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

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.

Reviews and syntheses: Physical and biogeochemical processes associated with upwelling in the Indian Ocean

“…Long-term trends were stronger during the SWM than during the NEM, with increased wind speeds for Tanga and Zanzibar in the north but a decline in the maximum wind speed for Mtwara in the south and constant maximum wind speeds for Dar es Salaam. A coastal upwelling index (CUI) based on SST output from a coupled biophysical-climatological model by Halo et al (2020) showed a moderate and steady linear increase in upwelling for Tanga over a 23-year period , in line with the regional increase in wind speed observed by Mahongo et al (2012).…”

“…This small upwelling cell has been observed in both monsoons (Fig. 11), suggesting it is a regular occurrence (Halo et al, 2020).…”

“…Along the coast of Tanzania, both the NEM winds and shear instabilities between the EACC and the chain of islands along the coast have been attributed as responsible physical mechanisms driving upwelling in the region, as suggested by a modelling study by Halo et al (2020). Roberts (2015) suggested elevated chlorophyll a concentrations in the lee (downstream) of the island of Zanzibar, in particular, and to a lesser extent off Pemba Island, measured during a survey in 2007, were a consequence of localized upwelling induced by an island wake (Roberts, 2015).…”

High-frequency dynamics of pH, dissolved oxygen, and temperature in the coastal ecosystems of the Tanga-Pemba Seascape: implications for upwelling-enhanced ocean acidification and deoxygenation

“…These conditions place resident marine organisms and coastal ecosystems at a high risk of future ocean acidification and deoxygenation (Cornwall et al, 2023;Kroeker et al, 2023). Although the extensive impacts of upwelling events on pH, DO, and temperature dynamics are well-documented globally (Rixen et al, 2012;Schulz et al, 2019;Kroeker et al, 2023), this information is largely unknown in coastal upwelling areas of the Western Indian Ocean (WIO), particularly in the Tanga-Pemba Seascape (T-PS), known to experience upwelling during the northeast monsoon season (Halo et al, 2020;Kyewalyanga et al, 2020). This information is crucial for informing the future of global change biology in the WIO.…”

“…The objective of this study was twofold. Firstly, to understand high-frequency dynamics in pH, DO, and temperature in the open ocean of the T-PS during the northeast monsoon, when upwelling events are thought to occur (Halo et al, 2020;Kyewalyanga et al, 2020), as well as the 24-hour high-frequency dynamics of these variables in mangrove, seagrass, and coral reef ecosystems within the seascape. Secondly, to understand the susceptibility of mangrove, seagrass, and coral reef ecosystems to lower pH and DO conditions induced by upwelling events and whether some of these ecosystems offer any degree of mitigation under such events.…”