Ocean Observing and Climate
OVAC Theme 2 - Investigating Rainfall Variability Using Satellites
The figure above shows rainfall occurrence in the equatorial Pacific and the commensurate changes in Sea Surface Temperature (SST). Left-hand column shows the climatological mean seasonal cycle for the years 1993 to 1996, whilst the other columns show the mean fields for individual 2-month periods throughout 1997 to 1999. Changes in SST are indicated by the thick (28 degC) and thin (24 degC) white contours.
El Niño is a major ocean-atmosphere phenomenon which affects the climate of the equatorial Pacific Ocean together with the countries bordering this region and others which are further afield. One of its impacts on the climate is a modification of precipitation patterns and we have studied this using an algorithm developed to determine rainfall from the dual-frequency measurements of the TOPEX satellite altimeter The diagram shows the climatological seasonal cycle of rainfall for the period 1993 to 1996 (left hand column) and the changes that occur during a recent period 1997 to 1999 in the middle of which there was a strong El Niño. The white contours show the associated changes in Sea Surface Temperature (SST).
The primary feature in the climatological fields is the Inter Tropical Convergence Zone, a zonal belt of high rainfall, centered just north of the Equator, which spans the basin. The onset of the El Niño in Mar-Apr 1997 is marked by an increase in the size of the western warm pool (the region with SST > 28°C). It is followed by a broadening and southward migration of the ITCZ and an increase in the frequency of rainfall within it, with the area of most intense rainfall shifting from the west of the Pacific to the central region. The change in rainfall is most pronounced in the first 4 months of 1998, subsequently the region of enhanced rainfall retracts in response to the transition from El Niño to La Niña conditions. This study has demonstrated the capacity of altimetry based rainfall determinations to characterise variations in this component of the freshwater forcing of the oceans. Subsequent research within this area will focus on obtaining improved estimates of precipitation by combining retrievals from various satellites and using the resulting fields in analyses of the freshwater forcing in other regions.
For more information, see Quartly et al., 2000.
Links to OVAC Theme 2 Research
Rainfall Variability Atlantic Freshwater SST Anomalies Flux Parameterisations
