New data on the sea current system in the Somali region, Indian Ocean

Abstract

Direct measurements of current velocities conducted during the International World Ocean Circulation Experiment (WOCE) yielded new data on spatiotemporal characteristics of a current system off the Somali coast. It is shown that background circulation within the domain of the western boundary current in the Indian Ocean forms a regional anticyclonic gyre similar in general patterns to its counterparts in the Atlantic and Pacific oceans. However, the Indian Ocean is distinguished from other oceans by superimposition of the alternating monsoon signal with the prevalent annual harmonic on the background dynamic structure. It is established that abnormally high velocities in the central part of the Somali Current (up to 2 m/s) during the southwestern monsoon period is related to superimposition of unidirectional background and summer monsoon components of this powerful western boundary current. The Indian Ocean segment adjacent to the northeastern coast of the African continent is one of the most active regions of the planet with respect to climatic perturbations. The vast region of a cold surface layer in the southern Arabian Sea related to upwelling in the Somali Current system [2] substantially influences atmospheric processes and weather conditions in the spacious maritime region of southwestern Asia. There are also grounds to believe that inversion heat transfer from the atmosphere to the ocean observed during the summer monsoon in the Somali temperature anomaly zone acts as a damper in the generation of mobile cyclonic whirlwinds, preventing them from transforming into catastrophic hurricanes. All these and other climatically important peculiarities in the thermodynamic behavior of the ocean and atmosphere near the African Horn attract the constant attention of experts in meteorology and oceanography to this region [3‐7]. Nevertheless, despite a long history of investigations, many problems related particularly to variability in the Somali segment of the oceanic circulation system remain poorly studied so far. The hypothesis of reverse behavior of the Somali Current is one of the most intriguing scenarios of this variability [4, 8, 9]. It is known that the monsoon type of atmospheric circulation that prevails over the northwestern segment of the Indian Ocean influences the system of sea currents and determines the unique characteristic of the powerful jet stream that flows along the northeastern coast of Africa, i.e., annual periodicity in the alternating direction of the current velocity vector [9]. Most researchers believe that the intense western boundary transfer (the Somali Current) appears in the summer monsoon period (approximately from June to September), when a strong favorable southwestern wind accelerates the northeastern alongshore flow of surface waters up to velocities exceeding 300 cm/s. After the onset of the winter phase of the monsoon (November), the Somali Current presumably disappears [10] and gives way to a flow with the southern velocity component under influence of the northeastern wind. It is unclear, however, whether or not this scenario is real. Published materials [11‐13] provide evidence that a countercurrent (opposite to the Somali Current) appears in the subsurface layer at the end of the summer climatic season. The countercurrent also incorporates the surface layer water at the beginning of the winter monsoon period.