The likelihood of competing climate factors leads to several physical scenarios for the hurricane season. Some scenarios are more likely than others, and these are reflected in our stated probabilities of expected activity. Based on these scenarios, we estimate a 70% probability for each of the following seasonal ranges:
- 9-14 Named Storms,
- 4-7 Hurricanes
- 1-3 Major Hurricanes,
- An ACE range of 65%-130% of the median
These likely seasonal 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 range of activity seen in those past seasons
NOTE:
This outlook provides a general guide to the expected overall nature of the upcoming hurricane season.
This outlook is not a seasonal hurricane landfall forecast, and it does not imply levels of activity for any particular region.
Residents, businesses, and government agencies of coastal and near-coastal regions are urged to prepare for every hurricane season regardless of the seasonal outlook. It is not how busy a season is that matters for a particular location, but simply if that location is impacted – something which can happen in any year.
DISCUSSION
1. Science behind the 2009 Outlook
This Outlook is a general guide to the expected overall activity during the 2009 Atlantic hurricane season. The outlook not a seasonal hurricane landfall forecast, and it does not imply levels of activity for any particular area.
The 2009 seasonal hurricane outlook primarily reflects combined impacts from three climate factors. These are the tropical multi-decadal signal (which reflects conditions associated with the ongoing high-activity era), the El Niño/ La Niña cycle (ENSO), and tropical Atlantic SSTs.
A main climate factor guiding this outlook is the continuation of atmospheric conditions that have been in place since 1995 in association with the ongoing high-activity era in the Atlantic Basin. Many aspects of this multi-decadal signal are developing as expected, including an enhanced African monsoon system, higher-than-average pressure and easterly wind anomalies in the upper atmosphere over the tropical Atlantic Ocean and Africa, and westerly wind anomalies at 700-hPa across the tropical Atlantic Ocean.
A second factor guiding this outlook is the likelihood of either ENSO-neutral conditions (no El Niño or La Niña) or the development of El Niño prior to or during August-October (ASO-the peak months of the season). ENSO-neutral conditions are now present. However, there is a large spread in the computer forecast models used to predict ENSO, and most have historically shown little forecast skill at this time of the year. Most statistical models indicate a continuation of ENSO-neutral conditions through the summer and fall. However, some dynamical models predict that El Niño will develop. If El Niño does develop prior to or during ASO, atmospheric circulation patterns over the tropical Atlantic and Caribbean Sea could become less conducive for hurricane development (Gray 1984).
A third factor guiding the outlook is the possibility for the currently cooler-than-average SSTs in the eastern tropical Atlantic to persist through ASO. ASO SSTs in the eastern tropical Atlantic have not been below average since 1997. Cooler SSTs in that region are typically associated with a reduction in Atlantic hurricane activity.
2. Expected 2009 activity
With the potential for competing climate factors this hurricane season, several physical scenarios are possible, and the likelihood of these scenarios depends on the confidence in the expected strength and impacts of the climate factors. Presently, there is considerable uncertainty in the available SST forecasts for both the Atlantic and Pacific basins, which makes it difficult to be precise about the extent to which these factors might offset or even outweigh the multi-decadal signal.
Nonetheless, some physical scenarios are more likely than others, and these are reflected in our probabilities of season type. The outlook indicates a near-normal season is most likely (50%), which reflects the possibility that either a developing El Niño and/or cooler SSTs in the eastern tropical Atlantic could offset the multi-decadal signal. There is a 25% chance of an above-normal season, which reflects the ongoing high-activity era, the possibility that El Niño will not develop and impact the season, and the possibility that eastern Atlantic SSTs will return to normal. This 25% chance of an above-normal season is well below last years’ (65%) forecast.
Conversely, there is also a 25% chance of a below-normal hurricane season. This reflects the possibility that El Niño and/or cooler SSTs could significantly reduce the activity. It also reflects climate model forecasts from promising new models such as NOAA’s Climate Forecast System (CFS) and the European Center for Medium-Range Weather Forecasts Global Climate Model System 3, which are also indicating a below-normal season is possible.
An important measure of the total overall 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 scenarios, we estimate a 70% chance the 2009 seasonal ACE range will be 65%-130% of the median. A value above 117% of the median reflects an above-normal season, and a value below 75% reflects a below-normal season.
