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HOME > Expert Assessments > Climate Assessment > Seasonal Assessment: December 1999 - February 2000 > Summary

Seasonal Assessment DJF 1999-2000

Mature cold episode (La Niña) conditions prevailed during December-February (DJF) 1999-2000, with the associated effects dominating the global tropical and subtropical atmospheric circulation. The ongoing cold episode first developed in May-June 1998, reached mature conditions during DJF 1998/99, and then weakened through mid-1999. During the latter half of 1999 the cold episode again strengthened, with strong mature phase conditions dominating DJF 1999-2000.

Seasonal mean sea surface temperature (SST) anomalies were more than 1.0 oC below normal across the equatorial Pacific east of 165 oE. Within the season, monthly mean SST anomalies were more than 2 oC below normal at many locations in the equatorial Pacific. The Niņo 3.4 index in the central equatorial Pacific stayed at or below -1.5 throughout the season. The 21-month period from May 1998 to February 2000 was one of the longest continuous periods of cold SST conditions in the central tropical Pacific in the last 50 years. Similar long-lasting cold episodes were also observed during 1954-1956 and 1973-1976.

The subsurface thermal structure in the equatorial Pacific was highly persistent during the season with a deeper-than-normal thermocline in the west central and western Pacific and a much shallower-than-normal thermocline in the central and eastern equatorial Pacific. This resulted in an anomalous pattern of subsurface temperatures featuring positive anomalies (up to 5oC) in the western Pacific and negative anomalies (as low as -5oC ) in the east Pacific. This pattern greatly resembled conditions observed during DJF 1998/1999.

Consistent with ongoing cold episode conditions, tropical convection (as inferred by OLR) was enhanced across the winter monsoon region and suppressed over the central and eastern equatorial Pacific. The standardized outgoing longwave radiation (OLR) index in the central equatorial Pacific exceeded +2 during all three months, indicating greatly suppressed convection in that region. The DJF average OLR index was the highest value recorded during the period 1979-present. Seasonal precipitation totals ranged from 50-600 mm above normal over a large region encompassing the eastern Indian Ocean, Southeast Asia, northern Australia and the western equatorial Pacific. Seasonal rainfall totals were also excessive over northern South America and over southern Africa. In the latter region saturated soils and excessive rainfall from two tropical systems combined to produce disastrous flooding in Mozambique and portions of neighboring countries during February.

Seasonal mean lower-level and upper-level tropical wind anomalies exhibited features that are consistent with the precipitation anomalies in the tropical Pacific. At 850 hPa, easterly zonal wind anomalies were in excess of 6 m s-1 in the west-central equatorial Pacific, while westerly anomalies in excess of 3 m s-1 were observed over the central and eastern Indian Ocean. This low-level wind pattern implied strong anomalous low-level zonal convergence over Indonesia and Malaysia. At upper levels (200 hPa) westerly anomalies exceeded 10 m s-1 in the central and eastern equatorial Pacific, which together with the low-level easterly anomalies contributed to an enhanced Walker circulation with anomalous rising motion over southeast Asia and anomalous sinking motion over the central and eastern equatorial Pacific.

The tropical sea level pressure (SLP) anomaly pattern shows the well known cold episode characteristics with above normal pressure over the central and southeast tropical Pacific and below normal pressure over Southeast Asia. This SLP pattern was especially strong during February, contributing to the largest positive value (since 1958) of the equatorial SOI at 3.9.

The lower mid-latitudes and subtropics of both hemispheres exhibited above-normal 200-hPa heights, while below-normal heights prevailed throughout the Tropics. These features are consistent with La Niña conditions. In fact this pattern (sometimes referred to as the ‘global monsoon mode’) constitutes a recurring pattern associated with cold episode conditions, and has been present more or less continuously since mid-1998. In contrast to a highly persistent pattern of height anomalies in the Tropics, subtropics, and lower mid-latitudes, the high latitudes, especially in the Northern Hemisphere, exhibited considerable intraseasonal variability.   In the Pacific-North American sector, this variability was associated with a westward contraction of the mean jet stream, a northward shift of the westerlies relative to normal over the eastern Pacific, and  large month-to-month variability in the monthly values of the PNA index.  Also, considerable high-latitude retrogression of circulation features occurred during the season, as evident in the animation of 200-hPa heights.  At the middle and high latitudes, similar features can be found at 500 hPa and 200 hPa. Over the North Atlantic, the anomalous 500-hPa height pattern was associated with a strong positive phase of the North Atlantic Oscillation (NAO). This phase of the NAO was a contributing factor to the anomalous warmth over the United States. In fact, surface temperatures over the United States during DJF 1999/00  ranked as the warmest DJF period in the last 105 years. Accumulated seasonal precipitation, was below normal and was particularly dry (in the 10-20th percentile) along the southern tier of the United States and in the Northeast. Except for the Atlantic and Gulf states, where the moisture excesses from the above normal hurricane activity during 1999 lingered, widespread dry conditions dominated the rest of the country.

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Page last modified: August 10, 2007
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