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Tropical Cyclone Climatology
1. When is hurricane season ?
The Atlantic hurricane season is officially from 1 June to 30 November. There is nothing magical in these dates, and hurricanes have occurred outside of these six months, but these dates were selected to encompass over 97% of tropical activity. The Atlantic basin shows a very peaked season from August through October, with 78% of the tropical storm days, 87% of the minor (Saffir-Simpson Scale categories 1 and 2) hurricane days, and 96% of the major (Saffir-Simpson categories 3, 4 and 5) hurricane days occurring then (Landsea 1993). Maximum activity is in early to mid September. Once in a few years there may be a tropical cyclone occurring out of season - primarily in May or December. (For more detailed information, see "What is my chance of having a tropical storm or hurricane strike by each month?")
The Northeast Pacific basin has a broader peak with activity beginning in late May or early June and going until late October or early November with a peak in storminess in late August/early September. The season is officially 15 May 15 through 30 November.
The Central Pacific basin has a very peaked season from August through September. The hurricane season is officially from 1 June to 30 November.
The Northwest Pacific basin has tropical cyclones occurring all year round regularly though there is a distinct minimum in February and the first half of March. The main season goes from July to November with a peak in late August/early September.
The North Indian basin has a double peak of activity in May and November though tropical cyclones are seen from April to December. The severe cyclonic storms (>33 m/s winds [76 mph]) occur almost exclusively from April to June and late September to early December.
The Southwest Indian and Australian/Southeast Indian basins have very similar annual cycles with tropical cyclones beginning in late October/early November, reaching a double peak in activity - one in mid-January and one in mid-February to early March, and then ending in May. The Australian/Southeast Indian basin February lull in activity is a bit more pronounced than the Southwest Indian basin's lull.
The Australian/Southwest Pacific basin begin with tropical cyclone activity in late October/early November, reaches a single peak in late February/early March, and then fades out in early May.
Globally, September is the most active month and May is the least active month. (Neumann 1993)
2. How does El Nino-Southern Oscillation affect tropical cyclone activity around the globe ?
El Nino/Southern Oscillation (ENSO) - During El Nino events (ENSO warm phase), tropospheric vertical shear is increased inhibiting tropical cyclone genesis and intensification, primarily by causing the 200 mb (12 km or 8 mi) westerly winds to be stronger (Gray 1984). La Nina events (ENSO cold phase) enhances activity. Recently, Tang and Neelin (2004) also identified that changes to the moist static stability can also contribute toward hurricane changes due to ENSO, with a drier, more stable environment present during El Nino events.
Reference: Tang, B. H., and J. D. Neelin, 2004: ENSO Influence on Atlantic hurricanes via tropospheric warming. Geophys. Res. Lett.: Vol 31, L24204. "
The Australian/Southwest Pacific shows a pronounced shift back and forth of tropical cyclone activity with fewer tropical cyclones between 145° and 165°E and more from 165°E eastward across the South Pacific during El Nino (warm ENSO) events. There is also a smaller tendency to have the tropical cyclones originate a bit closer to the equator. The opposite would be true in La Nina (cold ENSO) events. See papers by Nicholls (1979), Revell and Goulter (1986), Dong (1988), and Nicholls (1992).
The Central Pacific basin (140°W to the dateline) appears to experience more tropical cyclone genesis during the El Nino year and more tropical cyclones tracking into the sub-region in the year following an El Nino (Schroeder and Yu 1995) , but this has not been completely documented yet.
The Northwest Pacific basin, similar to the Australian/Southwest Pacific basin, experiences a change in location of tropical cyclones without a total change in frequency. Pan (1981), Chan (1985), and Lander (1994) detailed that west of 160°E there were reduced numbers of tropical cyclone genesis with increased formations from 160°E to the dateline during El Nino events. The opposite occurred during La Nina events. Again there is also the tendency for the tropical cyclones to also form closer to the equator during El Nino events than average.
The eastern portion of the Northeast Pacific, the Southwest Indian, the Southeast Indian/Australian, and the North Indian basins have either shown little or a conflicting ENSO relationship and/or have not been looked at yet in sufficient detail.
