Abstract
Mesoscale sea surface temperature (SST) variability plays an important role in shaping local atmospheric boundary layers through thermodynamic processes. This study focuses on the upscaling effects of mesoscale SST gradients in sensitive areas on the southern Africa regional atmospheric circulation. Using regional atmospheric model sensitivity experiments which differ only in the mesoscale SST forcing characteristics (either the full spectrum of SST variability or only its large‐scale components are included), we first quantify the importance of SST gradients on regional atmospheric conditions. Agulhas eddies and meanders influence the vertical air column up to the troposphere, and mesoscale ocean patterns significantly modify incoming landwards moisture fluxes. The austral summer mean state is then modified in terms of air temperature, cloud cover and mean rainfall, with notable differences in tropical rainbands over southwestern Africa. Mesoscale SST variability favours tropical–extra‐tropical interactions and cloudband development over the continent. These results stress the importance of high‐resolution ocean forcing for accurate atmospheric simulations.
Highlights
Many interactions and coupling processes coexist over a large spectrum of temporal and spatial scales at the air–seaAbbreviations: ABFZ, Angola–Benguela frontal zone, BUS, Benguela upwelling system, CFSR, NCEP Climate Forecast System Reanalysis, DJF, austral summer months: December–January–February, ERA-Interim, European Reanalysis developed at ECMWF, ECMWF, European Centre for Medium-Range Weather Forecasts, ITCZ, Intertropical Convergence Zone, MABL, marine atmospheric boundary layer, MUR SST, multi-scale ultra-high resolution sea surface temperature, NCEP, National Centers for Environmental Prediction, NCEP-FNL, NCEP Final Reanalysis, OSTIA, Operational Sea Surface Temperature and Sea Ice Analysis, PBL, planetary boundary layer, SST, sea surface temperature, TOA, top-of-atmosphere, WRF, Weather Research and Forecasting model interface
We examine the influence of mesoscale SST patterns on the overlying atmosphere at a seasonal timescale, based on model experimentation with a regional Weather Research and Forecasting model (WRF) configuration
The integrated imprint of mesoscale SST variability on the atmospheric austral summer circulation over southern Africa has been examined through sensitivity experiments using the WRF model
Summary
The mitigation (development) of the vertical shear profile within the planetary boundary layer (PBL) contains strong near-surface signatures (i.e., the acceleration (deceleration) of the wind over warm (cold) waters) This SST/wind interaction satisfies a roughly linear relationship between the two fields, which has already been quantified over coherent mesoscale structures (e.g., Chelton et al, 2004; Small et al, 2008; O’Neill et al, 2010; Desbiolles et al, 2014). The large-scale influence of the ocean on southern African precipitation and general climate is fairly well documented (e.g., Mason, 1995; Reason and Mulenga, 1999; Behera and Yamagata, 2001; Reason et al, 2006), the influence of mesoscale SST gradients are poorly understood.
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