Barnston, A.G., H.M. Van den Dool, D.R. Rodenhuis, C.R. Ropelewski, V.E. Kousky, E.A. O'Lenic, R.E. Livezey, S.E. Zebiak, M.A. Cane, T.P. Barnett, N.E. Graham, M. Ji and A. Leetmaa, 1994: Long-Lead Seasonal Forecasts - Where Do We Stand. Bulletin of the American Meteorological Society, 75(11): 2097-2114.
The National Weather Service intends to begin routinely issuing long-lead forecasts of 3-month mean U. S. temperature and precipitation by the beginning of 1995. The ability to produce useful forecasts for certain seasons and regions at projection times of up to 1 yr is attributed to advances in data observing and processing, computer capability, and physical understanding-particularly, for tropical ocean-atmosphere phenomena. Because much of the skill of the forecasts comes from anomalies of tropical SST related to ENSO, we highlight here long-lead forecasts of the tropical Pacific SST itself, which have higher skill than the U.S forecasts that are made largely on their basis.
Biskamp, D., S. J. Camargo and B. D. Scott, 1994: Spectral Properties and Statistics of Resistive Drift-Wave Turbulence. Physics Letters A, 186(3): 239-244.
Resistive drift-wave turbulence is studied by high-resolution numerical simulation in the limit of small viscosity (high Reynolds numbers), such that the adiabaticity parameter C is the only relevant parameter. Energy spectra exhibit a maximum at some wave-number k0 and a power law behavior for k > k0. Statistics in this range are non-Gaussian indicating strong intermittency, but are perfectly Gaussian for k less than or similar to k0.
Bürger, G. and M. A. Cane, 1994: Interactive Kalman Filtering. Journal of Geophysical Research-Oceans, 99(C4): 8015-8031.
Data assimilation via the extended Kalman filter can become problematic when the assimilating model is strongly nonlinear, primarily in connection with sharp, ''switchlike'' changes between different regimes of the system. The filter seems too inert to follow those switches quickly enough, a fact that can lead to a complete failure when the switches occur often enough. In this paper we replace the key feature of the filter, the use of local linearity for the error model update, with a principle that uses a more global approach through the utilization of a set of preselected regimes. The method uses all regime error models simultaneously. Being mutually incompatible, a compromise between the different error models is found through the use of a weighting function that reflects the 'closeness' of the error model to the correct model. To test the interactive Kalman filter a series of numerical experiments is performed using the double-well system and the well-known Lorenz system, and the results are compared to the extended Kalman filter. It turns out that, depending on the set of preselected regimes, the performance is worse than, comparable to, or better than that of the extended Kalman filter.
Cane, M. A., G. Eshel and R. W. Buckland, 1994: Forecasting Zimbabwean Maize Yield Using Eastern Equatorial Pacific Sea-Surface Temperature. Nature, 370(6486): 204-205.
SOUTHERN Africa is subject to recurrent droughts which cause severe food shortages. There is considerable evidence(1) that El Niño(2) warm events in the Pacific Ocean are linked to below-average rainfall in southern Africa, and the 1991-92 El Niño event was accompanied by the worst drought in southern Africa this century, affecting nearly 100 million people. But although models can predict El Niño events a year in advance(3-6), the drought was not anticipated, increasing relief costs. Here we present data showing a strong correlation between an El Niño index and both rainfall and maize yield in Zimbabwe. Surprisingly, the correlation with maize yield is stronger than that with rainfall, with more than 60% of the variance in yield accounted for by sea surface temperatures in the eastern equatorial Pacific Ocean-half-way around the world. We also show that model predictions of the El Niño index provide accurate forecasts of maize yield in Zimbabwe, with lead times of up to a year. As maize is the most important food crop for the ten-nation Southern African Development Community region(7), we suggest that this approach could provide an effective early-warning system for southern African drought-induced famines.
Chen, D., A. J. Busalacchi and L. M. Rothstein, 1994: The Roles of Vertical Mixing, Solar-Radiation, and Wind Stress in a Model Simulation of the Sea-Surface Temperature Seasonal Cycle in the Tropical Pacific-Ocean. Journal of Geophysical Research-Oceans, 99(C10): 20345-20359.
