June-August 2005: Seasonal Climate Summary

By 

Dr. Gerald Bell
NOAA/ NWS/ Climate Prediction Center

TROPICS
Highlights 
1.	ENSO-neutral conditions
2.	Ongoing intraseasonal activity linked to Madden-Julian Oscillation (MJO)
3.	Another very active Atlantic hurricane season, suppressed East Pacific activity
4.	Above-average West African monsoon rains
TROPICAL PACIFIC
For the June-August 2005 (JJA) equatorial SSTs were near average across the central and east-central equatorial Pacific. A general decrease in surface and subsurface temperature anomalies was observed during the season, which reflects a continued evolution away from the weak El Niño conditions that ended in April. This evolution was accompanied by stronger-than-average easterly winds and near-average convection over the central equatorial Pacific. By the end of the season, SST departures in the Niño 3.4, Niño 3, and Niño 1+2 regions were near zero, while positive departures (+0.4°C) persisted in the Niño 4 region of the central equatorial Pacific. 
MJO activity was generally absent during June, but returned to a moderate strength during July and August, as indicated by equatorial time-longitude sections of Outgoing Longwave Radiation (OLR) and 200-hPa velocity potential. This MJO activity resulted in considerable intraseasonal variability in the Tropics and subtropics, which tended to modulate the timing for Atlantic tropical storm formation during the season.
ATLANTIC and EASTERN PACIFIC  HURRICANES
The Atlantic hurricane season runs from 1 June through 30 November, with the normal peak in activity occurring from mid-August through early October. The period June-September 2005 featured 17 tropical storms (TS), of which nine became hurricanes and five became major hurricanes (MH). Five of these systems made landfall along the Gulf Coast of the United States, two as tropical storms (Arlene and Cindy) and three as major hurricanes (Dennis, Katrina, and Rita). Hurricane Ophelia also affected the coastal regions of North Carolina, but did not officially make landfall.
The 2005 hurricane season featured record early-season activity, with seven tropical storms and two major hurricanes (Dennis and Emily) by the end of July. Two of the tropical storms (Arlene and Cindy) and MH Dennis made landfall along the U.S. Gulf Coast. TS Arlene made landfall on June 11th in extreme western Florida panhandle and then traveled northward along the Mississippi-Alabama border, producing rainfall totals of 100-125 mm throughout the region. It then moved northward through the Midwest and dissipated over Michigan on 13 June, producing rainfall totals of 75-100 mm along its path from western Tennessee through Indiana and into southern Michigan. TS Cindy made landfall in southeastern Louisiana on 6 July, and produced 100-200 mm of rainfall along the coastal areas of southeastern Louisiana, Mississippi, and Alabama.
MH Dennis then made landfall in Pensacola, Florida on 10 July as a category-3 hurricane with maximum sustained winds of 140 mph. It then moved through northeastern Louisiana and eventually dissipated in southern Illinois. The largest rainfall totals associated with MH Dennis averaged 150-200 mm across the central Florida Panhandle, western Georgia, and western Alabama. 
Five tropical storms then formed during August, with two becoming hurricanes and one becoming a major hurricane (Katrina). MH Katrina reached maximum sustained winds of 165 mph over the eastern Gulf of Mexico. It then weakened to 145 mph as is struck the eastern Louisiana and western Mississippi coasts on 29 August. Katrina produced massive flooding and widespread devastation from New Orleans, LA to well east of Biloxi, MS. Much of the massive flooding in New Orleans occurred as major levees were breached at several locations, which resulted in water from Lake Ponchartrain inundating the city. It is estimated that the MH Katrina is one of the worst natural disasters ever to strike the United States. 
Five additional hurricanes, of which two became major hurricanes (Maria and Rita), then formed during September. Hurricane Rita stuck the Gulf Coast region of eastern Texas and western Louisiana on 23 September, also causing widespread evacuations and extensive flooding. 
During the last four Atlantic hurricane seasons (2002-2005) 26 named storms have made landfall in the United States (7 in 2002, 5 in 2003, 9 in 2004, and 5 in 2005). Nineteen of these systems made landfall along the Gulf coast, defined as the region from the southeastern tip of Florida to the southern tip of Texas. Six of these systems hit the Gulf Coast in 2002, 4 struck in both 2003 and 2004, and five hit in 2005. Eleven of these 19 landfalling Gulf Coast systems struck as hurricanes. 
The above-average activity in 2005 partly reflects the overall increase in activity that began in 1995. Nine of 11 hurricane seasons were above normal during 1995-2005, compared to only three above-normal seasons during the previous 25-yr period 1970-1994. Because Atlantic hurricane seasons exhibit such strong multi-decadal variability, with extremes in activity typically lasting 20-30 years (Goldenberg et al. Science, 2001), it is likely that the present above-normal hurricane era will continue for at least the next decade. Since the United States experiences 2-3 landfalling hurricanes on average during above-normal seasons, it will be at an increased risk of hurricane landfalls for many years to come.
During 2005 several conditions known to favor above-normal Atlantic hurricane seasons were present across the tropical Atlantic. These include 1) an amplified 200-hPa subtropical ridge, 2) reduced vertical wind shear, which results from an expanded area of easterly winds in the upper atmosphere and weaker easterly trade winds in the lower atmosphere, 3) a configuration of the African easterly jet that favors hurricane development from tropical disturbances moving westward from the African coast, and 4) above-average sea-surface temperatures (1°C-2°C above average). Over the central and eastern tropical Atlantic these conditions have prevailed since 1995, and are associated with the ongoing active Atlantic phase of the multi-decadal signal (Bell et al., Bull. Amer. Meteor. Soc., May 2004; Bell and Chelliah, J. Climate 2005, In Press).
In contrast by the end of September 2005 the East Pacific hurricane season had featured 14 TS, 6H and 2 MH (Jova and Kenneth). These numbers are below the1971-2000 means of 15 TS, 9H, 4.4 MH. This represents a continuation of suppressed East Pacific hurricane seasons that began in 1995. 
WEST AFRICAN MONSOON SEASON
The rainy season in the Sahel and Sudan regions of western Africa normally lasts from June-October, with peak rains typically observed during August and September.  The June-August rainfall in these areas was well above average, with many areas recording totals above the 90th percentile of occurrences. The largest positive precipitation departures occurred during June and July, followed by near-normal rains in August. Farther south rainfall in the Equatorial Guinea region was considerably below average throughout the season, with totals in many areas below the 10th percentile of occurrences.
There is a very strong link between West African monsoon variability and Atlantic hurricane activity. The increased Atlantic hurricane activity since 1995 has been associated with a return to a stronger West African monsoon circulation and generally above-average precipitation in the African Sahel.

