Summary of 2007 atlantic tropical cyclone activity and verification of author’s seasonal and monthly forecasts
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- 8.5 October-November Discussion
- 9 Comparison of the last four hurricane seasons of 2004, 2005, 2006 and 2007
- 9.1 Why did October 2007 have low NTC activity when a La Niña was present
- 9.2 Why was October 2005 so active when the three Octobers of 2004, 2006 and 2007 were so inactive
- 9.3 Why was July 2005 so extremely active when the July periods of 2004, 2006 and 2007 were not
- 9.4 Why were the four months of July-August-September-October 2005 the most active four consecutive months on record
- 9.5 Why was August-September 2004 the most active two-month tropical cyclone period on record
- 9.6 Why were 2006 and 2007 so similar in overall TC activity when ENSO conditions in these two years were so different
- 10 Was Global Warming Responsible for the Large Upswing in 2004-2005 US Hurricane Landfalls
- YEARS Named Storms Hurricanes
It is interesting to note, for example, that September 2007 had twice as many named storm formations as did September 2004. However, September 2004 had more than three times the number of named storm days and nearly three times the level of Net Tropical Cyclone (NTC) activity that 2007 had. The average named storm in September 2004 lasted approximately 13 times longer than did the average named storm in September 2007. 8.5 October-November Discussion Approximately six times more tropical cyclone activity occurs in an average season in October than in November, so this discussion will focus on October. It is very difficult to explain what happened in October 2007. In general, La Niña conditions lead to very active Octobers, and a moderate La Niña was underway during this October. Table 13 displays the observed October NTC in La Niña years (defined as observed October Nino 3.4 < -0.5°C) in the active AMO years (defined as 1950-1969, 1995-2007). All La Niña years prior to 2007 had above-average October values of NTC (1950-2000 average October NTC value is 18). Obviously, NTC in October 2007 was well below the long-term average with only an NTC of 3 being observed during the month. But, clearly based upon all other La Niña Octobers prior to this October in an active AMO era, precedent dictated forecasting an active month. Table 13: Observed October Net Tropical Cyclone activity values in La Niña years during active AMO phases.
We are still struggling to understand why October was not more active. As noted earlier in the verification, vertical wind shear levels were well below their long-period averages across the Caribbean during the month. Typically, October storms develop and intensify in the Caribbean (Figure 19), and conditions were quite favorable for development in the Caribbean during October. 
Figure 19: Typical tracks of October Atlantic basin tropical cyclones. Note that the most typical formation area for storms in October is the western Caribbean. Figure courtesy of the National Oceanic and Atmospheric Administration (NOAA). Deep convection was also plentiful across the Caribbean throughout October. Brightness temperature values were cooler than normal throughout the month, indicating enhanced levels of intense thunderstorm activity (Figure 20). 
Figure 20: Caribbean brightness temperatures from January-October. Note that average brightness temperatures were much cooler than normal in October 2007, indicating enhanced levels of deep convection. Since unfavorable conditions for genesis do not seem to have been the inhibiting factor for the lack of activity in October 2007, we believe that it may be due to the size of the disturbances that formed, along with the proximity of land to where vorticity was most pronounced. A large cyclonic circulation was evident over most of the western Caribbean during the first half of October. This system drifted over land as it was beginning to get organized. Another large disturbance located over the Bahamas in early October had large amounts of deep convection and favorable upper-level winds for several days. However, this system was never able to concentrate its large-scale vorticity. Another small area of low pressure had a very organized low-level circulation but little deep convection as it tracked across the northern Gulf of Mexico. Convection finally began to fire as the system reached the coastline. The pronounced upper-level low near the islands in September persisted through October and may have also played a role in reducing October’s hurricane activity. More discussion on October 2007 follows in Section 9. The October over-forecast illustrates that there are certainly challenges left to be solved when it comes to predicting hurricane activity. Statistical genesis forecasts indicated well above-average chances of formation in October, and many of the dynamical models spun up several tropical cyclones during the month. The 2007 hurricane season illustrates that there are still many lessons to be learned about tropical cyclone genesis and intensification. The average November witnesses a total of 0.5 named storms, 0.3 hurricanes and 0.1 major hurricanes. The first two days of November witnessed the intensification of Noel into a hurricane followed by its extra-tropical transition. After Noel became an extra-tropical cyclone, no other tropical cyclone activity was recorded during the month. 8.6 Track Differences As evident from Figure 1, both major hurricanes in 2007 developed in the tropical Atlantic and had long westward tracks, staying south of the United States. Both Dean and Felix formed and remained at low latitudes throughout their life span. All tropical cyclones that formed south of 20°N (except for Noel) remained south of 20°N until they either made landfall or dissipated. The 2006 hurricane season was notable for its re-curvature, while 2007 was notable for its straight-moving systems. Figure 21 displays the difference in the 500 mb height pattern that was present from August 1 – September 10, 2007 from the 500 mb height pattern that was present from August 1 – September 10, 2006. The anomalous high heights (ridging) over the southeastern United States helped force both Dean and Felix to track westward across the Caribbean into Central America. 