The climatological seasonal cycle of sea surface temperature (SST) in the tropical Pacific is simulated using a newly developed upper ocean model. The roles of vertical mixing, solar radiation, and wind stress are investigated in a hierarchy of numerical experiments with various combinations of vertical mixing algorithms and surface-forcing products. It is found that the large SST annual cycle in the eastern equatorial Pacific is, to a large extent, controlled by the annually varying mixed layer depth which, in turn, is mainly determined by the competing effects of solar radiation and wind forcing. With the application of our hybrid vertical mixing scheme the model-simulated SST annual cycle is much improved in both amplitude and phase as compared to the case of a constant mixed layer depth. Beside the strong effects on vertical mixing, solar radiation is the primary heating term in the surface layer heat budget, and wind forcing influences SST by driving oceanic advective processes that redistribute heat in the upper ocean. For example, the SST seasonal cycle in the western equatorial Pacific basically follows the seminanual variation of solar heating, and the cycle in the central equatorial region is significantly affected by the zonal advective heat flux associated with the seasonally reversing South Equatorial Current. It has been shown in our experiments that the amount of heat flux modification needed to eliminate the annual mean SST errors in the model is, on average, no larger than the annual mean uncertainties among the various surface flux products used in this study. Whereas a bias correction is needed to account for remaining uncertainties in the annual mean heat flux, this study demonstrates that with proper treatment of mixed layer physics and realistic forcing functions the seasonal variability of SST is capable of being simulated successfully in response to external forcing without relying on a relaxation or damping formulation for the dominant surface heat flux contributions.
Chen, D., L. M. Rothstein and A. J. Busalacchi, 1994: A Hybrid Vertical Mixing Scheme and Its Application to Tropical Ocean Models. Journal of Physical Oceanography, 24(10): 2156-2179.
A novel hybrid vertical mixing scheme, based jointly on the Kraus-Turner-type mixed layer model and Price's dynamical instability model, is introduced to aid in parameterization of vertical turbulent mixing in numerical ocean models. The scheme is computationally efficient and is capable of simulating the three major mechanisms of vertical turbulent mixing in the upper ocean, that is, wind stirring, shear instability, and convective overturning.
Eshel, G., M. A. Cane and M. B. Blumenthal, 1994: Modes of Subsurface, Intermediate, and Deep-Water Renewal in the Red-Sea. Journal of Geophysical Research-Oceans, 99(C8): 15941-15952.
A linear box model of the Red Sea is presented. The model, which inverts hydrographic and He-3 data for the main features of the width-integrated circulation, is able to describe a large part of the seasonal and annual mean circulation. By combining and reconciling major elements of current theories of the Red Sea circulation, available flow measurements, and tracer data the model provides a unified conceptual framework for the understanding of the circulation of the Red Sea. The model forms deep water in the extreme north in two modes, as first suggested by Cember (1988). The convective mode renews the bottom water at an annual mean rate of approximately 0.04 Sv. In the winter the isopycnal mode water renews the uppermost part of the deep water mass. It flows to the south along the bottom of the pycnocline at an annual mean rate of 0.02-0.04 Sv. A new feature in the model's near-surface circulation scheme is the ventilation of the thermocline in the winter at a mean rate of approximately 0.04 Sv, also from the extreme north. There is also a middepth return flow to the north at an annual mean rate of approximately 0.07 Sv. This flow has been suggested but never rigorously quantified before.
Flagg, C. N., R. W. Houghton and L. J. Pietrafesa, 1994: Summertime Thermocline Salinity Maximum Intrusions in the Mid-Atlantic Bight. Deep-Sea Research Part II-Topical Studies in Oceanography, 41(2-3): 325-.
Mooring data with high vertical resolution have been combined with hydrographic data to investigate the character of high salinity thermocline intrusions in the shelf water-slope water frontal zone in the southern Mid-Atlantic Bight. The coincidence of current, temperature and salinity data has allowed a detailed investigation of how the intrusion process is initiated. The data show that the time scale for both the onset and duration of the intrusions is very short. Most intrusions at a single location seem to last no more than a day, while the onshore flow associated with many, but not all, intrusions accelerates very quickly attaining amplitudes of 10-20 cm s(-1). As has been noted in earlier studies, there is a correlation between the intrusions and upwelling favorable winds, but the magnitude of the wind stress is not sufficient to account for the onshore transport of the intrusions. The combination of current measurements with density profiles has allowed the calculation of gradient Richardson numbers associated with the intrusions. These calculations show that during the strong onshore flow periods there is significant mixing at the boundaries of the intrusions. At other times when the high salinity water is being passively advected along the shelf, there appears to be little shear turbulence at the intrusion boundaries. Continuous data coverage during the summer suggests that the intrusion process is actually fairly infrequent, apparently requiring an offshore preconditioning in which high salinity waters of the correct density are located just offshore in addition to upwelling favorable winds.
