June-August 2007: Seasonal Climate Summary

By 

Dr. Gerald Bell 
NOAA/ NWS/ Climate Prediction Center

TROPICS
Highlights 
1.	La Nina develops during August
2.	Dynamical models predicted La Niña onset better than the statistical models
3.	Below-normal East and Central Pacific hurricane activity, Near-normal Atlantic hurricane activity 
4.	Normal to above-normal Indian monsoon rainfall
5.	Above-average precipitation in the West African monsoon region 

La Nina develops during August
The June-August (JJA) 2007 season featured a transition from ENSO-neutral conditions to La Niña in the tropical Pacific Ocean. Throughout the season, sea-surface temperatures (SSTs) were below average in the east-central equatorial Pacific. The upper-ocean heat content (average temperatures in the upper 300 m of the ocean) was also below average in the central and east-central equatorial Pacific, and temperatures at thermocline depth ranged from 1°C to 3°C below average.  Also, low-level tropical easterly winds were stronger than average near the date line, and convection was suppressed throughout the central and eastern equatorial Pacific. 
During August, the negative sea surface temperature (SST) anomalies strengthened and expanded westward, extending from the coast of South America to the date line. Three of the Niño indices were cooler than −0.5 ºC throughout August, with only the Niño-4 index remaining near zero. By the last week in August, the Niño 3.4 index had dropped to −0.6ºC, and the Niño 3 index had dropped to −1.2ºC.  These conditions satisfied NOAA’s operational definition for La Niña. 
Dynamical models predicted La Niña onset better than statistical models 
The dynamically-based ENSO forecast models were much better as a group in predicting the onset of La Niña than the statistical models. For forecasts issued in June and July, most dynamical models indicated La Niña would develop within the next three months, while most statistical models predicted ENSO-neutral conditions through August. Also, several statistical models indicated only the possibility of weak La Niña conditions during the fall or winter.
However, several of the dynamical models predicted a stronger and more rapid cooling during June-July than actually occurred. These forecasts, combined with the under-predictions from the statistical models, made confident predictions of La Niña very difficult. For example, in early July NOAA indicated it was “reasonable to expect either a slower evolution toward La Niña conditions or the continuation of ENSO-neutral conditions.” In early August, NOAA stated “recent atmospheric conditions and model forecasts suggest a slightly greater than 50% chance of La Niña developing during the next couple of months.” La Niña developed in August.
The inability of nearly all ENSO forecast models to reliably predict La Niña and El Niño onsets during the summer and fall months remains problematic. Also, none of these models has been shown to reliably predict ENSO impacts at this time of the year.

Below-normal East and Central Pacific hurricane activity, Near-normal Atlantic hurricane activity 
The Eastern and central Pacific hurricane activity was below average during JJA, which is consistent with the combination of a developing La Niña and the ongoing below-normal hurricane era that began in 1995. The eastern Pacific season produced nine named storms during JJA, of which only three became hurricanes and one became a major hurricane. One tropical storm (Barbara) made landfall in North America during JJA, striking extreme southwestern Mexico as a tropical storm on 2 June. One tropical storm (Cosme) and one major hurricane (Flossie) moved westward into the central Pacific region, both staying well south of Hawaii.
The Atlantic hurricane season produced five named storms during JJA, two of which became category-5 Major hurricanes (Dean and Felix). For the first time in recorded history, two Category 5 hurricanes made landfall in the Atlantic Basin during the same season. Hurricane Dean struck the Yucatan Peninsula near Costa Maya on August 21 with 165 mph sustained winds. Hurricane Felix then made landfall near Punta Gorda, Nicaragua on September 2 with 160 mph sustained winds. 

Normal to above-normal Indian monsoon rainfall 
According to the India Meteorological Department, the seasonal rainfall from 1st June to 30th September was 105% of average for the country as a whole. Seasonal rainfall was 126% of average over South Peninsula, 108% of average in central India, and 104% of average in northeastern India. Rainfall was 15% below average in northwestern India. 

Above-average precipitation in West African monsoon region
The peak in the West African monsoon occurs from late July through September.
During August 2007, the region recorded above-average precipitation, with totals in much of the Sahel region ranking in the upper 70th percentile of occurrences. The African Sahel region has been receiving normal to above normal precipitation since 1995. 
The increased precipitation since that time has been associated with the warm phase of the Atlantic Multi-decadal Oscillation (AMO), and with below-average precipitation in the Amazon Basin. This combination of conditions is associated with one of the leading multi-decadal modes of tropical convective variability (Bell and Chelliah, 2006, J. Climate, pp. 590-612). These conditions have contributed to the current active Atlantic hurricane era, and to below-average hurricane activity in both the central Pacific and eastern Pacific regions. 

EXTRATROPICS
Highlights 
1.	US: Severe-exceptional drought in Southeast, moderate-to-severe drought in West
2.	Above-average precipitation in south-central US, and in southwest monsoon region
3.	Hot and dry in southeastern Europe and Mongolia

NORTHERN HEMISPHERE 

The 500-hPa circulation during JJA 2007 featured above-average heights in the middle latitudes, and a zonal wave-2 pattern at high latitudes. Prominent features of the circulation included above-average heights over the central North Pacific Ocean, central North America, Greenland and the polar region, and across northern Russia, and below-average heights over Canada, Scandinavia, and Siberia. This pattern was associated with above-average temperatures across the contiguous US, Alaska, Europe, and eastern Asia. Significant precipitation departures during the season included above-average totals in the southern Plains States, northwestern Europe, and much of northern Russia, and below-average totals in the southwestern and southeastern US, southeastern Europe, and Mongolia.

