December-February 2004-05: Seasonal Climate Summary

By 

Dr. Gerald Bell
NOAA/ NWS/ Climate Prediction Center

TROPICS
Highlights 
1.	A weak Pacific warm episode (El Niño) continues
2.	Ongoing intraseasonal variability dominates ENSO signal
TROPICAL PACIFIC
Positive sea surface temperature (SST) anomalies greater than +1.0°C (~2°F) were observed in the central equatorial Pacific during December-February (DJF), reflecting a continuation of weak warm (El Niño) episode conditions. However, the spatial coverage of the largest positive anomalies decreased as the season progressed, and by February anomalies greater than 1°C were confined to the date line. Aspects of the tropical rainfall pattern were consistent with past Pacific warm episodes. These included enhanced rainfall over the central equatorial Pacific during February, and suppressed rainfall over Indonesia and northern Australia, the Amazon basin, and southern Africa. 
Considerable intra-seasonal variability was also evident in the patterns of tropical convection, which modulated the ENSO signal across the tropical Pacific. This intraseasonal variability contributed to large fluctuations in atmospheric circulation anomalies over the eastern North Pacific and North America, and likely contributed to excess precipitation in California and the southwestern U.S. during January (see Extratropics). During the past six months, the intraseasonal activity has also been associated with periods of weaker-than-average easterlies that have initiated eastward-propagating oceanic Kelvin waves. These Kelvin waves have contributed to fluctuations in sea-surface temperatures over the central and eastern equatorial Pacific, and to fluctuations in the depth of the oceanic thermocline in these regions. 
EXTRATROPICS
Northern Hemisphere
Highlights 
1.	Zonally-symmetric height anomalies at 500-hPa
2.	Extremely wet in southwestern U.S.
3.	High-latitude blocking activity over the North Atlantic
4.	Continuation of exceptionally warm North Atlantic SSTs

The mean Northern Hemisphere 500-hPa height pattern during DJF 2004-05 featured a north-south dipole of anomalies that extended around the hemisphere and exhibited considerable zonal symmetry. This pattern consisted of a band of positive height anomalies between 40°N-70°N and a band of negative height anomalies between 25°N-40°N. These anomalies were associated with recurring blocking activity over the high latitudes of the eastern North Pacific and North Atlantic, and with enhanced upper-level westerlies across the southern U. S., the central North Atlantic and Europe. The resulting split-flow patterns contributed to large precipitation anomalies over both the western U.S. and Europe.

North America
The mean circulation during DJF 2004-05 featured above-average 500-hPa heights over most of North America, and below-average heights over the lower latitudes of the eastern North Pacific and southwestern United States. This anomaly pattern was associated with a split-flow configuration over the eastern North Pacific. The northern branch of the split flow was associated with recurring blocking activity over the high latitudes of the eastern North Pacific.  Within the southern branch of the split flow, the mean jet stream and associated storm track entered the United States over southern California, well south of their climatological mean position over the Pacific Northwest. These conditions resulted in significantly below-average precipitation (more than 250 mm below average) in the Pacific Northwestern U.S., and significantly above-average precipitation (200% above average) in the Southwestern and Inter-Mountain regions of the United States. These precipitation surpluses, combined with well above-average precipitation during September-November, led to a dramatic improvement in long-term drought conditions over large portions of the western United States.
In the Southwest, the enhanced storm track was often accompanied by a deep flow of tropical moisture from lower latitudes. During these periods, the East Asian jet stream undercut the blocking ridge farther north, bringing moisture-rich air and intense storm activity into southern California and the Southwest. For a major precipitation event that occurred during 7-11 January, Bell and Higgins (2005, http://www.cpc.ncep.noaa.gov/products/expert_assessment/california_assessment_2005.pdf) linked these conditions to an eastward propagation of deep tropical convection to the central Pacific in association with the MJO.
In the Midwest, a northward shift and strengthening of the mean jet stream between mid-December and mid-January allowed for a deep layer of moisture originating over the Gulf of Mexico to penetrate well north of normal. The interaction of this moisture-rich air with vigorous upper-level disturbances and strong frontal boundaries led to three major precipitation events during the period, which culminated in significant river flooding in many Midwest states. These same conditions led to periods of extreme warmth across the Midwest and East, with daily mean temperatures in many locations often exceeding 8°C above average.

North Atlantic and Europe
The mean 500-hpa circulation pattern during DJF 2004-05 featured above-average heights over the high latitudes of the North Atlantic, in association with a complete disappearance of the mean Icelandic Low. Similar circulation anomalies were observed during SON 2004, in association with a very persistent and recurring blocking pattern. These conditions were associated with unseasonably warm temperatures in Europe in both seasons, with departures of 2°-3°C observed across northern Europe during DJF. 
The North Atlantic also saw a continuation of exceptionally warm SSTs at both high latitudes and in the subtropics during DJF. This warmth reflects the ongoing warm phase of the Atlantic multi-decadal mode that began approximately in 1995 (Goldenberg et al. Science, 2001). It was consistent with above-average heights across the high latitudes of the North Atlantic, and with generally above-average temperatures in Europe.

Southern Hemisphere
Highlights 
1.	Continuation of above-average surface temperatures across the three continents,
2.	 Below-average precipitation over Australia

In the Southern Hemisphere the 500-hPa circulation pattern during DJF featured negative anomalies over much of the high latitudes, in the area from southeastern Australia to New Zealand, and flanking southern South America.  

South Pacific and South America
Above-average surface temperatures were again observed over large portions of South America during DJF. The largest departures were observed in the Amazon Basin, where mean temperatures were 1°-2°C above average. Much of the Amazon Basin also recorded below-average precipitation during the season, with totals in many areas falling within the lowest 10th percentile of occurrences. The largest negative precipitation anomalies were observed in eastern Brazil immediately upstream of an anomalous 200-hPa trough axis.

Australia
Much of Australia experienced anomalously warm and dry conditions for the DJF season as a whole.  The most extensive dryness occurred during February when precipitation totals were below the 30th percentile of occurrences across the continent. During that month the Tahiti-Darwin based Southern Oscillation Index (SOI) was more than 4.0 standard deviations below average, as anomalously high surface pressure covered most of Australia and exceptionally low pressure occurred at Tahiti. These conditions were accompanied by a larger-scale pattern of suppressed tropical convection throughout Indonesia and the western equatorial Pacific, and above-average precipitation over the central equatorial Pacific. 

Southern Africa
In southern Africa, the rainy season normally lasts from October to April. During DJF area-averaged precipitation totals for the monsoon region were generally near-normal. However, large precipitation deficits were recorded in Mozambique, mainly in response to significantly below-average precipitation during February. Those deficits reflected an equatorward contraction of the main region of tropical convection, which occurred in association with an anomalous upper-level cyclonic circulation across southern Africa.