Garzoli, S. L. and C. F. Giulivi, 1994: What Forces the Variability of the Southwestern Atlantic Boundary Currents. Deep-Sea Research Part I-Oceanographic Research Papers, 41(10): 1527-1550.
A marked variability in the location of the front originating at the confluence of the Brazil and Malvinas Currents has been observed from both surface and subsurface observations. Modeling experiments using climatological winds predict a seasonal variability on the latitude of separation of the Brazil Current from the coast. During the Confluence Program (November 1988-February 1990) and from data collected with an array of inverted echo sounders, the location of the confluence front and its variability was established. In this paper, the observed oceanic variability is analyzed simultaneously with the wind product from the European Center for Medium Weather Forecast (ECMWF) obtained for the period of the observations. The ECMWF data is validated against in situ indirect wind magnitude observations obtained from a sub-array of the Confluence deployments. The large-scale anomalies are explored through the comparison with the climatological winds field obtained from HELLERMAN and ROSENSTEIN (1983), Journal of Physical Oceanography, 13, 1093-1104. From the analysis it is concluded that the main source of variability of the Confluence front is the local wind forcing. There is a variability in the location of the front due to the seasonal cycle of the winds in the South Atlantic. In addition to this seasonal variability, the latitude of separation of the Brazil Current from the coast presents a marked interannual variability that is forced from anomalous wind patterns south of the Confluence. There is no apparent correlation between wind-forced pulses in the Antarctic Circumpolar Current and the observed anomalous northward penetration of the Malvinas Current.
Gordon, A. L., A. Ffield and A. G. Ilahude, 1994: Thermocline of the Flores and Banda Seas. Journal of Geophysical Research-Oceans, 99(C9): 18235-18242.
In December 1991, 30 conductivity-temperature-depth (CTD) stations to 1000 dbar were obtained from the R/V Baruna Jaya I in the Flores Sea, Banda Sea, and Alor-Wetar passage of the Indonesian sea. A salinity maximum within the interval 100-150 dbar and a salinity minimum within the interval 300-350 dbar mark water mass core layers derived from the North Pacific. They are drawn into the Flores Sea from the Makassar Strait, with subsequent flow into the Banda Sea, and are weakened en route by strong vertical mixing characteristic of the Indonesian seas. In the Flores Sea, water below 300 dbar becomes saltier with increased distance from the Makassar Strait, suggesting that an advective process may be drawing relatively salty water into the Flores Sea lower thermocline from the Banda Sea. The Banda-to-Flores Sea flow may be a consequence of vertical transfer of horizontal momentum produced by the same turbulent processes that are responsible for enhanced vertical mixing. The interocean transport profile may not correspond exactly with the Pacific-to-Indian Ocean pressure gradient profile, as deeper water is carried along with the through flow by the effects of eddy viscosity. The 550-m sill at the southern end of the Makassar Strait creates a situation where downward flux of momentum entrains deeper water that must be compensated by lower thermocline water drawn from the Banda Sea. Geostrophic transport relative to 1000 dbar in the Banda Sea shows not only a strong through flow transport in the upper 300 dbar (6.3 x 10(6) m3 s-1) but also a deeper flow toward the Flores Sea (1.5 x 10(6) m3 s-1 from 300 to 500 dbar and an additional 2.4 x 10(6) m3 s-1 from 500 to 1000 dbar). A simple model suggests that the magnitude of the deeper westward flow is proportional to the vertical eddy viscosity coefficient. Water mass analysis shows that either the South Pacific or Indian Ocean can provide the lower thermocline Banda Sea water.
Hoerling, M. P. and M. F. Ting, 1994: Organization of Extratropical Transients During El Niño. Journal of Climate, 7(5): 745-766.
