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James A. Carton

Abstract

POLYMODE data is used to produce a new analysis of multilevel streamfunctions in a (500 km)2 domain during July 1977–August 1978. The analysis is used to define initial and lateral boundary conditions, and verification fields for six predictability experiments. These experiments are designed to determine the accuracy of current techniques of forecasting and hindcasting the circulation in a limited domain and to define the sources of error.

A forecast based on persistence is shown to be reasonably accurate only for ten days. A quasi-geostrophic model with persistence boundary conditions can maintain the same accuracy in the inner (125 km)2 to 23 days. If the boundary information is updated, the mot-mean-square error can be maintained below 60% throughout the (500 km)2 region for at least 120 days. The initial state of the circulation only influences the first 30 days of a hindcast.

The instabilities in the circulation and inaccuracies in the specification of the boundary conditions place a lower bound on the error of streamfunction predictions of 35%. Thus a doubling of the accuracy of hindcasts and even greater improvements for forecasts appear to be possible if the accuracy of the model and boundary data can be improved.

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Jiande Wang and James A. Carton

Abstract

Here, seasonal heat transport in the North Pacific and North Atlantic Oceans is compared using a 49-year-long analysis based on data assimilation. In midlatitudes surface heat flux is largely balanced by seasonal storage, while equatorward of 15°N, divergence of heat transport balances seasonal storage. The seasonal cycle of heat transport in the Pacific is in phase with the annual migration of solar radiation, transporting heat from the warm hemisphere to the cool hemisphere. Analysis shows that the cycle is large with peak-to-peak shifts of 5 PW. To examine the cause of these large shifts, a vertical and zonal decomposition of the heat budget is carried out. Important contributions are found from the annual cycle of wind drift in the mixed layer and adiabatically compensating return flow, part of the vigorous shallow tropical overturning cell. The annual cycle of heat transport in the North Atlantic is also large. Here too, wind-driven transports play a role, although not as strongly as in the Pacific, and this is an important reason for the differences in heat transport between the basins. Analysis shows the extent to which seasonally varying geostrophic currents and seasonal diabatic effects are relatively more important in the Atlantic. Thus, although the annual cycle of zonally integrated mass transport in the mixed layer is only 1/5 as large, the time-averaged heat transport is nearly as large as in the Pacific. This difference in transport mechanics gives rise to changes in the phase of seasonal heat transport with latitude in the Atlantic.

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James A. Carton and Bohua Huang

Abstract

Sea surface temperature in the eastern equatorial Atlantic Ocean undergoes anomalous warming events of 1°–2°C every few years. The warm anomalies reach their maximum strength in Northern Hemisphere summer, when equatorial upwelling normally brings cold thermocline water to the surface. By compositing surface observations from a 28-year record, we are able to identify consistent features in anomalies of SST and winds. The composites show that the SST anomalies in northern summer are confined to the eastern equatorial region, with reduced zonal winds to the west and reduced northward trade winds to the east. Accompanying these changes in winds are enhanced convection near the equator caused by a southward shift and intensification of the intertropical convergence zone. Later in the year, SST south of the equator becomes elevated. As a result, by spring of the year following the equatorial anomaly, convection in the western side of the basin is much higher than normal.

To understand the ocean dynamics that give rise to these warm anomalies we examine a simulation of the ocean circulation during the 1980s. The authors find that the cause of the warm event in 1984 stretches back to the intense trade winds during the summer and fall of 1983. The unusual winds led to Ekman deepening of the thermocline in the west on both sides of the equator. Late in 1983 the trade winds in the west relaxed, which led to a surge of warm water eastward along the equatorial waveguide. The arrival of anomalous warm water deepened the thermocline throughout the eastern Gulf of Guinea in early 1984 and gradually spread southward and back into the interior basin throughout that year. Secondary factors in elevating equatorial SST were the local advection of warm surface water from the north and a reduction of advection of cool coastal water from the east. In contrast with 1984, the anomalous warming of 1988 seems to have been largely the result of changes in the equatorial winds during spring of the same year. These wind anomalies are likely, themselves, to have resulted from the increase in SST to the east. During both years anomalous deepening of the thermocline in the east (however it was caused) prevented the normal seasonal cooling of the equatorial waters and thus led to elevated SSTs. The eastward shift of heat also had important consequences for the coastal regions of southern Africa.

