration patterns from the Eulerian model and maps of particle positions from the Lagrangian model. Excellent agreement in both spatial distribution and temporal evolution of these quantities was found between the two models. Second, the circulation in the GON was simulated using the POM model. While using simplified forcing fields, the simulated circulation patterns in the GON reproduce many observed features. These include the flushing of the GON waters typically occurring in spring and the formation of a close cyclonic gyre (trapping and homogenizing tracers in the GON) in autumn. The circulation patterns are strongly influenced by both the surface wind stresses and bathymetry and only "remotely" by the Tyrrhenian circulation. For the biological application, the spatial and temporal evolution of passive tracers (e.g., nutrients) was simulated using the Eulerian approach and that of the zoo- and phytoplankton using the Lagrangian approach. These populations were assumed to follow a prey-predator relationship and were studied using a grid resolution of 1.5 km. At these scales, the biological and physical processes (e.g., grazing, phyto- and zooplankton growth rate, mesoscale eddies, horizontal turbulent diffusion), influence plankton heterogeneity and patchiness. In particular, the model results show that phytoplankton variability have spatial and temporal scales similar than those of the carrying capacity (considered here as the effect of a limiting nutrient), yet bigger than the flow turbulence due to diffusion processes. The zooplankton population on the other hand develops on smaller scales due to its longer time taken to mature. The temporal evolution of the two populations shows that they follow a cyclical pattern which is smooth at time when the magnitude of the mean flow is much larger than the amplitude of the turbulent component of the ocean current and "noisy" at time when the turbulent component is larger than the mean flow. The high frequency oscillations are caused by the turbulence of the background ocean current through which the parcels move. A time lag of 20 days between phytoplankton and zooplankton concentrations was simulated, with the zooplankton temporal variability smaller than that of the phytoplankton. These results show that the hybrid Eulerian-Lagrangian technique here implemented is an appropriate tool to investigate the complex interactions between physical and biological dynamics. (c) 2005 Elsevier Ltd. All rights reserved.
Hazeleger, W., C. Severijns, R. Seager and F. Molteni, 2005: Tropical Pacific-driven decadel energy transport variability. Journal of Climate, 18(12): 2037-2051.
The atmospheric energy transport variability associated with decadal sea surface temperature variability in the tropical Pacific is studied using an atmospheric primitive equation model coupled to a slab mixed layer. The decadal variability is prescribed as an anomalous surface heat flux that represents the reduced ocean heat transport in the tropical Pacific when it is anomalously warm. The atmospheric energy transport increases and compensates for the reduced ocean heat transport. Increased transport by the mean meridional overturning (i.e., the strengthening of the Hadley cells) causes increased poleward energy transport. The subtropical jets increase in strength and shift equatorward, and in the midlatitudes the transients are affected. NCEP-NCAR reanalysis data show that the warming of the tropical Pacific in the 1980s compared to the early 1970s seems to have caused very similar changes in atmospheric energy transport indicating that these atmospheric transport variations were driven from the tropical Pacific. To study the implication of these changes for the coupled climate system an ocean model is driven with winds obtained from the atmosphere model. The poleward ocean heat transport increased when simulated wind anomalies associated with decadal tropical Pacific variability were used, showing a negative feedback between decadal variations in the mean meridional circulation in the atmosphere and in the Pacific Ocean. The Hadley cells and subtropical cells act to stabilize each other on the decadal time scale.
Herweijer, C., R. Seager, M. Winton and A. Clement, 2005: Why ocean heat transport warms the global mean climate. Tellus Series a-Dynamic Meteorology and Oceanography, 57(4): 662-675.
Observational and modelling evidence suggest that poleward ocean heat transport (OHT) can vary in response to both natural climate variability and greenhouse warming. Recent modelling studies have shown that increased OHT warms both the tropical and global mean climates. Using two different coupled climate models with mixed-layer oceans, with and without OHT, along with a coupled model with a fixed-current ocean component in which the currents are uniformly reduced and increased by 50%, an attempt is made to explain why this may happen.
Hogg, N. G. and A. M. Thurnherr, 2005: A zonal pathway for NADW in the South Atlantic. Journal of Oceanography, 61(3): 493-507.
