June-August 2004: Seasonal Climate Summary

By 

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

TROPICS
Highlights 
1.	A weak Pacific warm episode (El Niño) develops
2.	Ongoing intraseasonal activity linked to Madden-Julian Oscillation (MJO)
3.	Record Atlantic hurricane activity for August, suppressed East Pacific activity
4.	Near- to below-average rains in the West African monsoon region
TROPICAL PACIFIC
Positive sea surface temperature (SST) anomalies strengthened across the central and east-central equatorial Pacific during June-August 2004, with August departures averaging 1°-1.5°C from the date line eastward to 130°W. This eastward expansion in the area of anomalous warmth indicates the early stages of a Pacific warm (El Niño) episode. 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, this anomalous warmth did contribute to farther eastward shifts in enhanced convection associated with the convectively active phase of the Madden Julian Oscillation (MJO). 
The MJO activity during June-August resulted in considerable week-to-week and month-to-month variability in the Tropics. For example, during mid-June through early July the easterlies weakened in many areas of the equatorial Pacific, as enhanced convection shifted eastward from the Indian Ocean to the western tropical Pacific. By mid-July the low-level winds and equatorial convection returned to near average in many areas of the equatorial Pacific. However, a strong oceanic Kelvin wave, initiated by the weaker-than-average easterly winds in June, propagated eastward resulting in a substantial deepening of the oceanic thermocline and an increase in the subsurface temperature anomalies in the central and eastern equatorial Pacific. This Kelvin wave reached the South American coast during late August, resulting in an increase in SSTs along the coasts of Ecuador and northern Peru. 
ATLANTIC and EAST 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. No tropical storms formed during June and July 2004, followed by record activity during August. Eight tropical storms (TS) formed during August, with five becoming hurricanes (H) and three becoming major hurricanes (MH) (Alex, Charlie, and Frances). This sets a record for the most major hurricanes during in that month. It is noted that the increased hurricane activity also continued through September, with a total of twelve tropical storms, seven hurricanes, and five major hurricanes, recorded by the end of the month. This makes 2004 one of the most active seasons in the record dating back to 1945.
For August-September 2004 combined, six named storms made landfall in the United States. Tropical storm Bonnie hit the Florida Panhandle on August 12th, MH Charlie hit the west coast of Florida as a Category-3 storm on August 13th, TS Gaston hit South Carolina on Aug. 30th, Hurricane Frances hit the east coast of Florida as a Category-2 storm on Sep. 5th, MH Ivan hit Alabama as a Category-4 storm on Sep. 16th, and MH Jeanne hit the east coast of Florida as a Category-3 storm on Sep. 26th.  
During the last three Atlantic hurricane seasons (2002-2004) seventeen named storms have made landfall in the United States (6 in 2002, 5 in 2003, and 6 in 2004). Thirteen 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. Five of these systems hit the Gulf Coast in both 2002 and 2003 each, and three hit during August-September 2004 (TS Bonnie, MH Charley, and MH Ivan).
The above-average activity in 2004 partly reflects the overall increase in activity that began in 1995. Eight of ten hurricane seasons were above normal during 1995-2004, 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 20-25 years (Goldenberg et al. Science, 2001), it is likely that the above-normal activity observed since 1995 will continue for at least the next several years. On average, the United States experiences 2-3 landfalling hurricanes during active seasons, and could therefore be at an increased risk of hurricane landfalls in upcoming years.
During 2004 several conditions known to favor above-normal Atlantic hurricane seasons were present across the tropical Atlantic. These include 1) an amplified subtropical ridge at upper levels, 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 (0.5°C-1.0°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).
In contrast by the end of September 2004 the East Pacific hurricane season had produced only 11 TS, 6H and 3 MH (Darby, Howard, and Javier). These numbers are 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.  During June-August, rainfall totals in these areas were generally near- to below- average. Farther south rainfall in the Equatorial Guinea region was considerably below average, with seasonal totals in some areas below the 10th percentile of occurrences.

EXTRATROPICS
Northern Hemisphere
Highlights 
1.	Persistent anomalous circulation over North America that began in April,
2.	Above-average temperatures in Alaska, the western U.S., and Europe
3.	Below-average temperatures across the eastern U.S. and Canada,
4.	Above-average precipitation in the southern and eastern U.S., and central Russia, and below-average precipitation in Alaska, the southwestern U.S., and southern Europe,
5.	Continuation of exceptionally warm SSTs in the North Atlantic at both high latitudes and the subtropics.

