September-November 2004: Seasonal Climate Summary

By 

Dr. Gerald Bell
NOAA/ NWS/ Climate Prediction Center

TROPICS
Highlights 
1.	A weak Pacific warm episode (El Niño) continues
2.	Ongoing intraseasonal activity linked to Madden-Julian Oscillation (MJO)
3.	Above-average Atlantic hurricane season, below-average East Pacific season
4.	Normal to below-normal West African monsoon season
TROPICAL PACIFIC
Positive sea surface temperature (SST) anomalies greater than +0.5°C (~1°F) persisted across most of the central and east-central equatorial Pacific during September-November (SON), with departures greater than +1°C (~2°F) observed by the end of November. This anomalous warmth and the most recent 5-month running mean value of the Southern Oscillation Index (-0.6) reflect weak warm (El Niño) episode conditions.
Despite the anomalously warm SSTs, there was little or no reflection in the average pattern of deep convection (precipitation) over the central and east-central equatorial Pacific. This indicates that the anomalously warm SSTs were not yet having a discernible influence on the atmospheric circulation. However, the anomalous warmth did contribute to farther eastward shifts in enhanced convection associated with the convectively active phase of the Madden Julian Oscillation (MJO). 
Considerable MJO activity continued during SON, resulting in substantial week-to-week and month-to-month variability in the Tropics. This activity has 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 temperature anomalies over the central and eastern equatorial Pacific, and to fluctuations in the depth of the oceanic thermocline in these regions. This variability has been superposed on 1) an upward trend in SST anomalies east of the date line and 2) a gradual increase and eastward shift in the upper-ocean heat content anomalies. 
ATLANTIC and EAST PACIFIC  HURRICANES
The 2004 Atlantic hurricane season was well above normal, with 15 named storms, 8 hurricanes (H), and 6 major hurricanes [MH, defined as categories 3-5 on the Saffir-Simpson scale]. Nine of these systems struck the continental United States, three as tropical storms (Bonnie, Hermine, and Matthew) and six as hurricanes (Alex, Charley, Frances, Gaston, Ivan and Jeanne). Three of the hurricanes (Charley, Ivan and Jeanne) hit as major hurricanes. 
Five named storms hit Florida, one as a tropical storm (Bonnie) and four as hurricanes (Charley, Frances, Ivan, and Jeanne). This ties the record with Texas (in 1886) for the most hurricanes to hit one state in a single season. Also, all three landfalling major hurricanes struck Florida, which is the most ever recorded for that state in a single season since accurate state records began in 1900. Total rainfall associated with the U.S. landfalling storms reached 500 mm or more across Florida, Georgia, and the western Carolinas, and accounted for 60%-80% of the total August-September rainfall in these regions.
Over the central and eastern tropical Atlantic, most aspects of the atmospheric circulation and sea-surface temperatures departures during the peak August-September months of the 2004 season can be attributed to the ongoing active Atlantic multi-decadal signal. These include 1) an amplified subtropical ridge at upper levels, 2) reduced vertical wind shear across the tropical Atlantic and Caribbean Sea, 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 (0.5°C-1.0°C above average). Area-averaged SSTs in this region during were almost two standard deviations above normal during SON, which is the second warmest value since 1950. 
The large number of landfalling Atlantic hurricanes was related to an amplified subtropical ridge over the western North Atlantic, which contributed to exceptionally low vertical wind shear across the Caribbean Sea and to an exceptionally focused easterly steering current confined almost entirely to the deep Tropics. These conditions caused many hurricanes to track farther south and west than normal, ultimately making landfall in Florida and the Gulf Coast region of the United States. 
Although weak El Niño conditions appeared by mid-August, the enhanced tropical convection typically associated with El Niño never became established. Consequently, the typical downstream atmospheric teleconnections, which would have had a suppressing influence on seasonal hurricane activity by increasing the vertical wind shear over the Caribbean Sea and western tropical Atlantic, did not develop.
In contrast, the 2004 East Pacific hurricane season produced 13 TS, 6H and 3 MH (Darby, Howard, and Javier). These numbers are well below the1971-2000 means of 15 TS, 9H, 4.4 MH. This represents a continuation of generally 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 2004 West African monsoon season featured average to below-average rainfall, with the largest deficits observed in the western Sahel region where totals were generally in the lowest 30th percentile of occurrences.

