Intertropical Convergence Zones during the Active Season in Daily Data

Chia-Chi Wang,Gudrun Magnusdottir

doi:10.1175/2007jas2518.1

Chia-Chi Wang, Gudrun Magnusdottir

Open Access

https://doi.org/10.1175/2007jas2518.1

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Abstract

Abstract Synoptic-scale variability of vorticity structures in the lower troposphere of the tropics is analyzed in 23 yr of daily averaged high-resolution reanalysis data. The vorticity structures can be divided into zonally elongated vorticity strips, classified as intertropical convergence zones (ITCZs), and more localized maxima, termed westward-propagating disturbances. A composite of such variability is presented for the east to central Pacific and for the east Atlantic/Africa region, both in summer. The composite in the east Pacific is zonally elongated and ITCZ-like, propagating westward over a number of days before dissipating. The spatial structure of the vorticity strip shows the characteristic cyclonic tilt into the latitudinal direction with time that is also seen in modeling experiments. The composite over the Atlantic/Africa region shows two active regions that are correlated on synoptic time scales. The disturbances in the southern region are better developed and longer lasting, even though the time and space scales are smaller than over the east Pacific. Overall, variability over the Atlantic is consistent with variability due to African easterly waves. The double ITCZ in spring in the east Pacific is different from the few earlier studies available. It is stronger south of the equator and located at 10°S, which is farther poleward than earlier studies have indicated. The northern branch that is weak in comparison is located at 5°N. The two branches of the double ITCZ tend to appear in tandem on the 2-week time scale.

Highlights

We are interested in synoptic variability of the intertropical convergence zone (ITCZ) and the closely associated westwardpropagating disturbances (WPDs) during the active season in the eastern Pacific
We have presented a composite picture of the ITCZ on synoptic time scales in the east to central Pacific (Fig. 5) during the active season
The ITCZ builds from the easternmost part of the basin westward while propagating and eventually dissipating

Summary

Introduction

We are interested in synoptic (time scale of 2–25 days) variability of the intertropical convergence zone (ITCZ) and the closely associated westwardpropagating disturbances (WPDs) during the active season in the eastern Pacific. It shows that even though we neglect data outside of a fairly restricted longitude range, the important parts of the map, such as the well-separated double maximum, The tropical Atlantic in summer on the 2–25-day time scale is dominated by AEWs that originate over the African continent. Even though many features of the spectrum are similar to the one in the eastern Pacific (Fig. 3), note that in this case the maximum spectrum is located at higher frequencies (and wavenumbers), from about 2.5 to 7.5 per day This shows that tropical synoptic activity in the Atlantic basin and over West Africa has a shorter time scale, which is consistent with the region being dominated by FIG. This is entirely consistent with previous studies and lends credence to our analysis of disturbances in the eastern Pacific, which by comparison are poorly documented on the synoptic time scale

Eastern Pacific in spring for both sides of the equator

Concluding remarks