September-November 2006: Seasonal Climate Summary

By 

Dr. Gerald Bell 
NOAA/ NWS/ Climate Prediction Center

TROPICS
1.	El Niño strengthens
2.	Intra-seasonal variability affects tropical atmospheric and oceanic circulation
3.	Reduced Atlantic hurricane activity related to El Niño
4.	Increased East Pacific hurricane activity related to El Niño

(Large portions of this “Tropics” section are taken directly from Bell and Halpert (2007) and from Bell et al. (2007), both of which will appear in the Bulletin of the American Meteorological Society in June 2007.
El Niño Strengthens
The September-November (SON) 2006 season featured the development of a moderate-strength El Niño (Pacific warm episode) across the central and east-central equatorial Pacific. El Nino episodes are often defined from a time series of area-averaged sea-surface temperature (SST) anomalies in the Niño-3.4 region (called the Niño 3.4 index), which spans the central and east-central equatorial Pacific between 170°W-120°W, 5°N-5°S. This index shows that El Niño developed during August and September and then strengthened, with Niño 3.4 temperatures increasing to more than +1.2°C above average during November. This warming was also seen in the pattern of subsurface temperatures at thermocline depth, which were generally 2°-3°C above average during the season.
The strong coupling between surface and sub-surface temperature anomalies is a well-known characteristic of El Niño, and reflects their common link to increased thermocline depths. During El Nino, weaker-than-average tropical easterlies result in decreased upwelling, which leads to increased thermocline depths and warmer waters. 
During SON, these conditions were associated with suppressed convection over the western tropical Pacific and Indonesia, and periods of enhanced convection over the central equatorial Pacific. This anomalous tropical convection led to several well-known El Niño impacts, including 1) below-average rains across northeastern Australia, Indonesia, and the western tropical Pacific, 2) fewer Atlantic hurricanes and more East Pacific hurricanes than in recent years, 3) above-average temperatures across the northern half of the United States during November-December, and 4) stormier and wetter than average conditions across the southeastern U.S. during November-December.

Intra-seasonal variability also affects tropical atmospheric and oceanic circulation
Tropical intra-seasonal variability was prominent during SON, as indicated by equatorial time-longitude sections of Outgoing Longwave Radiation (OLR) and 200-hPa velocity potential. One major contributor to the tropical intraseasonal variability is the Madden-Julian Oscillation (MJO), which is indicated in a time-longitude section by continuous propagation of the 200-hPa velocity potential anomalies around the globe. During April-December several periods of MJO-related suppressed convection and anomalously weak low-level easterly winds were evident over the western tropical Pacific. These conditions helped to initiate oceanic equatorial Kelvin waves, which are eastward propagating gravity waves featuring down-welling in the mixed-layer at their leading edge and upwelling in their wake. A typical eastward propagation rate for these waves is roughly 10° of longitude per week. 
Four major Kelvin waves occurred during April-December, with the amplitude of each wave exceeding that of its predecessor. Prior to and during El Niño’s development, these waves acted to significantly modulate the upper ocean heat content across the central and east-central equatorial Pacific. During September the warming associated with the down-welling phase of a particularly strong Kelvin wave prompted NOAA to report that El Nino had developed. This was followed during November and December by an even stronger Kelvin wave and additional warming. 

