Hurricanes start as low pressure disturbances. Winds from surrounding warm areas rush in to replace the rising, warm air in the low pressure system. As the systems move over warm waters they are strengthened by the warm air and moisture which rises and condenses high in the atmosphere forming thunderstorms. Warm air from the surrounding area rushes in to replace the rising air at an increased rate and the cycle continues. The center of the storm is an area of low pressure and is extremely calm compared to the outer areas of the storm especially the eye wall where the storm is strongest. The northeast quadrant of hurricanes in the northern Atlantic tends to be where the strongest winds are located.
Caribbean people are very familiar with the destructive power of tropical storms and hurricanes. The hurricane season is one of nervous anxiety for disaster managers and citizens alike. Hurricane Season 2010 is upon us and it seems like just yesterday we were able to breathe a sigh of relief that the 2009 season had ended. The 2009 season was quiet compared to some of the previous years that were record breaking, like 2005. Most of the region was spared any direct hits last year except for Guadeloupe according to Wikipedia (http://en.wikipedia.org/wiki/2009_Atlantic_hurricane_season). Wind shear from the El Niño event in the Pacific Ocean was linked to the quiet season. The team of Klotzbach and Gray (2009), one of the premier hurricane activity forecasters based at Colorado State University, forecasted an average season for 2009 due the predicted formation of an El Niño event in the Pacific Ocean and cool sea surface temperatures in the tropical Atlantic Ocean. The season turned out to have below average activity. Thus far, experts are predicting above normal activity for the 2010 season.
Predicting future hurricane seasons is inherently difficult given all the factors involved and their variability. Scientists examine past seasons for which records are available and the conditions that existed then, current conditions and future expected conditions and run mathematical models and computer simulations. Some predictions have already been made for the upcoming season and more will be released and/or revised as June approaches and as the season progresses. Klotzbach and Gray, Wilkens Weather Technologies and the National Oceanic and Atmospheric Administration (NOAA) have already issued forecasts for the season. Klotzbach and Gray recently revised their December 2009 predictions for the 2010 Atlantic hurricane season. In December, the prediction was for 11-16 named storms, 6-8 hurricanes and 3-5 major storms. In April 2010 these estimates were revised to 15 named storms, 8 hurricanes and 4 major storms. Their revision was based on the expected weakening of the current El Niño event in the Pacific Ocean, as illustrated by several statistical and dynamic models, and the observed warming of the tropical Atlantic Ocean in the area where storms usually develop. Klotzbach and Gray (2010, 4) point out that “[w]e believe that these two features will lead to favorable dynamic and thermodynamic conditions for hurricane formation and intensification.” They note that the unusually warm waters in the eastern subtropical Atlantic Ocean is due to cooler waters in the south Atlantic Ocean, weaker-than-normal high pressure in the Azores and lower sea level pressures. This situation results in reduced Northeast Trade winds and less mixing in the ocean hence, higher sea surface temperatures. These conditions are usually symbolic of an active hurricane season.
Wilkens Weather Technologies (2010), based in Houston, Texas, also foresees an active season. They predict 13 named storms, 9 hurricanes and 5 major storms. They list some of the factors influencing the formation of hurricanes as the sea surface temperatures in the Atlantic Basin, the presence of an El Niño event in the Pacific Ocean, dust from dust storms in Africa and even conditions in the Arctic region. Wilkens Weather Technologies (2010, 1) points out that “[s]ea surface temperature (SSTs), both in the Atlantic Ocean and in the equatorial Pacific Ocean, have a large impact on the tropical cyclone activity in the North Atlantic Basin each year.” Two reasons cited as to why last season was not very active were warmer sea surface temperatures in the Pacific associated with an El Niño event which tends to reduce storm formation in the Atlantic region due to wind shear as well as dust originating in Africa travelling west across the Atlantic Ocean. As Klotzbach and Gray, Wilkens Weather Technologies (2010) point to current El Niño conditions that are expected to subside in the coming months and abnormally high sea surface temperatures in a large area of the Atlantic Ocean where storms develop as indicators of an active season. They point out that currently the tropical, subtropical North Atlantic Ocean and the Caribbean Sea are displaying above normal sea surface temperatures and storms are likely to develop in those areas early in the season with other areas warming up later. However, they also indicate that there is a possibility that the observed warming in the tropical North Atlantic Ocean could dissipate during the season which could have a significant impact on the level of activity.
NOAA (2010) recently released their 2010 hurricane season predictions and they also foresee a very active season. They predict 14-23 named storms, 8-14 hurricanes and 3-7 major hurricanes. As with the other predictions, NOAA points to the above normal sea surface temperatures in the main development area in the Atlantic and dissipation of El Niño conditions in the Pacific as important factors influencing this years’ hurricane season. They also point to a multi-decadal variability in Atlantic basin hurricane activity where a set of features that result in increased hurricane activity are present. These features include reduce trade winds, a western Africa jet stream that favors storm development, reduced wind shear and low sea level pressure. A period of increased activity began in 1995 and since that time the majority of hurricane seasons have shown above normal activity and some have been extremely active. Similar conditions and cyclone activity were present during the 1950-1969 period. The period between 1971 and 1994, by contrast, displayed less activity. Additionally, they note that sea surface temperatures in the development area were 1.50C above normal in April and that record temperatures were experienced in the Caribbean in April.
