MJO-Related SST Variations over the Tropical Eastern Pacific during Northern Hemisphere Summer

Abstract

The Madden?Julian oscillation (MJO) significantly modulates eastern tropical Pacific sea surface temperatures (SSTs) during Northern Hemisphere summer. Typical SST variations during an MJO life cycle are 0.4°?0.5°C. Considerably higher variations occur with the strongest events (1°?2°C). The magnitudes of these variations are comparable to those associated with west Pacific MJO variability during NH winter. SST anomalies to the north of 4°N, where the strongest modulation of east Pacific convection by the MJO occurs, are 180° out of phase with equatorial anomalies. These SST variations are responsible for variations in surface saturation equivalent potential temperatures of greater than 3°C. The highest SSTs lead maximum convection by about 10 days, and SSTs begin to cool with the onset of enhanced convection. Off-equatorial SST anomalies are most likely due to variations in surface latent heat flux, although shortwave flux anomalies are important in the heart of the MJO convective region. Decreased surface evaporation and increased shortwave fluxes lead maximum SSTs by 10?15 days. Longwave and sensible heat fluxes do not significantly contribute to MJO-related SST variations. MJO-related SST changes on the equator cannot be explained by surface fluxes, and are most likely caused by ocean dynamics. The effects of eastern Pacific MJO SST anomalies on the surface wind field are explored using the diagnostic primitive equation model of Nigam and Chung. Intraseasonal SST anomalies induce enhanced surface convergence (divergence) about 10 days before maximum (minimum) convection. The combined effects of increased surface convergence and increased equivalent potential temperature may create favorable conditions for MJO convection during periods of warm SST anomalies. A model with interactive convection is needed to fully explore these SST?convection feedbacks.