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Gayana (Concepción)

versión impresa ISSN 0717-652Xversión On-line ISSN 0717-6538

Gayana (Concepc.) v.68 n.2 supl.TIProc Concepción  2004

http://dx.doi.org/10.4067/S0717-65382004000200039 

 

Gayana 68(2) supl. t.I. Proc. : 215-217, 2004 ISSN 0717-652X

CLIMATOLOGY AND INTERANNUAL VARIABILITY OF INTERTROPICAL CONVERGENCE ZONE IN THE EASTERN PACIFIC

 

José Garcés-Vargas1, Rodrigo Abarca del Río2 & Wolfgang Schneider3

1. Universidad de Concepción, Facultad de Ciencias Naturales y Oceanográficas, Departamento de Oceanografía, Cabina 5, Casilla 160-C, Concepción, Chile
2. Universidad de Concepción, Facultad de Ciencias Físicas y Matemáticas, Departamento de Geofísica, Casilla 160-C, Concepción, Chile
3. Universidad de Concepción, Centro de Investigación Oceanográfica en el Pacífico Sur-Oriental (COPAS), Casilla 160-C, Concepción, Chile
E-mails: ajgarces@udec.cl, broabarca@udec.cl, cwschneid@udec.cl.


ABSTRACT

Characteristics of the Intertropical Convergence Zone (ITCZ) in the eastern Pacific are derived from the monthly mean wind convergence of ERS-QuikSCAT fields for 1991-2004. Its climatology is computed over 1999-2004, and compared with those derived from monthly sea surface temperature, atmospheric water vapor, cloud liquid water, and rain rate from the Tropical Rain Measuring Mission (TRMM, 1999-2004).

Preliminary results indicated an excellent relationship between wind convergence and TRMM data for the northern branch of ITCZ (NITCZ). However, none of the TRMM fields show, as the ERS-QuikSCAT fields does the southern branch of ITCZ (SITCZ). Therefore, an especial emphasis is given over SITCZ, for instance only shown in wind convergence, with a permanent characteristic, although with a wide and narrow cover during February-March and October-November, respectively. TRMM data only shows the SITCZ during February to April.

We extend the analysis to interannual time scales, with a particular emphasis on El Niño-Southern Oscillation cycle. The analysis of the ERS-QuikSCAT fields shows that during El Niño events both NITCZ and SITCZ are united in the first months and SITCZ disappear in the rest, whereas during la Niña, the SITCZ tends to strength and permanence throughout the year.


 

INTRODUCTION

The ITCZ in the eastern Pacific has been studied with satellite data using several methodologies, e.g. highly reflective Cloud (Walliser and Gautier, 1993), precipitation (Zhang, 2001) and wind convergence (Liu and Xie, 2002; Zheng et al., 1997). These studies shows the NITCZ though with slightly differences, however without any recommendations about the best estimated with respect to wind convergence. However, important differences are found in the localization and cover of SITCZ and almost all the studies establish its absence during the second semester. On the other hand, the time span of the studies which investigates the interannual variations of ITCZ (through wind convergence) does not exceed 5 years. This study address precisely a better characterization and understanding of the ITCZ in the eastern Pacific with an special emphasis on the interannual variability of SITCZ during 1991-2004 using satellite data.

MATERIAL AND METHODS

The region analyzed is 30°N-20°S, 180°-70°W, and the period under inspection spans from August 1991 to December 2000 for wind convergence ERS1-2 (1 resolution) and from August 1999 to July 2004 for QuikSCAT (0.5 resolution). TRMM data fields (0.25 resolution, sea surface temperature, atmospheric water vapor, cloud liquid water, and rain rate) over the same time span than QuikSCAT's are also used. All data fields (QuikSCAT and TRMM ) are monthly, were interpolated to a 1 resolution, and then a climatology was computed covering from August 1999 to July 2004. For the research at interannual time scales, a latitudinal average of a region involving 110W - to - 91W from a mixed fields of ERS and QuikSCAT (1991-2004) were created by means of a linear regression which coefficients were obtained through the overlap period.

RESULTS

In the following some examples are shown. The figure 1a and 1b show the climatology of ITCZ for months March and April obtained from wind convergence (values greater than 0 s-1) and rain rate (values greater than 0.1 mm/hr), respectively. Both climatologies are similar; however, the extension of SITCZ obtained from winds convergence is wider close to the coast. On the other hand, during October-November total absence of SITCZ is observed in rain rate (Figure 2b). However during the same months in wind convergence a small SITCZ is found below of Galapagos Island between 110W-91W (Figure 2a). Similar results to those of rain rate were found for the same months for atmospheric water vapor and cloud liquid water (not shown). Given the differences of SITCZ between diverse satellite data, the interannual variability is focused especially in the zone between 110W-91W. The figure 3 shows latitude-time section of the wind convergence from ERS-QuikSCAT. During El Niño events according with the El Niño 3.4 index (Trenberth, 1997) the NITCZ and SITCZ are united during the first months of the year but the last one disappear through the last months of the year while in La Niña the SITCZ tends to strengthen and be permanent throughout the year.

Figure 1. Climatology of ITCZ for the months of March (left panel) and April (right panel) according a) wind convergence (QuikSCAT) and b) rain rate. In a) the contour of 0 s-1 is marked with thick solid black line, and dashed black lines represent positive wind convergence and b) thick black lines represent 0.1 mm/hr. Units are in a) s-1 and b) mm/hr, the color bars displayed right hand vertical each panel apply


Figure 2. Same as Fig. 1, but for October (left panel) and November (right panel).


Figure 3. Latitude-time section of the wind convergence (ERS-QuikSCAT, average between 91W-110W) characterizing the eastern Pacific. The contour of 0 s-1 is marked with thick solid black line, thin solid black lines represent positive wind convergence, and dashed black lines represent negative wind convergence (divergence). Units are in s-1, the right hand vertical color bar applies.

ACKNOWLEDGEMENTS

This work was supported by the DAAD (cod. A/99/14453) and MECESUP/UCO- 0002 scholarship program, and COPAS. ERS1-2 and QuickSCAT data were obtained from IFREMER/CERSAT. TMI data are produced by Remote Sensing Systems and sponsored by the NASA Earth Science REASoN DISCOVER Project.

REFERENCES

Liu, W.T., Xie, X.S., 2002. Double intertropical convergence zones - a new look using scatterometer. Geophysical Research Letters 29, doi:10.1029/2002GL015431. [         [ Links ]1]

Trenberth, K.E., 1997. The Definition of El Niño. Bulletin of the American Meteorological Society 78, 2771-2777. [         [ Links ]2]

Walliser, D.E., Gautier, C., 1993. A Satellite-Derived Climatology of the ITCZ. Journal of Climate 6, 2162-2174. [         [ Links ]3]

Zhang, C.D., 2001. Double ITCZs. Journal of Geophysical Research 106, 11785-11792. [         [ Links ]4]

Zheng, Q.A., Yan, X.H., Liu, W.T., Tang, W.Q., Kurz, D., 1997. Seasonal and interannual variability of atmospheric convergence zones in the tropical Pacific observed with ERS-1 scatterometer. Geophysical Research Letters 24, 261-263. [         [ Links ]5]

 

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