3. What may happen with tropical cyclone activity due to global warming ?
At the end of the 2004 Atlantic hurricane season, many scientists, reporters and policymakers looked for simple answers to explain the extent of the devastation, which totaled more than $40 billion according to the National Hurricane Center. Some prominent scientists proposed that the intense 2004 hurricane season and its considerable impacts, particularly in Florida, could be linked to global warming resulting from the emissions of greenhouse gases into the atmosphere (e.g., Harvard Medical School 2004; NCAR 2004). But the current state of climate science does not support so close a linkage.
Tropical cyclones can be thought of to a first approximation as a natural heat engine or Carnot cycle (Emanuel 1987). From this perspective global warming can theoretically influence the maximum potential intensity of tropical cyclones through alterations of the surface energy flux and/or the upper-level cold exhaust (Emanuel 1987, Lighthill et al. 1994, Henderson-Sellers et al. 1998). But no theoretical basis yet exists for projecting changes in tropical cyclone frequency, though empirical studies do provide some guidance as to the necessary thermodynamic and dynamic ingredients for tropical cyclogenesis (Gray 1968, 1979).
Since 1995 there has been an increase in the frequency and in particular the intensity of hurricanes in the Atlantic. The changes of the past decade are not so large as to clearly indicate that anything is going on other than the multi-decadal variability that has been well documented since at least 1900 (Gray et al. 1997; Landsea et al. 1999; Goldenberg et al. 2001). Consequently, in the absence of large or unprecedented trends, any effect of greenhouse gases on the behavior of hurricanes is necessarily very difficult to detect in the context of this documented variability. Perspectives on hurricanes are no doubt shaped by recent history, with relatively few major hurricanes observed in the 1970s, 80s and early 90s, compared with considerable activity during the 1940s, 50s and early 60s. The period from 1944 to 1950 was particularly active for Florida. During that period eleven hurricanes hit the state, at least one per year, resulting in the equivalent of billions of dollars in damage in each of those years (Pielke and Landsea 1998).
Globally there has been no increase in tropical cyclone frequency over at least the past several decades (Lander and Guard 1998; Elsner and Kocher 2000). In addition to a lack of theory for future changes in storm frequencies, the few global modeling results are contradictory (Henderson-Sellers et al 1998; IPCC 2001). Because historical and observational data on hurricanes and tropical cyclones are relatively robust, it is clear that storm frequency has not tracked recent tropical climate trends. Research on possible future changes in hurricane frequency due to global warming is ambiguous, with most studies suggesting that future changes will be regionally-dependent, and showing a lack of consistency in projecting an increase or decrease in the total global number of storms (Henderson-Sellers et al. 1998, Royer et al. 1998; Sugi et al. 2002). These studies give such contradictory results as to suggest that the state of understanding of tropical cyclogenesis provides too poor a foundation to base any projections about the future. While there is always some degree of uncertainty about the future and model-based results are often fickle, the state of current understanding is such that we should expect hurricanes frequencies in the future to have a great deal of year-to-year and decade-to- decade variation as has been observed over the past decades and longer.
The issue of trends in tropical cyclone intensity is more complicated, simply because there are many possible metrics of intensity (e.g., maximum potential intensity, average intensity, average storm lifetime, maximum storm lifetime, average wind speed, maximum sustained wind speed, maximum wind gust, Accumulated Cyclone Energy (ACE) and so on), and not all such metrics have been closely studied from the standpoint of historical trends, due to data limitations among other reasons. Statistical analysis of historical tropical cyclone intensity shows a robust relationship to the thermodynamic potential intensity (Emanuel, 2000), suggesting that increasing potential intensity should lead to an increase in the actual intensity of storms. The increasing potential intensity associated with global warming as predicted by global climate models (Emanuel, 1987) is consistent with the increase in modeled storm intensities in a warmer climate, as might be expected (Knutson and Tuleya 2004). But while observations of tropical and subtropical sea surface temperature have shown an overall increase of about 0.2 C (0.4 F) over the past ~50 years, there is only weak evidence of a systematic increase in potential intensity (Bister and Emanuel, 2002; Free et al., 2004). The limited studies that have addressed tropical cyclone intensity variations (Landsea et al. 1999; Chan and Liu 2004) show no significant secular trends during the decades of reliable records.