Several large deployments of neutrally buoyant floats took place within the Antarctic Intermediate (AAIW), North Atlantic Deep Water (NADW), and the Antarctic Bottom Water (AABW) of the South Atlantic in the 1990s and a number of hydrographic sections were occupied as well. Here we use the spatially and temporally averaged velocities measured by these floats, combined with the hydrographic section data and various estimates of regional current transports from moored current meter arrays, to determine the circulation of the three major subthermocline water masses in a zonal strip across the South Atlantic between the latitudes of 19 degrees S and 30 degrees S. We concentrate on this region because the historical literature suggests that it is where the Deep Western Boundary Current containing NADW bifurcates. In support of this notion, we rind that a net of about 5 Sv. of the 15-20 Sv that crosses 19 degrees S does continue zonally eastward at least as far as the Mid-Atlantic Ridge. Once across the ridge it takes a circuit to the north along the ridge flanks before returning to the south in the eastern half of the Angola Basin. The data suggest that the NADW then continues on into the Indian Ocean. This scheme is discussed in the context of distributions of dissolved oxygen, silicate and salinity. In spite of the many float-years of data that were collected in the region a surprising result is that their impact on the computed solutions is quite modest. Although the focus is on the NADW we also discuss the circulation for the AAIW and AABW layers.
Honda, M., Y. Kushnir, H. Nakamura, S. Yamane and S. E. Zebiak, 2005: Formation, mechanisms, and predictability of the Aleutian-Icelandic low seesaw in ensemble AGCM simulations. Journal of Climate, 18(9): 1423-1434.
The potential predictability associated with the remote influence of midlatitude tropospheric anomalies over the North Pacific or the North Atlantic via a seesawlike interannual oscillation between the surface Aleutian and Icelandic lows (AL and IL, respectively) is investigated. Data from a 24-member ensemble of 50-yr atmospheric general circulation model simulation forced with observed sea surface temperature (SST) conditions are analyzed by separating the total simulated fluctuations into the external component forced by the prescribed SST and the internal component generated by atmospheric internal dynamics. The AL-IL seesaw can be identified in both the external and internal components of the variability. In the external variability, determined through the ensemble mean, the seesaw is gradually formed from December to March through the development of a Pacific-North American (PNA) patternlike wave train, remotely forced by the El Ni (n) over tildeo-Southern Oscillation. The amplitudes of the externally forced North Atlantic anomalies are only about half as large as the North Pacific anomalies. The potential predictability of the Atlantic anomalies, defined as the ratio of the SST-forced variance to the total variance, does not exceed the 20% level. In the internal component of the variability, determined from the deviations of each ensemble member from the ensemble mean, the negative correlation between the AL and IL anomalies is modest but persistent through winter. It is confirmed that, regardless of the polarity of the AL-IL seesaw, the IL anomalies are formed through eastward wave activity propagation of the stationary Rossby wave train emanating from the AL region in the form of what may be called a '' PNAA pattern,'' the extension of the PNA-like wave train into the Atlantic. Thus, the midwinter development of North Pacific anomalies is found to be a necessary, though not sufficient, condition for the seesaw formation. The persistence of the North Pacific anomalies beyond a 1-month time span appears to augment the probability of the seesaw formation by sustaining eastward wave activity propagation to the North Atlantic.
Huang, H. P., A. W. Robertson and Y. Kushnir, 2005: Atlantic SST gradient and the influence of ENSO. Geophysical Research Letters, 32(20): doi:10.1029/2005GL023944.
The relationship between the boreal winter El Nino SST anomaly and boreal spring tropical Atlantic SST gradient (North Atlantic minus South Atlantic) is investigated using a long, detrended SST record. For both El Nino and La Nina, concordant cases (same sign for NINO3 index and Atlantic SST gradient) slightly dominate over discordant ones, reflecting the fact that the NINO3 index correlates more strongly with the North Atlantic than the South Atlantic SST anomaly. The ratio of the numbers of concordant and discordant cases is 4:3 overall, indicating strong non-ENSO influences on the Atlantic SST gradient. The composite of the concordant cases shows an SST anomaly in the North Atlantic with the same sign as NINO3 and an opposite-signed anomaly off the southwest coast of Africa resembling "Benguela Nino''. That of the discordant cases is dominated by a pre-existing SST anomaly with the same sign as NINO3 in the south-central South Atlantic.
Huang, H. P., R. Seager and Y. Kushnir, 2005: The 1976/77 transition in precipitation over the Americas and the influence of tropical sea surface temperature. Climate Dynamics, 24(7-8): 721-740.
