December-February 2005-06: Seasonal Climate Summary

By 

Drs. Gerald Bell and Vernon Kousky
NOAA/ NWS/ Climate Prediction Center

TROPICS
Highlights 
1.	Weak La Niña develops
2.	Continued very dry in equatorial eastern Africa
The patterns of anomalous ocean temperatures, atmospheric circulation and precipitation were consistent in indicating La Niña conditions in the tropical Pacific during the season. By the end of February equatorial SSTs were generally more than 0.5ºC below average between 165ºE and 115ºW. Negative SST departures were observed in all of the Niño regions, except for Niño 1+2.
Positive SST anomalies greater than +0.5ºC were restricted to the western equatorial Pacific between Indonesia and 155ºE, and to the extreme eastern equatorial Pacific. During February, positive SST anomalies increased in the extreme eastern equatorial Pacific and heavy rains occurred in some portions of Ecuador and northern Peru. Brief warming in the extreme eastern equatorial Pacific at this time of the year has been observed in previous years, including the La Niña years of 1999, 2000 and 2001. 
By February, above-average precipitation had developed over portions of Indonesia, the Philippines and along the South Pacific Convergence Zone (SPCZ), while below-average precipitation was observed over the central equatorial Pacific. Stronger-than-average low-level (850-hPa) easterly winds persisted over the central equatorial Pacific, and anomalous upper-level (200-hPa) cyclonic circulation centers were observed in both hemispheres. The equatorial subsurface temperature anomaly pattern (negative anomalies in the central and eastern Pacific and positive anomalies in the western Pacific), persisted during January-February 2006, and the basin-wide upper ocean heat content remained below-average. These atmospheric and oceanic features are consistent with ongoing La Niña conditions. 
In equatorial eastern Africa precipitation was significantly below-average across Kenya and Tanzania, with many areas recording totals in the lowest 10th percentile of occurrences. This area also recorded well below-average rainfall during September-November 2005. In both seasons, this dryness coincided with an area of large-scale anomalous sinking motion upstream of a persistent upper-level trough centered just east of the continent.
EXTRATROPICS
Highlights 
1.	Above-average temperatures across North America and the North Atlantic 
2.	Below-average temperatures across northern Eurasia
3.	Severe dryness continues in the southwestern U.S.
4.	Enhanced South African rainy season continues

NORTHERN HEMISPHERE 
Surface temperatures during DJF 2005-06 were generally 3°-5°C above average across central Canada and the north-central U.S, and generally 1°-2°C above average across the southern states. The most extensive warmth occurred in January when much of the U.S. and Canada recorded departures of +5°C to +8°C. The 500-hPa circulation pattern during January featured a combination of above-average heights over the east-central North Pacific and eastern Canada, and below-average heights over Alaska and western Canada. This anomaly pattern reflected a nearly zonal flow pattern across North America, in response to a disappearance of the mean upper-level ridge normally located over western North America and a marked weakening of the climatological mean Hudson Bay trough. This zonal flow produced near-record warmth throughout the continent. 
The circulation during DJF 2005-06 also featured periods with a pronounced eastward extension of the East Asian jet stream and an eastward shift of the jet exit region to just upstream of the northwestern United States. This jet stream pattern resulted in a focused storm track that brought exceptionally heavy precipitation to the northwestern United States. These storms did not affect the southwestern United States, where seasonal precipitation totals were generally below the 10th percentile of occurrence. Ongoing precipitation deficits have resulted in severe drought conditions across most of southern Arizona and western New Mexico.
 Anomalous warmth was also observed throughout the high latitudes and subtropical latitudes of the North Atlantic, with departures in the northern region generally greater than 0.5°C. This anomalous warmth reflects the ongoing warm phase of the Atlantic multi-decadal mode that began approximately in 1995 (Goldenberg et al. Science, 2001).
In contrast, seasonal temperatures were well below average across Europe and northern/ central Russia, with departures of -1°C to -2°C seen throughout the area. Exceptionally cold conditions occurred during January, when a large-amplitude ridge-trough pattern extended from the eastern Atlantic to central Russia. In association with this blocking ridge, the northern branch of the jet stream headed north of Scandinavia and then strongly southward into western Russia. At the same time the southern branch of the jet stream was situated over northern Africa. This circulation represented a nearly complete disappearance of the normal flow of marine air into Europe, along with a significant flow of polar air into Russia. This combination resulted in below-average temperatures across much of Eurasia, with the largest negative departures (-5° to -8°C) centered in south-central Russia and Kazakhstan where temperatures were below the 10th percentile of occurrences.

SOUTHERN HEMISPHERE 
The DJF 2005-06 mean 500-hPa circulation pattern featured below-average heights in the middle latitudes and above-average heights in the polar region. It also featured persistent positive height anomalies over southeastern Australia. For Australia, this ridge delineated anomalous cool and wet conditions in the west from anomalously warm and dry conditions in the east.
In South America, below-average precipitation was again observed over the eastern Amazon Basin and southeastern Brazil. This dryness is consistent with an overall tendency for below-average precipitation in the Amazon Basin since 1995. Farther south, exceptionally wet conditions prevailed across southern South America, with many areas recording seasonal totals above the 90th percentile of occurrences. Most of this enhanced precipitation occurred during January and February, and was focused along an upper-level confluence zone located downstream of an exceptionally strong mid-Pacific trough. Increased storminess in this area, combined with large-scale ascending motion in the associated left jet entrance region, were likely contributors to this enhanced precipitation.
The South African rainy season normally lasts from October through April. This region recorded above-average precipitation during DJF 2005-06, with the most significant wetness occurring in the normally arid regions of southwestern Africa. An enhanced South African rainy season is consistent with ongoing La Niña conditions.