1.
Introduction
Recently, it is recognized that the monsoon system exists in South America and that the unique uplift- ing of the Andes modulate the regional monsoon cli- mate (Zhou and Lau 1998).  The South American Monsoon System (SAMS) shows three monsoon-like features (Nogues-Paegle and co authors 2002). (1) It shows the monsoon-type surface low pressure/upper- level anticyclone configuration and intense low-level jet (LLJ) stream of moisture from the Atlantic Ocean. (2) It is affected by large-scale land-sea surface tem- perature contrasts, as well as by terrain elevations and land surface conditions. (3) It includes shifts in regional precipitation from low or relatively low to very intence. The first spaceborne Precipitation Radar (PR) on board the Tropical Rainfall Measuring Mission (TRMM) satellite has capability to observe precipita- tion distribution even for the oceanic and high moun- tain region.  The La Plata basin became known as one of the regions which have unique characteristics of precipitation.  Figure 1a shows monthly mean rain- fall in 1998–2003 derived from PR2A25.  There are about 2000–3000 mm yr-1 rainfall over the La Plata basin as the same as over the Intertropical Conver- gence Zone (ITCZ), the maritime continent, the cen¬ tral Africa and the western part of the Amazon basin. Rainfall frequency over the La Plata basin is rela- tively smaller than that over the ITCZ and the mar- itime continent but comparable to the mid-latitude precipitation zones (Fig. 1b). However, the La Plata basin represents one of the highest conditional rain rate in other tropics and subtropics area (Fig.  1c). Moreover, the fraction of heavy rain (> 30 mm h- 1 ) is the particularly highest (Fig. 1d). In other words, the heavy rain easily occurs over the La Plata basin. The purpose of this study is to show the seasonal variation of precipitation in the La Plata basin using high-resolution data derived from TRMM PR. We also examined the atmospheric fields when the heavy rainfall occurs.
2.
Data
This study mainly used the near surface rain rate data of TRMM-PR2A25 version 5.0.  The analysed period is from January 1998 to December 2003, and the analysed area is the South America continent (40°S-0°, 80°W-30°W) (Fig.  2) including the La Plata basin.  To examine the seasonal variation of precipitation in the La Plata basin, PR2A25 data are averaged into 0.2° by 0.2° grid boxes, and monthly mean rain, rain frequency, conditional rain rate (ac- cumulated rain amount devided by the time when it rains) and the fraction of precipitation samples of 30 mm h-1 or more to the all precipitation samples are calculated. We also used relative humidity, tempera- ture, and geopotential height data from the National Centers for Environmental Prediction/Department of Energy Atmospheric Model Intercomparison Project 2 (NCEP/DOE AMIP-2) reanalysis. For topo- graphic data, the Global 30 Arc Second Elevation Data Set (GTOP30) dataset of the U.S Geological Survey was utilized.
3.
Results
The seasonal variation of precipitation in the La Plata basin was examined. Figure 3 shows the sea- sonaly mean daily rainfall over the South American Continent. In the austral summer season (DJF), pre- cipitation is widely distributed around the Amazon basin and the La Plata basin, and a high value is also indicated in the vicinity of the foothill of the An- des. On the other hand, in the austral winter season (JJA), the large rainfall area is limited to the Amazon basin and an eastrn part of the La Plata basin. This seasonal variation was also seen from the conditional rain rate and the fraction of the heavy rain (30 mm h− 1 or more) to the all rain (not shown). Focused on the La Plata basin, the seasonal variation of precip- itation was examined in longitude-time cross section of mean rain rate (Fig. 4a) and conditional rain rate (Fig. 4b). The averaged area is 40°S–35°S (Fig. 2 black rectangle). While more than 5 mm day-1 pre- cipitation appears through the year in the eastern part of the La Plata basin (the east from 56°W), a clear seasonal variation appears in the western part of the La Plata basin (the west from 56°W). Inter- estingly, the conditional rain rate is higher during the period before the mature rainy season than the pe- riod after the mature rainy season, though mean daily rainfall hardly changes among the two period. This rainfall characteristic is similar to the Asian mon- soon precipitation characteristic which suggests con- ditional rain rate is higher in the pre-monsoon season than the muture monsoon season (Hirose and Naka- mura 2002). Figure 5 shows the monthly averaged wind velocity and wind direction, and the geopotential height at 850 h Pa from 1998 to 2003 by the NCEP2 reanalysis data. In June (Fig. 5a), a low-level jet flows through the Amazon basin, and the direction changed from easterly to northwesterly by the Andes, then the jet goes through the La Plata basin. In December (Fig. 5b), while in the north of 20°S the low-level jet flows similarly in June, in the south of 20°S the wind di- rection changed from northwest to northeast in the La Plata basin.  The low pressure is formed locally around the Altiplano region (20°S, 65°W), and the direction of the low-level jet changed according to the low pressure.  The monthly averaged moisture flux (not shown) shows similar to the averaged wind field. As mentioned above, while in the winter sea- son wet low-level jet flows from northwest to south east directly in the La Plata region, in the summer season the wind direction changed from northwest to northeast according to the local formation of the low pressure at the Altiplano region.  This northeast wind brings a large amount of water vapor into the western part of the La Plata basin, and causes much rainfall in the austral summer. The scatter plot of the monthly averaged surface pressure at the Altiplano region (22°S, 65°W) and the monthly averaged zonal wind component at the La Plata region (27.5°S–25°S, 65°W–62.5°W) (not shown) shows that when the surface pressure in the Altiplano region decreases, the easterly wind compo- nent at the La Plata region becomes strong.  From the scatter plot of the monthly averaged zonal wind component and monthly averaged daily rainfall at the La Plata region (not shown), it is shown that when the easterly wind component is strong, the daily rain- fall becomes strong at the La Plata region. In addi- tion, the scatter plot of the monthly averaged surface pressure at the Altiplano region and the monthly av- eraged daily rainfall at the La Plata region (Fig. 6) shows that when the surface pressure decreases at the Altiplano region, rainfall in the La Plata region in- creases. As menthiond above, it is shown that the rainfall in the La Plata basin increases according to the local low level pressure at highly elevated land area which arises from the intense surface heating in summer season.
4.
Summary and Conclusion
Seasonal variation of precipitation and atmospheric field are investigated around the La Plata basin where the heavy rain events highly occur compared with all the tropical and the subtropical region in the world. From the correlation of monthly averaged surface pressure at the Altiplano region, wind and rainfall at the La Plata basin, it is suggested that the rainfall in the La Plata basin increases according to the local low level pressure at highly elevated land area decreases. In summer season, the locally formated surface lows at the Altiplano region changes the direction of wet low-level jet from northwest to northeast in the La Plata basin.  This northeasterly wind transports a large mount of moisture to the dried foothill of the eastern slope of the Andes. The wet air makes the state of the atmosphere unstable, and causes convec¬ tive hevy rain frequently. The low pressure over the Altiplano region and South America low-level jet features in rainy season are similar to the features of the Tibetan low and the southwesterlies flow in the monsoon season over the east Asia, respectively.  The climate of South America and the climate of east Asia also have the common features.  But the distinct difference is found in (1) the direction of a very high mountain range, that is, the Andes mountain range from north to south, while the Himalayas range from east to west, (2) the sur- face condition on the west side of two systems, that is, cold pool of the eastern South Pacific versus strong thermal contrast between South Asia and the Indian Ocean.  The unique geographical features and sur- face conditions in South America might produce the heavy precipitation in the La Plata basin.