July 2011, Vol. 238 No. 7

Features

Active Hurricane Season Forecast For 2011 On U.S. Gulf Coast

Over the past two years, the U.S. has been fortunate with regards to land falling tropical systems. In 2009, only two tropical systems made landfall along the Gulf Coast. Claudette made landfall as a weak tropical storm and Hurricane Ida weakened to tropical storm status as it approached the Mississippi Delta in early November.

In 2010, no tropical systems made landfall along the U.S. Gulf Coast. The year 2008, however, was a different story. Five tropical systems made land fall along the U.S. Gulf coast, three of which were hurricanes. Hurricanes Gustav and Ike, both strong Category 2 hurricanes, caused a shutdown of significant portions of the pipeline infrastructure, processing and production along the Gulf Coast in East Texas and Louisiana. The Minerals Management Service estimated that 2,127 of the 3,800 total oil and natural gas production platforms in the Gulf of Mexico were exposed to hurricane conditions, with winds greater than 74 miles per hour, from Gustav and Ike. A total of 60 platforms were destroyed as a result of these two storms. One oil and 14 natural gas pipeline systems reported damage from these two hurricanes.

Another active hurricane season is forecast for the Atlantic basin in 2011. The U.S. Gulf Coast will be at a higher-than-normal risk for impacts from these tropical systems. The possibility of higher risk also comes at the busiest part of the hurricane season, late August through mid-September. These factors will put pipeline systems at a higher risk for potential damage during this upcoming season.

Atlantic Multidecadal Oscillation
Recent research is indicating that cycles of tropical activity are associated with the Atlantic Multidecadal Oscillation (AMO). The AMO is a naturally occurring pattern over the Atlantic basin typified by variations in both the sea surface temperature and sea level pressure fields. It is theorized that the AMO is linked to fluctuations in the strength of the large-scale oceanic circulation across the Atlantic Ocean. When the circulation is stronger than normal, the AMO tends to be in its warm phase. The warm phase of the AMO is associated with weaker trade winds and warmer sea surface temperatures (SSTs).

The AMO has been in a warm phase since 1995 and this has coincided with more active tropical seasons across the North Atlantic basin. The AMO is on a 23-month run of positive values, i.e., the warm phase. The last cold AMO phase values were in early 2009, which preceded an Atlantic tropical season with nine named storms, three hurricanes and two major hurricanes. While the lifespan of an individual phase is uncertain, a complete cycle is estimated to last roughly 60-70 years. The implication is that more active tropical seasons for the North Atlantic basin may last another 10-20 years. This is one of the key factors in Wilkens Weather’s 2011 hurricane season outlook.

Sea Surface Temperatures
Sea surface temperatures (SSTs), in the Atlantic Ocean and in the equatorial Pacific Ocean, have a large impact on the tropical cyclone activity across the North Atlantic basin each year. For the Atlantic Ocean, warmer SSTs lead to 1) higher evaporation rates, 2) a more active Intertropical Convergence Zone (ITCZ) and 3) a northward shift in the ITCZ. These factors combine to yield a more active tropical season. Across the equatorial Pacific Ocean, the variations in SSTs are known as El Niño and La Niña.

During a La Niña event, the vertical wind shear (i.e., the difference between the upper-level and lower-level winds) is reduced across the North Atlantic basin. In this reduced shear environment, the potential for tropical development across the North Atlantic basin increases and decreases for El Niño events.

The equatorial Pacific is transitioning out of its La Niña phase. Both climate forecasts and oceanographic measurements indicate the region will continue to transition to a neutral state over the next several months. Across the tropical North Atlantic Ocean, the above-normal SSTs currently in place are forecast to last through the 2011 hurricane season. This outlook is consistent with the warm phase of the AMO, the recent pattern for North Atlantic.

The September forecast shows sea surface temperatures rising to over 1°C above normal across the central and western Caribbean Sea. From a hurricane formation standpoint, this area is one of two major areas of storm formation during these months; the other is near the Cape Verde Islands. The warmer SST forecast for the Caribbean Sea may cause an above-average number of storms to develop in this region. Caribbean storms generally track in a westerly direction toward the Yucatan Peninsula or the Gulf of Mexico. Both of these factors could result in a larger risk to the pipelines and gathering systems in these areas.