Four observed El Nino-Southern Oscillation (ENSO) events are studied to determine the mechanisms responsible for the anomalous extratropical atmospheric circulation during northern winter. A parallel analysis of a GCM's response to El Nino is performed in order to assess if similar mechanisms are operative in the model atmosphere. The observed stationary wave anomalies over the Pacific/North American (PNA) region are found to be similar during the four winters despite appreciable differences in sea surface temperatures. The anomalous transient vorticity fluxes are remarkably robust over the North Pacific during each event, with an eastward extension of the climatological storm track leading to strong cyclonic forcing near 40-degrees-N, 150-degrees-W. This forcing is in phase with the seasonal mean Aleutian trough anomaly suggesting the importance of eddy-mean flow interactions. By comparison, the intersample variability of the GCM response over the PNA region is found to exceed the observed inter-El Nino variability. This stems primarily from a large variability in the model's anomalous transients over the North Pacific.
Houghton, R. W., C. N. Flagg and L. J. Pietrafesa, 1994: Shelf-Slope Water Frontal Structure, Motion and Eddy Heat-Flux in the Southern Middle Atlantic Bight. Deep-Sea Research Part II-Topical Studies in Oceanography, 41(2-3): 273-306.
Results of an analysis of SEEP-II temperature and velocity data to investigate the shelf-slope frontal structure, motion and eddy heat flux in the southern Middle Atlantic Bight are presented. In the winter and spring the inclination of the frontal boundary and the vertical velocity shear, when averaged over periods greater than a week, approximately satisfy the Margules' equation. Associated with the locally wind driven cross-shelf excursions (similar to 20 km) of the foot of the front are vertical displacements of the frontal boundary due to vorticity constraints imposed by the bottom topography. Some of the smaller-scale frontal motions suggest wave-like disturbances propagating southward along the continental margin. In April 1988 the most severe wind event recorded during SEEP-II displaced the foot of the front into the upper slope region and injected shelf water into the interior of the slope water column.
Hunt, B. G., S. E. Zebiak and M. A. Cane, 1994: Experimental Predications of Climatic Variability for Lead Times of 12 Months. International Journal of Climatology, 14(5): 507-526.
Interannual variability of present-day climate has been clearly linked to the occurrence of large-scale sea-surface temperature anomalies, particularly in the Pacific Ocean. Prediction of such anomalies many seasons in advance by using relatively simple, coupled oceanic-atmospheric models provides a basis for predicting the atmospheric climate over the same period. This is achieved by inserting the sea-surface temperature anomalies predicted by the coupled model at appropriate time intervals into a global climatic model and integrating forward in time. Prediction experiments for 1991 and 1992 have been conducted on this basis using sea-surface temperature anomalies predicted for the Pacific Ocean' Global and regional rainfall outcomes are presented here, although the predictions are not expected to be valid for the whole globe. Detailed results are given for Australia and southern Africa, which experienced severe drought conditions in 1991 and 1992. Useful skill, with statistical significance for predictions when substantial rainfall anomalies occurred, was obtained for these regions, indicating the potential utility of the method. Considerable scope exists for improvements in the technique presented here, particularly in the global climatic model used, so that increasing accuracy should result as the technique is developed. Operational implementation of this method is essentially straightforward.
Krupitsky, A. and M. A. Cane, 1994: On Topographic Pressure Drag in a Zonal Channel. Journal of Marine Research, 52(1): 1-23.
The effect of bottom topography H on the barotropic transport in a periodic zonal channel is studied. An asymptotic approximation is found for the zonal transport on an f-plane and a beta-plane when all f/H isolines are blocked by the zonal walls. It is shown that to leading order, the zonal channel transport is independent of friction. In this it is similar to the Sverdrup transport in a basin. To leading order, the transport is proportional to the bottom topographic wavelength, and inversely proportional to the height of the topography and to R, the range of values of f/H that exists on both sides of the channel. For sufficiently high topography the transport varies inversely with the topographic height squared. The analytic results are verified by numerical experiments.
Kushnir, Y., 1994: Interdecadal Variations in North-Atlantic Sea-Surface Temperature and Associated Atmospheric Conditions. Journal of Climate, 7(1): 141-157.
