December-February 2008-09: Seasonal Climate Summary

By 

Dr. Gerald Bell 
NOAA/ NWS/ Climate Prediction Center

TROPICS
Highlights 
1.	La Niña re-develops

La Niña re-develops
	The December-February (DJF) 2008-09 season was highlighted by the re-development of La Niña for a second consecutive year. La Niña episodes are often defined by the Niño-3.4 index, which measures the area-averaged sea surface temperature (SST) anomalies in the east-central equatorial Pacific Ocean between 170°W-120°W, 5°N-5°S.  This index was -0.8°C for the entire DJF season, with the largest negative value (-1.1 °C recorded in early January). 
	Although La Niña had previously ended in June 2008 according to NOAA, its associated pattern of suppressed convection had persisted near the date line. Also, the low-level easterly winds had remained enhanced in the western and central tropical Pacific Ocean. Later in the year (November and December) a subsequent strengthening of these tropical easterlies led to a rapid cooling of SSTs across the central and east-central equatorial Pacific, and to the re-emergence of La Niña late in December.
During DJF 2008-09, all oceanic and atmospheric anomalies typical of La Niña were present. Below the ocean surface, the equatorial thermocline was anomalously shallow in the eastern Pacific in association with enhanced upwelling related to the enhanced tropical easterly winds. As a result, the anomalously cool ocean temperatures extended from the surface to a depth of 150-200 m, with the largest temperature departures (-3 to -5˚C) seen at thermocline depth. 
La Niña’s strength and atmospheric impacts are strongly linked to the patterns of anomalous tropical convection. During DJF, the anomalous pattern of suppressed convection near the date line and enhanced convection over Indonesia was in place. This pattern was also consistent with the enhanced low-level easterlies and with anomalous upper-level westerlies across the central and western equatorial Pacific. This combination reflects an enhanced equatorial Walker circulation that was confined to the western half of the Pacific Ocean, which is typical of La Niña. 
In both hemispheres the subtropical circulation during DJF also reflected La Niña, with cyclonic anomalies over the central and east-central Pacific and anticyclonic anomalies over Australasia. This pattern reflected enhanced mid-Pacific troughs, and a westward retraction of the subtropical ridges toward the western Pacific. In the Northern Hemisphere, these conditions were associated with a westward retraction of the East Asian jet core, and both a broadening and westward shift of the mean North American ridge into the Gulf of Alaska. In the Southern Hemisphere, the main circulation anomalies associated with La Niña included above average heights over the central extratropical South Pacific and below average heights over the high latitudes of the eastern South Pacific. 
Typical La Niña temperature and precipitation impacts during DJF included:
1.	Enhanced precipitation across Indonesia, northern Australia, the South African monsoon region, and the central United States;
2.	Suppressed precipitation over the central equatorial Pacific, the southern tier of the United States, and southeastern South America;
3.	Warmer than average temperatures in the south-central United States; 
4.	Cooler than average temperatures across western and central Canada.

EXTRATROPICS
Highlights 
1.	La Niña affects temperature and precipitation in North America
2.	Split-flow pattern favors cool and wet in southern Europe, warm in the North
3.	Exceptionally warm and dry in China