North America
The period April-August 2004 featured exceptionally warm and dry conditions from Alaska to the southwestern U.S., with record temperatures set at many locations in Alaska. For the state as a whole, this was the warmest April-August period in the 1950-2004 record. In the southwestern U.S., below-average precipitation occurred during June-August in association with a suppressed monsoon season. 
The April-August period also featured well below-average temperatures across the eastern half of Canada and the central/ eastern United States. Central Canada experienced its third coolest such season in the record, with mean temperatures averaging 2°-3°C below normal. Across the central and eastern U.S., the most significantly below-average temperatures occurred during June-August. Several significant cold-air outbreaks occurred during this period, in association with an amplified upper-level ridge over western North America/ Alaska and an amplified trough extending southward into the central or eastern United States. These conditions favored the southward penetration of cold fronts well into the deep South. In Texas, the exceptional southward penetration of the cold fronts was also related to a strong southward funneling of the cold air along the eastern slopes of the Rocky Mountains. These frontal systems also brought below-average temperatures to much of the eastern United States, and contributed to above-average precipitation from Texas northeastward to New England. 
On average, the anomalous temperature and precipitation patterns over North America during April-August were related to a persistent blocking ridge over Alaska and northwestern Canada, a westward shift and strengthening of the mean trough over central/ eastern Canada, and a persistent jet stream in the Great Lakes region. During the second half of June and the latter part of July, these conditions were associated with a westward shift of the mean summertime ridge that normally extends from Texas northward to south-central Canada. This mean ridge not only influenced the summertime temperature and precipitation patterns over North America, but also helped to regulate the strength of the Southwest U.S. monsoon system. Overall, this monsoon system was weaker than average during June-August.
While it is not possible to attribute a specific cause or causes of the persistent April-August circulation, three factors likely contributed to its longevity. These include a persistent zonal wave-3 pattern at high latitudes, and a pronounced stratospheric extension of the high-latitude height anomalies, with the blocking ridge over Alaska and the downstream amplified trough extending past the 30-hPa level. They also include a highly diffluent trough over eastern Siberia, and a westward retraction of the East Asian jet core and jet exit region, which favored the persistence of a blocking ridge over western North America.

North Atlantic and Europe
Over the North Atlantic the mean 500-hpa circulation during June-August featured above-average heights at high latitudes and below-average heights from Canada eastward to Great Britain and Scandinavia. This circulation was associated with enhanced westerlies across northern Europe. It was also associated with an above average strength of the subtropical ridge across southern Europe, situated along the equatorward flank of the enhanced jet stream.
This circulation contributed to a continuation of anomalous warmth throughout Europe. It also contributed to a dipole pattern of precipitation anomalies over the continent, with northern Europe experiencing generally above-average precipitation and southern Europe/northern Africa experiencing below-average precipitation.
Also over the North Atlantic, the season featured a continuation of exceptionally warm SSTs at both high latitudes and the subtropics, in association with the ongoing warm phase of the Atlantic multi-decadal mode that began approximately in 1995 (Goldenberg et al. Science, 2001). This anomalous warmth is 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.

Southern Hemisphere
Highlights 
1.	Zonally-symmetric pattern of anticyclonic circulation anomalies in middle latitudes
2.	Above-average surface temperatures across the three continents,
3.	 Below-average precipitation in Amazon Basin, Central America, and eastern Australia. 

The mean Southern Hemisphere circulation during June-August featured below-average heights at upper levels across Antarctica, and generally above-average heights in the middle latitudes. These conditions were associated with considerable zonal-symmetry of the anticyclonic streamfunction anomalies in the middle latitudes, and contributed to warmer than average surface temperatures across the three continents.

South/ Central America
Over South America the upper-level circulation featured cyclonic streamfunction anomalies over the Amazon Basin and anticyclonic streamfunction anomalies at higher latitudes. In the Tropics this anomaly pattern reflected a weakening of the upper-level “monsoon” ridge, and was consistent below-average precipitation in the northern Amazon Basin and Central America. In both regions June-August precipitation totals were below the 10th percentile of occurrences. The Amazon Basin also experienced warmer than normal conditions during the season, with mean temperatures ranging from 1°-2°C above average.
In the middle latitudes the anomalous upper-level anticyclonic circulation was associated with reduced jet stream winds, and a reduction in the number of synoptic-scale troughs and major cold frontal passages east of the Andes Mountains. These conditions contributed to warmer than normal conditions across extratropical South America, with seasonal temperatures in many regions ranging from 1°-2°C above average.

Eastern Australia
An amplified upper-level subtropical ridge covered most of Australia during June-August, resulting in warmer than average temperatures over much of the continent. Downstream of the mean ridge axis, eastern Australia experienced below-average precipitation, with seasonal totals generally below the 30th percentile of occurrences.