Evidence is presented for a distinct pattern of ocean-atmosphere relationship associated with interdecadal variability in the North Atlantic region. Using a century of surface marine observations it is shown that middle- and high-latitude sea surface temperature (SST) display a long-term fluctuation with negative anomalies before 1920, and during the 1970s and 1980s. Positive SST conditions prevailed from about 1930 to 1960. The pattern of interdecadal SST variability is constructed by subtracting the average field during 15 cold years from that during a similar interval of warm years. The early-century warming and the more recent cooling display a similar spatial pattern. In both cases the pattern is basin scale and largely of one polarity, with maxima in the vicinity of Iceland, in the Labrador Sea, and northeast of Bermuda.
Latif, M., T. P. Barnett, M. A. Cane, M. Flugel, N. E. Graham, H. Vonstorch, J. S. Xu and S. E. Zebiak, 1994: A Review of ENSO Prediction Studies. Climate Dynamics, 9(4-5): 167-179.
A hierarchy of ENSO (El Nino/Southern Oscillation) prediction schemes has been developed which includes statistical schemes and physical models. The statistical models are, in general, based on advanced statistical techniques and can be classified into models which use either low-frequency variations in the atmosphere (sea level pressure or surface wind) or upper ocean heat content as predictors. The physical models consist of coupled ocean-atmosphere models of varying degrees of complexity, ranging from simplified coupled models of the 'shallow water'-type to coupled general circulation models. All models, statistical and physical, perform considerably better than the persistence forecast on predicting typical indices of ENSO on lead times of 6 to 12 months. The most successful prediction schemes, the fully physical coupled ocean-atmosphere models, show significant prediction abilities at lead times exceeding one year period. We therefore conclude that ENSO is predictable at least one year in advance. However, all of this applies to gross indices of ENSO such as the Southern Oscillation Index. Despite the demonstrated predictability, little is known about the predictability of specific features known to be associated with ENSO (e.g. Indian Monsoon rainfall, Southern African drought, or even off-equatorial sea surface temperature). Nor has the relative importance for prediction of different regional anomalies or different physical processes yet been established. A seasonal dependence in predictability is well established, but the processes responsible for it are not fully understood.
McPhee, M. G. and D. G. Martinson, 1994: Turbulent Mixing under Drifting Pack Ice in the Weddell Sea. Science, 263(5144): 218-221.
By providing cold, dense water that sinks and mixes to fill the abyssal world ocean, high-latitude air-sea-ice interaction is the main conduit through which the deep ocean communicates with the rest of the climate system. A key element in modeling and predicting oceanic impact on climate is understanding the processes that control the near surface exchange of heat, salt, and momentum. In 1992, the United States-Russian Ice Station Weddell-1 traversed the western Weddell Sea during the onset of winter, providing a platform for direct measurement of turbulent heat flux and Reynolds stress in the upper ocean. Data from a storm early in the drift indicated (i) well-formed Ekman spirals (in both velocity and turbulent stress); (ii) high correlation between mixed layer heat flux and temperature gradients; (iii) that eddy viscosity and eddy thermal diffusivity were similar, about 0.02 square meters per second; and (iv) that the significant turbulent length scale (2 to 3 meters through most of the boundary layer) was proportional to the wavelength at the peak in the weighted vertical velocity spectrum. The measurements were consistent with a simple model in which the bulk eddy viscosity in the neutrally buoyant mixed layer is proportional to kinematic boundary stress divided by the Coriolis parameter.
Ou, H. W., 1994: Flow near a Continental Boundary Driven by an Oceanic Jet. Journal of Physical Oceanography, 24(5): 966-978.
To provide possible dynamical interpretations of the Gulf Stream-induced circulation in the Middle Atlantic Bight (MAB), the inshore flow driven by a steady and straight jet in a homogeneous ocean is considered via similarity solutions.
Pietrafesa, L. J., J. M. Morrison, M. P. Mccann, J. Churchill, E. Bohm and R. W. Houghton, 1994: Water Mass Linkages between the Middle and South-Atlantic Bights. Deep-Sea Research Part II-Topical Studies in Oceanography, 41(2-3): 365-.