Sahel Africa Rainfall And African Dust
Massive clouds of dust are transported across the Atlantic Ocean during the summer months after being lifted from the western reaches of the Sahara and Sahel deserts. These clouds may be seen on visible satellite images during hurricane season and the dust fallout is experienced as far west as the Caribbean Sea.

The role that the Sahel rainfall plays in the hurricane season forecast is that dust acts as an inhibitor to potential tropical development for several reasons. When drier winter and spring seasons are experienced across the Sahel, more dust is lifted into the atmosphere, thereby negatively impacting tropical development. Conversely, a wet winter and spring will result in less dust moving across the Atlantic, thereby minimizing this negative influence.

The La Niña phase across the equatorial Pacific typically brings heavy rainfall and flooding to West Africa during the winter months which has been the case this past winter. As noted above, the heavier rain will result in less dust being transported across the Atlantic and less of an impact of the hurricane season.

Arctic Oscillation – Impact On Storm Tracks
While the Arctic Oscillation (AO) does not play a role in the number of named storms forecast, it is a factor in evaluating the probability of a land-falling storm. The AO is an atmospheric circulation pattern between the polar and middle latitudes (above 45°N) that strengthens and weakens the winds around the Arctic. Typically, this pattern gains more attention during the winter months as it is used in the prediction of cold air outbreaks. However, the AO remains a primary factor in North American weather through the summer months.

The long-range forecast shows a below-normal temperature pattern extending from the Canadian Prairies to the Great Lakes for June through August. This pattern is similar to the temperature pattern associated with a negative AO phase for these months.

If these below-normal temperatures are indeed a result of a negative AO phase, then the probability for a tropical system in the Gulf of Mexico or western Caribbean Sea is lower than normal for June through August. The negative AO phase is associated with lower-than-normal pressures across the central Atlantic. The weaker subtropical ridge would allow tropical systems to turn north prior to making landfall along the East Coast.

The AO has generally been in a negative phase since summer 2009. The continuation of the negative phase through at least summer 2011 is not unlikely and may be implied in the forecast model output. Therefore, Wilkens Weather anticipates that early season (June through August) tropical systems will be more likely to remain in the North Atlantic rather than move into the Caribbean Sea or the Gulf of Mexico. The exception would be storms that form over the Caribbean Sea or the Gulf of Mexico.

Things are forecast to change in September. First, the below-normal temperatures that were shown in the June through August output are no longer as evident for September. Second, the SSTs across the central and eastern Caribbean Sea are forecast to rise to at least 1°C above normal. The combination of these factors would lead to more tropical systems being formed over the Caribbean Sea and these would be more likely to move in a westerly fashion toward the Yucatan Peninsula and Gulf of Mexico. In addition, September is typically the most active month of the hurricane season. These two factors could increase the amount of tropical activity across the Gulf of Mexico during the upcoming season.

Conclusion
The Wilkens Weather team has examined all of the factors discussed in this report and our forecast for the 2011 North Atlantic hurricane season is presented in Table 1.

Wilkens Weather Team North Atlantic Hurricane Season Forecast For 2011

The forecast of a more active season, specifically across the Gulf Coast, will expose pipeline and gathering systems to a higher risk for damage. While planning and preparation for a more active hurricane season does not remove all risks, it will help reduce the impacts of the inevitable damage.

Author
Aaron Studwell is a senior meteorologist for Wilkens Weather Technologies, Houston. His work includes worldwide tropical forecasting, conducting research to improve the accuracy of tropical forecasts and coordinating business development efforts. He has more than 10 years of experience in energy weather forecasting and natural gas analysis. He was one of the first meteorologists to work in the energy trading sector. He graduated from the University of Michigan with a B.S. degree in aerospace engineering and from Texas A&M University with a master’s degree in meteorology. He can be reached at 713-430-7100 or aaron.studwell@wilkensweather.com.