Climate patterns similar to those expected this year have historically produced a wide range of activity, and have been associated with both near-normal and above-normal seasons. Allowing for uncertainties, we estimate a 60%-70% chance of occurrence for each of the following ranges of activity:
- 12-16 named storms,
- 6-9 Hurricanes,
- 2-5 Major Hurricanes,
- An ACE range 100%-210% of the median
These likely ranges have been observed in about two-thirds of past seasons having similar climate conditions to those expected this year. They do not represent the total ranges of activity seen in those past seasons.
Most of the 2008 activity is expected to take place during August through October (ASO), the peak months of the Atlantic hurricane season.
The Climate Prediction Center will issue an update to this outlook in early August, at the start of the peak months of the Atlantic hurricane season.
DISCUSSION
1. Expected 2008 Activity
This Outlook is a general guide to the expected overall activity for the 2008 Atlantic hurricane season. It is not a seasonal hurricane landfall forecast, and it does not imply levels of activity for any particular area
The expected conditions during the 2008 Atlantic hurricane season are related to two main climate signals: 1) the continuation of conditions (called the multi-decadal signal) that have been conducive to above-normal Atlantic hurricane activity since 1995 , including above-normal sea-surface temperatures in the eastern tropical Atlantic Ocean, and 2) a possible La Niña influence or ENSO-neutral conditions during the peak months (August-October) of the season.
Historically, seasons with climate patterns similar to those expected this year have produced a wide range of activity, and have been associated with both near-normal and above-normal seasons. This outlook considers the historical distribution of activity for these climate factors, uncertainties in the La Niña impacts, and the possibility of other unpredictable factors also influencing the season. Based on these factors, we estimate a 90% chance of a near-normal to above-normal 2008 Atlantic hurricane season. While an above-normal season is most likely (65% chance), there is a significant 25% chance of a near-normal season and a 10% chance of a below-normal season. See NOAA definitions of above-, near-, and below-normal seasons.
An important measure of the total seasonal activity is NOAA’s Accumulated Cyclone Energy (ACE) index, which accounts for the collective intensity and duration of named storms and hurricanes during the season. Based on the above factors, we estimate a 60%-70% chance the 2008 seasonal ACE range will be 100%-210% of the median. This range can be satisfied even if the numbers of named storms, hurricanes, or major hurricanes fall outside their likely ranges. If La Niña persists, the probability increases that the activity could be at or above the high end of the indicated ACE range.
The likely (60%-70% chance) ranges of activity for 2008 (each of which is seen in about two-thirds of similar seasons in the historical record): are 12-16 Named Storms, 6-9 Hurricanes, and 2-5 Major Hurricanes. Most of this activity is expected during August through October, the peak months of the Atlantic hurricane season.
2. Expected Climate Conditions – Active multi-decadal signal, La Niña impacts possible
a. Expected continuation of active Atlantic hurricane era
Atlantic hurricane seasons exhibit prolonged periods lasting decades of generally above-normal or below-normal activity. These fluctuations in hurricane activity result almost entirely from differences in the number of hurricanes and major hurricanes forming from tropical storms first named in the Main Development Region (MDR), which spans the tropical Atlantic Ocean and Caribbean Sea between 30oW-87.5oW and 10oN-21.5oN (Goldenberg et al. 2001).
The current active hurricane era began in 1995. Hurricane seasons during 1995-2007 have averaged 14.5 named storms, 8 hurricanes, and 4 major hurricanes, with an average ACE index of 167% of the median. NOAA classifies nine of the thirteen seasons since 1995 as above normal, with seven being hyperactive (ACE > 175% of median). Only four seasons since 1995 have not been above normal. These include the three El Niño years (1997, 2002, and 2006) and the 2007 season.
