HR: 11:05h
AN: OS31D-04    [PDF]
TI: Impact of Atmospheric Intraseasonal Variability in the Equatorial Indian Ocean: Low-Frequency Rectification in Equatorial Surface Currents
AU: * Han, W
EM: whan@enso.colorado.edu
AF: PAOS, University of Colorado, Campus Box 311, Boulder, Co 80309 United States
AB: An ocean general circulation model (OGCM) is used to investigate the low-frequency rectification of atmospheric intraseasonal variability (10--90 day periods) in surface currents of the equatorial Indian Ocean. A hierarchy of OGCM solutions are found in an actual tropical Indian Ocean basin for the period of 1988--2001. To isolate the contribution from various nonlinear processes, a 4$\frac{1}{2}$-layer intermediate ocean model is also used. Results from the OGCM solutions suggest that the intraseasonal atmospheric forcing causes episodical, swift eastward surface currents during spring and fall. These eastward currents are commonly referred to as the Wyrtki Jets (WJs). Of particular interest is that except for this intraseasonal behavior there are low frequency rectification in zonal surface currents. Amplitudes of the spring and fall WJs and westward surface flow during January--March are weakened by as much as 15-25 cm s$^{-1}$ with intraseasonal forcing. These rectified currents exhibit significant interannual variabilities and they result primarily from intraseasonal wind forcing. Processes that determine the rectification are: asymmetric response of mixed layer depth to easterly and westerly winds, entrainment and advection of subsurface water into the surface mixed layer, and horizontal advection. During an intraseasonal event, westerly wind deepens whereas easterly wind shoals the surface mixed layer. A net, westward current is produced over an event mean because easterly wind acts onto a thinner surface mixed layer whereas westerly wind acts onto a thicker one. This effect is especially strong and dominates other processes during spring and fall, when the seasonal westerly winds are strong, the WJs are intense, and mixed layers are relatively thick. 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. Zonal and meridional advections contribute somewhat to the WJs weakening near the WJs maxima. 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 act to reduce the westward surface flow. This eastward rectification results largely from the vertical advection and entrainment of the EUC. During the Indian Ocean Zonal Mode events, surface winds are weak easterlies and EUC reappear during summer and fall. The rectified currents are westward in the OGCM solutions, because effects of the asymmetric response dominate the entrainment and vertical advection of the EUC.
DE: 4500 OCEANOGRAPHY: PHYSICAL
DE: 4512 Currents
SC: Ocean Sciences [OS]
MN: 2003 Fall Meeting