Reduced Atlantic hurricane activity related to El Niño
The 2006 Atlantic hurricane season produced ten tropical storms (TS), five hurricanes (H) and two major hurricanes (MH) [categories 3-5 on the Saffir- Simpson scale]. The National Oceanic and Atmospheric Administration’s (NOAA’s) seasonal ACE index (Bell et al. 2000) was 90% of the median, indicating a near-normal season. 
The activity was reduced compared to the previous several seasons, largely in response to the rapid development of El Niño during the summer months. Although several models indicated that El Nino might develop during the second half of 2006, none of them predicted its rapid development during August and September. As a result, NOAA’s seasonal hurricane outlooks over-predicted the activity.
The large-scale El Niño signal during ASO 2006 is evident in the patterns of anomalous 200-hPa velocity potential and streamfunction. In the tropics velocity potential anomalies are related to the upper-level divergent circulation and anomalous convection. Negative velocity potential anomalies across the central and east-central equatorial Pacific reflect the El Niño-related enhanced convection and anomalous upper-level divergence. Positive values over both the western Pacific/ Indonesia region and the Caribbean Sea reflect compensating anomalous upper-level convergence and suppressed convection. Another characteristic El Niño signature is anomalous upper-level ridges (positive streamfunction anomalies in NH, negative in SH) in the subtropics of both hemispheres flanking the region of enhanced convection over the central equatorial Pacific. However, this feature was not evident until October.
El Niño’s typical impacts over the western tropical Atlantic Ocean and Caribbean Sea include increased vertical wind shear between 200-850-hPa and anomalous sinking motion in the middle and upper atmosphere. During ASO 2006 El Niño appears to have suppressed TC activity mainly by contributing to anomalous upper-level convergence and sinking motion across the Caribbean Sea. This suppressing influence was particularly notable during September and October, when only one TC developed over the Caribbean Sea despite low wind shear (< 8 m s-1) and anomalously warm sea-surface temperatures (SST). During September these same conditions, but with no anomalous sinking motion, led to the formation of several hurricanes and major hurricanes over the central and eastern tropical Atlantic Ocean. 
For the entire Main Development Region (MDR), which encompasses the tropical Atlantic Ocean and Caribbean Sea (Goldenberg and Shapiro 1996), area-averaged SSTs during ASO were 0.68°C above average, the second warmest in the historical record dating back to 1871. The reduced activity despite this anomalous warmth is consistent with previous findings indicating that local atmospheric circulation anomalies, rather than local SST anomalies, are the dominant contributor to seasonal fluctuations in Atlantic hurricane activity (Shapiro and Goldenberg 1998, Bell and Chelliah 2006).
The vertical wind shear pattern during ASO 2006 primarily reflected anomalously weak shear (relative to the period 1971-2000) throughout the MDR in association with the ongoing active hurricane era. A modest El Niño-related increase in wind shear is perhaps evident over the western Caribbean Sea, but only when the departures are calculated relative to the current active era. 
As with the 200-hPa streamfunction anomalies, there is little indication that El Niño affected the vertical wind shear prior to October. Even then, there is little evidence that the increased shear contributed to the shut-down in activity. For example, the main suppressing influence from strong vertical shear (> 8 ms-1) occurred in August, prior to the time when El Niño began affecting this field. During September and October, the shear was quite weak across the Caribbean Sea, and was therefore not a suppressing factor
Although El Niño appears to be the primary cause for the reduced TC activity, highly variable circulation features not linked to El Niño also helped to suppress the activity. For example, during August increased vertical wind shear in association with an enhanced mid-oceanic trough led to reduced activity across the central MDR. During September a deep trough near the U.S. east coast contributed to anomalous sinking motion over the Gulf of Mexico. During October, an enhanced upper-level ridge over the south-central U.S. and western Gulf of Mexico contributed to the anomalous sinking motion over the Gulf of Mexico and also accentuated the sinking motion over Caribbean Sea. These conditions resulted in the third earliest end (following 1983 and 1993) to seasonal activity since routine daily satellite coverage began in 1966.
Only three Atlantic tropical storms, Alberto, Beryl, and Ernesto, struck the continental U.S. during 2006. Also, this was the first year since 2001 that no hurricanes struck the continental United States. This represents a sharp drop in strikes compared to the preceding four years (2002-2005), when an average of seven named storms and three hurricanes per season struck the continental United States. In September, (four of the five 2006 hurricanes formed in September) the deep trough near the U.S. east coast was critical in steering hurricanes out to sea long before they reached the coast. The overall suppression of activity over the western part of the Atlantic Basin due in part to El Niño also contributed to fewer U. S. strikes. 
The reduced 2006 activity occurred in an otherwise active hurricane era that began in 1995. During the 12-yr period 1995-2006, seasons have averaged 14.4 TS, 8.2 H, and 3.9 MH, with an average ACE value of 171% of the median. This value is just shy of the 175% threshold used by NOAA to define a hyperactive season. Nine of these twelve seasons are classified by NOAA as above normal, with the exceptions being the three El Niño years of 1997, 2002, and 2006. Only one season during this 12-yr period, corresponding to the extremely strong 1997 El Niño, is classified as below-normal. 
Despite the reduced 2006 activity, key oceanic and atmospheric features continued to reflect the ongoing active Atlantic hurricane era. A main contributing factor to this active era is the tropical multi-decadal signal, which reflects the leading modes of tropical convective rainfall variability occurring on multi-decadal time scales (Bell and Chelliah 2006). The ongoing warm phase of the Atlantic multi-decadal mode (Goldenberg et al. 2001), indicated by a continuation of above average SSTs in the North Atlantic, is one important aspect of the tropical multi-decadal signal. Some of this persistent anomalous warmth has also been linked to increasing global temperatures over the last 100 years (Santer et al. 2006). 
Other very important and inter-related aspects of the tropical multi-decadal signal and the ongoing active hurricane era that were also in place during 2006 included 1) an enhanced West African monsoon system (Landsea and Gray 1992) and suppressed convection in the Amazon Basin, 2) enhanced upper-level ridges in both hemispheres over the Atlantic Ocean, 3) reduced wind shear across the tropical Atlantic, 4) an enhanced tropical easterly jet at 200-hPa and reduced tropical easterlies at 700-hPa across the tropical Atlantic, and 5) increased cyclonic relative vorticity across the eastern MDR. In light of these conditions, there is no indication the current active hurricane era has ended. Instead, the El Niño-related reduction in activity merely represents a short-term break (similar to the1997 and 2002 El Niño years) in an ongoing string of active hurricane seasons that began in 1995.
Increased East Pacific hurricane activity related to El Niño
The 2006 East Pacific hurricane season featured 18 tropical storms, 10 hurricanes, and 5 major hurricanes. These values are above the 1971-2000 averages of 15.4 tropical storms, 9 hurricanes, and 4.4 major hurricanes. The seasonal ACE value was 132% of the median, which falls into the near-normal range of activity as defined by NOAA.
The 2006 ACE index was more than double the average of the past four seasons (61% of the median). Since the beginning of the current inactive East Pacific hurricane era that began in 1995, only four seasons have NOT been below normal. Three of those four seasons correspond to the El Niño years of 1997, 2002, and 2006. Also since 1995, only one east Pacific hurricane season has been above normal. That season corresponds to the very strong El Niño of 1997.