Time and frequency domain analyses are used to relate coastal meteorological data with 7 years of daily surface temperature and salinity collected at three coastal light stations; offshore of the mouth of Chesapeake Bay, Virginia, on Diamond Shoals, at Cape Hatteras, North Carolina and on Frying Pan Shoals, off Cape Fear, North Carolina. Salinity fluctuations at Diamond Shoals are highly correlated with alongshore wind stress, implying wind driven advection of the front between Virginia Coastal Water (VCW) and Carolina Coastal Water (CCW) across Diamond Shoals. The data collected at Diamond Shoals indicate that more than half the time there is significant encroachment of Mid Atlantic Bight water into the South Atlantic Bight around Cape Hatteras, contrary to the notion that VCW is entirely entrained into the Gulf Stream. In fact, VCW can appear as far south as Frying Pan Shoals, thereby extending across the entire North Carolina Capes inner to mid shelf. Temperature and salinity time series also indicate that water masses overlying Diamond Shoals respond quickly to cross-shelf winds. Cross-shelf wind stress is significantly correlated with surface water temperature at Diamond Shoals, for periods between 2 and 12 days. Changes in temperature can be brought about by wind-driven cross-shelf circulation and by wind-induced upwelling. Seasurface temperature satellite (AVHRR) imagery taken during the SEEP II confirm these concepts.
Seager, R. and M. B. Blumenthal, 1994: Modeling Tropical Pacific Sea-Surface Temperature with Satellite-Derived Solar Radiative Forcing. Journal of Climate, 7(12): 1943-1957.
Two independent datasets for the solar radiation at the surface derived from satellites are compared. The data derived from the Earth Radiation Budget Experiment (ERBE) is for the net solar radiation at the surface whereas the International Satellite Cloud Climatology Project (ISCCP) data is for the downward flux only and was corrected with a space- and time-varying albedo. The ISCCP net flux is at all times higher than the ERBE flux. The difference can be divided into an offset that decreases with latitude and another component that correlates with high tropical cloud cover. With this latter exception the two datasets provide spatial patterns of solar flux that are very similar.
Seager, R. and S. E. Zebiak, 1994: Convective Interaction with Dynamics in a Linear Primitive Equation Model. Journal of the Atmospheric Sciences, 51(10): 1307-1331.
A new global atmosphere model purpose designed for climate studies is introduced. The model is solved in terms of the normal modes of the linearized primitive equations on a sphere, which allows use of long time steps without introducing computational instability or phase errors of the linear wave components. The model is tested by attempting to simulate the tropical intraseasonal oscillation using an idealized sea surface temperature distribution. Simple treatments of radiation and boundary-layer processes are used together with the much more complete Betts-Miller convection scheme. The Betts-Miller scheme maintains the atmosphere in a state of near neutrality to reversible saturated ascent. It is found that for different values of the surface evaporation time scale, either the evaporation-wind feedback mechanism postulated by Neelin et al. and Emmanuel or low-level convergence of moisture can create eastward propagating deep convective modes. In general, both mechanisms seem important, but it is the latter mechanism that provides phase speeds more in line with observations. Moisture convergence in this model works to erode the low-level equivalent potential temperature inversion that is ubiquitous in nonconvecting regions, thus triggering convection. In contrast to CISK models, changes in boundary-layer equivalent potential temperature are essential in this model to create propagating modes.
Sennechael, N., C. Frankignoul and M. A. Cane, 1994: An Adaptive Procedure for Tuning a Sea-Surface Temperature Model. Journal of Physical Oceanography, 24(11): 2288-2305.
To determine the value of the adjustable parameters of an ocean model required to optimally fit the observations, an adaptive inverse method is developed and applied to a sea surface temperature (SST) model of the tropical Atlantic. The best-fit calculation is performed by minimizing a misfit between observed and simulated data, which depends on the observational and the modeling errors. An adaptive procedure is designed in which the model being tuned is also used to construct a model of the observational errors. This is done by performing the optimization on the mean seasonal cycle and using the SST anomalies obtained for different years and plausible forcing fields as additional information to construct a sample estimate of the observational error covariance matrix. Assuming idealized modeling errors, the procedure is applied to the SST model of Blumenthal and Cane, yielding refined estimates for several models and heat flux parameters. The simulation of the mean annual SST is improved, but not the simulation of seasonal and interannual variability. The model-observation discrepancies remain too large to be solely attributed to atmospheric and oceanic data uncertainties and are linked to the model's rudimentary geometry and its incorrect representation of SST cooling by upwelling. The existence of larger model deficiencies than was originally assumed in the model errors is confirmed by a statistical test of the correctness of the assumptions in the inverse calculation.