Because the global earth system is highly complicated, until a relationship between actual storm intensity and tropical climate change is clearly demonstrated, it would be premature to conclude that such a link exists or is significant (from the standpoints of either event or outcome risk) in the context of variability. Additionally, even if a relationship were to be found between trends in sea surface temperature and various measures of tropical cyclone intensity, this would not necessarily mean that the storms of 2004 or their associated damages could be attributed directly or indirectly to increasing greenhouse gas emissions.
Looking to the future, global modeling studies suggest the potential for relatively small changes in tropical cyclone intensities related to global warming. Early theoretical work suggested an increase of about 10% in wind speed for a 2 C (4 F) increase in tropical sea surface temperature (Emanuel, 1987). A 2004 study from the Geophysical Fluid Dynamics Laboratory in Princeton, N.J., that utilized a mesoscale model downscaled from coupled global climate model runs indicated the possibility of a 5% increase in the wind speeds of hurricanes by 2080 (Knutson and Tuleya 2004; cf. IPCC 2001). Michaels et al. 2005 suggest that even this 5% increase may be overstated and that a more realistic projection is on the order of only half of that amount. Even if one accepts that the Knutson and Tuleya results are in the right ballpark, these would imply that changes to hurricane intensity on the order of 0.5-1.0 m/s (1-2 mph) may be occurring today. This value is exceedingly small in the context of the more than doubling in numbers of major hurricanes between quiet and active decadal periods in the Atlantic (Goldenberg et al. 2001). Moreover, such a change in intensities would not be observable with today's combination of aircraft reconnaissance and satellite based intensity estimates, which only resolves wind speeds of individual tropical cyclones to - at best - 2.5 m/s (5 mph) increments.
- Modeling and theoretical studies suggest hurricanes will have no major changes in WHERE they form or occur.
- Preliminary analyses hint that globally only small to no change in the NUMBER of hurricanes may occur and that regionally there may be areas that have small increases or small decreases in frequency (on order of +/- 10%).
- The PEAK and AVERAGE INTENSITY of tropical cyclones may increase by about 5% in wind speeds.
- Storm total RAINFALL may also increase on the order of about 5% more precipication.
These are hypothesized changes that may occur around the end of the 21st Century, when a doubling in the amount of carbon dioxide in the atmosphere may be observed. Changes seen today are likely to be on the order of a 1% alteration in frequency, intensity and rainfall in hurricanes - not even measurable by today's observational techniques.
Overall, these man-made alterations are quite tiny compared to the observed large natural hurricane variability. The Atlantic basin activity has cycles with about 3.5 major hurricanes a year in active periods and about 1.5 majors annually in quiet periods, with each lasting 25-40 years. Moreover, as Knutson and Tuleya stated in their 2004 Journal of Climate article:
"CO2-induced tropical cyclone intensity changes are unlikely to be detectable in historical observations and will probably not be detectable for decades to come."
However, more study is needed to better understand the complex interaction between these storms and the tropical atmosphere/ocean as well as to extend our knowledge of hurricane climate variations back in time as much as possible with both historical reconstructions and paleotempestology methods.
(Much of this writeup is from Pielke et al. 2005.)
- Bister, M. and K. A. Emanuel, 2002: Low frequency variability of tropical cyclone potential intensity, 1: Interannual to interdecadal variability. J. Geophys. Res., 107 (4801), doi:10.1029/2001JD000776
- Chan, J. C. L, and S. L. Liu. 2004: Global warming and western North Pacific typhoon activity from an observational perspective. J. Climate: 17. 4590-4602.
- Elsner, J. B., and B. Kocher, 2000: Global tropical cyclone activity: A link to the North Atlantic Oscillation. Geophysical Research Letters, 27:129-132.
- Emanuel, K., 1987: The dependence of hurricane intensity on climate. Nature, 326, 483-485.
- Emanuel, K., 2000: A statistical analysis of tropical cyclone intensity. Mon. Wea. Rev., 128, 1139-1152.
