Biophysical modelling of coastal upwelling variability and circulation along the Tanzanian and Kenyan coasts

Issufo Halo,Philip Sagero,Majuto Manyilizu,Shigalla B Mahongo

doi:10.4314/wiojms.si2020.1.5

Abstract

Ocean circulation, upwelling phenomena and chlorophyll-a concentrations were investigated within the framework of numerical model simulations with 1/12° nested horizontal grid-size, in the tropical western Indian Ocean, along the coasts of Tanzania and Kenya. Ekman driven upwelling exhibited high levels of spatial and temporal variability in the region, characterized by a more vigorous occurrence/intensification during the Northeast than the Southwest Monsoon season. A similar trend was observed for chlorophyll-a distribution, but with an additional strong contribution during the inter-monsoon period from March to April. Trend analysis of a SST-derived coastal upwelling index (CUI) computed over the Pemba Channel and offshore of the East African Coastal Current (EACC), for 24 years (1990 - 2013), revealed a general linear relation of the form CUI(yr) = 2.4x10-7yr – 285, with a steady small annual increase of the upwelling phenomena by 0.0024/year ≃ 4% during the whole period of the simulation, which could be attributed to documented increasing trends of wind intensity and water volume transport in the region. The CUI exhibited the two most dominant peaks of variabilities on the range of annual and semi-annual timescales. The wind-stress southward component and the easting/westing veering of the northward EACC at 6°S revealed that these parameters were moderate and significantly correlated with the CUI (r = - 0.53 and 0.52, p<0.05) respectively, further suggesting its intensification during the Northeast Monsoon season.

Highlights

Coastal upwelling events have been the subject of high interest among physical and biological oceanographers for many decades (Brink et al, 1983; Bakun, 1998; Durand et al, 1998; Capet et al, 2004; Echevin et al, 2005)
Coastal upwelling along the Tanzanian and Kenyan continental shelves can be visually diagnosed from thermal expressions of high resolution sea surface temperature (SST) products (able to resolve detailed patterns of spatial and temporal scale variabilities, for example fronts, (SST gradients), and filaments as depicted in Fig. 1) simulated by the Regional Ocean Modelling Systems (ROMS) with a horizontal grid-resolution of 1/12°, for the 2nd January of year 8
Its main core lies at the offshore side of the chain of islands formed by Mafia, and Unguja and Pemba (Zanzibar archipelago), whereby parts of the flow upslopes onto the narrow continental shelf and recirculates around the islands, as documented by Mahongo and Shaghude (2014), Roberts (2015), and Mayorga-Adame et al (2016)

Summary

Introduction

Coastal upwelling events have been the subject of high interest among physical and biological oceanographers for many decades (Brink et al, 1983; Bakun, 1998; Durand et al, 1998; Capet et al, 2004; Echevin et al, 2005). As a result of the on-going global concerns about climate change (Garcia-Reyes and Mahongo (this issue)) and associated perturbations on the patterns of general atmospheric and oceanic circulations, many scientific studies have directed their attention towards understanding how such changes may impact the strength/ intensity of upwelling systems around the world (Garcia-Reyes et al, 2015). Recently have these concerns been raised in the western Indian Ocean and awareness among the marine scientific community and relevant stakeholders has been raised (Roberts, 2015). Its main core lies at the offshore side of the chain of islands formed by Mafia, and Unguja and Pemba (Zanzibar archipelago), whereby parts of the flow upslopes onto the narrow continental shelf and recirculates around the islands, as documented by Mahongo and Shaghude (2014), Roberts (2015), and Mayorga-Adame et al (2016)

Methods

Results

Conclusion