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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

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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, 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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Martina Ricko, James A. Carton, and Charon Birkett

Abstract

The availability of satellite estimates of rainfall and lake levels offers exciting new opportunities to estimate the hydrologic properties of lake systems. Combined with simple basin models, connections to climatic variations can then be explored with a focus on a future ability to predict changes in storage volume for water resources or natural hazards concerns. This study examines the capability of a simple basin model to estimate variations in water level for 12 tropical lakes and reservoirs during a 16-yr remotely sensed observation period (1992–2007). The model is constructed with two empirical parameters: effective catchment to lake area ratio and time delay between freshwater flux and lake level response. Rainfall datasets, one reanalysis and two satellite-based observational products, and two radar-altimetry-derived lake level datasets are explored and cross checked. Good agreement is observed between the two lake level datasets with the lowest correlations occurring for the two small lakes Kainji and Tana (0.87 and 0.89). Fitting observations to the simple basin model provides a set of delay times between rainfall and level rise ranging up to 105 days and effective catchment to lake ratios ranging between 2 and 27. For 9 of 12 lakes and reservoirs the observational rainfall products provide a better fit to observed lake levels than the reanalysis rainfall product. But for most of the records any of the rainfall products provide reasonable lake level estimates, a result which opens up the possibility of using rainfall to create seasonal forecasts of future lake levels and hindcasts of past lake levels. The limitations of the observation sets and the two-parameter model are discussed.

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Semyon A. Grodsky and James A. Carton

Abstract

Recent observations from the QuikSCAT and Tropical Rainfall Measuring Mission satellites, as well as a longer record of Special Sensor Microwave Imager winds are used to investigate the existence and dynamics of a Southern Hemisphere partner to the intertropical convergence zone in the tropical Atlantic Ocean. The southern intertropical convergence zone extends eastward from the coast of Brazil in the latitude band 10°–3°S and is associated with seasonal precipitation exceeding 6 cm month−1 during peak months over a part of the ocean characterized by high surface salinity. It appears in austral winter when cool equatorial upwelling causes an anomalous northeastward pressure gradient to develop in the planetary boundary layer close to the equator. The result is a zonal band of surface wind convergence that exceeds 10−6 s−1, with rainfall stronger than 2 mm day−1, and an associated decrease in ocean surface salinity of 0.2 parts per thousand.

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Benjamin S. Giese and James A. Carton

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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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