- Free, M., M. Bister and K. Emanuel, 2004: Potential intensity of tropical cyclones: Comparison of results from radiosonde and reanalysis data. J. Climate, 17, 1722-1727
Goldenberg, S.B., C.W. Landsea, A.M. Mestas-Nunez, and W.M. Gray, 2001. The recent increase in Atlantic hurricane activity : Causes and implications Science. 293:474-479
- Gray, W. M., 1968: Global view of the origin of tropical disturbances and storms. Mon. Wea. Rev., 96, 669-700.
- Gray, W.M. (1979): "Hurricanes: Their formation, structure and likely role in the tropical circulation", Meteorology Over Tropical Oceans. D. B. Shaw (Ed.), Roy. Meteor. Soc.,
- James Glaisher House, Grenville Place, Bracknell, Berkshire, RG12 1BX, pp.155-218.
Gray, W.M., J.D. Sheaffer, and C.W. Landsea, 1997: Climate trends associated with multidecadal variability of Atlantic hurricane activity. "Hurricanes: Climate and Socioeconomic Impacts." H.F. Diaz and R.S. Pulwarty, Eds., Springer--Verlag, New York, 15-53.
- Harvard Medical School, 2004. Press release: Experts to warn global warming likely to continue spurring more outbreaks of intense hurricane, 21 October, http://www.med.harvard.edu/chge/hurricanespress.html [A full transcript of the press conference can be found here: http://www.ucar.edu/news/record/transcripts/hurricanes102104.shtml]
- Henderson-Sellers, A., H. Zhang, G. Berz, K. Emanuel, W. Gray, C. Landsea, G. Holland, J. Lighthill, S-L. Shieh, P. Webster, and K. McGuffie, 1998. Tropical cyclones and global climate change: a post-IPCC assessment. Bulletin of the American Meteorological Society, 79:9-38.
- IPCC, 2001. Climate Change 2001 - The Scientific Basis. Contribution of Working Group I to the Third Assessment Report of the Intergovernmental Panel on Climate Change. J. H. Houghton, Y. Ding, D. J. Griggs, M. Nogue, P. J. van der Linden, X. Dai, K. Maskell and C. A. Johnson (Eds.), Cambridge University Press, 881 pp.
- Knutson T. R., and R. E. Tuleya, 2004: Impact of CO2-Induced Warming on Simulated Hurricane Intensity and Precipitation: Sensitivity to the Choice of Climate Model and Convective Parameterization. Journal of Climate, 17:3477-3495.
- Lander, M. A., and C. P. Guard, 1998: A look a global tropical cyclone activity during 1995: Contrasting high Atlantic activity with low activity in other basins. Monthly Weather Review, 126:1163-1173
- Landsea, C.W., Pielke, Jr., R.A., Mestas-Nunez, A.M., and Knaff, J.A., 1999: Atlantic basin hurricanes: Indices of climatic changes, Climatic Change, 42:89-129.
- Lighthill, J., G. J. Holland, W. M. Gray, C. Landsea, K. Emanuel, G. Craig, J. Evans, Y. Kurihara, and C. P. Guard, 1994: Global climate change and tropical cyclones. Bull. Amer. Meteor. Soc., 75, 2147-2157.
- Michaels, P. J., P. C. Knappenberger, and C. W. Landsea, 2005: Comments on impacts of CO2-induced warming on simulated hurricane intensity and precipitation: Sensitivity to the choice of climate model and convective scheme. _J. Climate_, (in press).
- NCAR (National Center for Atmospheric Research), 2004. Hurricanes and climate change: Is there a connection?, NCAR Staff Notes Monthly, October, http://www.ucar.edu/communications/staffnotes/0410/hurricane.html
- Pielke, Jr., R. A., and Landsea, C.W., 1998: Normalized U.S. hurricane damage, 1925- 1995. Weather and Forecasting, 13:621-631.
- Pielke, Jr., R. A., C. Landsea, K. Emanuel, M. Mayfield, J. Laver and R. Pasch, 2005: Hurricanes and global warming. _Bulletin of the American Meteorological Society_, in press.