Most major features of the interdecadal shift in boreal winter-spring precipitation over the American continents associated with the 1976-1977 transition are reproduced in atmospheric general circulation model (GCM) simulations forced with observed sea surface temperature (SST). The GCM runs forced with global and tropical Pacific SSTs produce similar multidecadal changes in precipitation, indicating the dominant influence of tropical Pacific SST. Companion experiments indicate that the shift in mean conditions in the tropical Pacific is responsible for these changes. The observed and simulated "post- minus pre-1976" difference in Jan-May precipitation is wet over Mexico and the southwest U.S., dry over the Amazon, wet over sub-Amazonian South America, and dry over the southern tip of South America. This pattern is not dramatically different from a typical El Niño-induced response in precipitation. Although the interdecadal (post- minus pre-1976) and interannual (El Niño-La Niña) SST anomalies differ in detail, they produce a common tropics-wide tropospheric warmth that may explain the similarity in the precipitation anomaly patterns for these two time scales. An analysis of local moisture budget shows that, except for Mexico and the southwest U.S. where the interdecadal shift in precipitation is balanced by evaporation, elsewhere over the Americas it is balanced by a shift in low-level moisture convergence. Moreover, the moisture convergence is due mainly to the change in low-level wind divergence that is linked to low-level ascent and descent.
Huwald, H., L. B. Tremblay and H. Blatter, 2005: A multilayer sigma-coordinate thermodynamic sea ice model: Validation against Surface Heat Budget of the Arctic Ocean (SHEBA)/Sea Ice Model Intercomparison Project Part 2 (SIMIP2) data. Journal of Geophysical Research-Oceans, 110(C5): doi:10.1029/2004JC002328.
 A new multilayer sigma-coordinate thermodynamic sea ice model is presented. The model employs a coordinate transformation which maps the thickness of the snow and ice slabs onto unity intervals and thus enables automatic relayering when the snow or ice thickness changes. This is done through an advection term which naturally appears in the transformed energy equation. Unlike previous approaches, the model conserves the total energy per layer (Jm(-2) as opposed to Jm(-3)), which takes into account the changes in internal energy associated with thickness changes. This model was then tested against observational data from the Surface Heat Budget of the Arctic Ocean (SHEBA) experiment in the context of the Sea Ice Model Intercomparison Project, Part 2, Thermodynamics (SIMIP2). In general, the model reproduces the observed internal snow-ice temperature and the ice thickness evolution very well. Results show that the ice thickness evolution is very sensitive to the ocean heat flux (F-ocn) and the thickness of the snow cover in winter. Given that the spatial variability in snow depth at small scale is large, the specification of the snow depth temporal evolution is crucial for an intercomparison project. Since F-ocn in SIMIP2 is calculated as a residual of the observed basal growth rates and heat conduction, the salinity of newly formed ice used in the simulations must be consistent with that used to derive F-ocn. Simulated and observed snow surface and snow-ice interface temperatures suggest that not enough heat is conducted through the snow layer even when using a snow thermal conductivity as large as 0.50 Wm(-1) K-1 ( value derived from observed snow and ice internal temperature profiles). A surface energy budget of simulated and observed energy fluxes confirms this finding.
Huwald, H., L. B. Tremblay and H. Blatter, 2005: Reconciling different observational data sets from Surface Heat Budget of the Arctic Ocean (SHEBA) for model validation purposes. Journal of Geophysical Research-Oceans, 110(C5): doi:10.1029/2003JC002221.
 Observations from the Surface Heat Budget of the Arctic Ocean (SHEBA) are analyzed to develop a consistent data set suitable for the validation of snow and sea ice components used in climate models. Since the snow depth is a crucial variable to properly determine the ice thickness evolution, several methods are tested to estimate the actual snow depth at the exact location of the measured internal temperatures. Snow and ice thickness gauge measurements show high spatial variability at small spatial scales. Consequently, individual measurements of snow/ice thickness are not representative of the thickness at the locations where temperature profiles were measured. Observed skin temperatures and snow internal temperature profiles suggest that the mean winter snow cover at the reference mass balance site was thicker by 11 cm when compared with gauge observations at a small distance from that reference site. The mean winter snow cover thickness measured at the SHEBA mass balance site, Pittsburgh, is larger by a factor of 2.3 when compared to the snow depth derived from precipitation measurements. Assuming continuity of heat fluxes at the snow-ice interface, an effective snow thermal conductivity of 0.50 Wm(-1) K-1 is calculated. This is significantly higher than values generally used in climate models (0.31 Wm(-1) K-1) or derived from in situ measurements (0.14 Wm(-1) K-1) at SHEBA. Ocean heat fluxes, inferred from ice thickness and internal temperature measurements at various sites, are very consistent and match reasonably well those derived from turbulence measurements and a bulk formulation. A heat budget of surface fluxes shows a mean annual net imbalance of 1.5 Wm(-2), with a mean energy deficit of 3.5 Wm(-2) during winter and a mean surplus of 6.4 Wm(-2) during summer.