Figure 21: 500 mb geopotential height difference between August 1 – September 10, 2007 and August 1 – September 10, 2006. As noted previously, there tended to be an anomalous upper-level low across the central Atlantic during most of September and October. This upper low tended to re-curve storms towards the north and sheared them apart before they left the tropics. 9 Comparison of the last four hurricane seasons of 2004, 2005, 2006 and 2007 The last four hurricane seasons have been notable for:
The character of the hurricane activity in 2004 and 2005 seasons was dissimilar. Nearly all TC activity during the 2004 season was concentrated in the two months of August and September. No previous hurricane season has had so much activity concentrated in August-September. By contrast, 2005, the most active hurricane season on record, had elevated levels of activity from June through December. There had never been a previous season with as much TC activity before August as the season of 2005. Yet, even though well above the long-period average, August-September NTC in 2005 was only half of the August-September NTC in 2004. In addition to the fact that both 2006 and 2007 rendered little damage in the United States, both the 2006 and 2007 seasons had similar levels of total Atlantic basin activity. Both seasons had two major hurricanes. 2006 had five hurricanes, while 2007 witnessed six hurricanes. The 2006 and 2007 hurricane seasons were similar to 2004 in that nearly all activity was concentrated in August and September. However, NTC in 2006 and 2007 in the two months of August-September was only about a third of that experienced in August-September 2004. Table 14 lists the amount of Net Tropical Cyclone (NTC) activity by month from June-October for the last four years. Nearly all TC activity in the three seasons of 2004, 2006, and 2007 occurred during the months of August and September. Even though 2006 and 2007 had very similar amounts of August-September NTC activity (77 for 2006 and 81 for 2007), ENSO conditions for these two seasons was very different. An El Niño was present during these months in 2006, while a La Niña event was present in 2007. We seldom see similar amounts of hurricane activity in two seasons with such large differences in ENSO conditions. There were compensating negative factors in the Atlantic which prevented the 2007 season from being as active as we expected it to be given the fact that we had a moderately strong La Niña this year. Table 14 – The last four years of Net Tropical Cyclone (NTC) activity by month from June-October.
To understand the activity variations in these last four seasons we must answer the following questions.
Although it was impossible for us to have well predicted each of these individual seasons and individual monthly variations in NTC it is possible in post analysis to suggest likely physical causes for these differences. The following is our current best suggested explanation for the six questions listed above. 9.1 Why did October 2007 have low NTC activity when a La Niña was present? The 2007 October period did not have the usual teleconnection patterns from the eastern Pacific which occur in most La Niña years. A nearly stationary strong baroclinic trough extended from the middle latitudes at 50°W into the deep tropics near central Venezuela. This trough increased low latitude wind shear to its east, preventing systems from the east penetrating through it. The 2007 October monthly hemispheric westerly wind pattern showed a five wave stationary pattern. This near stationary wave pattern produced a strong trough and broadly unfavorable cyclonic conditions at upper tropospheric levels throughout the whole western Atlantic. Low latitude tropical systems moving through this trough encountered strong upper level northeasterly winds on the back side of the trough. These northeasterly winds inhibited cyclone formation and/or maintenance by disrupting upper-level outflow. Southern upper level winds to the west of a system are important ingredient in aiding upper-level outflow. This strong and deep baroclinic trough thus led to broad-scale upper tropospheric cyclonic conditions which are known to be unfavorable for tropical cyclone formation. In addition, 200 mb zonal wind conditions within the western Atlantic were stronger out of the west than in the typical La Niña year. 9.2 Why was October 2005 so active when the three Octobers of 2004, 2006 and 2007 were so inactive? The 200 mb westerlies and the 200 mb minus 850 mb zonal wind shear across the Caribbean and the tropical Atlantic in October 2005 were weaker than in the three other years. Caribbean low level winds were significantly stronger from the west – giving favorable higher low-level vorticity during October 2005 than occurred during October 2004, 2006 and 2007. Upper level anticyclonic vorticity in October 2005 was also more favorable than in the three other years. In addition, October tropical Atlantic SSTs were higher in 2005 than they were in 2004, 2006 or 2007. 9.3 Why was July 2005 so extremely active when the July periods of 2004, 2006 and 2007 were not? July 2005 saw the formation of two major hurricanes and three other tropical cyclones. Since reliable records began in the mid 1940s, no other July has seen this amount of activity. Sea surface temperatures (SST) in the tropical Atlantic in July 2005 were higher than in any year since 1950. In addition to SST, a number of other factors led to July 2005 being the most active July on record. Both low level cyclonic vorticity and upper level anti-cyclonic vorticity in the western half of the Atlantic basin were the highest on record according to the NOAA/NCEP reanalysis data records which extend back to 1948. 