March-May 2006: Seasonal Climate Summary

By 

Dr. Gerald Bell
NOAA/ NWS/ Climate Prediction Center

TROPICS
Highlights 
1.	La Niña episode fades in April

La Niña conditions continued during March 2006 as negative SST anomalies (below        -0.5ºC) covered most of the equatorial Pacific east of the date line. The associated pattern of suppressed convection also persisted over the central equatorial Pacific, and above-average convection was again observed over Indonesia, the Philippines, northern Australia, and Hawaii. Consistent with these conditions, the pattern of stronger-than-average low-level (850-hPa) easterly winds continued over the central equatorial Pacific, and anomalous upper-level (200-hPa) cyclonic circulation centers at 200-hPa were observed in the subtropics of both hemispheres over the central Pacific. The equatorial subsurface temperature anomaly pattern (negative anomalies in the central and eastern Pacific and positive anomalies in the western Pacific) also persisted during March 2006, and the basin-wide upper ocean heat content remained below-average. 
During April, SSTs and sub-surface ocean temperatures returned to near-average across the equatorial Pacific, indicating a return to ENSO-neutral conditions. However, the previously established patterns of tropical rainfall and atmospheric circulation lingered during April and even into May, affecting downstream precipitation patterns in both hemispheres.
EXTRATROPICS
Highlights 
1.	La Niña-related circulation persists during March-May 2006 
2.	Extremely wet in Hawaii
3.	Dryness continues in the southwestern U.S. and along the U.S. Gulf Coast
4.	Enhanced South African rainy season continues 

NORTHERN HEMISPHERE 
The mean 500-hPa circulation pattern during March-May (MAM) 2006 featured above-average heights across the central U.S. and Canada, over the high latitudes of the central North Pacific, and from southeastern Europe to China, and negative height anomalies over the eastern North Pacific and high latitudes of Eurasia. Key aspects of the large-scale circulation were consistent with Pacific cold episode conditions. In particular, suppressed convection near date line was associated with cyclonic streamfunction anomalies at 200-hPa in the subtropics of both hemispheres, while enhanced convection over Indonesia was associated with anticyclonic streamfunction anomalies over the western subtropical North Pacific and southeastern Asia. 
This pattern reflected a pronounced westward retraction of the East Asian jet core, and a westward shift of the associated jet exit region to well west of Hawaii. Record rainfall and severe flooding in Hawaii during the season is attributed to its relative position just downstream of the mean jet exit region. In the extratropics, these conditions were accompanied by positive 500-hPa height anomalies across the high latitudes of the North Pacific and the central U.S, and negative height anomalies over the eastern North Pacific. This pattern reflects a pronounced westward shift of the mean upper-level ridge and trough axes, and is typical of Pacific cold episode conditions. 
During MAM 2006 this circulation was associated with a markedly reduced strength of the mean Hudson Bay Low, which contributed to above-average temperatures over much of North America. The largest departures covered central Canada where seasonal temperatures were 3°-5°C above average, and the central United States where temperatures were 1°-3°C above average. The anomalous circulation was also associated with below-average precipitation in the southwestern, Gulf Coast, and mid-Atlantic regions of the United States. Ongoing precipitation deficits resulted in severe drought conditions across most of southern Arizona and western New Mexico. The southwest and mid-Atlantic regions have experienced below-average precipitation in every month since January 2006, and the Gulf Coast region has experienced below-average precipitation in every month since September 2005.
Across Eurasia, a pattern of negative 500-hPa height anomalies at high latitudes and positive height anomalies in the middle latitudes prevailed during March and April. This pattern contributed to well below-average temperatures at high latitudes and above-average temperatures in the middle latitudes from Portugal to China.
Anomalous warmth was also observed throughout the high latitudes and subtropical latitudes of the North Atlantic, with departures generally greater than +0.5°C. This pattern is partly associated with the ongoing warm phase of the Atlantic multi-decadal mode that began approximately in 1995 (Goldenberg et al. Science, 2001). 

SOUTHERN HEMISPHERE 
The 500-hPa circulation during MAM 2006 featured an anomalous zonal wave-3 pattern, with positive height anomalies over the south-central portions of the three ocean basins, and negative height anomalies southeast of Australia, over the high latitudes of the eastern South Pacific, and south of Africa. 
In Australia, below-average 500-hPa heights and surface temperature were observed during April and May. Seasonal precipitation was generally above average in the extreme northeast in association with the Pacific cold episode, and below average in both the southwest and southeast. In the southwest drier than average conditions were recorded throughout the season. In the southeast much of the suppressed rainfall occurred during May, when the region was situated in an area of large-scale descending motion between the mean upper-level ridge and trough axes. 
Southeastern South America also recorded below-average precipitation during MAM 2006. The most significant dryness occurred during May when the region was situated in an area of large-scale descending motion between the mean upper-level ridge and trough axes. 
In southern Africa the rainy season normally lasts from October through April. This region recorded above-average precipitation throughout MAM 2006. For the entire 2005-06 rainy season, precipitation was above average in every month since October 2005. An enhanced South African rainy season is consistent with La Niña conditions.