The role of inertial instability in determining the location and strength of near‐equatorial convection
Authors: Tomas Robert A. , Webster Peter J. 
Published: February 2007

There are two major organized cloud configurations in the vicinity of the equator. Where there is a small cross‐equatorial surface pressure gradient, convection is close to the equator and is generally tied to the location of the lowest sea‐level pressure (SLP) and warmest sea‐surface temperature (SST), in agreement with arguments based upon simple thermodynamical considerations. However, when there is a substantial cross‐equatorial pressure gradient, such as occurs in the monsoon regions, organized convection appears off the equator in the summer hemisphere, equatorward of the SLP minimum and not necessarily collocated with the warmest SSTs. Thus, in this instance, simple thermodynamical considerations alone cannot explain 2he location of the convection. In this situation, the zero absolute vorticity contour (η = 0) also lies in the summer hemisphere. Therefore, between the equator and the η = 0 contour is a region of locally‐anticyclonic absolute vorticity and an inertially unstable regime. It is argued that the convection results from the low‐level divergence‐convergence doublet centred about the η = 0 contour which is the mitigating response to the inertial instability. The associated latitude‐height secondary circulation should provide subsidence (suppressed convection) over the equator and rising motion (enhanced convection) to the north of the zero absolute vorticity contour.

Signatures of the inertial instability predicted by theory are found in observations supporting the hypothesis. Wherever a strong cross‐equatorial pressure gradient exists, the η = 0 contour bisects a maximum in the divergent wind field. Divergence is found equatorward of the zero contour and convergence on the poleward side. Latitude‐height cross sections show strong local meridional circulations with maximum rising motion on the poleward side of η = 0. As the regions where the rising motions occur are conditionally unstable, there is deep convection and the vertical circulations extend throughout the troposphere. It is noted that the intensity of the off‐equator convection is deeper (and probably stronger) than convection located at the equator. This is probably because the convection associated with the inertial instability is more efficient. Necessary conditions for the location of near‐equatorial convection are listed.

Arguments are presented whereby inertial instability is established as the cause, rather than an effect, of off‐equatorial convection. These include an outline of the sequence of processes leading up to the convection. The factors that limit the encroachment of the η = 0 contour into the summer hemisphere are discussed and an explanation for the existence of the low‐level westerly monsoon wind maximum is suggested. The possible role played by the instability mechanism (or the lack of it) in coupled model simulations that produce seasonally migrating and/or double ITCZs in the eastern Pacific Ocean is discussed. Finally, it is proposed that the instability mechanism is important in the initiation of westward‐moving disturbances found in the eastern Pacific and in determining active and break periods in the summer Indian monsoon.