Westward Propagation of Annual Sea Level and Wind Signals in the Western Pacific Ocean

Abstract

The annual cycle of sea level variability is examined for the period 1976?1985 at seven stations that lie along the mere axis (near 7°N)) of the north equatorial countercurrent (NECC) sea level trough in the western Pacific. The technique of complex demodulation is used to describe the year to year modulation of the phase and amplitude of the annual cycle of sea level along this line. This technique reveals differences in the character of the annual cycle that are related to the occurrence of the El Niño/Southern Oscillation (ENSO) phenomenon. During non-ENSO time periods, the amplitude of the annual cycle averages 4?5 centimeters and increases towards the west at approximately 1 mm per degree of longitude. The annual cycle during thew periods also shows a phase propagation towards the west at about 50 cm/s, which is comparable to the Rossby wave phase speed at this latitude. During the ENSO events, the amplitude of the annual cycle averages 7?8 centimeters and increases towards the west at about 2 mm per degree of longitude. During these times the annual cycle is nearly in phase from the dateline to the coast of the Philippines. Interannual modulations of the amplitude of the annual cycle of wind in this region are similar to those seen in the sea level field, but no westward phase propagation is observed. A global analysis of the FGGE winds reveals a westward propagating zonal wind component that is obscured at the latitude of the NECC trough by a larger zonally uniform signal. The zonally uniform signal varies with latitude, following the solar heating, and accounts for the zonal mean phase of the annual cycle. The propagating component has a phase speed of about 50 cm/s and has maximum amplitude at 7.5°N. The energy source for this wind signal appears to be in the monsoon region of the Indian Ocean. It is hypothesized that the propagating annual sea level signal observed in the western Pacific results from resonant forcing by this westward propagating feature of the wind field.