Abstract
Abstract. A diagnostic analysis of the climatological annual mean and seasonal cycle of the Angola–Benguela Frontal Zone (ABFZ) is performed by applying an ocean frontogenetic function (OFGF) to the ocean mixing layer (OML). The OFGF reveals that the meridional confluence and vertical tilting terms are the most dominant contributors to the frontogenesis of the ABFZ. The ABFZ shows a well-pronounced semiannual cycle with two maximum (minimum) peaks in April–May and November–December (February–March and July–August). The development of the two maxima of frontogenesis is due to two different physical processes: enhanced tilting from March to April and meridional confluence from September to October. The strong meridional confluence in September to October is closely related to the seasonal southward intrusion of tropical warm water to the ABFZ that seems to be associated with the development of the Angola Dome northwest of the ABFZ. The strong tilting effect from March to April is attributed to the meridional gradient of vertical velocities, whose effect is amplified in this period due to increasing stratification and shallow OML depth. The proposed OFGF can be viewed as a tool to diagnose the performance of coupled general circulation models (CGCMs) that generally fail at realistically simulating the position of the ABFZ, which leading to huge warm biases in the southeastern Atlantic.
Highlights
The Angola-Benguela Frontal Zone (ABFZ, see Fig. 1), situated off the coast of Angola and Namibia, is a key oceanic feature in the southeastern Atlantic Ocean
Because the vertical turbulent mixing term at the mixed-layer base Qb is represented according to K-profile parameterization in ocean–atmosphere general circulation models (OAGCMs), it will not be explicitly addressed in this study as it is not possible to estimate it from the reanalysis outputs
We have shown that in terms of climatological annual-mean, confluence effect (CONF) and tilting effect (TILT) of the ocean frontogenetic function (OFGF) were the main sources for the Angola–Benguela Frontal Zone (ABFZ) generation
Summary
The Angola-Benguela Frontal Zone (ABFZ, see Fig. 1), situated off the coast of Angola and Namibia, is a key oceanic feature in the southeastern Atlantic Ocean. A dynamical diagnosis for the SST front in the north of the Atlantic cold tongue (e.g., Hasternrath and Lamb, 1978; Giordani et al, 2013) was proposed by Giordani and Caniaux (2014, hereafter referred to as GC2014) The frontogenetic function they use is, in general, adapted to explore sources of frontogenesis of atmospheric synoptic-scale cyclones in the extratropics (e.g., Keyser et al, 1988; Giordani and Caniaux, 2001). This system provides 6hourly data with a 0.5◦ horizontal resolution and 70 vertical layers for ocean This resolution is relatively coarse compared to the resolution of simulations performed with regional ocean models in a forced mode using wind forcing from satellite products. In this paper we will analyze daily means (the procedure of data post-processing is given in the Supplement) and utilize the CFSR outputs of velocity (horizontal and vertical), potential temperature, net surface heat flux, OML depth, and sea surface height (SSH)
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