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Rick Lumpkin, Semyon A. Grodsky, Luca Centurioni, Marie-Helene Rio, James A. Carton, and Dongkyu Lee

Abstract

Satellite-tracked drifting buoys of the Global Drifter Program have drogues, centered at 15-m depth, to minimize direct wind forcing and Stokes drift. Drogue presence has historically been determined from submergence or tether strain records. However, recent studies have revealed that a significant fraction of drifters believed to be drogued have actually lost their drogues, a problem that peaked in the mid-2000s before the majority of drifters in the global array switched from submergence to tether strain sensors. In this study, a methodology is applied to the data to automatically reanalyze drogue presence based on anomalous downwind ageostrophic motion. Results indicate that the downwind slip of undrogued drifters is approximately 50% higher than previously believed. The reanalyzed results no longer exhibit the dramatic and spurious interannual variations seen in the original data. These results, along with information from submergence/tether strain and transmission frequency variations, are now being used to conduct a systematic manual reevaluation of drogue presence for each drifter in the post-1992 dataset.

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Ching-Yee Chang, James A. Carton, Semyon A. Grodsky, and Sumant Nigam

Abstract

The Community Climate System Model version 3 (CCSM3) has a dipolelike pattern with a cold bias in the northern Tropics and a warm bias in the southeastern Tropics, which is reminiscent of the observed pattern of climate variability in boreal spring. Along the equator, in contrast, in boreal spring CCSM3 exhibits striking westerly winds with easterly winds in the upper troposphere, in turn reminiscent of the observed pattern of climate variability in boreal summer. The westerly winds cause a deepening of the eastern thermocline that keeps the east warm despite enhanced coastal upwelling. Thus, the bias in the seasonal cycle of the coupled model appears to project at least partially onto the spatial patterns of natural climate variability in this sector.

Information about the origin of the bias in CCSM3 is deduced from a comparison of CCSM3 with a simulation using specified historical SST to force the Community Atmospheric Model version 3 (CAM3). The patterns of bias in CAM3 resemble those apparent in CCSM3, including the appearance of substantially intensified subtropical bands of sea level pressure (SLP), indicating that the problem may be traced to difficulties in the atmospheric component model. Positive SLP bias also appears in the western tropical region, which may be related to deficient Amazonian precipitation. The positive SLP bias seems to be the cause of the anomalous westerly trade winds in boreal spring, and those in turn appear to be responsible for the anomalous deepening of the thermocline in the southeastern Tropics.

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Semyon A. Grodsky, James A. Carton, Sumant Nigam, and Yuko M. Okumura

Abstract

This paper focuses on diagnosing biases in the seasonal climate of the tropical Atlantic in the twentieth-century simulation of the Community Climate System Model, version 4 (CCSM4). The biases appear in both atmospheric and oceanic components. Mean sea level pressure is erroneously high by a few millibars in the subtropical highs and erroneously low in the polar lows (similar to CCSM3). As a result, surface winds in the tropics are ~1 m s−1 too strong. Excess winds cause excess cooling and depressed SSTs north of the equator. However, south of the equator SST is erroneously high due to the presence of additional warming effects. The region of highest SST bias is close to southern Africa near the mean latitude of the Angola–Benguela Front (ABF). Comparison of CCSM4 to ocean simulations of various resolutions suggests that insufficient horizontal resolution leads to the insufficient northward transport of cool water along this coast and an erroneous southward stretching of the ABF. A similar problem arises in the coupled model if the atmospheric component produces alongshore winds that are too weak. Erroneously warm coastal SSTs spread westward through a combination of advection and positive air–sea feedback involving marine stratocumulus clouds.

This study thus highlights three aspects to improve to reduce bias in coupled simulations of the tropical Atlantic: 1) large-scale atmospheric pressure fields; 2) the parameterization of stratocumulus clouds; and 3) the processes, including winds and ocean model resolution, that lead to errors in seasonal SST along southwestern Africa. Improvements of the latter require horizontal resolution much finer than the 1° currently used in many climate models.

