March-May 2007: Seasonal Climate Summary

By 

Dr. Gerald Bell 
NOAA/ NWS/ Climate Prediction Center

TROPICS
Highlights 
1.	ENSO-neutral conditions across the tropical Pacific Ocean

ENSO-neutral conditions across the tropical Pacific Ocean
The March-May (MAM) 2007 season featured ENSO-neutral conditions, with slightly above average sea-surface temperatures (SSTs) in the central equatorial Pacific Ocean, and below average SSTs in the eastern equatorial Pacific. Anomalous cooling was evident between 150°W and the South American coast during the season, with the Niño-3 region SST index dropping from -0.2°C to -0.7°C, and the Niño-1+2 region SST index dropping from -0.5°C to -1.9°C.
These conditions were associated with a persistent pattern of enhanced convection west of the date line and suppressed convection east of the date line, along with enhanced low-level easterly winds (more than 3.0 m s-1 above average) over the central equatorial Pacific. The 200-hPa circulation showed a strong response to this distribution of anomalous tropical convection, as indicated by anomalous ridges in both hemispheres west of the date line, and anomalous troughs in both hemispheres east of the date line.
These conditions followed the demise during January of a weak El Niño episode, along with additional cooling during February across the eastern equatorial Pacific. This cooling is evident in all of the Niño region indices, with the Niño-4 index dropping 0.6°C during January-February, the Niño 3.4 index dropping 1.1°C, the Niño 3 index dropping 1.2°C, and the Niño 1+2 index dropping 1.0°C. This evolution was associated with a shoaling of the oceanic thermocline in response to the upwelling phase of an equatorial oceanic Kelvin wave, which contributed to a sharp drop in both upper-oceanic heat content and sub-surface ocean temperatures at thermocline depth.

EXTRATROPICS
Highlights 
1.	North Pacific jet stream reflects suppressed convection near the date line 
2.	Warm in U.S. during March and May
3.	Extreme drought develops in the southwestern and southeastern United States
4.	Exceptional warmth in Eurasia

NORTHERN HEMISPHERE 

The 500-hPa circulation pattern during MAM featured positive height anomalies across North America, the central North Pacific, the high latitudes of the North Atlantic, and most of Eurasia, and negative height anomalies over Scandinavia and the polar region. This pattern was associated with above-average temperatures across the central United States and all of Eurasia, and below-average temperatures in Alaska and western Canada. Significant precipitation departures during the season included above-average totals in the Plains States, western Canada, and central Russia, and below-average totals in the southwestern and southeastern U.S., western Alaska, and portions of southwestern Russia.

North Pacific jet stream reflects suppressed convection near the date line
In the subtropics, the 200-hPa circulation during March and April reflected the pattern of anomalous convection across the equatorial Pacific Ocean, with enhanced ridges flanking the region of increased convection and enhanced mid-Pacific troughs flanking the region of suppressed convection east of the date line. These conditions are associated with a pronounced split flow configuration in the exit region of the wintertime East Asian/ North Pacific jet stream, often favoring high-latitude blocking over the central North Pacific. During March, anomalous northerly flow downstream of such a blocking ridge brought exceptionally cold temperatures to Alaska and western Canada, where departures were generally in the lowest 10th percentile of occurrences.

Warm in U.S. during March and May
Temperatures across the central United States were generally 1°C to 2°C above average during MAM. The most significant departures (+3° to +4°C) occurred in the Midwestern and Plains states occurred during March, in association with an extensive anomalous 500-hPa ridge. The Midwest also recorded anomalously warm temperatures (generally 2°-3°C above average) during May, in association with a strong 500-hPa ridge over the eastern half of North America.

Moderate to extreme drought develops across the southwestern and southeastern United States
During March and May, positive 500-hPa height anomalies were situated over both the western and eastern U.S. and a broad trough was located over the southern Plains States. This circulation contributed to exceptionally low precipitation totals (less than 25% of normal) in both the southwestern and southeastern U.S., and to exceptionally high totals (more than 200% of normal) from Texas northward to south-central Canada.
For the southwestern and southeastern United States, these ongoing precipitation deficits led to the development of moderate to extreme drought. By the end of the season extreme drought covered large portions of Mississippi, Alabama, Georgia, Tennessee, and Florida. Extreme drought also covered the southern half of California, northern Arizona, southern Nevada, and western Wyoming. 

Exceptional warmth in Eurasia
Nearly all of Eurasia recorded above-average temperatures during MAM, with the largest seasonal departures (+2°C to +3°C) recorded in Europe and Siberia. However, the areas experiencing the largest temperature departures varied considerably from month to month. During March the largest departures (exceeding +4°C) occurred in western Russia and northeastern Europe. During April similar departures were observed mainly in western Europe and central Asia. In May, the largest departures (+2°C to +3°C) were observed in southeastern Europe and southwestern Russia. 
  
SOUTHERN HEMISPHERE 
1.	South African rainy season: Below-average in March and above average in April
2.	Anomalously warm and dry in eastern Australia
3.	Anomalously warm and dry in southeastern Brazil 

The 500-hPa circulation exhibited a substantial zonal symmetry during MAM 2007, with positive height anomalies across Antarctica and large portions of the subtropics, and negative height anomalies in the middle latitudes. The most significant temperature departures during the season included anomalous warmth in eastern Australia, all of South Africa, and Brazil, and anomalously cool conditions in western Australia and southern Argentina. The significant precipitation departures included above-average totals in northwestern Australia and central Argentina, and below-average totals in eastern Australia and southeastern Brazil.

South African rainy season: Below-average in March and above average in April
The South Africa rainy season extends from October to April. During March precipitation was slightly below average for the region as a whole, with the most significant deficits occurring in northern Mozambique. During April precipitation was well above average, with the largest surpluses occurring in eastern South Africa and southern Mozambique. For the entire 2006-07 rainy season, area-averaged totals were below-normal in October, February and March, near-normal in November and December, and above normal in January and April.

Anomalously warm and dry in eastern Australia 
Eastern Australia experienced anomalously warm and dry conditions during MAM, with temperature departures across the eastern half of the continent averaging +1°C to +3°C, and precipitation totals along the east coast generally falling into in the lowest 10th percentile of occurrences. The anomalous warmth occurred throughout the season in association with persistent positive height anomalies at 500-hPa. During May, the above-average temperatures were also partly related to a disappearance of the normal onshore flow of cooler air from the Great Australian Bight. 
The prevalence of positive 500-hPa height anomalies also contributed to long periods of below-average precipitation. The most significant departures were observed in the Northeast and East during April and May, when totals were generally within the lowest 30th percentile of occurrences.

Anomalously warm and dry in southeastern Brazil 
Anomalously warm and dry conditions were observed across southeastern Brazil during MAM, in association with an amplified 200-hPa ridge over the region. This ridge was situated downstream of an amplified mid-Pacific trough, which was partly related to the suppressed convection over the central equatorial Pacific. For southeastern Brazil, the most significant precipitation deficits were observed in March and April, in association with anomalous sinking motion immediately downstream of the upper-level ridge axis.