23 October 2018

Seasonal Cycle and Annual Reversal of the Somali Current in an Eddy-Resolving Global Ocean Model

Dynamics of the Somali Current seasonal cycle.

Science

To better understand the dynamical drivers of the seasonal cycle of the Somali Current (SC), an atypical western boundary current that reverses seasonally off the northeast coast of Africa, momentum budget analyses were carried out over its length. The budget results indicated that the annual cycle and timing of the SC reversal differ depending on the location along the African coast. 

Impact

The Somali Current (SC) is important to the regional climate and ecology (due to upwelling wedges that develop in the boreal summer) and potentially impacts Indian Monsoon precipitation. 

Summary

The Somali Current (SC) reverses seasonally; originally its reversal was associated with that of the Indian Monsoon winds. Observations, however, showed an early reversal (northward flow) of the SC between 5°S-10°N prior to the onset of the southwest (SW) monsoon. Privacy hampered further investigation, leading to a reliance on numerical ocean models instead. Here, we investigate the dynamics of the seasonal cycle of the SC using full momentum budgets, within two degrees of the East African coast (5°S-10°N), using output from an atmospheric reanalysis-forced global 0.1° Parallel Ocean Program and CICE (ocean/sea ice) simulation for 2005-2009. Results indicate that the annual cycle and timing of the SC reversal differ depending on location along the coast. From 2°-5°N, the flow directly follows the monsoon wind reversal, while from 5°-10°N a remotely-generated annual Rossby wave and life cycle of Great Whirl extend the duration of northward flow before and after the occurrence of SW monsoon winds. The nonlinear term is of leading order importance when the Great Whirl is present, in contrast to the leading‐order geostrophic balance between 2°N and 5°N. To the south of 2°N, the reversal is affected by the northward-flowing East African Coastal Current, where equatorial dynamics prevail.

Contact
Julie McClean
Scripps Institution of Oceanography (SIO)
Publications
Wang, H, J McClean, L Talley, and S Yeager.  2018.  "Seasonal Cycle and Annual Reversal of the Somali Current in an Eddy-Resolving Global Ocean Model."  Journal of Geophysical Research: Oceans 123(9): 6562-6580, doi:10.1029/2018jc013975.