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 temperat ure (SST) is observed in the eastern Pacific Ocean following events of stro ng surface winds associated with the Madden-Julian oscillation (MJO). This intraseasonal SST pattern straddles at the equator with its longitudinal sc ales of 2-5 X 10(3) km and meridional scales of about 500 km. The amplitude of the perturbations is 0.5 degreesC or greater. Positive and negative per turbations sometimes follow one another. They show tendencies of both eastw ard and westward movement. Such equatorially elongated perturbations in SST in the eastern Pacific are hypothesized to be caused by intraseasonal ocea nic Kelvin waves forced by the MJO over the western/central Pacific. As the Kelvin waves propagate eastward, changes in the vertical temperature gradi ent in the upper ocean due to the fluctuations in the depth of the thermocl ine modify the thermal effect of the equatorial upwelling. As a result, mix ed layer and surface temperatures may fluctuate. The observational basis fo r this hypothesis is presented through an empirical analysis of intraseason al perturbations in SST, surface wind forcing, the depth of the thermocline , and the vertical temperature gradient of the upper ocean along the equato r. The intraseasonal components of these fields fluctuate in coherence on i nterannual timescales. A possible implication of the observations to the in terannual variability in the Pacific is proposed.