The Low-Frequency Variability of the Southern Ocean Circulation

Abstract: Long time series of sea surface height (SSH), sea surface temperature, and wind stress curl are used to determine the main modes of low-frequency variability of the Southern Ocean (SO) circulation. The dominant mode is a trend of increasing SSH at an average rate of 3.3 mm yr(-1). Similar trends have been reported in previous studies and the analysis indicates that the tendency of sea level increase over the SO has become more spatially homogeneous during the last decade, with changes in the increasing rate in 2000 and 2006. The other modes consist of stationary, basin-type modes, and an eastward-propagating wave. The stationary modes are particularly dominant in the Indian and Atlantic Ocean basins, where their spatial structure appears to be shaped by the basin geometry and the bottom topography. The wavelike patterns travel eastward with a period of approximately 10 years. Two waves were identified in the analysis: a complete cycle between 1997 and 2007 and a second cycle starting in 2000. Such waves have rarely been mentioned or identified in studies using recent satellite-derived SSH products.Keywords: In-situ, Signals, Expansion, Temperature, Sea level change, Wind, Antarctic circumpolar wave, Hemisphere annular mode, Scales, satellit

Equatorial Atlantic Ocean dynamics in a coupled ocean–atmosphere model simulation

The ocean temperatures and zonal currents at the equatorial Atlantic simulated by an improved version of the Brazilian earth system model (BESM), with changes in the cloud cover scheme and optical properties of the atmospheric component, are analyzed and compared to those obtained from a previous version of BESM and also from other seven selected CMIP5 models. It is shown that this updated version of BESM, despite some persistent biases, more accurately represents the surface temperature variation at the Equator and the equatorial thermocline east–west slope. These improvements are associated to a more realistic seasonal cycle achieved for the Atlantic equatorial undercurrent, as well as sea surface temperatures and zonal wind stress. The better simulation of the equatorial undercurrent is, in its turn, credited to a more realistic representation of the surface wind position and strength at the tropical Atlantic by the coupled model. With many of the systematic errors noticed in the previous version of the model alleviated, this version of BESM can be considered as a useful tool for modelers involved in Atlantic variability studies

The tropical Atlantic observing system

The tropical Atlantic is home to multiple coupled climate variations covering a wide range of timescales and impacting societally relevant phenomena such as continental rainfall, Atlantic hurricane activity, oceanic biological productivity, and atmospheric circulation in the equatorial Pacific. The tropical Atlantic also connects the southern and northern branches of the Atlantic meridional overturning circulation and receives freshwater input from some of the world’s largest rivers. To address these diverse, unique, and interconnected research challenges, a rich network of ocean observations has developed, building on the backbone of the Prediction and Research Moored Array in the Tropical Atlantic (PIRATA). This network has evolved naturally over time and out of necessity in order to address the most important outstanding scientific questions and to improve predictions of tropical Atlantic severe weather and global climate variability and change. The tropical Atlantic observing system is motivated by goals to understand and better predict phenomena such as tropical Atlantic interannual to decadal variability and climate change; multidecadal variability and its links to the meridional overturning circulation; air-sea fluxes of CO2 and their implications for the fate of anthropogenic CO2; the Amazon River plume and its interactions with biogeochemistry, vertical mixing, and hurricanes; the highly productive eastern boundary and equatorial upwelling systems; and oceanic oxygen minimum zones, their impacts on biogeochemical cycles and marine ecosystems, and their feedbacks to climate. Past success of the tropical Atlantic observing system is the result of an international commitment to sustained observations and scientific cooperation, a willingness to evolve with changing research and monitoring needs, and a desire to share data openly with the scientific community and operational centers. The observing system must continue to evolve in order to meet an expanding set of research priorities and operational challenges. This paper discusses the tropical Atlantic observing system, including emerging scientific questions that demand sustained ocean observations, the potential for further integration of the observing system, and the requirements for sustaining and enhancing the tropical Atlantic observing system

Simulação dos efeitos da passagem do ciclone Catarina sobre o nível do mar

Neste trabalho, o modelo de circulação geral oceânica MICOM é forçado por campos de tensões de vento representando a passagem do ciclone Catarina, a fim de mostrar a influência deste ciclone na elevação da superfície do mar. Os resultados obtidos são comparados com um experimento de controle, sem ciclone. Os distúrbios no nível do mar, em áreas afastadas da costa, persistem depois da passagem do ciclone. Próximo à costa, o processo de ajuste é mais rápido, de maneira que os distúrbios desaparecem em poucos dias. Entretanto, as ondas internas geradas no oceano aparentemente também causam distúrbios na área costeira, efeito notado alguns dias depois da dissipação do ciclone. ABSTRACT: In this study, the ocean general circulation model MICOM is forced by wind stress fields representing the Catarina cyclone passage, in order to show the influence of this cyclone in the sea surface elevation. The obtained results are compared to a control experiment, with no cyclone. The sea level disturbances, in areas far from the coast, persist after the cyclone passage. Near the coast, the adjustment process is faster, so the disturbances disappear in few days. However, the internal waves generated in the ocean apparently also cause disturbances in the coastal area, effect that is noted some days after the dissipation of the cyclone

