The Tropical Rain Measuring Mission (TRMM) satellite marked its third anniversary at the end of November 2000, while still recording reliable data. This month's article takes El Niño and La Niña as examples of long-term changes in tropical rainfall and sea surface temperature, both of which are observed by the TRMM satellite.
Third anniversary of TRMM
The Japan-U.S. joint TRMM project celebrated its third anniversary on November 28, 2000. On that day, many experts in "rainfall" from Japan, the U.S. and Europe held a symposium and discussed achievements of TRMM scientific activities and future rainfall observation missions.
TRMM carries the world’s first spaceborne Precipitation Radar (PR), jointly developed by the Communications Research Laboratory (CRL) and NASDA, with four NASA-developed sensors. With the combination of these sensors, TRMM, which is called a "flying rain gauge," is designed for globally and quantitatively understanding and obtaining the three-dimensional distribution of rainfall in tropical regions.
Although TRMM has gone beyond its initially planned mission time of three years and two months, the precipitation data is steadily being recorded even at the present. Monitoring global rainfall for a long time enables us to understand not only relatively short-term phenomena, such as localized heavy rain, but also changes in rainfall from the perspective of a long-term period. This article introduces long-term changes in rainfall, using as examples El Niño and La Niña, which are large-scale coupled atmospheric and oceanic variations in tropical regions.
 Last El Niño and La Niña in the 20th century
TRMM observed the abrupt termination of El Niño and subsequent development of La Niña in the early summer of 1998. La Niña is an opposite phenomenon to El Niño, which occurs when sea surface temperatures in the central and eastern equatorial Pacific Ocean are higher than those of a normal year. It is well known that El Niño causes various abnormal weather conditions and that, to a lesser extent, La Niña is also a trigger of abnormality.
Figure 1 is the time-longitude diagram of sea surface temperatures in the Pacific Ocean around the equator, which were retrieved from TRMM Microwave Imager (TMI) data. This diagram shows that the sea surface temperatures declined rapidly in the central and eastern equatorial Pacific Ocean in the middle of May 1998. In response to this change, El Niño dramatically transformed into La Niña (changes indicated by red turning to blue in the diagram).
Simultaneous measurements of the tropical atmosphere and ocean
The features of El Niño and La Niña do not only represent changes in sea surface temperature. It is known that El Niño and La Niña cause changes in rainfall amounts and their distribution globally, because sea surface temperature in the tropical Pacific Ocean affects large-scale atmospheric circulation. Since rainfall can be measured only on the ground, however, it is not easy to discover quantities of global rainfall trends on the vast oceans.
Figure 2 shows the year-to-year anomaly of monthly average precipitation imaged by TRMM (left column) and monthly average sea surface temperature imaged by TMI (right column) from that of a normal year. Since multiple sensors are mounted on TRMM, precipitation data sets integrated with these observation data are also made available. The precipitation presented here represents a monthly average precipitation product (3B43), which was created by combining TRMM data, geostationary meteorological satellite data and ground rain gauge data with the aim of providing the best precipitation profiles.
According to Figure 2, El Niño was still active in January 1998, immediately after launch of TRMM. The figure also shows that around the eastern equator where sea surface temperature was rising (shown in red) there was more rain than during a normal year, while to the contrary, there was less rain than during a normal year on the northern and western equator (shown in blue).
One year later, in January 1999, El Niño ceased and La Niña began to develop. This caused a totally different distribution of precipitation on the tropical Pacific Ocean, compared to that in the same month of the previous year. Precipitation on the Pacific Ocean around the central equator decreased in proportion to a decrease in sea surface temperatures, while precipitation increased in the Pacific Ocean around the western equator where the sea surface temperatures were close to those during a normal year.
After one more year, in January 2000, La Niña still continued to prevail. As of December 2000, La Niña was continuing, but getting weaker.
From observation to prediction
In the tropical Pacific Ocean where La Niña is getting weaker, the question is whether La Niña will cease and El Niño will regenerate. We cannot make predictions based only on satellite observation data, but attempts have recently been made to predict oncoming periods of El Niño and La Niña in the long term by assimilating various satellite data into numerical weather forecast models.
Figure 3 shows long-term predictions of anomaly in sea surface temperatures where El Niño events are monitored in the Pacific Ocean around the western equator. These predictions were provided by the National Centers for Environmental Prediction (NCEP) in January 2001. Although prediction results vary according to available numerical models, the comprehensive prediction (orange line) suggests that a weak El Niño could occur in the winter between 2001 and 2002. Continuous atmospheric and oceanic observations using such earth observation satellites as TRMM are expected to contribute to improving the accuracy of long-term predictions.
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