Khatiwala, S., M. Visbeck and M. A. Cane, 2005: Accelerated simulation of passive tracers in ocean circulation models. Ocean Modelling, 9(1): 51-69.
A novel strategy is proposed for the efficient simulation of geochemical tracers In ocean models. The method captures the tracer advection and diffusion in a general circulation model (GCM) without any alteration (or even knowledge) of the GCM code. In comparison with offline tracer models. the proposed method is considerably more efficient and automatically includes all parameterizations of unresolved processes present in the most sophisticated GCMs. A comparison with a global configuration of the MIT GCM shows that the scheme can capture the complex three-dimensional transport of a state-of-the-art GCM. A key advantage of the proposed technique is the ability to directly compute steady-state solutions, a facility particularly well-suited to tracers such as natural radiocarbon. This capability is applied to develop a novel algorithm for accelerating the dynamical adjustment of ocean models. (C) 2004 Elsevier Ltd. All rights reserved.
Khodri, M., M. A. Cane, G. Kukla, J. Gavin and P. Braconnot, 2005: The impact of precession changes on the Arctic climate during the last interglacial-glacial transition. Earth and Planetary Science Letters, 236(1-2): 285-304.
Three sensitivity experiments using an Ocean Atmosphere General Circulation Model (OAGCM) are conducted to simulate the climate impact of precession. The relative contributions of components of the hydrological cycle including the albedo of Arctic sea ice, advection of atmospheric water vapor and sea surface temperature to the summer Arctic melt process are evaluated. Timing of the perihelion is varied in each experiment with meteorological spring (SP), winter (WP) and autumn (AP) perihelion corresponding to conditions at 110, 115 and 120 ky BP, respectively. Obliquity is unchanged at the 115 ky level which is lower than today. The experiments are assessed relative to the present day control, which has been shown to simulate current conditions based on observations. In the SP experiment, top of the atmosphere (TOA) insolation is weaker than today between the summer solstice and autumnal equinox.
Landman, W. A., A. Seth and S. J. Camargo, 2005: The effect of regional climate model domain choice on the simulation of tropical cyclone-like vortices in the southwestern Indian Ocean. Journal of Climate, 18(8): 1263-1274.
A regional climate model is tested for several domain configurations over the southwestern Indian Ocean to examine the ability of the model to reproduce observed cyclones and their landfalling tracks. The interaction between large-scale and local terrain forcing of tropical storms approaching and transiting the island landmass of Madagascar makes the southwestern Indian Ocean a unique and interesting study area. In addition, tropical cyclones across the southern Indian Ocean re likely to be significantly affected by the large-scale zonal flow. Therefore, the effects of model domain si e and the positioning of its lateral boundaries on the simulation of tropical cyclone-like vortices and their tracks on a seasonal time scale are investigated. Four tropical cyclones, which occurred over the southwestern Indian Ocean in January of the years 1995-97, are studied, and four domains are tested. The regional climate model is driven by atmospheric lateral boundary conditions that are derived from large-scale meteorological analyses. The use of analyzed boundary forcing enables comparison with observed cyclones in these tests. Simulations are performed using a 60-km horizontal resolution and for an extended time integration of about 6 weeks. Results show that the positioning of the eastern boundary of the regional model domain is of major importance in the life cycle of simulated tropical cyclone-like vortices: a vortex entering through the eastern boundary of the regional model is generally well simulated. The size of the domain also has a bearing on the ability of the regional model to simulate vortices in the Mozambique Channel, and the island landmass of Madagascar additionally influences storm tracks. These results show that the regional model can produce cyclonelike vortices and their tracks (with some deficiencies) given analyzed lateral boundary forcing. Statistical analyses of GCM-driven nested model ensemble integrations are now required to further address predictive skill of cyclones in the southwestern Indian Ocean and to test if the model can realistically simulate tropical storm genesis as opposed to advecting existing tropical disturbances entering through the model boundaries.
