Causes and evolution of the southeastern tropical Atlantic warm event in early 2016

• Published in: Climate dynamics Vol. 53; no. 1-2; pp. 261 - 274
• Main Authors: Lübbecke, Joke F., Brandt, Peter, Dengler, Marcus, Kopte, Robert, Lüdke, Jan, Richter, Ingo, Sena Martins, Meike, Tchipalanga, Pedro C. M.
• Format: Journal Article
• Language: English
• Published: Berlin/Heidelberg Springer Berlin Heidelberg 01.07.2019; Springer; Springer Nature B.V
• Subjects: Anomalies; Barrier layers; Climatic changes; Climatology; Density stratification; Earth and Environmental Science; Earth Sciences; Entrainment; Evolution; Freshwater; Geophysics/Geodesy; Heat loss; Inland water environment; Kelvin waves; Latent heat; Local precipitation; Mixed layer; Natural history; Ocean circulation; Ocean currents; Ocean temperature; Oceanography; Oceans; Precipitation; Reduction; River discharge; River flow; Rivers; Satellite observation; Satellites; Sea surface; Sea surface temperature; Sea surface temperature anomalies; Stratification; Surface mixed layer; Surface salinity; Surface temperature; Surface water; Temperature anomalies; Trapped waves; Tropical climate; Upper ocean; Upwelling; Wind; Winds; Namibia; Angola; Benguela Niño; Tropical Atlantic Ocean; Interannual variability; Sea surface temperature anomalies
• ISSN: 0930-7575; 1432-0894
• DOI: 10.1007/s00382-018-4582-8

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.