EXTRATROPICS
NORTHERN HEMISPHERE 
1.	Considerable variability in U.S. temperature and precipitation anomalies
2.	Excessive warmth continues in Europe and across the North Atlantic 
3.	Excessive warmth continues in China

Considerable variability in U.S. temperature and precipitation anomalies
The mean 500-hPa circulation pattern during SON 2006 featured above-average heights over the high latitudes of the North Pacific and Europe, and below-average heights over the eastern U.S., the eastern North Atlantic Ocean, and the eastern Caribbean Sea. One prominent feature of the overall circulation pattern was the persistence of below-average heights extending from the eastern U.S. to Europe during September and October. Enhanced upper-level westerlies associated with this pattern helped steer Atlantic hurricanes out to sea well before reaching the coastal United States. This pattern was associated with below-average temperatures across the eastern half of the U.S., and above-average temperatures across western Canada, Alaska, and Europe.  It was also associated with above-average precipitation in portions of the north-central and northeastern parts of the U.S., where seasonal precipitation totals were in the upper 90th percentile of occurrences. A reversal in November of the height anomaly pattern across the U.S. brought above-average temperatures to much of the country, and below-average temperatures to much of Canada and Alaska.

Excessive warmth continues in Europe and across the North Atlantic
The 500-hPa circulation during SON featured a trough over the eastern North Atlantic and a ridge over Europe. This pattern was associated with an anomalous southwesterly flow of marine air into the continent, which resulted in well above-average temperatures (+2° to +4°C departures) throughout Europe and Scandinavia. The persistent upper-level ridge also led to below-average precipitation across much of Europe, with the most significant deficits occurring in eastern Europe where seasonal totals were in the lowest 10th percentile of occurrences.
In the Atlantic Ocean, seasonal SSTs were again well above average in both the subtropical and high latitudes. This warmth is consistent with the ongoing warm phase of the Atlantic Multi-decadal Oscillation (AMO, Goldenberg et al. 2001), and with the overall upward trend in ocean temperatures observed during the last thirty years. Atlantic Ocean temperatures have been well above average in these regions since the current active Atlantic hurricane era began in 1995.