United States
The 500-hPa circulation during JJA 2007 featured above-average heights across the US, which occurred in association with above-average heights throughout the middle latitudes. In the US, the largest positive height anomalies were centered in the upper Midwest, and reflected an above average strength of the mean summertime ridge. Positive height anomalies were also especially persistent over the western US. This pattern led to above-average temperatures across much of the country, with the largest departures observed in the western Intermountain region (1°-3°C) and the southeast (1°-2°C). It also contributed to below-average precipitation across the southeastern quadrant of the US, where drought worsened in response to the combination of exceptional heat and ongoing precipitation deficits.
In the Southwest, the monsoon season extends from late June through early August. The strength of the monsoon can be strongly regulated by the upper-level circulation features. The persistent upper-level ridge over the southwestern US during JJA 2007 was associated with above average monsoon rains in Arizona and western New Mexico. The largest surpluses (>175% of normal) occurred in Arizona during July, when the mean upper-level ridge was situated west of normal over the Rocky Mountains.
In the southern Plains, Texas and Oklahoma also recorded well above-average precipitation during JJA, with seasonal totals in many locations exceeding 500 mm (more than 200% of normal). The most significant surpluses occurred during June and July, in association with a variable upper-level trough and an enhanced flow of moisture from the Gulf of Mexico.

Severe-exceptional drought in Southeast, moderate-to-severe drought in West
By the end of August 2007, extreme to exceptional drought had developed across much of the southeastern US. The areas most affected included northeastern Mississippi, Alabama, western Georgia, Tennessee, Kentucky, and North Carolina. In the West, extreme drought covered the southern California, western Arizona, central Idaho, and portions of Montana, Wyoming, and western Nebraska. Moderate to severe drought covered much of the remainder of the West.

Hot and dry in southeastern Europe and Mongolia
Most of Europe, western Russia, and eastern Asia recorded another season of above-average temperatures. The largest departures (2°-3°C) were observed in southeastern Europe, while departures of 1°-2°C covered eastern Europe and eastern Mongolia. 
In Europe, JJA 2007 featured a sharp transition between warmer and wetter conditions in the northwest, and warmer and drier conditions in the southeast. For the season as a whole, precipitation totals in northeastern Europe were in the upper 90th percentile of occurrences, while totals in southeastern Europe were in the lowest 30th percentile of occurrences. This pattern was most prominent during July, and occurred in association with a strong trough situated over northeastern Europe. 
In central and eastern Mongolia, seasonal precipitation totals were in the lowest 10th percentile of occurrences. Anomalously warm and dry conditions in this region were associated with a persistent pattern of above-average heights at 500-hPa during much of the season.

SOUTHERN HEMISPHERE 
1.	Circulation reflects la Nina
2.	Below-average temperatures across central South America
3.	Below-average temperatures in northern Australia, dry across southern half of Australia

The 500-hPa circulation exhibited a substantial zonal symmetry during JJA 2007, with positive height anomalies across Antarctica and large portions of the subtropics, and negative height anomalies in the middle latitudes. This pattern is very similar to that observed last season. Significant temperature departures included anomalously cool conditions across central and southern Argentina, and across northern Australia. Significant precipitation departures included below-average totals across the southern half of Australia.
At 200-hPa, the circulation over the South Pacific Ocean reflected cyclonic streamfunction anomalies in the tropics and anticyclonic streamfunction anomalies in the extratropics along 30°S. This pattern was associated with reduced jet stream winds across the eastern South Pacific between 30°S-40°S, and a westward retraction of the South Pacific jet stream toward New Zealand. These conditions are consistent with suppressed convection across the central and east-central equatorial Pacific partly in response to La Niña.

Below-average temperatures across central South America
Wintertime temperatures were below average across central and southern South America, with the largest departures (-1° to -2°C) observed in central Argentina and Chile. Much of this signal occurred during July and August, when temperatures in central Argentina averaged 3°-4°C below average. In July, a very unusual cold air outbreak brought snow to Buenos Aires, Argentina for the first time in 89 years. 
During July, a strong ridge-trough pattern flanking southern South America brought below-average temperatures to the southern half of the continent. During August, extratropical South America was influenced by a strong upper-level trough, with a substantial southeastward transport of cold air evident upstream of the trough axis. These conditions also contributed to well below-average temperatures across the southern half of the continent, with many areas recording values in the lowest 10th percentile of occurrences. 

Below-average temperatures in northern Australia, dry across southern Australia
	In northern Australia temperatures were generally 1° to 2°C below average during JJA 2007. The most significant departures were observed in June, when temperatures were generally 2° to 3°C below average. In southern Australia, the JJA season featured well below-average precipitation, with totals generally in the lowest 10th percentile of occurrences.
During June the 500-hPa circulation across Australia featured a ridge in the southwest and a strong trough in the southeast. This circulation brought an anomalous southerly flow of cool marine air at 850-hPa into the continent. As a result, nearly the entire country recorded below-average temperatures, and departures across much of the interior were in the lowest 10th percentile of occurrences. 
Perhaps the most significant circulation anomalies occurred in July, when the 500-hPa circulation featured an anomalous trough-ridge-trough pattern, with the ridge located over the middle of the continent and the troughs centered just off the west and east coasts. This pattern led to a continuation of below-average temperatures in northeastern Australia. It also led to well below-average precipitation in the East, where large areas recorded totals in the lowest 10th percentile of occurrences.