Causes and evolution of the southeastern tropical Atlantic warm event in early 2016
A strong warm event occurred in the southeastern tropical Atlantic Ocean off Angola and Namibia in January and February 2016 with sea surface temperature anomalies reaching 3 °C. In contrast to classical Benguela Niño events, the analysis of various direct observations indicates that the warming was not predominantly forced by an equatorial Kelvin wave exciting a coastally trapped wave but instead resulted from a combination of local processes that are related to (1) a weakening of the alongshore, i.e. mainly southerly, winds and (2) enhanced freshwater input through local precipitation and river discharge. Consistent with the weakened winds, we find a reduction in latent heat loss from the ocean and a poleward surface current anomaly. The surface freshening, which is detected in satellite observations of sea surface salinity, caused a very shallow mixed layer and enhanced upper ocean stratification. This is supported by the analysis of the velocity structure of the Angola Current at 11°S, which shows that at the time of the event subsurface velocities were directed northward while surface velocities were directed southward. The shallow layer of warm and fresh surface water was thus advected poleward by the surface current. In addition, a reduction of the local upwelling and the formation of a barrier layer that inhibits the entrainment of cool subsurface waters into the surface mixed layer might have contributed to the warm surface anomaly. The sudden termination of the warm event was accompanied by a re-intensification of southerly winds in March.
KeywordsTropical Atlantic Ocean Interannual variability Sea surface temperature anomalies Benguela Niño
This study is a contribution of the SFB 754 supported by the Deutsche Forschungsgemeinschaft. It was further supported by the German Federal Ministry of Education and Research as part of the SACUS II project (03F0751A) as well as the BANINO project (03F0795A, 03F0795C) and as part of the SMOS Next project (50 EE 1610); by the European Union 7th Framework Programme (FP7 2007–2013) under grant agreement 603521 PREFACE project, and by the Deutsche Forschungsgemeinschaft as part of the cooperative project FOR1740. The L3_DEBIAS_LOCEAN_v2 Sea Surface Salinity maps have been produced by LOCEAN/IPSL (UMR CNRS/UPMC/IRD/MNHN) laboratory and ACRI-st company that participate to the Ocean Salinity Expertise Center (CECOS) of Centre Aval de Traitement des Donnees SMOS (CATDS). This product is distributed by the Ocean Salinity Expertise Center (CECOS) of the CNES-IFREMER Centre Aval de Traitement des Donnees SMOS (CATDS), at IFREMER, Plouzane (France). ePIRATA data have been obtained from http://www.aoml.noaa.gov/phod/epirata. GPCP data has been provided by the NOAA/OAR/ESRL PSD, Boulder, Colorado, USA, on their web site at http://www.esrl.noaa.gov/psd/. TropFlux data for heat flux, provided by the Indian National Centre for Ocean Information Services (INCOIS) have been downloaded from http://www.incois.gov.in/tropflux_datasets/data/monthly. QuikScat (or SeaWinds) and ASCAT data are produced by Remote Sensing Systems and sponsored by the NASA Ocean Vector Winds Science Team. Data are available at http://www.remss.com. Furthermore, the study has been conducted using E.U. Copernicus Marine Service Information by accessing altimetry data available at http://marine.copernicus.eu/. The OSCAR (Ocean Surface Current Analysis Real-time) data were obtained from the NASA EOSDIS Physical Oceanography Distributed Active Archive Center (PO.DAAC) at the Jet Propulsion Laboratory, Pasadena, CA ( https://doi.org/10.5067/OSCAR-03D01). Argo float profiles were obtained from Ifremer via ftp://ftp.ifremer.fr/ifremer/argo/geo/atlantic_ocean/2016. These data were collected and made freely available by the International Argo Program and the national programs that contribute to it (http://www.argo.ucsd.edu, http://argo.jcommops.org). The Argo program is part of the Global Ocean Observing System.
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