The equatorial eastern Pacific during boreal autumn features strong meridional gradients in SST and persistent cross-equatorial boundary layer flow into an ITCZ that can fluctuate in latitude by several degrees. The EPIC 2001 field experiment during September-October 2001 sampled deep convection, boundary layer structure, regional circulations, and atmosphere-ocean feedbacks in this region, with a particular focus on the 95 W line. We examine this region using a set of 8 EPIC airborne transects along 95 W from the equatorial cold tongue to 12 N, coupled with TAO buoy measurements.

We find that although the southerly flow is persistent, it varies considerably in intensity and in its northern terminus. We define the ITCZ as the latitude at which the meridional surface wind component reverses.

The boundary layer moist entropy in this southerly flow increases as the air passes over warmer SSTs in its northward movement, eventually becoming large enough for deep conditional instability. Deep convection is observed on and near the ITCZ when the southerly flow is strong (> 9 m/s) in regions where the following conditions are satisfied:

1. At least some conditional instability exists.

2. The convective inhibition is small.

3. The mid-troposphere isn't too dry.

The failure to meet any one of these conditions can inhibit deep convection in this region.

In conclusion, we find that deep ITCZ convection in the equatorial eastern Pacific develops when strong surface heat flux (as produced by strong southerly flow) occurs in conjunction with a thermodynamically favorable environment.