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James A. Carton, Xianhe Cao, Benjamin S. Giese, and Arlindo M. Da Silva

Abstract

The mechanisms regulating interannual and decadal variations of sea surface temperature (SST) in the tropical Atlantic are examined. Observed variations of sea surface temperature are typically in the range of 0.3°–0.5°C and are linked to fluctuations in rainfall on both the African and South American continents. The authors use a numerical model to simulate the observed time series of sea surface temperature for the period 1960–1989. Based on the results, experiments are conducted to determine the relative importance of heat flux and momentum forcing. Two dominant timescales for variability of SST are identified: a decadal timescale that is controlled by latent heat flux anomalies and is primarily responsible for SST anomalies off the equator and an equatorial mode with a timescale of 2–5 years that is dominated by dynamical processes. The interhemispheric gradient of anomalous SST (the SST dipole) is primarily linked to the former process and thus results from the gradual strengthening and weakening of the trade wind system of the two hemispheres.

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Semyon A. Grodsky, Abderrahim Bentamy, James A. Carton, and Rachel T. Pinker

Abstract

Weekly average satellite-based estimates of latent heat flux (LHTFL) are used to characterize spatial patterns and temporal variability in the intraseasonal band (periods shorter than 3 months). As expected, the major portion of intraseasonal variability of LHTFL is due to winds, but spatial variability of humidity and SST are also important. The strongest intraseasonal variability of LHTFL is observed at the midlatitudes. It weakens toward the equator, reflecting weak variance of intraseasonal winds at low latitudes. It also decreases at high latitudes, reflecting the effect of decreased SST and the related decrease of time-mean humidity difference between heights z = 10 m and z = 0 m. Within the midlatitude belts the intraseasonal variability of LHTFL is locally stronger (up to 50 W m−2) in regions of major SST fronts (like the Gulf Stream and Agulhas). Here it is forced by passing storms and is locally amplified by unstable air over warm SSTs. Although weaker in amplitude (but still significant), intraseasonal variability of LHTFL is observed in the tropical Indian and Pacific Oceans due to wind and humidity perturbations produced by the Madden–Julian oscillations. In this tropical region intraseasonal LHTFL and incoming solar radiation vary out of phase so that evaporation increases just below the convective clusters.

Over much of the interior ocean where the surface heat flux dominates the ocean mixed layer heat budget, intraseasonal SST cools in response to anomalously strong upward intraseasonal LHTFL. This response varies geographically, in part because of geographic variations of mixed layer depth and the resulting variations in thermal inertia. In contrast, in the eastern tropical Pacific and Atlantic cold tongue regions intraseasonal SST and LHTFL are positively correlated. This surprising result occurs because in these equatorial upwelling areas SST is controlled by advection rather than by surface fluxes. Here LHTFL responds to rather than drives SST.

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Stephen G. Penny, David W. Behringer, James A. Carton, and Eugenia Kalnay

Abstract

Seasonal forecasting with a coupled model requires accurate initial conditions for the ocean. A hybrid data assimilation has been implemented within the National Centers for Environmental Prediction (NCEP) Global Ocean Data Assimilation System (GODAS) as a future replacement of the operational three-dimensional variational data assimilation (3DVar) method. This Hybrid-GODAS provides improved representation of model uncertainties by using a combination of dynamic and static background error covariances, and by using an ensemble forced by different realizations of atmospheric surface conditions. An observing system simulation experiment (OSSE) is presented spanning January 1991 to January 1999, with a bias imposed on the surface forcing conditions to emulate an imperfect model. The OSSE compares the 3DVar used by the NCEP Climate Forecast System (CFSv2) with the new hybrid, using simulated in situ ocean observations corresponding to those used for the NCEP Climate Forecast System Reanalysis (CFSR).