EXTRATROPICS
Highlights 
1.	Above-average temperatures cover all land areas except western Canada 
2.	Above-average temperatures at both high and low latitudes of the North Atlantic.
3.	Drought eases in Montana and Wyoming
4.	Below-average southwestern U.S. monsoon rains
5.	Below-average precipitation and long-term drought in western Europe

NORTHERN HEMISPHERE 
Above-average temperatures were observed in all NH land areas during JJA, with the exception of western Canada. The largest positive temperature departures (2°-3°C) were observed over the upper Midwestern U.S., with departures of 1°-2°C seen across the eastern U.S., Europe, and most of eastern Asia. Exceptionally warm SSTs were also observed throughout the high latitudes and subtropical latitudes of the North Atlantic, with departures in both regions exceeding the 90th percentile. This anomalous warmth reflects the ongoing warm phase of the Atlantic multi-decadal mode that began approximately in 1995 (Goldenberg et al. Science, 2001).
Precipitation highlights during the season included above-average rainfall in both the northwestern and southeastern U.S., and below-average precipitation in portions of the midwestern U.S. and western Europe. For the drought-stricken states of Montana and Wyoming seasonal precipitation totals were generally above the 70th percentile of occurrences. The most significant departures occurred during June when totals exceeded 175% of normal in both states. This rainfall helped to alleviate severe long-term drought conditions in both states, with only isolated areas of moderate drought remaining by the end of the season.
Rainfall in the southeastern U.S. was above average during JJA, mainly in response to the landfalling tropical systems noted above. In contrast, rainfall was below average in New Mexico and large portions of Mexico in association with a below-average strength of the monsoon rains. These anomalies were associated with a pronounced eastward shift of the mean summertime ridge toward eastern North America, and a resulting suppressed monsoon ridge over the southwestern United States.
Western Europe has experienced severe drought in response to below-average precipitation that began in October 2004. For the entire period of 1 October 2004 – 13 September 2005, totals were generally 200 mm-350 mm below average across western Europe, with the largest deficits of 500 mm – 850 mm recorded in northern Portugal. During JJA 2005, precipitation was generally below the 30th percentile of occurrences across central and southern Europe, and below the 10th percentile in Portugal and western Spain. Much of this seasonal deficit occurred during June and August when the region was situated beneath or immediately downstream of a persistent upper-level ridge.

SOUTHERN HEMISPHERE 
Above-average temperatures were observed across South America during JJA, with the exception of the extreme south. In tropical South America, seasonal temperatures were 1°-2°C above average, in association with below-average precipitation throughout the Amazon Basin and a corresponding weaker than average strength of the 200-hpa subtropical ridge. Over southern South America, below-average temperatures were associated with above-average precipitation and increased storminess, which occurred in response to a persistent upper-level trough and enhanced jet stream winds just upstream of the continent. Cooler-than-average conditions were also observed in western Australia during the season in association with above-average precipitation throughout the region.