Abstract
In-situ observation, climate reanalyses, and satellite remote sensing are used to study the annual cycle of turbulent latent heat flux (LHF) in the Agulhas Current system. We assess if the datasets do represent the intense exchange of moisture that occurs above the Agulhas Current and the Retroflection region, especially the new reanalyses as the former, the National Centers for Environmental Prediction Reanalysis 2 (NCEP2) and the European Centre for Medium-Range Weather Forecast (ECMWF) reanalysis second-generation reanalysis (ERA-40) have lower sea and less distinct surface temperature (SST) in the Agulhas Current system due to their low spatial resolution thus do not adequately represent the Agulhas Current LHF. We use monthly fields of LHF, SST, surface wind speed, saturated specific humidity at the sea surface (Qss), and specific humidity at 10 m (Qa). The Climate Forecast System Reanalysis (CFSR), the European Centre for Medium-Range Weather Forecast fifth generation (ERA-5), and the Modern-Era Retrospective analysis for Research and Applications version-2 (MERRA-2) are similar to the air–sea turbulent fluxes (SEAFLUX) and do represent the signature of the Agulhas Current. ERA-Interim underestimates the LHF due to lower surface wind speeds than other datasets. The observation-based National Oceanography Center Southampton (NOCS) dataset is different from all other datasets. The highest LHF of 250 W/m2 is found in the Retroflection in winter. The lowest LHF (~100 W/m2) is off Port Elizabeth in summer. East of the Agulhas Current, Qss-Qa is the main driver of the amplitude of the annual cycle of LHF, while it is the wind speed in the Retroflection and both Qss-Qa and wind speed in between. The difference in LHF between product are due to differences in Qss-Qa wind speed and resolution of datasets.
Highlights
The greater Agulhas Current system is composed of the core of the Agulhas Current, which is about 219 km wide near 34◦S [1]; the Agulhas Retroflection region with a loop diameter of 350 km [2]; and the Agulhas Return Current that meanders back in an eastward direction [3] (Figure 1)
This sea and less distinct surface temperature (SST) gradient is at the origin of the high turbulent flux of sensible and latent heat observed above the current and creates a wall of moisture above the current with distinct cloud lines observed at time above it [3,4]
The latent heat flux (LHF), as well as marine boundary layer modification, were measured above the core of the Agulhas Current, the Retroflection region and the Agulhas Return Current [4,5,12,13,14,15,16]. These measurements show that the LHF, which is akin to the turbulent flux of moisture at the air–sea interface, is higher in the Agulhas Current system compared to the surrounding ocean
Summary
The greater Agulhas Current system is composed of the core of the Agulhas Current, which is about 219 km wide near 34◦S [1]; the Agulhas Retroflection region with a loop diameter of 350 km [2]; and the Agulhas Return Current that meanders back in an eastward direction [3] (Figure 1). The Agulhas current takes warm water poleward creating a distinct signature in SST along its path This SST gradient is at the origin of the high turbulent flux of sensible and latent heat observed above the current and creates a wall of moisture above the current with distinct cloud lines observed at time above it [3,4]. The LHF, as well as marine boundary layer modification, were measured above the core of the Agulhas Current, the Retroflection region and the Agulhas Return Current [4,5,12,13,14,15,16] These measurements show that the LHF, which is akin to the turbulent flux of moisture at the air–sea interface, is higher in the Agulhas Current system compared to the surrounding ocean. Cloud lines above the Agulhas CReumrortee nSetnas.r2e0t1h9,e1r1e, sxuFlOtsRoPfEEthReRsEtVroIEnWg exchange of moisture and mixing occurring in this region [3,35o,1f73]3
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