Characterizing the ITCZ in the central to eastern Pacific on synoptic timescales in observations

On seasonal timescales, the ITCZ has been assumed to be a feature nearly in steady state. The ITCZ is where the northeasterly trades converge with the southeasterly trade winds. Air rises in deep convection, moves poleward in the upper troposphere, and sinks in the subtropics where the subtropical highs are located. The relatively dry, warm air then flows back to lower latitudes close to the surface, collecting moisture along the way, and converges into the ITCZ to complete the circulation cell. This closed circulation is called the ``Hadley cell", and the ITCZ represents its rising branch. The location of the ITCZ migrates seasonally with the sun, mostly controlled by the latitudinal variation in sea surface temperature.

The picture of the ITCZ in the tropical central to east Pacific is however quite different on synoptic timescales (several days). From day to day, the ITCZ is highly dynamic and changeable as seen in visible and infrared geostationary satellite images (GOES) that measure cloud reflectivity and cloud top temperature, respectively. It may be quite narrow and stretched over up to 90\deg in longitude for a day or two, developing undulations whose amplitude may increase over the next few days until the entire ITCZ breaks down into smaller disturbances some of which may strengthen with time as they move away, while others dissipate. The ITCZ may then reform in the original location, the entire lifecycle taking on the order of 10 days. The synoptic behavior of the ITCZ has lead to the hypothesis that the ITCZ may carry the seeds of its own instability, i.e., the thermal forcing induced by the convection produces a positive potential vorticity strip in the lower troposphere that leads to the reversal in the latitudinal gradient of PV thus satisfying the necessary conditions for barotropic/baroclinic instability. This mechanism for ITCZ breakdown is called the vortex roll-up mechanism to distinguish it from the other ITCZ breakdown mechanism, that of interaction with westward propagating disturbances (WPDs).

In this study we examine the synoptic evolution of the ITCZ in the tropical Pacific (east of the dateline) during the active season (July through October) over the past five years, using a variety of data, some of which has only recently become available. Our goal is to quantify the frequency of ITCZ breakdown and understand its role in producing tropical disturbances since this is one of the areas where numerous tropical cyclones are formed. Additional motivation for the study is: 1) This is a data sparse region in terms of the observational network, therefore not much is known about the dynamic nature of the ITCZ. 2) Even though major population centers are not in the region aside from the west coast of the Americas, tropical disturbances are of fundamental importance in terms of their role in transporting moisture out of the tropics. 3) The ITCZ represents a horizontal mixing barrier, but a deep/shallow ITCZ is also important in terms of fast mixing in the vertical (deep/shallow).

We examine ITCZ breakdown in terms of the mechanism of breakdown (due to vortex roll-up or/and WPDs), whether it is a deep or shallow ITCZ and connection to the MJO.