2004 LOM Workshop Tuesday 2:20 - 2:40 p.m.
Impact of atmospheric intraseasonal variability in the Indian Ocean: low-frequency rectification in equatorial surface current and transport
Weiqing Han, Peter Webster, Roger Lukas, Peter Hacker, and Aixue Hu
Univeristy of Colorado at Boulder
whan@enso.colorado.edu
ABSTRACT
An ocean general circulation model, the HYbrid Coordinate Ocean Model (HYCOM), is used to investigate the low-frequency (period longer than 90 days) rectification of atmospheric intraseasonal variability (10--90 day periods) in zonal surface current and transport of the equatorial Indian Ocean. A hierarchy of HYCOM solutions are found in an actual tropical Indian Ocean basin for the period of 1988--2001. To help identify nonlinear processes, a linear continuously stratified model is also used. To isolate the contribution from various nonlinear processes, a 4.5-layer intermediate ocean model is applied. Results from HYCOM solutions suggest that intraseasonal atmospheric forcing acts to weaken the equatorial zonal surface currents. Amplitudes of the spring and fall eastward surface jets, the Wyrtki Jets (WJs), and the westward surface current during January--March are reduced by as much as 15-25 cm/s by intraseasonal atmospheric forcing. Important processes that cause the rectification are: asymmetric response of mixed layer depth to easterly and westerly winds, entrainment, and upwelling. During an intraseasonal event, westerly wind deepens whereas easterly wind shoals the surface mixed layer. A net, westward current is generated over an event mean because easterly wind acts onto a thinner surface mixed layer whereas westerly wind acts onto a thicker one. Meanwhile, the strong, episodical westerly winds enhance the entrainment of the slower subsurface water into the surface mixed layer, also tend to weaken the WJs. In contrast, during January--March when the seasonal winds are equatorial easterlies, surface currents are westward, equatorial undercurrents (EUC) develop, and mixed layers are thin. The rectified surface currents are eastward, which reduce the westward surface flow. This eastward rectification results largely from the vertical advection and entrainment of the EUC. In the layers above the thermocline, the rectified zonal transports are westward when the WJs are significantly weakened. The total rectified zonal transports over the entire water column, however, are eastward with an amplitude of 3-15 Sv. This is because westerly winds generate equatorial downwelling, advecting the surface eastward momentum downward and giving an eastward subsurface current. Easterly winds cause equatorial upwelling and produce an eastward pressure gradient force that drives an eastward subsurface current. This eastward subsurface current is advected upward due to upwelling. The mean effect over an intraseasonal event is to create an eastward transport in the water column. Results from this paper have important implication for understanding climate variability because modification of WJs strength and transport can affect the SST and heat storage in the eastern Indian Ocean warm pool region.
LOM Users' Workshop, February 9-11, 2004