Coastal gradients in False Bay, south of Cape Town: what insights can be gained from mesoscale reanalysis?

Mark R Jury

doi:10.5194/os-16-1545-2020

Abstract

Abstract. Mesoscale datasets are used to study coastal gradients in the marine climate and oceanography in False Bay, south of Cape Town. Building on past work, satellite and ocean–atmosphere reanalyses are used to gain new insights into the mean structure, circulation and meteorological features. HYCOM v3 hindcasts represent a coastward reduction of mixing that enhances stratification and productivity inshore. The mean summer currents are westward 0.4 m s−1 along the shelf edge and weakly clockwise within False Bay. The marine climate is dominated by southeasterly winds that accelerate over the mountains south of Cape Town and fan out producing dry weather. Virtual buoy time series in December 2012–February 2013 exhibit weather-pulsed upwelling in early summer interspersed with quiescent spells in late summer. Intercomparisons between model, satellite and station data build confidence that coupled reanalyses yield opportunities to study air–sea interactions in coastal zones with complex topography. The 0.083∘ HYCOM reanalysis has 16 data points in False Bay, just adequate to resolve the coastal gradient and its impacts on ocean productivity.

Highlights

The coastal zone south of Cape Town, South Africa, is comprised of linear sandy beaches and a semi-enclosed bay surrounded by mountains (Fig. 1a, b)
We demonstrate that mesoscale reanalysis offers valuable new insights into the coastal gradient in summer climate and physical oceanography south of Cape Town
We focus on the summer of December 2012 to February 2013, which coincides with VIIRS reflectance, Jason-1 and Jason-2 altimeter, and ASCAT-A and ASCAT-B scatterometer coverage that better constrains the physical oceanography

Summary

Introduction

The coastal zone south of Cape Town, South Africa, is comprised of linear sandy beaches and a semi-enclosed bay surrounded by mountains (Fig. 1a, b). The shelf oceanography exhibits a range of conditions from seasonally pulsed upwelling events (Shannon and Field, 1985; Lutjeharms and Stockton, 1991; Largier et al, 1992; Dufois and Rouault, 2012) to warm-water intrusions from the Agulhas Current, creating great biological diversity (Shannon et al, 1985; Griffiths et al, 2010). The upper ocean circulation tends to be northwestward and pulsed at subseasonal timescales by passing weather, shelf waves, warm rings and tides (Grundlingh and Larger, 1991; Nelson et al, 1991). The high-pressure cells of the South Atlantic and South Indian Ocean tend to join in summer and produce dry weather and upwelling-favourable winds from the southeast that are shallow and diverted around the > 1000 m mountains of Cape Hangklip and the Cape Peninsula (Fig. 1a, b). Winds entering False Bay become channelled N–S and tend to induce standing clockwise rotors in the upper ocean (deVos et al, 2014), which are pulsed by geostrophic currents across the mouth

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