9.4 Why were the four months of July-August-September-October 2005 the most active four consecutive months on record? The above-discussed factors which made July and October 2005 so active were also generally applicable to the months of August and September. It is unusual to have such favorable conditions for tropical cyclone development and intensification extending for four consecutive months. 9.5 Why was August-September 2004 the most active two-month tropical cyclone period on record? The eastern Atlantic SST pattern off of the African coast during August-September was observed to be the highest on record. Extremely favorable lower and upper tropospheric wind patterns also existed in the western half of the tropical Atlantic. These favorable August-September West Atlantic conditions even exceeded those of 2005 for this two month period. As previously discussed (Klotzbach and Gray 2006), the eastern Atlantic monsoon trough in August-September 2004 was unusually strong, leading to the generation of many tropical disturbances. The positioning of a mid-latitude ridge over southeastern Canada assured that most of the storms that formed in August-September of 2004 in the central Atlantic would have long westward tracks. These long tracks led to the great buildup of NTC values in these months. 9.6 Why were 2006 and 2007 so similar in overall TC activity when ENSO conditions in these two years were so different? This was due to the fact that, despite La Niña conditions in the Pacific, conditions in the Atlantic in 2007 were more unfavorable than the observed Atlantic conditions during the El Nino year of 2006. The unfavorable 2007 Atlantic conditions tended to cancel out the positive influence of the La Niña in 2007. The SST conditions of the eastern tropical Atlantic were significantly colder than in 2006. Despite general La Niña conditions in 2007, the central Atlantic showed stronger upper-level westerly winds and more upper-level cyclonic vorticity in 2007 compared to 2006. 10 Was Global Warming Responsible for the Large Upswing in 2004-2005 US Hurricane Landfalls? The U.S. landfall of major hurricanes Dennis, Katrina, Rita and Wilma in 2005 and the four Florida landfalling hurricanes of 2004 (Charley, Frances, Ivan and Jeanne) raised questions about the possible role that global warming played in these two unusually destructive seasons. The global warming arguments have been given much attention by many media references to recent papers claiming to show such a linkage. Despite the global warming of the sea surface that has taken place over the last 3 decades, the global numbers of hurricanes and their intensity have not shown increases in recent years except for the Atlantic (Klotzbach 2006). The Atlantic has seen a very large increase in major hurricanes during the 13-year period of 1995-2007 (average 3.8 per year) in comparison to the prior 25-year period of 1970-1994 (average 1.5 per year). This large increase in Atlantic major hurricanes is primarily a result of the multi-decadal increase in the Atlantic Ocean thermohaline circulation (THC) that is not directly related to global temperature increase. Changes in ocean salinity are believed to be the driving mechanism. These multi-decadal changes have also been termed the Atlantic Multidecadal Oscillation (AMO). There have been similar past periods (1940s-1950s) when the Atlantic was just as active as in recent years. For instance, when we compare Atlantic basin hurricane numbers over the 15-year period from 1990-2004 with an earlier 15-year period (1950-1964), we see no difference in hurricane frequency or intensity even though the global surface temperatures were cooler and there was a general global cooling during 1950-1964 as compared with global warming during 1990-2004. Although global surface temperatures have increased over the last century and over the last 30 years, there is no reliable data available to indicate increased hurricane frequency or intensity in any of the globe’s seven tropical cyclone basins besides the Atlantic. Meteorologists who study tropical cyclones have no valid physical theory as to why hurricane frequency or intensity would necessarily be altered significantly by small amounts (< ±1oC) of global mean temperature change. In a global warming or global cooling world, the atmosphere’s upper air temperatures will warm or cool in unison with the sea surface temperatures. Vertical lapse rates will not be significantly altered. We have no plausible physical reasons for believing that Atlantic hurricane frequency or intensity will change significantly if global ocean temperatures continue to rise. For instance, in the quarter-century period from 1945-1969 when the globe was undergoing a weak cooling trend, the Atlantic basin experienced 80 major (Cat 3-4-5) hurricanes and 201 major hurricane days. By contrast, in a similar 25-year period from 1970-1994 when the globe was undergoing a general warming trend, there were only 38 major hurricanes (48% as many) and 63 major hurricane days (31% as many). Atlantic