Most of the islands in the eastern Caribbean are emerging from a drought which began last year and have been experiencing high temperatures. These conditions are likely associated with reduced trade winds and higher sea surface temperatures.
There have been some memorable storms and hurricanes in the past few decades that may be etched into people’s minds in the Caribbean, not only for the local and regional devastation but for the impact elsewhere. Andrew in the early 1990s, Mitch in 1998 and more recently Ivan and who can forget Katrina. Haiti had to deal with a series of storms, Guadeloupe experienced a direct hit last year and several islands have felt the effects of passing systems. No doubt people may want to associate climate change to the cyclone activity of the past few decades because there has been much talk on the topic. Klotzbach and Gray (2010) report that even though there has been an increase in cyclonic activity in the Atlantic Basin over the past 15 years compared to the 1970 -1994 period it is not unlike the activity experienced during the period 1950 to 1964 when the conditions of the Atlantic Ocean were similar to those being observed currently. They acknowledge that global temperatures have increased but note that there is no reliable link between the temperature increases and cyclonic activity. They link the variability in activity in the Atlantic Basin to the strength of the Thermohaline Circulation (THC) which affects the trade winds, wind shear and sea surface temperature. As a result, the current temperature anomaly is believed to have less to do with climate change. The THC is a wind driven, saline-dependent conveyor belt that transfers warm surface waters from tropical areas towards the poles where it cools, sinks and flows deep in oceans back towards the warm areas where it rises and the cycle continues. Klotzbach and Gray (2010) contend that the activity seen in the past recent seasons were within the expected ranges of variation.
The United States Climate Change Science Program, in a report from 2008 examined weather extremes in a changing climate Kunkel et al (2008) note that cyclonic activity has increased in the Atlantic Ocean, but globally numbers have been stable since satellite use began in the late 1960s. They acknowledge, however, that there is significant difficulty in establishing trends due to a lack of reliable historical data. The increased cyclone activity in the Atlantic Basin has occurred when sea surface temperatures were increasing. Historically, data was collected primarily for forecasting and not only until the 1970s was data collected for evaluating climate variability, so establishing trends is difficult from a relatively short four decade period. Additionally, monitoring, tracking and forecasting hurricanes is relatively new and is not consistent from region to region and agency to agency this, therefore, makes recognizing trends complicated. It was also indicated that there is evidence to support a shift of the development area in the Atlantic Basin and acknowledge decade to decade fluctuations in activity. Additionally, the report points to some studies that have recently linked greenhouse gases and aerosol to observed changes.
While some are reluctant to link climate change to increased hurricane activity others attest to the relationship. The Union of Concerned Scientists (2006) on their website notes that “[r]ecent scientific evidence suggests a link between the destructive power (or intensity) of hurricanes and higher ocean temperatures, driven in large part by global warming.” They point out that warmer tropical oceans and increased water vapor over oceans have contributed to more intense storms. They cite two studies that note that the destructiveness of cyclones has increased globally over the past three decades and that there has been an increase in stronger storms during the same period when oceans were observed to be warming.
The Intergovernmental Panel on Climate Change (IPCC) in its Fourth Assessment Report (2007) recognizes the role of El Niño and variability in ocean basins in the distribution of hurricanes across the globe. They note that a reduction of activity in one area is usually accompanied by an increase in another area. They, however, acknowledge that more storms have been attaining Categories 4 and 5 since the 1970s and that the Power Dissipation Index, a measure of intensity, duration, and frequency has increased due to increases in duration and intensity. The report (2007, 692) indicates that “[i]n the Caribbean, hurricane activity was greater from the 1930s to the 1960s, in comparison with the 1970s and 1980s and the first half of the 1990s. Beginning with 1995, all but two Atlantic hurricane seasons have been above normal (relative to the 1981-2000 baseline). The exceptions are the two El Niño years of 1997 and 2002.” It is likely that Hurricane Season 2009 will be added as a year with below normal activity due to El Niño in future reports.
The advancement of scientific methods and equipment in conjunction with computer technology will make predicting more reliable as more and more data will be gathered from each passing season, hence, more data will be available for analysis and comparison. One of the agencies in the United States responsible for hurricane monitoring, tracking and forecasting, NOAA, will begin using a new hurricane scale, the Saffir-Simpson Hurricane Wind Scale, this season. The new scale will account for variables that affect storm surge in an effort to better predict flooding according to a February 2010 release from NOAA. In the past, storm surge used to be estimated based on the category of the storm but this system was flawed. The release notes that in 2008 there was a 15 to 20 feet storm surge associated with Hurricane Ike, a Category 2 storm, while the Category 4 storm, Charley, produce only 6 to 7 feet surges. The new scale will take into account intensity, size, motion, barometric pressure, depth of near shore area and topography of coastal areas.
Variability in tropical cyclone activity is to be expected as climate is variable. As discussed, oscillations within the Atlantic Basin may play a role in the sea surface temperature anomalies that are linked to hurricane activity. Most experts acknowledge that El Niño significantly impacts storm development in the Atlantic Basin. As the debate continues on climate change and the effects of anthropogenic carbon, so will the debate on the variability observed in tropical cyclone activity. Perhaps in the future with improved technology, more reliable data and when more is known about the complex relationship between cyclones and the water bodies over which they develop and human impacts on climate will experts be more willing to attest to the climate change-cyclone activity link. Until such time, those prone to tropical storms and hurricanes will likely be paying close attention to real-time weather forecasts between June 1st and November 30th , hoping for below predicted activity.