Intraseasonal Perturbations in Sea Surface Temperatures of the Equatorial Eastern Pacific: Induced by 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), anomalous SSTs emerge in the far eastern Pacific. Their amplitudes are 0.5C or greater, their longitudinal scales are 2000 - 5000 km, and their meridional scales are 500 km. They are centered at the equator and tend to expand westward. Such equatorially elongated perturbations in SST in the eastern Pacific are hypothesized to be caused by intraseasonal oceanic Kelvin waves forced by the MJO in 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 fluctuate following each MJO event. The observational basis for this hypothesis is presented through an empirical analysis of observed intraseasonal perturbations in SST, surface wind forcing, the depth of the equatorial thermocline, and the vertical temperature gradient of the upper ocean. The analysis results show statistical coherence among these fields which all undergo seasonal and interannual fluctuations. The westward movement of the perturbations in SST, however, is not explained on the basis of the observations.