This high level of activity contrasts sharply to the 1971-1994 period of generally below-normal hurricane seasons (Goldenberg et al. 2001), which averaged only 8.5 named storms, 5 hurricanes, and 1.5 major hurricanes with an average ACE index of only 75% of the median. One-half of those seasons were below normal, only three were above normal (1980, 1988, 1989), and none were hyperactive. Time series of key atmospheric wind parameters highlight the dramatic differences between these above-normal and below-normal hurricane eras.
The regional atmospheric circulation anomalies that contribute to these long-period fluctuations in hurricane activity are strongly linked to the tropics-wide multi-decadal signal (Bell and Chelliah, 2006). A change in phase of that signal accounts for the transition in 1995 from the below-normal era to the above normal era. The multi-decadal signal is again a major factor guiding the 2008 outlook. Three key features of this signal associated with the current active hurricane era are: 1) a stronger West African monsoon system, 2) below-average convection in the Amazon Basin, and 3) warmer than average tropical Atlantic sea-surface temperatures.
Other ongoing regional features expected during the 2008 hurricane season include 1) lower surface air pressure and increased moisture across the tropical Atlantic, 2) an amplified ridge at upper levels across the central and eastern subtropical North Atlantic, 3) reduced vertical wind shear in the deep tropics over the central North Atlantic, which results from an expanded area of easterly winds in the upper atmosphere (green arrows) and weaker easterly trade winds in the lower atmosphere (dark blue arrows), and 4) weaker easterly winds in the middle and lower atmosphere, resulting in a configuration of the African easterly jet (wavy blue arrow) that favors hurricane development from tropical waves moving westward from the African coast.
b. Possible La Niña influence
The second key predictor for the 2008 Atlantic hurricane season is the possibility that the La Niña-related patterns of tropical convection and winds will persist, and therefore may be conducive to increased Atlantic hurricane activity. As discussed by Gray (1984), La Niña favors more Atlantic hurricanes and El Niño favors fewer hurricanes. The combination of La Niña and an active hurricane era increases the probability of an above-normal season.
Presently, La Niña is indicated by below average sea-surface temperatures across the central and east-central equatorial Pacific. La Niña is dominating the atmospheric convection and low-level winds in these regions as well, with suppressed convection over the central and eastern Pacific and enhanced convection over the western Pacific. There has been a tremendous tropics-wide response in the upper-level (200-hPa) atmospheric winds to this anomalous convection, with easterly anomalies extending across the eastern tropical Pacific Ocean, the tropical Atlantic Ocean, and northern Africa. If these anomalies persist through the summer, they would reinforce the multi-decadal signal and increase the probability of an above-normal and even hyperactive season.
In the latest ENSO Diagnostics Discussion released 8 May 2008, NOAA forecasters stated that La Niña has weakened since February, and that a transition to ENSO-neutral conditions is possible during June-July just prior to the peak months of the Atlantic hurricane season. This evolution is typical for La Niña, which often dissipates during the late spring or summer.
There is considerable spread and uncertainty among the climate models regarding how strong the La Niña influence will be on the Atlantic hurricane season. Most models are predicting ENSO-neutral conditions (neither El Niño nor La Niña) during the summer, with sea-surface temperature anomalies in the central and east-central equatorial Pacific between -0.5oC to 0.5oC. However, most of these models have historically shown little-to-no skill at this time of the year.
Historically, we have almost never (only once in 100 years) seen a La Niña transition from its current present strength to an El Niño by ASO. Therefore, there is a likelihood that the current La Niña patterns of tropical convection and winds will persist and affect the hurricane season, even if La Niña dissipates.
c. Atlantic Sea-surface temperatures (SSTs)
We also expect above-normal sea-surface temperatures (SSTs) in the eastern tropical Atlantic Ocean during 2008. SSTs are already well above average in that region and along the west coast of northern Africa.
In contrast, SSTs over the central and western MDR were below average during the last several months in association with the combination of La Niña and a persistent pattern of anomalous northeasterly and easterly surface winds. This wind pattern recently ended, and the SSTs quickly returned toward normal in these regions. Consistent with past La Niña episodes, and with the ongoing tropical multi-decadal signal, above-average temperatures are expected to return in the central and western MDR as the summer progresses.