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James A. Carton and Eric C. Hackert

Abstract

A hydrographic dataset based on data from the SEQUAL/FOCAL experiment is used to determine the errors of a numerical simulation of the complete temperature and velocity fields of the tropical Atlantic during the two-year period 1983–84. To improve the accuracy of the analysis we develop an application of four-dimensional data assimilation. In this analysis the thermal fields of the model are updated once a month using sea surface temperature measurements and observed temperature profiles.

Much of the paper describes comparisons between differing analyses using data assimilation and the numerical simulation, and verification of these with temperature and velocity data from moored instruments. Assimilation of the temperature observations improves the accuracy of the temperature analysis. The amplitude of seasonal changes in the meridional thermal gradient is doubled at 38°W, bringing the analysis closer to the observed thermal gradient. At 28°W the improved is less dramatic. The zonal thermal gradient at the equator is increased, but the month-to-month variability also significantly.

Comparisons are made with temperature and velocity measurements at midbasin mooring sites. Here assimilation sharpens and lowers the thermocline and reduces long-term trends in the thermal field. Assimilation also improves some features of the velocity field such as the depth and eastward penetration of the undercurrent core and the strength of the North Equatorial Countercurrent.

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Jung-Moon Yoo and James A. Carton

Abstract

Annual and interannual variations of the moisture and freshwater budgets are examined in the tropical Atlantic Ocean and the Caribbean Sea. The seasonal moisture budget (EP) is calculated by estimating precipitation from 11 years of outgoing longwave radiation data (1974–85), and subtracting evaporation estimated from surface data. Consistent with previous estimates, we find annual mean deficits of freshwater in the tropical Atlantic and Caribbean Sea.

The seasonal cycle of freshwater flux in both regions has strong annual and semiannual variations caused by shifts of the intertropical convergence zone (ITCZ). In the tropical Atlantic 20°S–20°N, monthly rainfall varies by 3 cm/month with the strongest rainfall occurring in May and October. Significant inconsistencies between results of the present study and the seasonal rainfall estimates of Dorman and Bourke are found. Evaporation varies by half as much as rainfall, while runoff has little seasonal fluctuation. The annual cycle of the net moisture balance dominates most of the tropical Atlantic region except near the annual mean position of the ITCZ at 5°N. In the Caribbean Sea, the freshwater flux is greatest in June and September.

The interannual variability of freshwater flux during the period 1974 to 1979 is examined. Seasonal or interannual variations of rainfall account for two-thirds of the variations of the freshwater flux in the tropical Atlantic. The least rainfall in the region during the 11-year record occurred in the Niño years of 1976–77 and 1982–83, while the wettest year was 1984.

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Bohua Huang, James A. Carton, and J. Shukla

Abstract

The authors have examined a numerical simulation of the tropical Atlantic Ocean circulation forced by the European Centre for Medium-Range Weather Forecasts (ECMWF) surface wind stress during 1980–88. The mean state and annual cycle of the ocean are realistically simulated by the model. The simulated interannual variability of sea surface temperature (SST) is also remarkably consistent with the observations, particularly the observed patterns of the basinwide warm/cold periods and the variation of dipole pattern and the associated meridional SST gradient. Discrepancies between observed and simulated SST anomalies are large early in the simulation, which seems caused by errors in the ECMWF wind analysis during that period.

