Dynamical features associated with 4–8 day variability in the OLR in the east Pacific Oceans

We will present results from our diagnostic study that investigates the causes and associated mechanisms that are responsible for the interannual variations in the strength and the location of the Intertropical Convergence Zones (ITCZ's) in the east Pacific and Atlantic Oceans. Understanding the interannual variability of the ITCZ's is important for several reasons. The convection within the convergence zones is responsible for a significant portion of the total heating that drives the large scale circulation in the tropics. Thus, variability in this heating is associated with variability in this circulation. Secondly, regional anomalies in the warm season precipitation over the land areas adjacent to the ITCZ's is thought to result from intensification, or weakening, or subtle shifts in the features that determine the rainfall climatology. Included among these features are the ITCZ's.

The results presented will examine the influence of several factors upon ITCZ variability. These include:

(1) SST and SST gradient anomalies; (2) Variability in the intensity and frequency of transient disturbances; (3) Variability in the large-scale, cross-equator pressure gradient.

There are some significant differences in the causes of ITCZ variability in the two regions examined. For example, ENSO accounts for much of the SST variability in the Pacific (and in turn, upon the ITCZ through its influence on the boundary layer moisture, temperature and pressure distribution), but no single mode of SST variability dominates in the Atlantic region. There are also causes of interannual variability that two regions share in common. For example, precipitation in both regions is influenced by the transient disturbances known as "easterly waves". Variations in the strength and frequency of these disturbances over the course of a month to a season will contribute to the interannual variability in the amount of precipitation. Also, in both regions there is a strong cross-equatorial boundary layer pressure gradients during the boreal summer season. Variations in the strength of this pressure gradient, owing to the influence of both the northern and southern hemispheres, can influence the strength and location of convection in the ITCZ's through variability in the cross-equator divergent flow in the boundary layer (Tomas and Webster, 1997; Tomas, Holton and Webster, 1999).

Tomas, Holton and Webster, 1999: The influence of cross-equatorial pressure gradients on the location of near-equatorial convection, Quart. J. Roy. Meteor. Soc., 125, pp. 1107-1127.

Tomas and Webster, 1997: The role of inertial instability in determining the location and strength of near-equatorial convection, Quart. J. Roy. Meteor. Soc., 123, pp. 1445-1482.