Excessive warmth continues in China
The 200-hPa circulation during SON featured a below average strength of the Asian monsoon ridge, consistent with suppressed convection across Indonesia and the eastern half of the equatorial Indian Ocean. During October and November, the anomalous upper-level trough was centered over northern India and western China, and was associated with anomalous southwesterly flow and exceptionally warm surface temperature across China. Temperatures across central and northern China generally averaged 2°-3C during the period, and were in the upper 90th percentile of occurrences. For the season as a whole, surface temperatures were 2°-3°C above average across central and northern China. Most of China also recorded above-average temperatures during the JJA season.

SOUTHERN HEMISPHERE 
1.	Warmer and drier than average in Australia
2.	Below-average rains in southern Africa
3.	Extensive Antarctic ozone hole

The 200-hPa circulation during SON featured anticyclonic streamfunction anomalies across the subtropics, which is consistent with ongoing El Niño conditions. In the middle and high latitudes, the 500-hPa circulation featured above-average heights across south-central Australia and below-average heights across the high latitudes of the central and eastern South Pacific. 

Warmer and drier than average in Australia
The overall pattern of below-average precipitation observed from across Indonesia, southeastern Asia, and northern Australia is consistent with El Niño. In Australia, a persistent mid- and upper-level ridge accentuated this dryness throughout the eastern half of the continent, with seasonal totals in many areas falling below the lowest 10th percentile of occurrences. This persistent ridge also contributed to above-average temperatures across the country, with many regions recording seasonal values of 1°-2°C above average.

South African rainy season
The southern Africa rainy season extends from October to April. Precipitation in the region was below-average in October and near-average in November. The South African rainy season tends to be drier-than-average during El Niño.
 
Extensive Antarctic ozone hole 
The 2006 Antarctic ozone hole developed in late August and remained extensive through the end of October. The ozone hole was near record size from late September through October, exceeding 25 million km2 in September and early October, and 20 million km2 in late October. An extensive ozone hole is consistent with anomalously low stratospheric temperatures, and with an exceptionally large SH circumpolar vortex. Temperatures at both 2 hPa and 10 hPa were among the lowest on record from August through mid-October, which led to a record large spatial extent of polar stratospheric clouds (PSCs). PSC’s are a main mechanism by which the ozone hole is created. 

References:
Bell, G. D., and co-authors, 1999: Climate Assessment for 1998. Bull. Amer. Meteor. Soc., 80, S1-S48.
Bell, G. D., and co-authors, 2000: Climate Assessment for 1999. Bull. Amer. Meteor. Soc., 81, S1-S50.
Bell, G. D., and co-authors, 2001: Climate Assessment for 2000. Bull. Amer. Meteor. Soc., 82, S1-S55.
Bell, G. D., and co-authors, 2004: The 2003 North Atlantic Hurricane Season: A Climate Perspective. State of the Climate in 2003. A. M. Waple and J. H. Lawrimore, Eds. Bull. Amer. Meteor. Soc., 85, S1-S68.
Bell, G. D., and co-authors 2005: The 2004 North Atlantic Hurricane Season: A Climate Perspective. State of the Climate in 2004. A. M. Waple and J. H. Lawrimore, Eds. Bull. Amer. Meteor. Soc., 85, S1-S68.
Bell, G. D., and co-authors 2006: The 2005 North Atlantic Hurricane Season: A Climate Perspective. State of the Climate in 2005. A. M. Waple and J. H. Lawrimore, Eds. Bull. Amer. Meteor. Soc., 86, S1-S68.
Bell, G. D., and co-authors 2007: The 2006 North Atlantic Hurricane Season: A Climate Perspective. State of the Climate in 2006. A. M. Waple and J. H. Lawrimore, Eds. Bull. Amer. Meteor. Soc., 87, S1-S68.
Bell, G. D., and M. S. Halpert 2007: El Niño and the tropical Pacific. State of the Climate in 2006. A. M. Waple and J. H. Lawrimore, Eds. Bull. Amer. Meteor. Soc., 87, S1-S68.
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.
Landsea, C. W., and W. M. Gray, 1992: The strong association between Western Sahel monsoon rainfall and intense Atlantic hurricanes. J. Climate, 5, 435-453.
Shapiro, L. J., and S. B. Goldenberg, 1998: Atlantic sea surface temperatures and tropical cyclone formation. J. Climate, 11, 578-590.
Santer,B. D., and co-authors, 2006: Forced and unforced ocean temperature changes in Atlantic and Pacific tropical cyclogenesis regions. PNAS, 103, 13905-13910.