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The ITCZ in the Central and Eastern Pacific on Synoptic Time Scales

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The ITCZ in the Central and Eastern Pacific on Synoptic Time Scales MAY 2006 WANG AND MAGNUSDOTTIR 1405 The ITCZ in the Central and Eastern Pacific on Synoptic Time Scales CHIA-CHI WANG AND GUDRUN MAGNUSDOTTIR Department of Earth System Science, University of California, Irvine, Irvine, California (Manuscript received 23 June 2005, in final form 15 September 2005) ABSTRACT The ITCZ in the central and eastern Pacific on synoptic time scales is highly dynamic. The active season extends roughly from May through October. During the active season, the ITCZ continuously breaks down and re-forms, and produces a series of tropical disturbances. The life span of the ITCZ varies from several days to 3 weeks. Sixty-five cases of ITCZ breakdown have been visually identified over five active seasons (1999–2003) in three independent datasets. ITCZ breakdown can be triggered by two mechanisms: 1) interaction with westward-propagating disturbances (WPDs) and 2) the vortex rollup (VR) mechanism. Results show that the frequency of occurrence of ITCZ breakdown from these two mechanisms is the same. The VR mecha- nism may have been neglected because the produced disturbances are rather weak and they may dissipate quickly. The ITCZ shows a strong tendency to re-form within 1–2 days in the same location. The ITCZ may break down via the VR mechanism without any other support, and thus it may continuously generate numerous tropical disturbances throughout the season. There are two main differences between the two mechanisms: 1) The WPDs-induced ITCZ breakdown tends to create one or two vortices that may be of tropical depression strength. The VR-induced ITCZ breakdown generates several nearly equal-sized weak disturbances. 2) The WPDs tend to disturb the ITCZ in the eastern Pacific only. Disturbances generally move along the Mexican coast after shedding off from the ITCZ and do not further disturb the ITCZ in the central Pacific. Therefore, the VR mechanism is observed more clearly and is the dominating mechanism for ITCZ breakdown in the central Pacific. 1. Introduction the ocean, while in the eastern Pacific, the ITCZ is narrow and long, generally being located at the south- On seasonal time scales, the intertropical conver- ern boundary of the eastern Pacific warm pool, north of gence zone (ITCZ) is thought to be a feature nearly in the strongest meridional gradient of sea surface tempera- steady state. It is the place where the northeasterly and ture (Raymond et al. 2003; Yin and Albrecht 2000). southeasterly trade winds converge. Air rises in deep The picture of the ITCZ is quite different on synoptic convection, moves poleward in the upper troposphere, time scales (order of 10 days) from the seasonal time and sinks in the subtropics where the subtropical highs scale (order of 100 days). From day to day, the ITCZ is are located. The air then flows back to lower latitudes highly dynamic and changeable as seen in visible and in the lower troposphere and converges into the ITCZ infrared geostationary satellite images [Geostationary to complete the circulation. This closed circulation is Operational Environmental Satellite (GOES)] that called the “Hadley cell,” and the ITCZ is its rising measure cloud reflectivity and cloud-top temperature, branch. In the western Pacific, the location of the ITCZ respectively (Fig. 1). It may be quite narrow (2° lati- migrates seasonally with the sun. However, in the east- tude) and stretched over 70° in longitude for a few days, ern Pacific, the ITCZ tends to stay in the Northern developing undulations whose amplitude may increase Hemisphere (NH) year-round. The shape of the ITCZ over the next few days until the entire ITCZ breaks varies with longitude. In the western Pacific, the ITCZ down into smaller disturbances, some of which may is broader in latitude mainly due to the warm pool in strengthen with time as they move away, while others dissipate. The ITCZ may then re-form in the original location, the entire life cycle taking on the order of 10 Corresponding author address: Chia-chi Wang, Department of Earth System Science, University of California, Irvine, Irvine, CA days. The fields of precipitation and low-level conver- 92697-3100. gence are both too noisy to characterize the instanta- E-mail: [email protected] neous ITCZ, even though both fields are used to de- © 2006 American Meteorological Society MWR3130 1406 MONTHLY WEATHER REVIEW VOLUME 134 scribe the monthly or seasonal mean ITCZ. We find and Magnusdottir (2005) simulated the mechanism for that low-level vorticity describes the ITCZ well in in- both shallow and deep ITCZ breakdown in a fully stantaneous data. It shows up as a narrow strip of posi- three-dimensional dynamical model and examined the tive vorticity near the surface [in Quick Scatterometer effect of various background flows. The VR mechanism (QuikSCAT) data] or at 975 hPa [in National Centers is suggested to be an efficient mechanism to pool vor- for Environmental Prediction (NCEP) data]. ticity in the tropical atmosphere, which represents the Deep and shallow ITCZs are considered in our in- early stage of cyclogenesis. vestigation. Shallow convection or shallow Hadley cell An easterly wave in the tropical east Pacific may be has been observed in several independent datasets traced as far back as the Atlantic basin using 700-hPa (e.g., Zhang et al. 2004). Its rising branch is in the ITCZ meridional wind perturbations (or surface pressure) as for the conventional Hadley cell, but its divergent and can be distinguished from the locally generated outflow lies just above the top of the planetary bound- disturbances. However, both types are considered dis- ary layer. The strength and depth of a shallow ITCZ turbances external to the ITCZ in the central and east- changes seasonally. It is strongest and deepest in fall/ ern Pacific. In a series of modeling studies by Schubert winter at about 3-km height, and shallowest in spring at and collaborators (Hack et al. 1989; Schubert et al. about 1-km height. Murakami et al. (1992) and Zhang 1991; Guinn and Schubert 1993; Nieto Ferreira and et al. (2004) suggest that the shallow convection is most Schubert 1997) and in Wang and Magnusdottir (2005), robust when deep convection is absent. In GOES sat- the term “ITCZ breakdown” was used for the events ellite images, the shallow ITCZ is detected in visible specifically triggered by the VR mechanism. In this pa- images but not in the infrared ones, whereas the deep per, we name the mechanism “the vortex rollup mecha- ITCZ shows up both in visible and infrared satellite nism” to avoid confusion. images. ITCZ breakdown has been largely ignored due to the An example of ITCZ breakdown from September difficulty in identifying the event in conventional mea- 2000 is shown in Fig. 1, which shows visible and infrared surements, especially in the tropical central to eastern GOES images from the tropical Pacific. On 19 Septem- Pacific where observations are sparse. This gives the ber ITCZ convection was shallow, with an easterly impression that the observed ITCZ breakdown event wave appearing on the right edge of the image, which (late July to early August 1988, in the central to eastern did not disturb the ITCZ but just moved northwestward Pacific) shown in Hack et al. (1989), Schubert et al. along the coast. The ITCZ intensified into a deep ITCZ (1991), and Nieto Ferreira and Schubert (1997) was an 2 days later (21 September) and started undulating isolated event, which it was not. The difficulty is mainly from its western end. The ITCZ broke into three pieces associated with horizontal resolution of observational on 23 September (due to the vortex rollup mechanism products. The horizontal resolution of conventional re- that will be described in the following paragraph). The analysis data (2.5° ϫ 2.5°) is not fine enough to depict produced disturbances were rather weak and not well ITCZ breakdown events. We have analyzed NCEP re- organized. They moved toward high latitudes and dis- analysis data (2.5° ϫ 2.5°) and did not locate any ITCZ sipated. On 29 September, an ITCZ re-formed in the breakdown events. In our modeling study (Wang and same region around 10°N. Magnusdottir 2005), we found that a horizontal resolu- ITCZ breakdown events can be triggered by interac- tion of at least 1.1° ϫ 1.1° was required in a primitive tions with westward-propagating disturbances (WPDs), equation model to produce strong horizontal wind which include easterly waves that originated over the shear to allow the VR mechanism to be initiated and to Atlantic (e.g., Avila and Clark 1989, and references resolve the vorticity filaments. In addition, in data-poor therein) and tropical disturbances that may have been regions, such as the tropical central to eastern Pacific, excited by flow going over the Central American moun- reanalysis data are often polluted by model assump- tains (e.g., Zehnder and Powell 1999, and references tions. With the high-quality remote sensing datasets, therein). ITCZ breakdown may also occur due to local such as QuikSCAT winds, supplemented by older sat- instability of the heating-induced, lower-tropospheric, ellite datasets, we are able to study the process of ITCZ vorticity strip (Charney 1963; Hack et al. 1989; Schu- breakdown in the real atmosphere. Wang and Magnus- bert et al. 1991, Guinn and Schubert 1993; Nieto Fer- dottir (2005) assumed that ITCZ breakdown occurs in reira and Schubert 1997; Wang and Magnusdottir summer and fall, coincident with the hurricane season. 2005), which is called the vortex rollup (VR) mecha- Here, we examine the entire year to confirm the extent nism in this paper. Nieto Ferreira and Schubert (1997) of the active season for ITCZ breakdown. We then simulated the VR mechanism in shallow-water (baro- focus on the active seasons in 1999–2003.
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