Extended Abstract (120K)15B.4
ITCZ breakdown: an observational study over the tropical East Pacific
Chia-chi Wang, University of California, Irvine, CA; and G. Magnusdottir
An observational study of Intertropical Convergence Zone (ITCZ) breakdown using multiple remote sensing datasets over the tropical East Pacific will be presented. The ITCZ has been observed to undulate and break into a series of disturbances on the timescale of less than two weeks. This process is referred to as ITCZ breakdown and is suggested to be one way of pooling vorticity in the tropical atmosphere. The produced disturbances either dissipate or move away and a new ITCZ may re-form in the same location. Wayne Schubert and his group simulated this process by using simple dynamical models and suggested that the ITCZ carries the seeds of its own destruction. However, since they often showed the same ITCZ breakdown case (26 July - 12 August 1988) in their papers, it may have created the impression that this was an isolated occurrence, which it is not.
From our preliminary analysis of (1) National Center for Environmental Prediction (NCEP) global high resolution tropospheric analysis data from the summer/fall 2000 and (2) visible and infrared satellite images from GOES-west from summer/fall 2002 and 2003, we found that the ITCZ in the tropical East Pacific went through several breakdowns during the active season (summer/fall) in those three years. This confirmed the Schubert et al. hypothesis and also motivated this observational study.
We will examine the active season thoroughly using the following datasets: visible and infrared images from GOES-west (Jul. 1992-present), ECMWF TOGA global advanced operational spectral analysis data (Jan. 1985-present), QuikSCAT scatterometer winds (Sept. 1999-present), and Tropical Rainfall Measuring Mission (TRMM) microwave data (Dec. 1997-present) to produce a climatology of ITCZ breakdown events. We determine ITCZ breakdown visually from the satellite images and from PV fields on 310 K isentropic surface in the ECMWF data. Some studies have shown that the convection in the ITCZ in the East Pacific is rather shallow (eg. Gu and Zhang 2001a,b). Thus, we use scatterometer winds to help determine a shallow breakdown event. The TRMM microwave data provide information on the environment of a breakdown event.
Two cases that will be presented occurred during 2003/Oct/10-18 and 2000/Aug/23-31. The year 2003 case is a very shallow ITCZ breakdown. The produced disturbances dissipated in a day and no named tropical cyclones formed. On the contrary, the year 2000 case was deeper and a named tropical cyclone, John, developed in the central Pacific after the ITCZ broke down.
This study will address the following questions that have not been answered yet: (1) what is the number of ITCZ breakdown events in the tropical East Pacific (for time period considered), (2) what is the ratio of strong (with named storms produced) and weak events, and (3) what is the interaction between the surrounding environment and ITCZ breakdown?
Session 15B, Large-scale circulation II
Thursday, 6 May 2004, 3:45 PM-5:15 PM, Napoleon I Room
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