The Feedback between Equatorial Convection and Local Radiative and Evaporative Processes: The Implications for Intraseasonal Oscillations

Abstract

Existing theories of the Madden?Julian oscillation neglect the feedback between the modification of sea surface temperature by the convection and development of a convective cluster itself. The authors show that the convection-generated SST gradient plays an important role in cluster propagation and development. The relative importance of radiative and evaporative fluxes in SST regulation is also discussed. Various Tropical Ocean Global Atmosphere Coupled Ocean?Atmosphere Response Experiment and Central Equatorial Pacific Experiment observation platforms are used to estimate the effects of equatorial convection on SST changes during March 1993. The data include drifting buoys and TAO-buoy array measurements, combined with the Navy Operational Global Atmospheric Prediction System analyzed surface wind fields and Geostationary Meteorological Satellite cloud-top temperatures. It is shown that during the equatorial convection episode SST is decreasing under and to the west of the convective heat source due to the large wind velocities and solar flux reduction. To the east of the source, in the convergence region of a Kelvin wave, low wind speeds and high insolation cause the SST to increase. The data are used to formulate an empirical relationship between wind speed and the 24-h SST change on the equator. Although formulated in terms of wind speed, this relationship implicitly includes radiative effects. This equation is then used in a global circulation model to examine the effect of SST feedback on the behavior of equatorial convection. A series of experiments is performed using an R15 general circulation model of the ?aquaplanet? with a zonally symmetric SST distribution. In the case with fixed SSTs, equatorial wind fluctuations have the character of waves propagating around the globe with a phase speed of about 20 m s?1. When the effect of SST modification is included, the fluctuations slow down and become more organized. In addition, a 40?60-day peak appears in the spectral analysis of equatorial precipitation.