September-November 2005: Seasonal Climate Summary

By 

Dr. Gerald Bell
NOAA/ NWS/ Climate Prediction Center

TROPICS
Highlights 
1.	ENSO-neutral conditions
2.	Another very active Atlantic hurricane season, suppressed East Pacific activity
3.	Above-average West African monsoon rains
TROPICAL PACIFIC
ENSO-neutral conditions continued during September-November (SON) 2005, with near-average to slightly below-average SSTs evident across the east-central equatorial Pacific. A general decrease in surface and subsurface temperature anomalies continued during the season, reflecting a continued evolution toward weak La Niña conditions. By the end of the season, the Niño 3 region index was -0.7. However, slightly above-average SSTs persisted in the central equatorial Pacific, as indicated by a +0.3 value in the Niño 4 region. 
MJO activity occurred throughout the season, as indicated by equatorial time-longitude sections of Outgoing Longwave Radiation (OLR) and 200-hPa velocity potential. This activity resulted in considerable intraseasonal variability in the Tropics and subtropics, and tended to modulate the timing for Atlantic tropical storm formation throughout the season.
ATLANTIC and EASTERN PACIFIC  HURRICANE SEASONS	
The Atlantic hurricane season runs from 1 June through 30 November, with the normal peak in activity occurring from mid-August through early October. During SON, 14 tropical storms (TS) formed, of which ten became hurricanes (H) and four became major hurricanes (MH). Three of these systems made landfall in the United States, one as a tropical storm (Tammy) and two as major hurricanes (Rita and Wilma). Hurricane Ophelia also affected the coastal regions of North Carolina, but did not officially make landfall. Also during the season, TS Alpha made landfall in eastern Mexico and MH Beta made landfall in Nicaragua. 
	The 2005 Atlantic hurricane season broke many tropical cyclone records. The season featured a record 27 TS, a record 15 hurricanes, a record three category-5 hurricanes, a record estimated Accumulated Cyclone Energy (ACE) index [Bell et al. 2000, Bull. Amer. Meteor. Soc., 81, S1-S51] of 285% of the median, and an ACE contribution of 131% of the median from named storms forming outside the Main Development region [MDR, consisting of the tropical Atlantic and Caribbean Sea south of 21.5°N]. The season also featured a record fifteen landfalling named storms in the Atlantic Basin, a record four landfalling U.S. major hurricanes, a record seven named storms during June-July, and a record 10 named storms forming after 1 October. The seven major hurricanes during the season is one shy of the record set in 1950.
Two tropical storms, two hurricanes, and four major hurricanes struck the United States during 2005. The combined rainfall totals from these eight systems shows very heavy rains extending well inland from the coast. The largest totals exceeded 400 mm (16 inches) in a swath extending from southeastern Louisiana/ southwestern Mississippi to northern Mississippi. This swath was associated with Hurricane Katrina, which struck the central Gulf Coast in late August. Katrina, one of the worst natural disasters to ever strike the United States, made the 2005 season the costliest in the nation’s history. Elsewhere, three tropical storms and three hurricanes struck Mexico, one tropical storm made landfall in the Dominican Republic, and one hurricane struck Nicaragua. 
The National Oceanic and Atmospheric Administration’s (NOAA) seasonal Atlantic hurricane outlook issued in May 2005 indicated a high (70%) likelihood the season would be above normal. The updated outlook issued in early August indicated a 95%-100% chance of an above-normal season, with the possibility of near-record activity. That outlook called for 18-21 named storms (previous record was 21), 9-11 hurricanes (previous record was 12), and 5-7 major hurricanes (record is 8). It also called for an ACE index in the range of 180%-270% of the median (previous record was 277% in 1950).
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). 
Many of these regional conditions are consistent with other very active hurricane seasons. However, both the amplitude and spatial extent of these anomalies were generally larger than seen in other active seasons. These conditions are attributed to four main factors: 1) the ongoing tropical multi-decadal signal, 2) long periods of anomalous upper-level convergence and suppressed convection over the central tropical Pacific reminiscent of La Niña conditions, 3) record warm sea-surface temperatures across the MDR, and (4) exceptionally conducive upper-level and lower-level wind and air pressure patterns over the western Atlantic and Gulf of Mexico.
A detailed analysis of the record 2005 Atlantic hurricane season can be found on NOAA’s Climate Prediction Center web site at http://www.cpc.ncep.noaa.gov/products/outlooks/hurricane-archive.shtml. Related articles are also scheduled for the “State of the Climate during 2005” article to appear in the June 2006 issue of the Bulletin of the American Meteorological Society. Additional analysis of the climate patterns controlling Atlantic hurricane activity is presented by Bell and Chelliah (2006, J. Climate, 15 February issue) in an article entitled “Leading Tropical Modes Associated with Interannual and Multi-Decadal Fluctuations in North Atlantic Hurricane Activity.”
The active 2005 season 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 25-40 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, the country remains at an increased risk of hurricane landfalls for many years to come.
The 2005 East Pacific (EP) hurricane season featured 15 tropical storms, of which seven became hurricanes and two became major hurricanes. These numbers are at or below the1971-2000 means of 15 TS, 9H, 4.4 MH. The seasonal ACE index was approximately 84% of the long-term median value, indicating a continuation of suppressed EP hurricane seasons that began in 1995. 
El Niño acts to lower the vertical shear and thus favors above-normal EP hurricane seasons, while La Niña increases the shear which suppresses seasonal activity. These affects are opposite to those for the North Atlantic hurricane seasons. Although no La Niña was present during the 2005 season, prolonged period of suppressed convection were evident near the date line and likely contributed to the reduced EP activity.
The ENSO signal is often superimposed upon a strong lower-frequency signal that lasts for decades at a time. For example, the period 1971-1994 featured generally above-normal EP seasons, while the period 1995-present has featured well below-normal activity. These multi-decadal fluctuations in activity are opposite to those in the Atlantic Basin. During 2005, the multi-decadal signal featured above-normal shear over the eastern Pacific and below-normal shear across the tropical Atlantic. During 2005 this signal, combined with the La Niña-like pattern of tropical convection, resulted in the combination of a suppressed EP season and an enhanced Atlantic hurricane season. 
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 occurring during August and September.  The September 2005 rains were near average. The October rains were well above average with many areas recording totals above the 90th percentile of occurrences. Overall, the 2005 rainy season was slightly above average, with the main precipitation surpluses occurring in June and October, and below-average totals occurring in July and August.
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 
2.	Above-average temperatures at both high and low latitudes of the North Atlantic.
3.	Severe dryness develops in portions of southwestern U.S.
4.	Significant dryness continues across northern Europe/ western Russia
5.	Significant dryness in equatorial eastern Africa
6.	Strong onset to the South African rainy season