3. Uncertainties in the Outlook, and a look back at 2007
The likely (60%-70% chance) ACE range for the 2008 Atlantic hurricane season reflects three inter-related sources of uncertainty: 1) When La Niña will dissipate, 2) the likelihood that the current La Niña patterns of tropical convection and winds will persist into ASO, even if La Niña dissipates, and 3) the likely strength of those patterns. These reflect the considerable spread in forecasts from the available ENSO prediction models.
Promising new climate models that can explicitly predict seasonal tropical cyclone activity in the Atlantic are also suggesting an active season. However, these new methods tend to have limited skill at this time of year because of the large uncertainties in the ENSO predictions being utilized.
Another uncertainty is the possibility of lesser climate factors also influencing the seasonal activity. Bell et al. (2008) identified two features that led to less-than-predicted seasonal hurricane activity in 2007. First, despite a strengthening La Niña during ASO 2007, the typical La Niña signal in the upper-tropospheric circulation was notably absent across the tropical North Pacific and MDR. As a result, La Niña did not weaken the upper-level trough and reduce the vertical wind shear in the MDR as expected. Second, that upper-level trough was enhanced in association with a very persistent ridge over eastern North America. This circulation led to increased vertical wind shear and anomalous mid-level sinking motion across the central and western MDR, two factors known to inhibit hurricane formation.
The analysis by Bell et al. (2008) shows that the lack of a La Niña signal was due in part to suppressed convection over Indonesia and the eastern tropical Indian Ocean. They state “These anomalies are opposite to the typical La Niña signal, and indicate that the total La Niña forcing and resulting upper-tropospheric circulation anomalies were much weaker than would normally be expected for the observed Pacific cooling. These conditions were associated with a record-strength pattern that also included enhanced convection over the western equatorial Indian Ocean, and enhanced convection across India and the Southeast Asian monsoon regions. This entire pattern is more typical of El Niño, as was seen in 2006. The observations suggest this climate signal may have overwhelmed the upper-tropospheric circulation anomalies normally associated with La Niña, thus negating La Niña’s normally enhancing influence on the 2007 Atlantic hurricane season. Conversely, this same pattern may have enhanced El Niño’s suppressing influence on the 2006 Atlantic hurricane season (Bell et al. 2007)
NOAA FORECASTERS
Climate Prediction Center
Dr. Gerry Bell, Meteorologist; Gerry.Bell@noaa.gov
Dr. Jae Schemm, Meteorologist; Jae.Schemm@noaa.gov
Dr. Tingzhuang Yan, Meteorologist; Tingzhuang.Yan@noaa.gov
National Hurricane Center
Eric Blake, Hurricane Specialist; Eric.S.Blake@noaa.gov
Todd Kimberlain, Meteorologist; Todd Kimberlain@noaa.gov
Dr. Chris Landsea, Meteorologist; Chris.Landsea@noaa.gov
Dr. Richard Pasch, Hurricane Specialist; Richard.J.Pasch@noaa.gov
Hurricane Research Division
Stanley Goldenberg, Meteorologist; Stanley.Goldenberg@noaa.gov
REFERENCES
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.
Bell, G. D., and Co-authors 2007: The 2006 Atlantic Hurricane Season: A Climate Perspective. State of the Climate in 2006. A. M. Waple and J. H. Lawrimore, Eds. Bull. Amer. Meteor. Soc., 88, S1-S78.
Bell, G. D., and Co-authors 2008: The 2007 Atlantic Hurricane Season: A Climate Perspective. State of the Climate in 2007. A. M. Waple and J. H. Lawrimore, Eds. Bull. Amer. Meteor. Soc., 89, S1-S78.
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.
Gray, W. M., 1984: Atlantic seasonal hurricane frequency: Part I: El Niño and 30-mb quasi-bienniel oscillation influences. Mon. Wea. Rev., 112, 1649-1668.
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