Ou, H. W., 2005: Dynamics of dense water descending a continental slope. Journal of Physical Oceanography, 35(8): 1318-1328.
A reduced-gravity model is used to examine the dynamics of dense water descending a continental slope. The model solves for the geostrophically adjusted state before it is subjected to significant frictional decay. For such bottom-mounted flow, it is argued that frictional torque would dominate the net vorticity balance to equalize the edge flows, resulting in double velocity cores. Constrained by the geostrophic balance, the dense water thus may settle only over a concave bottom and is sheetlike, covering typically the whole slope rise. As such, the adjustment is characterized by a spreading rather than sinking of the layer-with little descent of the upper edge but a swift downslope current propelling the lower edge. Through the mechanical energy balance, it is found in addition that a greater density anomaly would increase the total entrainment flux to more strongly dilute the original anomaly, yielding a product water that is less varied in the water-mass properties. Model predictions compare favorably with some observed dense outflows, in support of the entrainment and friction control of the geostrophic adjustment.
Ou, H. W., 2005: On the cooling of a buoyant boundary current. Deep-Sea Research Part Ii-Topical Studies in Oceanography, 52(11-13): 1662-1670.
Through a steady-state reduced-gravity model, we examine the downstream evolution of a buoyant boundary current as it is subjected to surface cooling. It is round that the adverse pressure gradient associated with the diminishing buoyancy is countered by falling pressure head, so the overall strength of the current as measured by the (transport-weighted) mean square velocity remains unchanged. This constancy also applies to the cross-stream difference or the square velocity because of the vorticity constraint, which leads to the general deduction that the net current shear is enhanced regardless of its upstream sign. As a consequence. if the upstream flow contain, near-shore and offshore branches that are comparable in strength, this parity would persist downstream, but if the near-shore branch is weaker to begin with, it may be stagnated by cooling, with the ensuing generation of anti-cyclonic eddies, Oil account of the geostrophic balance, the buoyant layer narrows as the square root of the buoyancy the same rate as the failing pressure head, but more rapid than that of the local deformation radius. Some of the model predictions are compared with observations from the Tsushima Current in the Japan East Sea. (c) 2005 Elsevier Ltd. All rights reserved.
Previdi, M. and D. E. Veron, 2005: North Atlantic Oscillation-related climate variability in a regional atmospheric model. Journal of Geophysical Research-Atmospheres, 110(D16): -.
 Climate variability over the Greenland ice sheet associated with the North Atlantic Oscillation (NAO) is simulated with the Polar MM5 (PMM5) regional climate model (RCM). Two five-member monthly ensembles, each representative of high or low NAO extremes, are made, with data from the European Centre for Medium-Range Weather Forecasts 40-year reanalysis (ERA-40) used to provide the forcing for the RCM runs. Ensemble differences between the high and low NAO cases are discussed, and model output is compared to ERA-40 and to observations from automatic weather stations over Greenland. PMM5 is shown to reproduce several well-known features of NAO-related climate variability in the Greenland region, including colder near-surface air temperatures, lower sea level pressure, stronger westerly winds south of Greenland, and reduced precipitation over western Greenland and the Labrador Sea during high NAO months.
Robertson, R., 2005: Baroclinic and barotropic tides in the Ross Sea. Antarctic Science, 17(1): 107-120.
The barotropic and baroclinic tides in the Ross Sea were simulated using a primitive equation, sigma-coordinate model, the Regional Ocean Model System (ROMS), for four tidal constituents, M-2, S-2, K-1, and O-1. Small elevation amplitudes were predicted over most of the basin, with a combined standard deviation of 30-50 cm. Larger amplitudes, with standard deviations ranging from 50-70 cm, occurred deep within the ice shelf cavity, over the continental slope, and over Iselin Bank. Most of the elevation response was associated with the diurnal constituents (K-1 and O-1), as was most of the depth-independent (barotropic) velocity response. Baroclinic tides were generated at locations of steep topography for the semidiurnal constituents, but not the diurnal. Diurnal continental shelf waves were generated by the diurnal tides and found to amplify the semidiurnal elevations and baroclinic tidal velocities over the continental slope. Comparisons with observations in both elevation and velocities showed very good agreement for the semidiurnal constituents (M-2 and S-2) and moderate agreement for the diurnal constituents (K-1 and O-1). The disagreement for the diurnal constituents was associated with diurnal frequency continental shelf waves, which were overexcited along the shelf break. The baroclinic tides induced both small-scale horizontal and vertical shear in the velocity fields in the Ross Sea.