NORTHERN HEMISPHERE 
Surface temperatures were generally 1°-2°C above average in all NH land areas during SON 2005, with the largest departures of 3°-5°C observed in northwestern Russia. This anomalous warmth was associated with positive height anomalies in northwestern Eurasia throughout the season.
 Anomalously warm SSTs were also observed throughout the high latitudes and subtropical latitudes of the North Atlantic, with departures in both regions generally greater than 0.5°C. This 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 totals in the northwestern U.S., and below-average totals across the southern U.S., northern Europe, and western Russia. In the United States, seasonal totals were generally 25%-50% of normal in Texas, Oklahoma, and Arizona, as long-term drought conditions became established in each region. This suppressed precipitation is consistent with a persistent storm track farther north across the Pacific Northwest, which resulted in generally above-average precipitation from Washington State to Maine.
From northeastern Europe to east of the Caspian Sea, SON precipitation was in the lowest 10th percentile of occurrences. Across northern Europe, the below-average precipitation was linked to a persistent pattern of positive 500-hPa height anomalies mainly during September and October. This pattern was associated with a pronounced split-flow over the eastern North Atlantic and Europe, with the northern branch of the flow entering the continent over Scandinavia, and the southern branch entering the continent over the Mediterranean Sea. The resulting lack of storminess led to below-average precipitation across northern Europe. These conditions, combined with a persistent upper-level ridge during November, also led to below-average seasonal precipitation across western Russia. 

SOUTHERN HEMISPHERE 
The mean 500-hPa circulation pattern during SON 2005 exhibited an anomalous zonal-wave 3 pattern, with positive height anomalies south of South America and New Zealand, and also over the central Indian Ocean, and negative height anomalies southeast of Australia, over the high latitudes of the central South Pacific, and south of Africa.
In Australia, seasonal temperatures were generally 1°-2°C above average. The most anomalous warmth occurred during September and October when temperatures were 2°-3°C above average over large areas, and many areas recorded values in the highest 10th percentile of occurrences. Seasonal precipitation was near-average in both western and eastern Australia, and above average across the central part of the continent. Much of this above-average precipitation occurred during October.
In equatorial eastern Africa, SON precipitation was significantly below-average across northern Mozambique, Kenya, Tanzania, and southern Madagascar, with many areas recording totals in the lowest 10th percentile of occurrences. This region coincided with an area of large-scale anomalous sinking motion centered between an anomalous upper-level ridge over central South Africa and an anomalous upper-level trough just east of the continent. In contrast, rainfall was above average beneath the mean upper-level ridge over South Africa, indicating a strong onset to the 2005-06 South African rainy season. 
In tropical South America, seasonal temperatures were 0.5°-2°C above average, in association with below-average precipitation in the Amazon Basin. Similar conditions were also observed during June-August. Rainfall in the eastern Amazon Basin has been generally below-average since 1995. At the same time rainfall has been generally above-average in the West African monsoon region. Bell and Chelliah (2006) show this dipole pattern of convective rainfall anomalies is an important contributor to the circulation anomalies across the tropical North Atlantic and eastern Pacific. These anomalies have favored very active Atlantic hurricane seasons since 1995, along with below-average strengths of the East Pacific hurricane seasons.

References
Bell, G. D., and co-authors, 2000: Climate Assessment for 1999. Bull. Amer. Meteor. Soc., 81, S1-S50.

Bell, G. D., and M. Chelliah, 2006: Leading tropical modes associated with interannual and multi-decadal fluctuations in North Atlantic hurricane activity. J. Climate. 19, 590-612.

Goldenberg, S. B., C. W. Landsea, A. M. Mestas-Nuñez, and W. M. Gray, 2001: The recent increase in Atlantic hurricane activity: Causes and implications. Science, 293, 474-479.