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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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Zengxi Zhou and James A. Carton

Abstract

This study examines the impact of the moisture exchange between the oceanic and atmospheric boundary layers on instabilities of the atmosphere–ocean system. Wind speed-sensitive evaporation can affect these instabilities in two ways. First, it can change atmospheric heating and thus modify the atmospheric wind response. Second, it can change the mixed layer heat budget and thus affect SST. Here the authors show that wind speed-sensitive evaporation produces a new unstable, westward-propagating SST mode with a growth rate of (4 month)−1 for standard parameters. These two processes, alternatively, act to stabilize the leading unstable mixed SST–dynamics mode if each is considered separately. However, the strongest instability of this mixed SST–dynamics mode occurs when the first process is relatively weak and the second is strong. The authors extend the work to consider the impact of wind speed-sensitive evaporation on the intermediate coupled model of Zebiak and Cane. The results from this model are similar to those obtained in the free mode analysis.

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Jiande Wang and James A. Carton

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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 Anthony Santorelli

Abstract

This paper examines nine analyses of global ocean 0-/700-m temperature and heat content during the 43-yr period of warming, 1960–2002. Among the analyses are two that are independent of any numerical model, six that rely on sequential data assimilation, including an ocean general circulation model, and one that uses four-dimensional variational data assimilation (4DVAR), including an ocean general circulation model and its adjoint. Most analyses show gradual warming of the global ocean with an ensemble trend of 0.77 × 108 J m−2 (10 yr)−1 (=0.24 W m−2) as the result of rapid warming in the early 1970s and again beginning around 1990. One proposed explanation for these variations is the effect of volcanic eruptions in 1963 and 1982. Examination of this hypothesis suggests that while there is an oceanic signal, it is insufficient to explain the observed heat content variations.

A second potential cause of decadal variations in global heat content is the uncorrelated contribution of heat content variations in individual ocean basins. The subtropical North Atlantic is warming at twice the global average, with accelerated warming in the 1960s and again beginning in the late 1980s and extending through the end of the record. The Barents Sea region of the Arctic Ocean and the Gulf of Mexico have also warmed, while the western subpolar North Atlantic has cooled. Heat content variability in the North Pacific differs significantly from the North Atlantic. There the spatial and temporal patterns are consistent with the decadal variability previously identified through observational and modeling studies examining SST and surface winds. In the Southern Hemisphere large heat content anomalies are evident, and while there is substantial disagreement among analyses on average the band of latitudes at 30°–60°S contribute significantly to the global warming trend. Thus, the uncorrelated contributions of heat content variations in the individual basins are a major contributor to global heat content variations.

A third potential contributor to global heat content variations is the effect of time-dependent bias in the set of historical observations. This last possibility is examined by comparing the analyses to the unbiased salinity–temperature–depth dataset and finding a very substantial warm bias in all analyses in the 1970s relative to the latter decades. This warm bias may well explain the rapid increase in analysis heat content in the early 1970s, but not the more recent increase, which began in the early 1990s.

Finally, this study provides information about the similarities and differences between analyses that are independent of a model and those that use sequential assimilation and 4DVAR. The comparisons provide considerable encouragement for the use of the sequential analyses for climate research despite the presence of erroneous variability (also present in the no-model analyses) resulting from instrument bias. The strengths and weaknesses of each analysis need to be considered for a given application.

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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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Jiande Wang and James A. Carton

Abstract

Climate variability in the tropical Atlantic sector as represented in six atmospheric general circulation models is examined. On the annual mean, most simulations overestimate wind stress away from the equator although much of the variability can be accounted for by differences in drag formulations. Most models produce excessive latent heat flux as a consequence of errors in boundary layer humidity. Systematic errors are also evident in precipitation and surface wind divergence fields. The seasonal cycle of the zonal trade winds is stronger than observed in most simulations, while the meridional component is well represented.

Next interannual variability is considered, focusing on two tropical patterns (Atlantic Niño and interhemispheric modes). The directions of the surface wind anomalies in the models are found to be generally similar to observations, although the magnitude of the wind stress response varies greatly among models. However, all models fail to reproduce the wind–latent heat feedback believed to be essential to interannual variability in this basin.

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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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Benjamin S. Giese and James A. Carton

Abstract

Forty-four years of mechanical and expendable bathythermograph observations are assimilated into a general circulation model of the Pacific Ocean. The model is run from 1950 through 1993 with forcing at the surface from observed monthly mean wind stress and temperature. The resulting analysis is used to describe the spatial and temporal patterns of variability at interannual and decadal periods. Interannual variability has its largest surface temperature expression in the Tropics and decadal variability has its largest surface temperature expression in the midlatitude Pacific. However, there are important interannual surface temperature changes that occur in the midlatitude Pacific and there are important decadal surface temperature changes in the Tropics.

An empirical orthogonal function (EOF) analysis of model data that has been bandpass filtered to retain energy at periods of 1–5 yr and at periods greater than 5 yr is presented. The results suggest that interannual variability is dominated by a positive feedback mechanism in the Tropics and a negative feedback mechanism in the midlatitude ocean, resulting in larger anomalies in the Tropics. A second EOF analysis of model data that has been low-pass filtered to retain periods greater than 5 yr reveals patterns of wind and surface temperature anomalies that have strikingly similar structure to the interannual patterns; however, the sequencing between the first and second EOFs is different. Even though there are large decadal anomalies of wind stress in the Tropics, the largest anomalies of surface temperature and ocean heat content occur at mid- and high latitudes. The EOF analysis indicates that decadal variability has a positive feedback mechanism that operates in the midlatitude ocean and a negative feedback mechanism that operates in the Tropics, so that the largest temperature anomalies are in midlatitudes. Previous studies have cited the contribution of heat flux anomalies as the primary cause of decadal surface temperature anomalies. These model studies indicate that meridional advection of heat is at least as important. The timing of interannual and decadal changes in the atmosphere and in the ocean suggests that the atmosphere plays an important role connecting these phenomena. One interpretation of the results is that interannual and decadal variability are manifestations of the same climate phenomena but have crucially different feedback mechanisms.

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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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Benjamin S. Giese and James A. Carton

Abstract

A coupled ocean-atmosphere model is used to investigate the seasonal cycle of sea surface temperature and wind stress in the Tropics. A control run is presented that gives a realistic annual cycle with a cold tongue in the eastern Pacific and Atlantic Oceans. In an attempt to isolate the mechanisms responsible for the particular annual cycle that is observed, the authors conducted a series of numerical experiments in which they alter the solar forcing. These experiments include changing the longitude of perihelion, increasing the heat capacity of land, and changing the length of the solar year. The results demonstrate that the date of perihelion and land heating do not, by themselves control the annual cycle. However, there is a natural timescale for the development of the annual cycle. When the solar year is shortened to just 6 months, the seasonal variations of climate remain similar in timing to the control run except that they are weaker. When the solar year is lengthened to 18 months, surface temperature in the eastern Pacific develops a prominent semiannual cycle. The semiannual cycle results from the ITCZ crossing the equator into the Southern Hemisphere and the development of a Northern Hemisphere cold tongue during northern winter. The meridional winds maintain an annual cycle, while the zonal winds have a semiannual component. The Atlantic maintains an annual cycle in all variables regardless of changes in the length of the solar year. A final experiment addresses the factors determining the season in which upwelling occurs. In this experiment the sun is maintained perpetually over the equator (simulating March or September conditions). In this case the atmosphere and ocean move toward September conditions, with a Southern Hemisphere cold tongue and convection north of the equator.

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