Also based on the above scenarios, the likely (70% chance) ranges of activity for 2009: are 9-14 named storms, 4-7 hurricanes, and 1-3 major hurricanes.
3. Further analysis of the main climate factors and uncertainties
a. The Ongoing High Activity Era in the Atlantic Basin
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 high-activity era began in 1995. Hurricane seasons during 1995-2008 have averaged about 15 named storms, 8 hurricanes, and 4 major hurricanes, with an ACE index of 167% of the median. NOAA classifies ten of the fourteen seasons since 1995 as above normal, with seven being hyperactive (ACE > 175% of median). Only four seasons since 1995 have not been above normal, which include three El Niño years (1997, 2002, and 2006) and the 2007 season.
This high level of activity since 1995 contrasts sharply to the 1971-1994 low-activity era (Goldenberg et al. 2001), which averaged only 8.5 named storms, 5 hurricanes, and 1.5 major hurricanes with an 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.
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 the multi-decadal signal accounts for the transition in 1995 from the low-activity to the high-activity era. The conditions associated with the multi-decadal signal are again a major factor guiding the 2009 outlook. A key feature of this signal now developing is an enhanced African monsoon system.
Other regional features of the multi-decadal signal now in place include 1) an amplified ridge at upper levels across the central and eastern subtropical North Atlantic, 2) extensive easterly wind anomalies in the upper atmosphere and weaker easterly winds in the middle atmosphere across the tropical Atlantic. In the MDR, these conditions typically lead to reduced vertical wind shear and a conducive 700-hPa circulation, which typically results in a configuration of the African easterly jet that is more conducive to hurricane development from tropical waves moving westward from the African coast.
b. ENSO (The El Niño/ La Niña cycle)
During a high-activity era such as the present, the occurrence of La Niña greatly increases the probability of an above-normal season, and the occurrence of El Niño often results in a near-normal or occasionally below-normal season. An important source of uncertainty in the 2009 outlook is the large spread in the computer model forecasts for ENSO. Most statistical models indicate a continuation of ENSO-neutral conditions through the summer, including ASO. However, some dynamical models are predicting El Niño to develop. Our outlook ranges reflect this uncertainty in whether or not El Niño develops.
c. Atlantic SSTs and trade winds
Seasonal Atlantic hurricane activity can be sensitive to SST departures in the eastern tropical Atlantic. Below average SSTs in that region are usually associated with a less conducive wind pattern coming off the west coast of Africa and with less conducive thermodynamics, both of which can suppress hurricane activity.
SSTs in the eastern tropical Atlantic are currently cooler than average. At the same time, SSTs are warmer-than-average south of the equator. This pattern is defined as the negative phase of the Atlantic Meridional Mode (Chiang and Vimont 2004), and is not conducive to an active hurricane season. For example, it is presently associated with a weaker Atlantic ITCZ that is shifted south of normal. These conditions can sometimes be quite persistent, since the associated northeasterly wind anomalies can help to maintain the cooler SSTs in the eastern tropical Atlantic. The 2009 outlook reflects the likelihood of below- to near-average SSTs in the eastern tropical Atlantic during ASO. However, there is considerable uncertainty in forecasting SSTs for this region so far in advance.
Also of interest is an extensive region of enhanced trade winds across much of the MDR. These winds developed in association with an extensive and persistent mid-latitude ridge over the central and eastern North Atlantic during February-April, and thus should be distinguished from the pattern in the equatorial eastern Atlantic. These mid-latitude conditions were particularly prominent during April, and have remained so during the last 30 days. It is too early to predict whether these conditions will continue into ASO.
NOAA FORECASTERS
Climate Prediction Center
Dr. Gerry Bell, Meteorologist; Gerry.Bell@noaa.gov
Dr. Jae Schemm, Meteorologist; Jae.Schemm@noaa.gov
National Hurricane Center
Eric Blake, Hurricane Specialist; Eric.S.Blake@noaa.gov
Todd Kimberlain, Hurricane Specialist; 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. of Climate. 19, 590-612.
Chiang, J. C. H., and D. J. Vimont: Analogous Pacific and Atlantic meridional modes of tropical atmosphere-ocean variability. J. of Climate, 17, 4143-4158
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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