NORTHERN HEMISPHERE 

La Niña affects temperatures and precipitation in North America
	One key feature of La Niña is a westward retraction of deep tropical convection to the Indonesia sector, which results in 1) a westward retraction of the East Asian jet core toward Japan and 2) a westward shift in the major downstream ridge and trough positions over North America. These conditions are reflected in the well-known set of La Niña-related circulation anomalies, which includes positive height anomalies at both 500-hPa and 200-hPa over southeastern Asia and the high latitudes of the central and eastern North Pacific, and negative height anomalies over the subtropical central North Pacific and central North America. 
These changes in the mean circulation, and in the associated storm track, can affect temperature and precipitation patterns across North America. For example, during DJF 2008-09 the mean 200-hPa ridge which is normally situated over western North America was shifted westward to the Gulf of Alaska, and the mean Hudson Bay trough was generally extended westward toward central North America. This pattern contributed to exceptionally cold temperatures across western and central Canada in the region between the mean ridge and trough axes, with seasonal departures in some areas averaging -2˚-3˚C.  It also affected the seasonal mean precipitation, with well below-average totals observed over western North America in the area immediately downstream of the mean ridge axis, and above average totals observed across the north-central United States.
La Niña also typically contributes to warmer and drier than average conditions across the southern tier of the United States. During DJF 2008-09, precipitation was below average from Texas to North Carolina, with the most significant deficits observed in Texas (and Mexico) where totals were in the lowest 10th percentile of occurrences. As a result, extreme to exceptional drought had developed across southern Texas by the end of the season. Drought conditions also continued in southwestern Louisiana, Georgia, Florida, and the southern Appalachian region.

Split-flow pattern favors cool and wet in southern Europe, warm in the North 
The circulation across Europe was quite persistent during DJF 2008-09, with positive height anomalies mainly covering Scandinavia and negative height anomalies extending across southern Europe. These conditions were associated with a pronounced split-flow pattern, with the northern branch of westerly winds confined to northern Scandinavia and the southern branch of westerly winds and main jet core entering the continent near the latitude of the Mediterranean Sea. In contrast, the climatological mean jet stream enters the continent much farther north in the vicinity of Great Britain.
This split flow pattern led to a north-south dipole pattern of temperature anomalies, with above average temperatures covering northern Europe and northwestern Russia and below average temperatures seen across central and southern Europe. The pronounced southward shift of the mean jet core also led to increased storminess and enhanced precipitation over large parts of southern Europe, the Mediterranean Sea, and northern Africa.

Exceptionally warm and dry in China 
Temperatures during DJF 2008-09 were generally 1˚-2˚C above average across much of central and southern Asia. In addition, seasonal precipitation was below average from eastern India to southeastern China, with totals generally in the lowest 10th percentile of occurrences. Eastern China experienced below average precipitation in all three months, with the most significant and widespread deficits occurring during February, in combination with excessive warmth.
As noted in CPCs monthly Climate Diagnostics Bulletin, “the 500-hPa height pattern during February featured persistent negative anomalies across central and eastern Siberia, and positive anomalies across central and eastern Asia. This pattern partly reflected a strong positive phase of the WP teleconnection pattern (2.2 standard deviations above normal). It was associated with a north-south dipole of temperature anomalies, with Siberia and northern Asia recording well below average temperatures, while central and southern Asia recorded significantly above average temperatures with departures generally in the upper 90th percentile of occurrences. Across southern and central China, the persistent positive height anomalies also led to a third consecutive month of below average precipitation.”

SOUTHERN HEMISPHERE 
1.	La Niña contributes to enhanced rainfall across South African monsoon region
2.	La Niña contributes to below average precipitation in southeastern South America

The 500-hPa circulation during DJF 2008-09 generally featured above average heights across the central South Pacific and central South Atlantic Oceans, and negative height anomalies over Antarctica. Over the South Pacific, these anomalies are consistent with La Niña.

La Niña contributes to enhanced rainfall across South African monsoon region
In southern Africa, the rainy season normally lasts from October through April. The region received above-average precipitation during DJF 2008-09, with many areas recording seasonal totals in the upper 70th percentile of occurrences.  The most statistically significant departures were observed in the western part of the monsoon region (Namibia), where rainfall is normally much lower than in the east. La Niña typically favors an enhanced South African monsoon.

La Niña contributes to below average precipitation in southeastern South America 
	La Niña also typically favors below average precipitation in southeastern South America. During DJF 2008-09, totals in this region were generally in the lowest 30th percentile of occurrences, with some areas recording totals in the lowest 10th percentile of occurrences. 
This reduced precipitation is related to: 1) positive 500-hPa height anomalies across the central and eastern South Pacific with a mean ridge situated west of the continent; and 2) a poleward shift of the mean storm track in association with the deep trough over the high latitudes of the eastern South Pacific.