Observed variability of the East Pacific ITCZ

Tropical convection is notoriously difficult to track and analyze, limiting investigations into the processes which control modulation. Part of the challenge is due to diverse influences on convection by both large and local scale processes, meaning deep convection may occur in well ordered patterns as well as anomalous isolated cloud systems.

The Intertropical Convergence Zone (ITCZ) can be thought of as an organized weather feature, therefore enabling the convection within its envelope to be tracked and its character assessed. The ITCZ is most obviously defined in terms of an elongated zonal band of clouds. It is particularly consistent and well defined in the eastern to central Pacific, from May through October. Its shape and the ‘envelope' which encompasses the cloudy region is easily identifiable by eye but until now has not been studied extensively in observations. Previous studies have relied strongly on a temperature threshold of cold cloud (as identified in satellite IR images), which does not take into account waning and re-establishment of convection and can include isolated storms that are not part of the larger cloudy zone.

Here we will present results from a long (1980-2008), 3 hourly dataset of ITCZ location and extent. The data is obtained using a statistical model that uses satellite data as input, discussed in a parallel presentation (Magnusdottir et al). A significant advantage of the dataset is its high temporal sampling rate of ITCZ location and extent that allows investigation of synoptic variability and interaction with climatic processes. The stretch of the dataset over multiple years allows intraseasonal variability in one year to be compared to other years.

In this presentation we will discuss the most significant findings on the intraseasonal interactions within the East Pacific ITCZ. The results will be shown from both a convective prospective (looking at the IR variability within the ITCZ envelope) as well as the overall characteristics of the ITCZ envelope itself, such as zonal propagation. The diurnal cycle of convection has been found to be very significant within the ITCZ envelope, and the ITCZ is observed to ‘pulse' accordingly in its overall extent. We shall present results on this and other time scales of variation that exist in the data.