March-May 2005: Seasonal Climate Summary

By 

Dr. Gerald Bell
NOAA/ NWS/ Climate Prediction Center

TROPICS
Highlights 
1.	Weak Pacific warm episode (El Niño) ends
2.	MJO continues to influence tropical convection patterns
TROPICAL PACIFIC
Positive sea surface temperature (SST) anomalies of 0.5°-1.0°C (1°-2°F) were observed in the central equatorial Pacific during March-May (MAM) 2005. The largest anomalies were observed during March, and weakened as the season progressed. During April and May, the positive SST departures disappeared from the east-central equatorial Pacific, and by May the main area of above-average SSTs (0.5°-1.0°C) had shifted to west of the date line. This evolution reflected a disappearance of the weak El Niño episode that had prevailed since August 2004. Aspects of the tropical rainfall pattern during MAM remained consistent with past Pacific warm episodes, including enhanced rainfall over the central equatorial Pacific and suppressed rainfall over Indonesia and northern Australia. 
Considerable intra-seasonal variability was again evident in the patterns of tropical convection and ocean temperature anomalies during MAM, which strongly masked the weak ENSO signal across the tropical Pacific. During the past nine 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. 
One example of this variability occurred during April and May. During April, suppressed convection associated with the MJO moved eastward from the Indian Ocean to the western equatorial Pacific. The associated low-level westerly wind anomalies triggered an equatorial Kelvin wave, which propagated to the eastern Pacific during May. The down-welling phase of this wave produced a deepening of the oceanic thermocline and anomalously warm ocean temperatures to a depth of approximately150 m. This anomalous warmth soon dissipated as the upwelling phase of the Kelvin wave passed by a few weeks later. Failure to account for this Kelvin wave, especially in the eastern Pacific, led to speculation that El Niño was strengthening when it had already dissipated.

EXTRATROPICS
Northern Hemisphere
Highlights 
1.	Continuation of above-average precipitation in the western U.S.
2.	Exceptionally warm across Canada, the North Atlantic, and central/ eastern Asia
3.	Persistent positive phases of NAO and PNA teleconnection patterns 
4.	Continued high-latitude blocking activity over the North Atlantic 

The mean Northern Hemisphere 500-hPa height pattern during MAM 2005 featured above-average heights from Alaska to Greenland, and below-average heights across the central Pacific and Atlantic Oceans. This anomaly pattern was associated with recurring high-latitude blocking activity, and with and southward shifts of mean jet streams, over the eastern North Pacific and North Atlantic.

North America
The mean circulation during MAM featured above-average 500-hPa heights across Alaska and Canada, and below-average heights over the eastern half of the United States. This anomaly pattern projected strongly onto the positive phase (+1.3) of the Pacific/ North American (PNA) teleconnection pattern.

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 and Alaska.  Within the southern branch of the split flow, the mean jet stream and associated storm track entered the United States over California, well south of their climatological mean position over the Pacific Northwest. Similar circulation anomalies were observed during DJF 1004-2005. Farther east, the pattern of negative height anomalies over eastern North America during MAM was associated with an amplification of the mean Hudson Bay Low, and with anomalous northwesterly and northerly flow into the eastern United States.

For much of the western United States, these conditions resulted significantly above-average precipitation (175% or more above average). These surpluses, combined with well above-average precipitation during December-February, helped to eliminate long-term drought conditions over large portions of the western United States. In contrast, portions of the Midwest and south-central U.S. experienced well below-average precipitation during MAM, with totals from eastern Texas to Michigan in the lowest 10th percentile of occurrences. This region was situated in an area of large-scale descending motion immediately upstream of the mean 500-hPa trough axis.

The prominent seasonal surface temperature anomalies during MAM included above-average temperatures (2°-3°C above average) in eastern Alaska/ western Canada and eastern Canada, and  below-average temperatures (1°-2°C below average) in the eastern U.S. This pattern of surface temperature departures was consistent with the 500-hPa height anomalies.

North Atlantic and Europe
The mean 500-hpa circulation pattern during MAM 2005 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 also observed in the previous two seasons, in association with a very persistent and recurring blocking pattern. Farther south, below-average heights during MAM extended from the eastern U.S. to Europe, while above-average heights spanned the tropical North Atlantic. This north-south tri-pole pattern of height anomalies reflected a combination of the negative phase (-1.1) of the North Atlantic Oscillation (NAO) and positive phase (+1.4) of the East Atlantic teleconnection pattern.

This circulation was associated with above-average temperatures throughout Europe, with the largest departures averaging 1°-2°C occurring across Southern Europe. This area also experienced drier than average conditions during the season, in association with above-average 500-hPa heights throughout the region. Much of this seasonal rainfall deficit occurred during April and May, when precipitation totals were generally within the lowest 30% percentile of occurrences.

The North Atlantic also saw a continuation of exceptionally warm SSTs at both high latitudes and in the subtropics during MAM. This warmth reflects the ongoing warm phase of the Atlantic multi-decadal mode that began approximately in 1995 (Goldenberg et al. Science, 2001). The warmth was also consistent with above-average heights across the high latitudes of the North Atlantic, and with generally above-average temperatures in Europe.

Central Asia
Seasonal mean surface temperatures were 2-3°C above average across central and eastern Asia. This warmth was associated with a broad area of above-average heights at 500-hPa, along with above-average surface pressures. The most anomalous warmth occurred during March, when temperatures were more than 4°C above average over large portions of central Asia in association with the persistent upper-level ridge.

Southern Hemisphere
Highlights 
1.	Continuation of above-average temperatures across much of the three continents,
2.	 Exceptionally dry in eastern Australia

In the Southern Hemisphere, the 500-hPa circulation pattern during MAM 2005 featured above-average heights over Australia and the extratropical North Pacific, and negative anomalies over southern South America.  

South Pacific and South America
Above-average surface temperatures were again observed over large portions of South America during MAM 2005. The largest departures were observed in the Amazon Basin and eastern Brazil, where seasonal mean temperatures were 1°-2°C above average. The Amazon Basin has experienced well above-average temperatures for the past three seasons.

Australia
Much of Australia experienced a continuation of warmer and drier than average conditions during MAM. Seasonal mean temperatures were 3°-4°C above average across interior Australia, and 2°-3°C above average across the south. Precipitation totals were in the lowest 10th percentile of occurrences throughout southeastern and central Australia, with the most significant deficits occurring during April and May. These conditions were associated with a persistent pattern of above-average 500-hPa heights and surface pressure throughout the southern part of the continent.