Seasonal Assessment MAM 2000

The long-lived tropical Pacific cold episode (La Niña) weakened in March-May 2000, but persisted through the season. Sea-surface temperature (SST) indices for the tropical Pacific  were near zero in the east Pacific and only slightly negative in the central Pacific. Seasonal average SST anomalies show continued but weakened La Niña conditions, compared to the previous season. The time-longitude plot of equatorial SSTs shows a double peak in the La Niña episode, first in late 1998 to early 1999 and again in late 1999 to early 2000. La Niña episodes that last two or more years are less common than shorter episodes, particularly for strong cold episodes, but they have occurred in the historical record. The previous last strong double-peak La Niña had extremes in 1973-74 and 1975-76. A weak extended cold episode also occurred in 1984–85. Cold episodes that last for three or more years have also been observed in the historical record. For example, cold episode conditions began in 1906 and persisted until 1911, with extremes in 1909 and early 1910. There were also weak negative anomalies in the equatorial Pacific for much of the period between 1942 and 1950, with extremes in late 1942 through early 1943 and again in early 1950. In those earlier extended episodes, the cold episode conditions were weak for most of the extended episode. There are no strong triple-peak cold episodes in the historical record.

The depth of the 20°C isotherm indicates that over the past two years, heat storage has been building in the western tropical Pacific, and by May 2000 positive subsurface temperature anomalies had begun to spread to the eastern-equatorial Pacific. Evolution and maintenance of La Niña depends on several factors besides subsurface temperatures, including atmospheric feedbacks affecting the strength of the easterly surface winds along the equatorial Pacific. However, the subsurface temperature anomalies are often the first sign of changes at the surface in the east-tropical Pacific because subsurface equatorial waters are typically brought to the surface (upwelled) by the winds in the equatorial Pacific. Warming of the subsurface eastern-equatorial Pacific limits the ability of a cold episode to re-establish itself, since any upwelled waters will be relatively warm.

Outgoing longwave radiation (OLR) anomalies for March-May 2000 indicate suppressed convection in the tropical Pacific near the date line, and enhanced convection over Indonesia, consistent with cold-episode conditions. However, there was also enhanced convection over parts of the eastern Pacific late in the season, reflecting positive SST anomalies in that region. The OLR anomalies also indicate suppressed convection in east-equatorial Africa and enhanced convection over northeast South America. The 850 mb wind anomalies show enhanced easterly winds in the central to western equatorial Pacific, over the region where negative SST anomalies persisted during the season. The strength of the easterly anomalies decreased over the season, and wind anomalies in the eastern-equatorial Pacific were weak or slightly westerly, reflecting the weakening cold-episode conditions. Late in the season strong west-southwest 850 hPa wind anomalies developed over the northwest Indian Ocean, suggesting an early onset of the Indian monsoon season.

For the contiguous United States, temperatures during the March-May season averaged above normal across the entire country, with the largest anomalies (more than 2°C above normal) observed over the Southwest and parts of the north-central region. On a monthly basis the largest anomalies occurred during March, with temperatures 3°-5°C above normal observed across parts of the Northeast, the Midwest and the Northern Plains. During April, much of the eastern half of the nation experienced slightly below-normal temperatures, with positive anomalies (up to +3°C) recorded over the Rocky Mountains, the Southwest, and along the West Coast. Temperatures were again above-normal from coast-to-coast during May, with the largest anomalies again observed over the Southwest.

Precipitation across the United States during the season featured above normal rainfall across the mid-Atlantic, the Northeast and the Great Lakes, and a continuation of drier-than-normal conditions in the Midwest and the Southeast. Rainfall was below normal across Florida and much of the Southeast during all three months, with deficits of more the 75 mm observed across much of this region during May and seasonal deficits of over 150 mm observed across Alabama and Georgia. Dryness along the Gulf Coast (Louisiana to Florida) and in the Midwest was a continuation of dry conditions that had persisted since 1999. In contrast, the above-normal rainfall in the Great Lakes region ended a prolonged period of below-normal rainfall that had also persisted from the previous year.

Global temperature anomalies for the season show that most of the Northern Hemisphere was warmer than usual, with largest anomalies in North America, Europe, and Siberia. Towards the end of the season the positive anomalies weakened and negative anomalies developed in northwest North America and western Russia. In the Southern Hemisphere the seasonal anomalies were on average positive, but much weaker than in the Northern Hemisphere. The largest positive Southern Hemisphere temperature anomalies were over the South Pacific and Indian Oceans. Temperatures over Australia were much below normal throughout the season, which was a continuation from the previous season.  Late in the season negative anomalies also developed over South Africa. The global-average temperature for the season was slightly above normal, but was smaller than in the previous season.

Global precipitation anomalies for the season were dominated by La Niña impacts. Precipitation was enhanced over Indonesia and suppressed over the central tropical Pacific. The Indian monsoon developed a little early, which resulted in a wetter-than-normal March-May season. Precipitation was greater than normal in a band extending from northeastern South America into the Atlantic. Early in the season southeast Africa was wetter than normal, but precipitation amounts were normal over the region late in the season.