Shaw, P. T., L. J. Pietrafesa, C. N. Flagg, R. W. Houghton and K. H. Su, 1994: Low-Frequency Oscillations on the Outer Shelf in the Southern Mid-Atlantic Bight. Deep-Sea Research Part II-Topical Studies in Oceanography, 41(2-3): 253-271.
Current meter data collected during the SEEP-II spring experiment off the coasts of Delaware, Maryland and Virginia were analyzed to examine the low-frequency phase propagation on the outer shelf and upper slope. In the low frequency band (periods between 24 and 12 days), there was a distinct offshore phase propagation in the data. The amplitude was nearly constant on the shelf and decreased offshore on the slope. In the vertical direction, there was an upward phase propagation on the shelf. These oscillations were probably topographic waves generated near the shelf-slope front. In the synoptic band (periods between 8 and 4.8 days), oscillation amplitudes were higher on the slope than on the shelf; the phase was nearly constant across the shelf and slope. The coherent motions were probably associated with the oscillations of the shelf-slope front. In the wind band with periods shorter than 4 days, the amplitude decreased rapidly offshore, similar to coastally trapped waves. Offshore veering of currents in the bottom boundary layer was observed at the 400 and 1000 m isobaths and was probably related to low-frequency oscillations at mid-depths.
Ting, M. F., 1994: Maintenance of Northern Summer Stationary Waves in a GCM. Journal of the Atmospheric Sciences, 51(22): 3286-3308.
The maintenance of northern summer stationary waves is studied with data from a 15-year integration of the general circulation model (GCM) experiment performed at the Geophysical Fluid Dynamics Laboratory. The model has relatively high resolution (rhomboidal 30 wavenumbers, 9 vertical levels) and simulates the summertime stationary waves reasonably well.
Tziperman, E., L. Stone, M. A. Cane and H. Jarosh, 1994: El Niño Chaos - Overlapping of Resonances between the Seasonal Cycle and the Pacific Ocean-Atmosphere Oscillator. Science, 264(5155): 72-74.
The El Nino-Southern Oscillation (ENSO) cycle is modeled as a low-order chaotic process driven by the seasonal cycle. A simple model suggests that the equatorial Pacific ocean-atmosphere oscillator can go into nonlinear resonance with the seasonal cycle and that with strong enough coupling between the ocean and the atmosphere, the system may become chaotic as a result of irregular jumping of the ocean-atmosphere system among different nonlinear resonances. An analysis of a time series from an ENSO prediction model is consistent with the low-order chaos mechanism.
Xue, Y., M. A. Cane, S. E. Zebiak and M. B. Blumenthal, 1994: On the Prediction of ENSO - a Study with a Low-Order Markov Model. Tellus Series a-Dynamic Meteorology and Oceanography, 46(4): 512-528.
A linear model best fit to the Zebiak and Cane (1987) ENSO forecast model (ZC) is used to study the model's prediction skill. Multivariate empirical orthogonal functions (MEOFs) obtained from the sea surface temperature anomaly, sea level and wind stress anomaly fields in a suite of 3-year forecast runs of ZC starting from the monthly initial conditions in the period January 1970 to December 1991, are used to construct a series of seasonally varying linear Markov models. It is found that the model with 18 MEOFs fits the original nonlinear model reasonably well and has comparable or better forecast skill. Assimilating the observed SST into the initial conditions further improves forecast skill at short lead times (< 9 months). The transient initial error growth in the model's prediction is attributed to the non-self-adjoint property as in Farrell and Blumenthal. Initial error grows fastest starting from spring and slowest starting from late summer and is sensitive to the initial error structures. Two singular vectors (SVs) of the linear evolution operator have significant transient growth dominating the total error growth. Since the optimal perturbation (fastest SV) has mostly high MEOF components, the error growth tends to be larger when there are more high mode components in the initial error fields. This result suggests a way to filter the initial condition fields: the MEOFs higher than the 18th in the initial fields are mostly noise and removing them improves prediction skill. The forecasts starting from late summer have the best predictability because the fastest growth season (summer) is just avoided. The well known, very rapid decline in forecast skill in the boreal spring (the ''spring barrier'') is here attributed to the smallness of the signal to be forecast: the standard deviation of the NINO3 SST anomaly is smallest in spring.
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