Seager, R., N. Harnik, W. A. Robinson, Y. Kushnir, M.F. Ting, H. P. Huang and J. Velez(Nakamura), 2005: Mechanisms of ENSO-forcing of hemispherically symmetric precipitation variability. Quarterly Journal of the Royal Meteorological Society, 131(608): 1501-1527.
The patterns of precipitation anomalies forced by the El Nino-Southern Oscillation during northern hemisphere winter and spring are remarkably hemispherically symmetric and, in the midlatitudes, have a prominent zonally symmetric component. Observations of global Precipitation variability and the moisture budget within atmospheric reanalyses are examined to argue that the zonally symmetric component is caused by interactions between transient eddies and tropical ly-forced changes in the subtropical jets. During El Niho events the jets strengthen in each hemisphere and shift equatorward. Changes in the subtropical jet influence the transient-eddy momentum fluxes and the eddy-driven mean meridional circulation. During El Nino events, eddy-driven ascent in the midlatitudes of each hemisphere is accompanied by low-level convergence and brings increased precipitation. These changes in the transient-eddy and stationary-eddy moisture fluxes almost exactly cancel each other and, in Ann, do not contribute to the zonal-mean precipitation anomalies. Propagation of anomalous stationary waves disrupts the zonal symmetry. Flow around the deeper Aleutian Low and the eastward extension of the Pacific jet stream supply the moisture for increased precipitation over the eastern North Pacific and the Western seaboard of the United States. while transient-eddy moisture convergence supplies the moisture for increased precipitation over the southern United States. In each case, increased precipitation is fundamentally caused by anomalous ascent forced by anomalous heat and vorticity fluxes.
Seager, R., Y. Kushnir, C. Herweijer, N. Naik(Henderson) and J. Velez(Nakamura), 2005: Modeling of tropical forcing of persistent droughts and pluvials over western North America: 1856-2000. Journal of Climate, 18(19): 4065-4088.
The causes of persistent droughts and wet periods, or pluvials, over western North America are examined in model simulations of the period from 1856 to 2000. The simulations used either (i) global sea surface temperature data as a lower boundary condition or (ii) observed data in just the tropical Pacific and computed the surface ocean temperature elsewhere with a simple ocean model. With both arrangements, the model was able to simulate many aspects of the low-frequency (periods greater than 6 yr) variations of precipitation over the Great Plains and in the American Southwest including much of the nineteenth-century variability, the droughts of the 1930s (the "Dust Bowl") and 1950s, and the very wet period in the 1990s. Results indicate that the persistent droughts and pluvials were ultimately forced by persistent variations of tropical Pacific surface ocean temperatures. It is argued that ocean temperature variations outside of the tropical Pacific, but forced from the tropical Pacific, act to strengthen the droughts and pluvials. The persistent precipitation variations are part of a pattern of global variations that have a strong hemispherically and zonally symmetric component, which is akin to interannual variability, and that can be explained in terms of interactions between tropical ocean temperature variations, the subtropical jets, transient eddies, and the eddy-driven mean meridional circulation. Rossby wave propagation poleward and eastward from the tropical Pacific heating anomalies disrupts the zonal symmetry, intensifying droughts and pluvials over North America. Both mechanisms of tropical driving of extratropical precipitation variations work in summer as well as winter and can explain the year-round nature of the precipitation variations. In addition, land-atmosphere interactions over North America appear important by (i) translating winter precipitation variations into summer evaporation and, hence, precipitation anomalies and (ii) shifting the northward flow of moisture around the North Atlantic subtropical anticyclone eastward from the Plains and Southwest to the eastern seaboard and western Atlantic Ocean.
Shaman, J., M. Cane and A. Kaplan, 2005: The relationship between tibetan snow depth, ENSO, river discharge and the monsoons of Bangladesh. International Journal of Remote Sensing, 26(17): 3735-3748.