The Hybrid-GODAS reduces errors for all prognostic model variables over the majority of the experiment duration, both globally and regionally. Compared to an ensemble Kalman filter (EnKF) used alone, the hybrid further reduces errors in the tropical Pacific. The hybrid eliminates growth in biases of temperature and salinity present in the EnKF and 3DVar, respectively. A preliminary reanalysis using real data shows that reductions in errors and biases are qualitatively similar to the results from the OSSE. The Hybrid-GODAS is currently being implemented as the ocean component in a prototype next-generation CFSv3, and will be used in studies by the Climate Prediction Center to evaluate impacts on ENSO prediction.

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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 S. Giese, Gilbert P. Compo, Niall C. Slowey, Prashant D. Sardeshmukh, James A. Carton, Sulagna Ray, and Jeffrey S. Whitaker

Abstract

El Niño is widely recognized as a source of global climate variability. However, because of limited ocean observations during the early part of the twentieth century, little is known about El Niño events prior to the 1950s. An ocean model, driven with surface boundary conditions from a recently completed atmospheric reanalysis of the first half of the twentieth century, is used to provide the first comprehensive description of the structure and evolution of the 1918/19 El Niño. In contrast with previous descriptions, the modeled El Niño is one of the strongest of the twentieth century, comparable in intensity to the prominent events of 1982/83 and 1997/98.

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Edwin K. Schneider, Zhengxin Zhu, Benjamin S. Giese, Bohua Huang, Ben P. Kirtman, J. Shukla, and James A. Carton

Abstract

Results from multiyear integrations of a coupled ocean–atmosphere general circulation model are described. The atmospheric component is a rhomboidal 15, 18-level version of the Center for Ocean–Land–Atmosphere Studies atmospheric general circulation model. The oceanic component is the Geophysical Fluid Dynamics Laboratory ocean model with a horizontal domain extending from 70°S to 65°N. The ocean model uses 1.5° horizontal resolution, with meridional resolution increasing to 0.5° near the equator, and 20 vertical levels, most in the upper 300 m. No flux adjustments are employed.

An initial multiyear integration showed significant climate drift in the tropical Pacific sea surface temperatures. Several modifications were made in the coupled model to reduce these errors. Changes were made to the atmospheric model cloudiness parameterizations, increasing solar radiation at the surface in the western equatorial Pacific and decreasing it in the eastern Pacific, that improved the simulation of the time-mean sea surface temperature. Large errors in the wind direction near the western coast of South America resulted in large mean SST errors in that region. A procedure to reduce these errors by extrapolating wind stress values away from the coast to coastal points was devised and implemented.

Results from the last 17 years of a 62-yr simulation are described. The model produces a reasonably realistic annual cycle of equatorial Pacific sea surface temperature. However, the upper-ocean thermal structure has serious errors. Interannual variability for tropical Pacific sea surface temperatures, precipitation, and sea level pressure that resemble the observed El Niño–Southern Oscillation (ENSO) in structure and evolution is found. However, differences from observed behavior are also evident. The mechanism responsible for the interannual variability appears to be similar to the delayed oscillator mechanism that occurs in the real climate system.

The structure of precipitation, sea level pressure, and geopotential anomalies associated with the tropical Pacific sea surface temperature interannual variability are isolated and described. The coupled model is capable of producing structures that are similar to those observed.

It is concluded that atmosphere–ocean general circulation models are beginning to capture some of the observed characteristics of the climatology of the tropical Pacific and the interannual variability associated with the El Niño–Southern Oscillation. Remaining obstacles to realistic simulations appear to include ocean model errors in the eastern equatorial Pacific, errors associated with cloud–radiation interactions, and perhaps errors associated with inadequate meridional resolution in the atmospheric model equatorial Pacific.

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James A. Carton, Stuart A. Cunningham, Eleanor Frajka-Williams, Young-Oh Kwon, David P. Marshall, and Rym Msadek
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