The Angola Current: Flow and hydrographic characteristics as observed at 11°S

Abstract

The eastern boundary circulation off the coast of Angola has been described only sparsely to date, although it is a key element in the understanding of the highly productive tropical marine ecosystem off Angola. Here, we report for the first time direct velocity observations of the Angola Current (AC) at ∼11°S collected between July 2013 and October 2015 in the depth range from 45 to 450 m. The measurements reveal an alongshore flow that is dominated by intraseasonal to seasonal variability with periodically alternating southward and northward velocities in the range of ±40 cm/s. During the observation period, a weak southward mean flow of 5–8 cm/s at 50 m depth is observed, with the southward current extending down to about 200 m depth. Corresponding mean southward transport of the AC is estimated to be 0.32 ± 0.046 Sv. An extensive set of hydrographic measurements is used to investigate the thermal structure and seasonality in the hydrography of the eastern boundary circulation. Within the depth range of the AC, the superposition of annual and semiannual harmonics explains a significant part of the total variability, although salinity in the near surface layer appears to be also impacted by year‐to‐year variability and/or short‐term freshening events. In the central water layer, temperature and salinity on isopycnals vary only weakly on seasonal to annual time scales. The available data set is further used to evaluate different reanalysis products particularly emphasizing the ocean's role in coupled climate model SST biases in the Eastern Tropical Atlantic.

Causes of the large warm bias in the Angola–Benguela Frontal Zone in the Norwegian Earth System Model

“…While O-CTL has a peak of southward current around 50 m depth, which is approximately consistent with the observations, NorESM and O-A-CTL-Wind have much shallower southward current at around 20-30 m depth. These profiles seem to be similar to those of reanalyses shown by Kopte et al (2017), and show that our model has a basic representation of the vertical structure of the Angola Current. We further consider vertical-longitude sections of meridional current velocity at 15°S, just north to the observed ABFZ.…”

“…The warm SST bias in the ABFZ is increased by about 2 °C (a quarter of NorESM full bias) when the local CAM4 surface wind errors are added to the COREv2-IAF forcing locally. MICOM, despite the coarse ~1° ocean model resolution, simulates a near surface, coastally trapped Angola Current with similarities to observations, but less intense than observed from mooring data at 11°S, 13°E (Kopte et al 2017). Adding the CAM4 negative wind stress curl bias drives a stronger surface Angola Current in the ABFZ and causes tropical warm water to penetrate farther south, consistent with Xu et al (2014).…”

“…6) with the ~2 years of observations from a single mooring at the same location ( Fig. 9 in Kopte et al 2017). Although O-CTL, O-A-CTLWind and NorESM indicate an Angola Current, compared to the in situ observations, all three underestimate the southward current (this is not unexpected given we comparing a ~2° grid cell with a point measurement).…”

N2O Emissions From the Northern Benguela Upwelling System

“…3.1.1. Although the 11 • S section laid between the Cunene cell and a weaker cell off Luanda (7 • S; Lutjeharms & Meeuvis, 1987), the lower N 2 O concentrations north and south of the section suggests that the zonal gradient of N 2 O resulted from local upwelling, likely driven by the passage of coastally trapped waves which lead to SST cooling in July/August (Kopte et al, 2017). It is noteworthy, however, that our measurements in the ATZ were conducted mostly out of the shelf.…”

“…It is noteworthy, however, that our measurements in the ATZ were conducted mostly out of the shelf. These waters correspond to the AC, which transports nutrient-rich O 2 -poor South Atlantic Central Waters from the equatorial region and the Angola Gyre southward (Kopte et al, 2017;Mohrholz et al, 2001;Shannon et al, 1987). Although the 11 • S section laid between the Cunene cell and a weaker cell off Luanda (7 • S; Lutjeharms & Meeuvis, 1987), the lower N 2 O concentrations north and south of the section suggests that the zonal gradient of N 2 O resulted from local upwelling, likely driven by the passage of coastally trapped waves which lead to SST cooling in July/August (Kopte et al, 2017).…”