EXTRATROPICS
Northern Hemisphere
Highlights 
1.	Warm and wet in U.S.
2.	Very warm across Europe and Asia
3.	Continuation of exceptionally warm North Atlantic SSTs
North America
The mean circulation pattern during SON featured a persistent pattern of above-average heights across the North Pacific, and in the area from central Canada to Europe, and a persistent upper-level trough over the southwestern United States. This circulation pattern was associated with an amplified storm track across the southern tier of the U.S., and a notable weakening of the mean Hudson Bay trough. These conditions contributed to above-average surface temperatures across the eastern half of the U.S., with seasonal mean temperatures in most areas ranging from 1°-2°C above average. 
The anomalous low-latitude storm track contributed to significantly above-average precipitation across the southern United States, with seasonal totals for many areas in the southwestern, south-central, and southeastern states exceeding 200 mm.  Precipitation totals in these areas generally exceeded 175% of normal, which was generally in the highest 90th percentile of occurrences. 

North Atlantic and Europe
Over the North Atlantic the mean 500-hpa circulation pattern during August-October featured above-average heights in association with a complete disappearance of the mean Icelandic Low. These conditions were associated with above-average temperatures throughout Europe, with seasonal mean values ranging from 2°-3°C above average over most of the continent. 
The North Atlantic also saw a continuation of exceptionally warm SSTs at both high latitudes and in the subtropics. This warmth reflects the ongoing warm phase of the Atlantic multi-decadal mode that began approximately in 1995 (Goldenberg et al. Science, 2001). This warmth was consistent with above-average heights across the high latitudes of the North Atlantic, with above-average temperatures across Europe, and with the above-normal Atlantic hurricane season.

Asia
Most of Asia recorded anomalously warm temperatures during SON, with the largest departures (2°-3°C above average) observed over the eastern half of Siberia. This warmth was especially prevalent during October and November, in association with persistent positive 500-hPa height anomalies throughout the region.

Southern Hemisphere
Highlights 
1.	Continuation of above-average surface temperatures across the three continents,
2.	 Above-average precipitation over central and southern South America

The Southern Hemisphere circulation pattern during September-November featured positive 500-hPa height anomalies over the subtropical latitudes of the central and east-central Pacific, across the central South Atlantic and Indian Oceans, and over Australia, and negative anomalies over the high latitudes of the South Pacific. These conditions contributed to a continuation of warmer than average surface temperatures across the three continents.

South Pacific and South America
Above-average surface temperatures were again observed over large portions of South America during SON. The largest departures were observed in central South America and the Amazon Basin, where mean temperatures were 1°-2°C above average. Much of central and southern South America also recorded above-average precipitation during the season. The most significant precipitation departures were observed in Peru and Chile, where totals in many areas exceeded the 90th percentile of occurrences.
In the middle latitudes the anomalously warm and wet conditions were associated with increased storm activity within the exit region of an enhanced South Pacific jet stream that was extended well east of its climatological mean position. This anomalous jet stream was related to a very persistent and enhanced subtropical ridge over the central and east-central Pacific.

Australia
An anomalous upper-level subtropical ridge again covered most of Australia during SON, resulting in a continuation of warmer than average temperatures over much of the continent. The largest monthly departures were observed during October, when mean temperatures were 3°-4°C above average over much of central and western Australia.

Southern Africa
Nearly all of Africa experienced above-average temperatures during SON. Over southern Africa mean temperatures were 1°-2°C above average, with the largest departures observed during October and November. This warmth is related to a persistent anomalous upper-level ridge located upstream of the continent.