Using satellite estimates of snow depth, we examine the interannual variability of the monsoon rains of Bangladesh, an area greatly affected by land surface hydrological processes including Himalayan snowpack size, snowmelt river flooding, and Bay of Bengal storm surge. For the twentieth century, we found Bangladesh monsoon rainfall (BMR) to be uncorrelated with the All-Indian Monsoon Index. This result is consistent with previous findings for shorter time records. We next used a short 9-year record of satellite estimates of April snow depth for the Himalayan region and concurrent seasonal El Nino-Southern Oscillation (ENSO) conditions in the equatorial Pacific to develop an empirical model that explains a high percentage of BMR interannual variability. Inclusion of late spring river discharge levels further improves the empirical model representation of BMR for June-September. These results, though with a limited length satellite record, suggest that BMR interannual variability is constrained by concurrent ENSO conditions, spring Himalayan snowpack size and land surface flooding. The same results could not be obtained from analyses using satellite estimates of snow cover. These findings stress the need for development of a quality longer record of satellite estimated snow depth. The twentieth-century analysis also indicates that BMR should be considered independently of Indian monsoon rainfall.
Sobel, A. H. and S. J. Camargo, 2005: Influence of western North Pacific tropical cyclones on their large-scale environment. Journal of the Atmospheric Sciences, 62(9): 3396-3407.
The authors investigate the influence of western North Pacific (WNP) tropical cyclones (TCs) on their large-scale environment by lag regressing various large-scale climate variables [atmospheric temperature, winds, relative vorticity, outgoing longwave radiation (OLR), column water vapor, and sea surface temperature (SST)] on an index of TC activity [accumulated cyclone energy (ACE)] on a weekly time scale. At all leads and lags out to several months, persistent, slowly evolving signals indicative of the El Nino-Southern Oscillation (ENSO) phenomenon are seen in all the variables, reflecting the known seasonal relationship of TCs in the WNP to ENSO. Superimposed on this are more rapidly evolving signals, at leads and lags of one or two weeks, directly associated with the TCs themselves. These include anomalies of positive low-level vorticity, negative OLR, and high column water vapor associated with anomalously positive ACE, found in the region where TCs most commonly form and develop. In the same region, lagging ACE by a week or two and so presumably reflecting the influence of TCs on the local environment, signals are found that might be expected to negatively influence the environment for later cyclogenesis. These signals include an SST reduction in the primary region of TC activity, and a reduction in column water vapor, and increase in OLR that may or may not be a result of the SST reduction.
Susanto, R. D. and A. L. Gordon, 2005: Velocity and transport of the Makassar Strait throughflow. Journal of Geophysical Research-Oceans, 110(C1): doi:10.1029/2004JC002425.
 Analyses of ocean current measurements from two moorings in the Makassar Strait (MAK-1, December 1996 to July 1998, and MAK-2, December 1996 to February 1998) with newly processed acoustic Doppler current profiler ( ADCP) data provide a new estimate of transport and vertical velocity structure for this important passageway of the Indonesian Throughflow. The 7-month record of the MAK-1 ADCP and the 3-month record of the MAK-2 ADCP, nominally set at depths of 150 m, are extrapolated to the end of the mooring period using a surface layer relationship to the shallowest current meter at 200 m and to the regional winds. The southward transport within Makassar Strait is confined for the most part to the upper 750 m, above the blocking topographic sill of Makassar Strait. The transport maximum occurs within the thermocline ( 100 - 300 m). After the temporal mean flow has been removed, the vertical structure of the along channel flow in the upper 750 m is decomposed using the empirical orthogonal function (EOF) method. The first two modes contain 91% of the total variance. For the entire mooring period, the total depth-integrated transport was 8.1 +/- 1.5 Sv ( Sv = 1 x 10(6) m(3)/s), with 7.9 +/- 1.2 Sv for calendar year1997. During the peak of 1997/1998 El Nino from September 1997 to mid-February 1998, the first mode time series displays northward flow, enhancing the vertical shear within the thermocline and reducing the mean throughflow to 4.6 +/- 0.9 Sv.
Thurnherr, A. M., L. C. St Laurent, K. G. Speer, J. M. Toole and J. R. Ledwell, 2005: Mixing associated with sills in a canyon on the midocean ridge flank. Journal of Physical Oceanography, 35(8): 1370-1381.
