Southward subsurface flow below the Somali Current

Abstract

The existence of a southward‐flowing current beneath the northern part of the seasonally reversing Somali Current is documented in a 2½‐year‐long time series of currents obtained at moored stations near 5°N about 30 km off the Somali coast. Its mean annual transport in the layer 150–600 m amounts to about 5×106 m3/s. The undercurrent has a pronounced seasonal cycle in phase with the near surface flow, suggesting a close coupling to the monsoonal wind forcing. With the spin‐up of the deepreaching northern Somali gyre after the onset of the southwest monsoon, the undercurrent is temporarily destroyed in the northern Somali Basin during June/July but is re‐established in August. The undercurrent does not reach 3°N but turns offshore north of that latitude.

Western Boundary Currents

“…Less is understood about the development of the Somali Current in response to the northeast monsoon, although it is clearly weaker and shallower and almost purely Ekman-driven (Figure 13.9b; Hastenrath and Greischar, 1991). Volume transports are about 5-10 Sv (Quadfasel and Schott, 1983). A southward undercurrent appears to persist throughout the winter, connected to eastward undercurrents along the equator (Jensen, 1991).…”

Seasonal variation of cross‐equatorial flow in the Somali Current

“…Also shown in Fig. 2a (dashed line) is the current profile at 5øN of the monthly mean from earlier moored current measurements [Quadfasel and Schott, 1983]; that profile is also augmented at the surface by the ship drift climatology, showing an apparently shallower northward current there. One has to be reminded here that northward currents at 5øN and further north are not a continuation of the crossequatorial flow but that the cross-equatorial flow turns offshore somewhere between the equator and 5øN and that the northern Somali branch is part of a separate northern gyre [Swallow et al, 1983].…”

“…While the response of the current system to the onset of the summer monsoon has been fairly intensively studied [Swallow et al, 1983;Schott and Quadfasel, 1982] including by direct current measurements near the equator [Leetmaa et al, 1982], not much is known about the response of the current to the winter monsoon, particularly regarding its vertical structure on the equator. From large-scale numerical modeling [Cox, 1970] and offequatorial measurements [Quadfasel and Schott, 1983] it was concluded that the Somali Current during winter would be the reverse of that during the fully developed summer monsoon situation, i.e., a continuous southward boundary current from 10øN to south of the equator, and monotonically decaying with depth; only weaker than during summer owing to the weaker wind forcing during the winter monsoon.…”

Current and thermohaline characteristics of the Arabian Sea during January 1998

“…According to previous studies it seems that the western flow into the Somali coast exists between 5˚N and 10˚N, with a southward flow from there to beyond the equator and a northward flow in the northern Somali Current region (Schott et al, 1990;Molinari et al, 1990). For the region from 6˚N to 10˚N, Quadfasel and Schott (1983) derived a southward undercurrent underneath the northward surface current from analysis of earlier hydrographic observations.…”