Seasonal Variability in the Equatorial Pacific

Five to nine years of observations from the Tropical Atmosphere Ocean array in the tropical Pacific are used to document the seasonal cycle of surface winds, zonal currents, SST, thermocline depth, and dynamic height. Along the equator, the normally westward surface zonal current reverses direction in late boreal spring and early summer. This seasonal variation in the zonal surface current propagates westward, as do seasonal variations in the Equatorial Undercurrent and zonal surface winds at the equator. At 5°N and 5°S, the seasonal variations in the 20°C isotherm and dynamic height also propagate westward. Conversely at the equator in the eastern and central Pacific, the variations in 20°C isotherm depth and dynamic height propagate eastward. These seasonal variations are interpreted by means of a simple dynamical model based on linear equatorial wave theory. Model results indicate that seasonal variability between 5°N and 5°S is dominated by wind-forced equatorial Kelvin waves and first meridional mode Rossby waves of the first and second baroclinic modes. The sum of the Rossby waves and Kelvin waves results in westward propagation in the equatorial zonal currents and off-equatorial thermal structure and, yet, eastward propagation in thermocline depth and dynamic height along the equator in the eastern and central Pacific.