To close the global overturning circulation, the production and sinking of dense water at high latitudes must be balanced elsewhere by buoyancy gain and upward vertical motion. Hydrographic and microstructure observations from the Brazil Basin in the South Atlantic Ocean indicate that most of the abyssal mixing there takes place on the topographically rough flank of the midocean ridge. In previous studies it has been suggested that the high level of abyssal mixing observed on the ridge flank is primarily caused by breaking internal waves forced by tidal currents. Here, the results from a detailed analysis of velocity, hydrographic, and microstructure data from a ridge-flank canyon are presented. Two-year-long current-meter records indicate that within the canyon there is a significant along-axial mean flow down the density gradient toward the ridge crest. Five hundred meters above the canyon floor the kinetic energy in the subinertial band exceeds that associated with the semidiurnal tides by approximately a factor of 2. The mean dissipation of kinetic energy inside the canyon exceeds that above the ridge-flank topography by approximately a factor of 5. The largest dissipation values were observed downstream of a narrow, 1000-m-high sill that extends across the full width of the canyon. Along the entire canyon, there is a strong association between the presence of sills and along-axial density gradients, while there is no similar association between the presence of depressions and density gradients. Together, these observations suggest that sill-related mixing contributes at least as much to the diapycnal buoyancy flux in the canyon as tidally forced internal-wave breaking, which is not expected to be associated preferentially with sills. While only approximate to 15% of the interfacial area between Antarctic Bottom Water and North Atlantic Deep Water in the Brazil Basin lie inside canyons, the available data suggest that approximately one-half of the diapycnal buoyancy fluxes take place there. In comparison, the region above the ridge-flank topography accounts for about one-third of the buoyancy fluxes. The apparent importance of sill-related processes for mixing in ridge-flank canyons is therefore of global significance, especially considering that such canyons occur on average every 50 km along 2/3 of the global midocean ridge system, and that sills partially block the canyon axes every few tens of kilometers.
Vranes, K. and A. L. Gordon, 2005: Comparison of Indonesian Throughflow transport observations, Makassar Strait to eastern Indian Ocean. Geophysical Research Letters, 32(10): doi:10.1029/2004GL022158.
The Pacific to Indian Ocean transport within the passageways of the Indonesian seas (ITF) varies on interannual and longer time scales associated with ENSO, the Asian monsoons and interannual climate variability of the Indian Ocean. Although direct current measurements of the ITF are of limited duration, none long enough to properly describe greater than annual variability of the ITF, observations indicate that the bulk of the ITF passes through Makassar Strait. The repeat expendable bathythermograph (XBT) IX1 section begun in 1983 provides the longest time series of the full ITF introduced into the Indian Ocean between northwest Australia and Java. We find that the surface to 600 dbar Makassar Strait transport, as measured by current meter moorings from December 1996 to July 1998, correlates at r = 0.77 +/- 0.14 with the geostrophic transport constructed from IX1 XBT data for that time interval, with a 98 day lag.
Zappa, C. J. and A. T. Jessup, 2005: High-resolution airborne infrared measurements of ocean skin temperature. IEEE Geoscience and Remote Sensing Letters, 2(2): doi:10.1109/LGRS.2004.841629.
Airborne measurements of ocean skin temperature T, are presented from the Coupled Boundary Layers, Air-Sea Transfer in Low Winds (CBLAST-Low) Pilot Experiment in August 2001 off Martha's Vineyard, MA. We used an infrared (IR) camera with a spatial resolution of 1 in or less and temperature resolution of roughly 0.02 degrees C. Using subframe sampling of the IR imagery, we achieve lower noise and higher spatial resolution than reported by previous investigators using IR radiometers. Fine-scale maps of T. exhibit horizontal variability over spatial scales ranging from O(10 km) down to O(1 m) that are related to atmospheric and subsurface phenomena under low to moderate wind conditions. Based on supporting measurements of wind and waves, we identify coherent ramp-like structures in T, with stratification breakdown and meandering streaky features with internal waves. Regional maps of T. show the standard deviation for the region is +/- 1.04 degrees C, while the meridional and zonal variability is 0.23 degrees C (.) km(-1) and 0.27 degrees C (.) km(-1), respectively. This temperature variability results in meridional and zonal scalar heat flux variability of 7.0 W (.) m(-2) (.) km(-1) and 7.6 W (.) m(-2) (.) km(-1), respectively. Our results demonstrate the potential for airborne IR imagery accompanied by high-quality ocean data to identify T-s features produced by subsurface circulation.
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