Coupled Ocean-Atmosphere variability of the South American Monsoon System

Climate variability over South America is strongly linked to SST over the tropical and South Atlantic Ocean. Two phenomena that cause large rainfall interannual variability over South America, and that have apparent links with SST anomalies over the tropical Atlantic are the Intertropical Convergence Zone (ITCZ) and the South Atlantic Convergence Zone (SACZ). Yet, the linkages of such convergence zones with SSTA are quite diverse, as the former seems to be strongly modulated by interhemispheric gradients of SSTA and the latter shows negative correlations with SSTA locally. Such are the conclusions of two-tiered approaches to simulate rainfall variability using AGCMs forced by prescribed SST fields globally. An alternative to the two-tier approach is the use of coupled ocean-atmosphere models (CGCM), which account for nonlinear effects of surface fluxes of momentum and heat on the determination of SST. Results of ongoing predictability experiments conducted at CPTEC using a global CGCM with eddy resolving ocean resolution over the tropical Atlantic indicates a gain of predictive skill over the area of the SACZ relative to AGCM skill measures over the same area. Yet, warm SST bias over eastern equatorial Atlantic in the CGCM forecasts remains an unsolved problem, which may be contributing to an apparent degradation of CGCM predictive skill of rainfall over northern Nordeste Brazil.Pages: 587-58

A avaliacao do desempenho de um OCGM forcado por um ACGM

Neste artigo sao apresentados resultados preliminares da componente oceanica do sistema acoplado oceano-atmosfera de previsao sazonal numerica usado no CPTEC. A destreza do modelo de circulacao geral oceanico (OGCM) e analisada quando o mesmo e forcado por ventos gerados previamente por um modelo de circulacao geral atmosferico (AGCM). Esses resultados sao comparados com uma simulacao oceanica forcada por ventos observados, o que representaria teoricamente o melhor desempenho que se pode obter de um OGCM. Os melhores resultados sao obtidos quando se forca o OGCM com tensoes de vento observada, o que nao e surpreendente. Porem os resultados mostram que a resolucao atmosferica usada na geracao das tensoes do vento tem papel fundamental no skill do modelo oceanico. O experimento forcado com tensoes do vento de maior resolucao (T62) apresenta melhores resultados do que o experimento forcado com mais baixa resolucao (T42)

The impact of ocean initialization for SST predictions over the tropical oceans

In this study, two different initial conditions for November 2004 are used in experiments with the Modular Ocean Model. The first initial condition is the steady state reached after a 33 year-simulation with reanalysis wind stresses. The second one is based on observed ocean data, obtained by data assimilation schemes and interpolated to the model grid. The objective is to point out which one is better to simulate the sea surface temperature (SST) in selected areas of the global-tropical ocean (180W 180E, 40S 40N). The analysis are based on differences between simulated and observed SST values. Apparently, the skill of the model on reproducing the SST increases with the second initial condition, based on observed ocean data, in some coastal areas and in the Central Pacific. On the other hand, the skill in the Tropical Atlantic, a region known by low SST predictability, are almost the same whatever initial condition is used.Pages: 563-56

Analyzing the Influence of the North Atlantic Ocean Variability on the Atlantic Meridional Mode on Decadal Time Scales

Important features of the Atlantic meridional mode (AMM) are not fully understood. We still do not know what determines its dominant decadal variability or the complex physical processes that sustain it. Using reanalysis datasets, we investigated the influence of the North Atlantic Ocean variability on the dominant decadal periodicity that characterizes the AMM. Statistical analyses demonstrated that the correlation between the sea surface temperature decadal variability in the Atlantic Ocean and the AMM time series characterizes the Atlantic multidecadal oscillation (AMO). This corroborates previous studies that demonstrated that the AMO precedes the AMM. A causal inference with a newly developed rigorous and quantitative causality analysis indicates that the AMO causes the AMM. To further understand the influence of the subsurface ocean on the AMM, the relationship between the ocean heat content (0–300 m) decadal variability and AMM was analyzed. The results show that although there is a significant zero-lag correlation between the ocean heat content in some regions of the North Atlantic (south of Greenland and in the eastern part of the North Atlantic) and the AMM, their cause-effect relationship on decadal time scales is unlikely. By correlating the AMO with the ocean heat content (0–300 m) decadal variability, the former precedes the latter; however, the causality analysis shows that the ocean heat content variability drives the AMO, corroborating several studies that point out the dominant role of the ocean heat transport convergence on AMO