sea-surface temperatures and hurricane activity do not necessarily follow global mean temperature trends. The most reliable long-period hurricane records we have are the measurements of US landfalling tropical cyclones since 1900 (Table 15). Although global mean ocean and Atlantic surface temperatures have increased by about 0.4oC between these two 50-year periods (1900-1949 compared with 1956-2005), the frequency of US landfall numbers actually shows a slight downward trend for the later period. If we chose to make a similar comparison between US landfall from the earlier 30-year period of 1900-1929 when global mean surface temperatures were estimated to be about 0.5°C colder than they were during the 30-year period from 1976-2005, we find exactly the same US hurricane landfall numbers (54 to 54) and major hurricane landfall numbers (21 to 21). We should not read too much into the two hurricane seasons of 2004-2005. The activity of these two years was unusual but well within natural bounds of hurricane variation. In addition, following the two very active seasons of 2004 and 2005, both 2006 and 2007 had slightly below-average and average activity, respectively, and only one Category 1 hurricane made United States landfall. Between 1966 and 2003, US major hurricane landfall numbers were below the long-term average. Of the 79 major hurricanes that formed in the Atlantic basin from 1966-2003, only 19 (24 percent) made US landfall. During the two seasons of 2004-2005, seven of 13 (54 percent) came ashore. Zero of the four major hurricanes that formed in 2006 and 2007 made US landfall. This is how nature sometimes works. What made the 2004-2005 seasons so unusually destructive was not the high frequency of major hurricanes but the high percentage of major hurricanes that were steered over the US coastline. The major US hurricane landfall events of 2004-2005 were primarily a result of the favorable upper-air steering currents present during these two years. Table 15: U.S. landfalling tropical cyclones by intensity during two 50-year periods.
Although 2005 had a record number of tropical cyclones (28 named storms, 15 hurricanes and 7 major hurricanes), this should not be taken as an indication of something beyond natural processes. There have been several other years with comparable hurricane activity to 2005. For instance, 1933 had 21 named storms in a year when there was no satellite or aircraft data. Records of 1933 show all 21 named storm had tracks west of 60oW where surface observations were more plentiful. If we eliminate all the named storms of 2005 whose tracks were entirely east of 60oW and therefore may have been missed given the technology available in 1933, we reduce the 2005 named storms by seven (to 21) – about the same number as was observed to occur in 1933. Utilizing the National Hurricanes Center’s best track database of hurricane records back to 1875, six previous seasons had more hurricane days than the 2005 season. These years were 1878, 1893, 1926, 1933, 1950 and 1995. Also, five prior seasons (1893, 1926, 1950, 1961 and 2004) had more major hurricane days. Finally, five previous seasons (1893, 1926, 1950, 1961 and 2004) had greater Hurricane Destruction Potential (HDP) values than 2005. HDP is the sum of the squares of all hurricane-force maximum winds and provides a cumulative measure of the net wind force generated by a season’s hurricanes. Although the 2005 hurricane season was certainly one of the most active on record, it is not as much of an outlier as many have indicated. Despite a slightly below-average season in 2006 and average activity in 2007, we believe that the Atlantic basin is currently in an active hurricane cycle associated with a strong thermohaline circulation and an active phase of the Atlantic Multidecadal Oscillation (AMO). This active cycle is expected to continue for another decade or two at which time we should enter a quieter Atlantic major hurricane period like we experienced during the quarter-century periods of 1970-1994 and 1901-1925. Atlantic hurricanes go through multi-decadal cycles. Cycles in Atlantic major hurricanes have been observationally traced back to the mid-19th century, and changes in the AMO have been inferred from Greenland paleo ice-core temperature measurements going back thousand of years. Directory: content -> documents documents -> Extended range forecast of atlantic seasonal hurricane activity and landfall strike probability for 2013 documents -> Extended range forecast of atlantic seasonal hurricane activity and u. S. Landfall strike probability for 2009 documents -> Extended range forecast of atlantic seasonal hurricane activity and u. S. Landfall strike probability for 2010 documents -> Summary of 2008 atlantic tropical cyclone activity and verification of author’s seasonal and monthly forecasts documents -> Extended range forecast of atlantic seasonal hurricane activity and u. S. Landfall strike probability for 2007 documents -> Extended range forecast of atlantic seasonal hurricane activity, individual monthly activity and u. S. Landfall strike probability for 2007 documents -> Forecast of atlantic hurricane activity for october-november 2007 and seasonal update through september documents -> European organisation for the safety of air navigation Download 0.5 Mb. Share with your friends:
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