June-August 2006: Seasonal Climate Summary

By 

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

TROPICS
Highlights 
1.	Weak Pacific warm episode (El Niño) develops by late August
2.	Intra-seasonal variability affects tropical circulation
3.	Atlantic hurricane activity is light through August
The June-August (JJA) 2006 season featured a transition into a weak Pacific warm episode (El Niño) by late August. By early July, weaker-than-average low-level equatorial easterly winds had developed across most of the equatorial Pacific, and in August the Southern Oscillation Index (SOI) was negative for the fourth consecutive month. As the season ended equatorial SST anomalies greater than +0.5ºC covered most of the central and east-central equatorial Pacific, with anomalies exceeding +1.0ºC in the central Pacific between 165ºE and 170ºW. Collectively, these conditions reflected a developing Pacific warm episode. Some El Niño impacts were evident in the pattern of tropical precipitation during August, including drier-than-average conditions across Indonesia, Malaysia and most of the Philippines. These areas are usually the first to experience ENSO-related impacts.
Tropical intra-seasonal variability was prominent during June-August (JJA) 2006, as indicated by equatorial time-longitude sections of Outgoing Longwave Radiation (OLR) and 200-hPa velocity potential. This variability contributed to the formation of strong equatorial Kelvin waves in the Pacific Ocean, which led to a progressive warming of the central and eastern equatorial Pacific. 
By the end of August, the Atlantic hurricane season had produced five tropical storms, with only one of these systems (H Florence) becoming a hurricane. The eastern Pacific hurricane season had produced twelve named storms, with eight becoming hurricanes. 
EXTRATROPICS
Highlights 
1.	Enhanced southwestern U.S. monsoon 
2.	Warm in Europe and south-central Russia
3.	Hot and dry in August across southern China
4.	Warm in central South America
5.	Dryness across southern Australia 

NORTHERN HEMISPHERE 
The mean 500-hPa circulation pattern during June-August (JJA) featured above-average heights in the middle latitudes, and below-average heights in the polar region. One prominent feature was a persistent ridge over the western U.S. during June and July, which contributed to well above average seasonal temperatures in that region. This ridge was also associated with enhanced monsoonal rainfall in the southwestern U.S. and northeastern Mexico. During both July and August, precipitation totals over large parts of the southwestern U.S. were more than 175% of normal, with seasonal totals in some areas exceeding the 90th percentile of occurrences. Above-average precipitation continued in these regions during August in association with a broad trough situated along the west coast of the United States. 
Another prominent feature of the mean 500-hPa circulation during JJA was a ridge extending southeastward from Scandinavia to the Caspian Sea. This feature contributed to significantly below-average precipitation across Scandinavia, with seasonal totals in many areas in the lowest 10th percentile of occurrences. The ridge was particularly prominent across Europe during June and July and in the vicinity of the Caspian Sea during June and August, resulting in above average seasonal surface temperatures in these regions.
Most of China also recorded above-average temperatures during JJA, in association with an anomalous eastward extension of the 200-hPa Asian monsoon ridge. This feature was particularly prominent during August, with much of southern China situated directly beneath the mean upper-level ridge axis and well south of the influence of mid-latitude weather systems and low-latitude frontal passages. These conditions contributed to exceptionally warm and dry weather across southwestern and south-central China, with rainfall totals in some areas in the lowest 10th percentile of occurrences.

SOUTHERN HEMISPHERE 
The mean 500-hPa circulation during JJA 2006 featured above-average heights south of the three continents, and negative height anomalies over the high latitudes of the central South Pacific, flanking South America, and over the central Indian Ocean. For South America, an anomalous anticyclonic circulation was situated over the central part of the continent during June and July, and the main belt of westerlies was shifted anomalously poleward. These conditions contributed to average temperatures across central South America. Extensive warmth during June was also associated with an anomalous low-level flow of tropical origin from the Amazon Basin.
In Australia, generally cooler than average conditions were observed in the central part of the continent during JJA, while anomalously warm and dry conditions were observed in the southwest and southeast. In these latter regions, seasonal precipitation totals were generally in the lowest 10th percentile of occurrences. The most extensive dryness occurred in June and August in association with anomalous large-scale descending motion situated upstream of the mean trough axis.