Inertial Instability Criterion over the Indian Ocean

Toma and Webster (a) and (b) 2010 argued that the characteristics of the mean Intertropical Convergence Zone (ITCZ) in the Eastern Pacific Ocean arise from instabilities associated with the strong cross-equatorial pressure gradient as a result of the latitudinal sea-surface temperature gradient. Furthermore, numerical experiments using the Weather Research and Forecasting regional model (WRF) indicated that instabilities of the mean ITCZ result in the in situ development of easterly waves.

The strongest pressure gradient and subsequent cross-equatorial flow occurs in the Indian Ocean, with large excursion of zero absolute vorticity contour to 15 N during the summer. However, while the Bay of Bengal shows variability at inertial frequency (6-8 days), the East Indian Ocean does not exhibit significant peak in the 4-8 day range. For inertial instability to occur either the cross- equatorial wind shear has to be very large or the vertical stability has to be very low (Tomas and Webster, 1999). We investigate the inertial instability criterion over various regions of the Indian Ocean and note that the inertial stability allows CAPE to build up slowly in the manner described by Stephens et al (2004) thus prescribing a time scale to intraseasonal monsoon variability and the MJO. Boreal winter conditions are also examined.

Stephens, G. L., P. J. Webster, R. H. Johnson, R. Engelen and T. L'Ecuyer, 2004: Observational Evidence for the Mutual Regulation of the Tropical Hydrological Cycle and Tropical Sea Surface Temperatures. J. Climate: 17(11), 2213-2224

Toma, V., P. J. Webster, 2010: Oscillations of the intertropical convergence zone and the genesis of easterly waves. I Theory and diagnostics. Clim. Dyn. doi: 10.1007/s00382-009-0584

Toma, V., P. J. Webster, 2010: Oscillations of the intertropical convergence zone and the gensis of eeasterly waves. II Numerical experiments. Clim. Dyn. doi: 10.1007/s00382-009-0585-9