Climatology of the sea surface temperature for the Southwesthern Atlantic Ocean

Este artigo apresenta uma nova climatologia do campo de temperatura da superfície do mar (TSM) para a bacia oceânica do Atlântico Sudoeste (10o S - 30o S, e 30o W - 60o W) referentes aos quadrados de Marsden 339, 375, 376, 411, 412, e 413. Para formar esta climatologia foram utilizados dados de navios armazenados no Banco Nacional de Dados Oceanográficos da Diretoria de Hidrografia e Navegação (BNDO/DHN) (http://www.dhn.mar.mil.br/) no período de 1960 a 1996. São apresentados todos os procedimentos de tratamento e análise de dados para o cálculo das médias mensais, sazonais, e as respectivas anomalias. Para o período considerado são mostrados os mapas mensais da distribuição espacial do número de observações, e os mapas das médias mensais, sazonais e anuais da TSM. Nos mapas de distribuição espacial do número de observações fica evidente a baixa densidade de observações em regiões fora da costa, enfatizando a necessidade da recorrência de outras fontes de dados como a fornecida por imagens de satélites. Os campos médios climatológicos de TSM trazem detalhes dos fortes gradientes na Zona de Confluência Oceânica do Atlântico Sul (ZCOAS), e que normalmente não são encontrados em outras climatologias como a de Da Silva et al. (1994) a partir do Comprehensive Ocean-Atmosphere Data Set (COADS). A migração sazonal da ZCOAS é perfeitamente identificada nos mapas mensais e sazonais, assim como a influência no inverno das águas de plataforma vindas do Estuário do Prata. This paper presents a new climatology of the sea surface temperature (SST) field for the Southwestern Atlantic oceanic basin (10o S - 30o S, and 30o W - 60o W) corresponding to Marsden's squares 339, 375, 376, 411, 412, and 413. To build this climatology, ships data stored in the National Bank of Oceanographic Data (NBOD) of the Brazilian Navy (http://www.dhn.mar.mil.br/) for the period from January 1960 to 1996 were used. The data treatment and analysis procedure used for the calculation of monthly and seasonal means, and the respective anomalies is presented. For the considered period, spatial distribution maps of number of observations, monthly, seasonal, and annual means are shown. The low density offshore observations is evident, emphasizing the necessity to search for another sources of data, as those supplied by satellites images, in order to complement in site measurements. The SST climatological mean fields show details of the strong gradients at the South Atlantic Oceanic Confluence Zone (SAOCZ), which are not usually found in other climatologies as da Silva et al., 1994 from the Comprehensive Ocean-Atmosphere Data Set (COADS). The SAOCZ seasonal migration is perfectly identified in the monthly and seasonal maps, as well as the influence during winter of the shelf waters coming from the Plata Estuary

Effects of the passage of atmospheric cyclones over the ocean in the Brazil-Malvinas confluence

on.The atmospheric region over the Western South Atlantic (WSA), between 15S and 50S, presents an intense cyclogenetic activity, with the frequent passage of cyclones over the ocean. Incidentaly, that is the region of confluence of two major western boundary currents, the Brazil Current and the Malvinas Current, and one of the most eddy-energetic regions of the oceans. As part of a numerical study of the oceanic circulation in the South Atlantic, a series of experiments with an implementation of the Miami Isopycnic Coordinate Ocean Model (MICOM) were run to investigate the effect of the passage of such atmospheric phenomena. In these experiments, the model was forced initially with monthly means of the wind stress. After reaching a state of statistical equilibrium, the amospheric perturbations were then introduced. Among the objectives of these experiments was an investigation of the effect of the relative size of the cyclones. For that, two main sets of simulations were explored. The diameter of the cylone in one case was twice the size of the other. The results show that the smaller cyclone generates perturbations in the sea level in a much smaller region, as compared with the larger one. However, the perturbations caused by the smaller cyclone last much longer than those produced by the larger one. Analises of the surface velocity divergence field show the appearance of wave trains along the tracks of the cyclones, with characteristics of those classified as ``Lee Waves'' by other authors. The results also show that a considerable part of the energy transfered to the ocean during the passage of the cyclones are transported towards the Brazil-Malvinas confluence region, were it remains trapped long after the cyclones have vanished. This might imply in a possible mechanism for the excitation of meso-scale instabilities in that regi