The low-frequency fluctuations of the meridional SST gradient associated with the dipole pattern during 1980–88 were caused by opposite SST anomalies between hemispheres, forced by out of phase fluctuations of the trade winds. Specifically, the southeast trade winds were anomalously strong during 1981–83 and weaker than normal during 1985–86 and 1987–88. The northeast trade winds, on the other hand, showed nearly opposite variation, being weak in 1980–83 and strong in 1985–86. In the northern ocean, SST was higher during 1980–83 but lower during 1985–86 as the local trade winds were weak and strong. On the other hand, as the southeast trades and the equatorial easterlies were strong in 1981–83, the slope of the thermocline was anomalously steep along the equator and both the South Equatorial Current and the Equatorial Undercurrent intensified. Forced by the anomalous equatorial easterlies, warm water diverged from the equator into the Tropics in the western ocean. In the southeastern portion of the basin, the thermocline was shallow and SST was anomalously low. When the southeast trade winds were weakened in 1984, warm water converged toward the equator from both hemispheres, and then shifted into the southeast part of the ocean. The heat anomalies were maintained there during 1985/86, when the southeast trades were weak, deepening the thermocline and causing anomalously high SST. Therefore, unlike those in the northern Tropics, SST fluctuations in the southeastern part of the ocean are related to the basinwide adjustment of the ocean and net heat transport across the equator in response to the equatorial wind anomalies.

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James A. Carton, Gennady Chepurin, and Xianhe Cao

Abstract

The authors explore the accuracy of a comprehensive 46-year retrospective analysis of upper-ocean temperature, salinity, and currents. The Simple Ocean Data Assimilation (SODA) analysis is global, spanning the latitude range 62°S–62°N. The SODA analysis has been constructed using optimal interpolation data assimilation combining numerical model forecasts with temperature and salinity profiles (MBT, XBT, CTD, and station), sea surface temperature, and altimeter sea level. To determine the accuracy of the analysis, the authors present a series of comparisons to independent observations at interannual and longer timescales and examine the structure of well-known climate features such as the annual cycle, El Niño, and the Pacific–North American (PNA) anomaly pattern.

A comparison to tide-gauge time series records shows that 25%–35% of the variance is explained by the analysis. Part of the variance that is not explained is due to unresolved mesoscale phenomena. Another part is due to errors in the rate of water mass formation and errors in salinity estimates. Comparisons are presented to altimeter sea level, WOCE global hydrographic sections, and to moored and surface drifter velocity. The results of these comparisons are quite encouraging. The differences are largest in the eddy production regions of the western boundary currents and the Antarctic Circumpolar Current. The differences are generally smaller in the Tropics, although the major equatorial currents are too broad and weak.

The strongest basin-scale signal at interannual periods is associated with El Niño. Examination of the zero-lag correlation of global heat content shows the eastern and western tropical Pacific to be out of phase (correlation −0.4 to −0.6). The eastern Indian Ocean is in phase with the western Pacific and thus is out of phase with the eastern Pacific. The North Pacific has a weak positive correlation with the eastern equatorial Pacific. Correlations between eastern Pacific heat content and Atlantic heat content at interannual periods are modest. At longer decadal periods the PNA wind pattern leads to broad patterns of correlation in heat content variability. Increases in heat content in the central North Pacific are associated with decreases in heat content in the subtropical Pacific and increases in the western tropical Pacific. Atlantic heat content is positively correlated with the central North Pacific.

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James A. Carton, Gennady Chepurin, Xianhe Cao, and Benjamin Giese

Abstract

The authors describe a 46-year global retrospective analysis of upper-ocean temperature, salinity, and currents. The analysis is an application of the Simple Ocean Data Assimilation (SODA) package. SODA uses an ocean model based on Geophysical Fluid Dynamics Laboratory MOM2 physics. Assimilated data includes temperature and salinity profiles from the World Ocean Atlas-94 (MBT, XBT, CTD, and station data), as well as additional hydrography, sea surface temperature, and altimeter sea level.

