Indian Ocean near-equatorial symmetric stability from satellite observations: an elusive connection to atmospheric convection

Abstract

The north–south symmetry of the low level near-equatorial atmospheric flow is examined in relation to the position of the Intertropical Convergence Zone (ITCZ) over the Indian Ocean with the aid of satellite and reanalysis data. Absolute vorticity (AV) and convergence fields on temporal scales of 10 days are derived from 7 years of Quick Scatterometer (QuikSCAT) observations and related statistically to organized convection deduced from outgoing longwave radiation (OLR) measured by the National Oceanic and Atmospheric Administration (NOAA) polar-orbiting satellites. The AV observations reveal a belt of symmetric instability on the summer side of the equator. The evolution of this symmetric instability with respect to the cross-equatorial flow (CEF), the location of the ITCZ, and monsoonal forcing is investigated. We identify three distinct dominant convective regimes. We show that during the active Indian monsoon season, symmetric instability and the displacement of the zero-AV line off the equator are directly linked to organized convection and the off-equatorial location of the ITCZ. The advective (inertial) regime interacts with and is controlled by monsoon and CEF. In periods of transition between summer and winter monsoon, the lower atmosphere is symmetrically stable and two other convective regimes can be identified: a double ITCZ, and localized convection near the equator.