Intraseasonal perturbations in Sea Surface Temperatures of the equatorial eastern Pacific and their association with the Madden-Julian Oscillation

A particular pattern of intraseasonal perturbations in sea surface temperature (SST) is observed in the eastern Pacific Ocean following events of strong surface winds associated with the Madden-Julian oscillation (MJO). This intraseasonal SST pattern straddles at the equator with its longitudinal scales of 2-5 × 10³ km and meridional scales of about 500 km. The amplitude of the perturbations is 0.5°C or greater. Positive and negative perturbations sometimes follow one another. They show tendencies of both eastward and westward movement. Such equatorially elongated perturbations in SST in the eastern Pacific are hypothesized to be caused by intraseasonal oceanic Kelvin waves forced by the MJO over the western/central Pacific. As the Kelvin waves propagate eastward, changes in the vertical temperature gradient in the upper ocean due to the fluctuations in the depth of the thermocline modify the thermal effect of the equatorial upwelling. As a result, mixed layer and surface temperatures may fluctuate. The observational basis for this hypothesis is presented through an empirical analysis of intraseasonal perturbations in SST, surface wind forcing, the depth of the thermocline, and the vertical temperature gradient of the upper ocean along the equator. The intraseasonal components of these fields fluctuate in coherence on interannual timescales. A possible implication of the observations to the interannual variability in the Pacific is proposed.