After reviewing the basic methodology the authors present experiments to examine the impact of trends in the wind field and model forecast bias (referred to in the engineering literature as “colored noise”). The authors believe these to be the major sources of error in the retrospective analysis. With detrended winds the analysis shows a pattern of warming in the subtropics and cooling in the Tropics and at high latitudes. Model forecast bias results partly from errors in surface forcing and partly from limitations of the model. Bias is of great concern in regions of thermocline water-mass formation. In the examples discussed here, the data assimilation has the effect of increasing production of these water masses and thus reducing bias.

Additional experiments examine the relative importance of winds versus subsurface updating. These experiments show that in the Tropics both winds and subsurface updating contribute to analysis temperature, while in midlatitudes the variability results mainly from the effects of subsurface updating.

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Allan R. Robinson, James A. Carton, Nadia Pinardi, and Christopher N. K. Mooers

Abstract

In order to perform real-time dynamical forecasts and hindcasts, three high-resolution hydrographic surveys were made of a (150 km)2 domain off northern California, providing two sets of initialization and verification fields. The data was objectively analyzed and regularly gridded for model compatibility. These maps initially show an anticyclonic eddy segment in the northeast and part of another in the northwest. Two weeks later only the northwest anticyclonic eddy remained, with the domain center dominated by a 0.6 m s−1 jet. Two weeks after that only a larger northwest eddy with fairly weak velocities remained. Numerical forecasts with persistent boundary conditions and forecast experiments with boundary conditions linearly interpolated between surveys were performed. The real-time forecast successfully predicted the formation of the zonal jet prior to its observation. Dynamical interpolation shows unambiguously that the two anticyclonic eddies have merged and formed a single eddy. Even the forecast with incorrect boundary conditions demonstrates the internal dynamical processes involved in the merger event.

Two examples are given of four-dimensional data assimilation: direct insertion and a backward-forward combination technique. These results justify the use of the dynamical forecasts as synoptic time series. Parameter sensitivity experiments were performed to determine the sensitivity of the model to physical parameters such as stratification, to explore the dynamical balance, and to choose a reference level. The dynamics were found to be controlled by horizontal nonlinear interactions. A reference level of 1550 m was chosen. A set of energy and vorticity equations, consistent with quasi-geostrophic dynamics, were evaluated term by term for the forecast experiments. The evolutions of the streamfunction and vorticity fields are shown to be a three-phase (merging, expanding, and relaxation) process. Available gravitational energy increases due to buoyancy work; the merger event is interpreted as a finite amplitude barotropic instability process.

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Benjamin K. Johnson, Frank O. Bryan, Semyon A. Grodsky, and James A. Carton

Abstract

Six subtropical salinity maxima (S max) exist: two each in the Pacific, Atlantic, and Indian Ocean basins. The north Indian (NI) S max lies in the Arabian Sea while the remaining five lie in the open ocean. The annual cycle of evaporation minus precipitation (EP) flux over the S max is asymmetric about the equator. Over the Northern Hemisphere S max, the semiannual harmonic is dominant (peaking in local summer and winter), while over the Southern Hemisphere S max, the annual harmonic is dominant (peaking in local winter). Regardless, the surface layer salinity for all six S max reaches a maximum in local fall and minimum in local spring. This study uses a multidecade integration of an eddy-resolving ocean circulation model to compute salinity budgets for each of the six S max. The NI S max budget is dominated by eddy advection related to the evolution of the seasonal monsoon. The five open-ocean S max budgets reveal a common annual cycle of vertical diffusive fluxes that peak in winter. These S max have regions on their eastward and poleward edges in which the vertical salinity gradient is destabilizing. These destabilizing gradients, in conjunction with wintertime surface cooling, generate a gradually deepening wintertime mixed layer. The vertical salinity gradient sharpens at the base of the mixed layer, making the water column susceptible to salt finger convection and enhancing vertical diffusive salinity fluxes out of the S max into the ocean interior. This process is also observed in Argo float profiles and is related to the formation regions of subtropical mode waters.

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