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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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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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Jung-Moon Yoo and James A. Carton

Abstract

We develop a Spatially dependent formula to estimate rainfall from satellite-derived outgoing longwave radiation (OLR) data and the height of the base of the trade-wind inversion. This formula has been constructed by comparing rainfall records from twelve islands in the tropical Atlantic with 11 years of OLR data. Zonal asymmetries due to the differing cloud types in the eastern and western Atlantic and the presence of Saharan sand in the cast are included.

The climatological winter and summer rainfall derived from the above formula concurs with ship observations described by Dorman and Bourke. However, during the spring and fall, OLR-derived rainfall is higher than observations by 2–4 mm day−1 in the intertropical convergence zone. The largest discrepancy occurs during the fall in the region west of 28°W. Interannual anomalies of rainfall computed using this technique are large enough to cause potentially important changes in ocean surface salinity.

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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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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, Stephen G. Penny, and Eugenia Kalnay

Abstract

This study extends recent ocean reanalysis comparisons to explore improvements to several next-generation products, the Simple Ocean Data Assimilation, version 3 (SODA3); the Estimating the Circulation and Climate of the Ocean, version 4, release 3 (ECCO4r3); and the Ocean Reanalysis System 5 (ORAS5), during their 23-yr period of overlap (1993–2015). The three reanalyses share similar historical hydrographic data, but the forcings, forward models, estimation algorithms, and bias correction methods are different. The study begins by comparing the reanalyses to independent analyses of historical SST, heat, and salt content, as well as examining the analysis-minus-observation misfits. While the misfits are generally small, they still reveal some systematic biases that are not present in the reference Hadley Center EN4 objective analysis. We next explore global trends in temperature averaged into three depth intervals: 0–300, 300–1000, and 1000–2000 m. We find considerable similarity in the spatial structure of the trends and their distribution among different ocean basins; however, the trends in global averages do differ by 30%–40%, which implies an equivalent level of disagreement in net surface heating rates. ECCO4r3 is distinct in having quite weak warming trends while ORAS5 has stronger trends that are noticeable in the deeper layers. To examine the performance of the reanalyses in the Arctic we explore representation of Atlantic Water variability on the Atlantic side of the Arctic and upper-halocline freshwater storage on the Pacific side of the Arctic. These comparisons are encouraging for the application of ocean reanalyses to track ocean climate variability and change at high northern latitudes.

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D. A. S. Patil, Brian R. Hunt, and James A. Carton

Abstract

Computational modeling is playing an increasingly vital role in the study of atmospheric–oceanic systems. Given the complexity of the models a fundamental question to ask is, How well does the output of one model agree with the evolution of another model or with the true system that is represented by observational data? Since observational data contain measurement noise, the question is placed in the framework of time series analysis from a dynamical systems perspective. That is, it is desired to know if the two, possibly noisy, time series were produced by similar physical processes.

In this paper simple graphical representations of the time series and the errors made by a simple predictive model of the time series (known as residual delay maps) are used to extract information about the nature of the time evolution of the system (in this paper referred to as the dynamics). Two different uses for these graphical representations are presented in this paper. First, a test for the comparison of two competing models or of a model and observational data is proposed. The utility of this test is that it is based on comparing the underlying dynamical processes rather than looking directly at differences between two datasets. An example of this test is provided by comparing station data and NCEP–NCAR reanalysis data on the Australian continent.

Second, the technique is applied to the global NCEP–NCAR reanalysis data. From this a composite image is created that effectively identifies regions of the atmosphere where the dynamics are strongly dependent on low-dimensional nonlinear processes. It is also shown how the transition between such regions can be depicted using residual delay maps. This allows for the investigation of the conjecture of Sugihara et al.: sites in the midlatitudes are significantly more nonlinear than sites in the Tropics.

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Hailong Liu, Semyon A. Grodsky, and James A. Carton

Abstract

A monthly gridded analysis of barrier-layer and compensated-layer width based on observed vertical profiles of temperature and salinity and covering the period 1960–2007 is explored for evidence of subseasonal variability and its causes. In the subtropics and midlatitudes this variability is mostly evident during the local cold season when barrier layers and compensated layers are present. There is significant variability of anomalous (nonseasonal) barrier-layer and compensated-layer width on interannual periods, while in the North Pacific longer-term changes are also detectable. In the winter North Pacific a salinity-stratified barrier layer exists at subpolar latitudes. Farther south along the Kuroshio Extension a compensated layer exists. The width of the barrier layer varies from year to year by up to 60 m while compensated-layer width varies by half as much. During the observation period the barrier-layer width decreased in response to a strengthening of the Aleutian low pressure system, the resulting strengthening of dry northerly winds, and a decrease of precipitation. In contrast, the compensated-layer width increased in response to this pressure system strengthening and related amplification of the midlatitude westerly winds, the resulting increase of net surface heat loss, and its effect on the temperature and salinity of the upper-ocean water masses. The tropical Pacific, Atlantic, and Indian Oceans all have permanent barrier layers. Their interannual variability is less than 20 m but is comparable in magnitude to the time mean barrier-layer width in these areas. In the tropical Pacific west of 160°E and in the eastern tropical Indian Ocean, the barrier-layer width changes by approximately 5 m in response to a 10-unit change in the Southern Oscillation index. It thickens during La Niñas as a result of the presence of abundant rainfall and thins during dry El Niños. Interannual variations of barrier-layer width in the equatorial Pacific are weak east of 160°E with an exception of the area surrounding the eastern edge of the warm pool. Here subduction of salty water contributes to locally stronger variations of barrier-layer width.

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James A. Carton, Semyon A. Grodsky, and Hailong Liu

Abstract

A new monthly uniformly gridded analysis of mixed layer properties based on the World Ocean Atlas 2005 global ocean dataset is used to examine interannual and longer changes in mixed layer properties during the 45-yr period 1960–2004. The analysis reveals substantial variability in the winter–spring depth of the mixed layer in the subtropics and midlatitudes. In the North Pacific an empirical orthogonal function analysis shows a pattern of mixed layer depth variability peaking in the central subtropics. This pattern occurs coincident with intensification of local surface winds and may be responsible for the SST changes associated with the Pacific decadal oscillation. Years with deep winter–spring mixed layers coincide with years in which winter–spring SST is low. In the North Atlantic a pattern of winter–spring mixed layer depth variability occurs that is not so obviously connected to local changes in winds or SST, suggesting that other processes such as advection are more important. Interestingly, at decadal periods the winter–spring mixed layers of both basins show trends, deepening by 10–40 m over the 45-yr period of this analysis. The long-term mixed layer deepening is even stronger (50–100 m) in the North Atlantic subpolar gyre. At tropical latitudes the boreal winter mixed layer varies in phase with the Southern Oscillation index, deepening in the eastern Pacific and shallowing in the western Pacific and eastern Indian Oceans during El Niños. In boreal summer the mixed layer in the Arabian Sea region of the western Indian Ocean varies in response to changes in the strength of the southwest monsoon.

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