- Royer, J.-F., F. Chauvin, B. Timbal, P. Araspin, and D. Grimal, 1998: A GCM study of impact of greenhouse gas increase on the frequency of occurrence of tropical cyclones, Climate Dynamics, 38:307-343
- Sugi, M., A. Noda, and N. Sato, 2002: Influence of the global warming on tropical cyclone climatology: An experiment with the JMA global model. Journal of the Meteorological Society of Japan, 80:249-272.
4. Are we getting stronger and more frequent hurricanes, typhoons, and tropical cyclones in the last several years?
Globally, no. However, for the Atlantic basin we have seen an increase in the number of strong hurricanes since 1995. We have had a record 33 hurricanes in the four years of 1995 to 1999 (accurate records for the Atlantic are thought to begin around 1944). The extreme impacts from Hurricanes Marilyn (1995), Opal (1995), Fran (1996), Georges (1998) and Mitch (1998) in the United States and throughout the Caribbean attest to the high amounts of Atlantic hurricane activity lately.
As discussed in the previous section, it is highly unlikely that global warming has (or will) contribute to a drastic change in the number or intensity of hurricanes. We have not observed a long-term increase in the intensity or frequency of Atlantic hurricanes. Actually, 1991-1994 marked the four quietest years on record (back to the mid-1940s) with just less than 4 hurricanes per year. Instead of seeing a long-term trend up or down, we do see a quasi-cyclic multi-decade regime that alternates between active and quiet phases for major Atlantic hurricanes on the scale of 25-40 years each (Gray 1990; Landsea 1993; Landsea et al. 1996). The quiet decades of the 1970s to the early 1990s for major Atlantic hurricanes were likely due to changes in the Atlantic Ocean sea surface temperature structure with cooler than usual waters in the North Atlantic. The reverse situation of a warm North Atlantic was present during the active late-1920s through the 1960s (Gray et al. 1997). It is quite possible that the extreme activity since 1995 marks the start of another active period that may last a total of 25-40 years. More research is needed to better understand these hurricane cycles.
For the region near Australia (105°-160°E, south of the equator), Nicholls (1992) identified a downward trend in the numbers of tropical cyclones, primarily from the mid-1980s onward. However, a portion of this trend is likely artificial as the forecasters in the region no longer classify weak systems as "cyclones" if the systems do not possess the traditional tropical cyclone inner-core structure, but have the band of maximum winds well-removed from the center (Nicholls et al. 1998). These changes in methodology around the mid-1980s have been prompted by improved access to and interpretation of digital satellite data, the installation of coastal and off-shore radar, and an increased understanding of the differentiation of tropical cyclones from other type of tropical weather systems. By considering only the moderate and intense tropical cyclones, this artificial bias in the cyclone record can be overcome. Even with the removal of this bias in the weak Australian tropical cyclones that the frequency of the remaining moderate and strong tropical cyclones has been reduced substantially over the years 1969/70-1995/96. Nicholls et al. (1998) attribute the decrease in moderate cyclones to the occurrence of more frequent El Nino occurrences during the 1980s and 1990s.
For the Northwest Pacific basin, Chan and Shi (1996) found that both the frequency of typhoons and the total number of tropical storms and typhoons have been increasing since about 1980. However, the increase was preceded by a nearly identical magnitude of decrease from about 1960 to 1980. It is unknown currently what has caused these decadal-scale changes in the Northwest Pacific typhoons.
For the remaining basins based upon data from the late 1960s onwards, the Northeast Pacific has experienced a significant upward trend in tropical cyclone frequency, the North Indian a significant downward trend, and no appreciable long-term variation was observed in the Southwest Indian and Southwest Pacific (east of 160°E) for the total number of tropical storm strength cyclones (from Neumann 1993). However, whether these represent longer term (> 30 years) or shorter term (on the scale of ten years) variability is completely unknown because of the lack of a long, reliable record.
5. Why do tropical cyclones occur primarily in the summer and autumn ?
The primary time of year for tropical cyclones is during the summer and autumn: July-October for the Northern Hemisphere and December-March for the Southern Hemisphere (though there are differences from basin to basin). The peak in summer/autumn is due to having all of the necessary ingredients become most favorable during this time of year: warm ocean waters (at least 26°C or 80°F), a tropical atmosphere that can quite easily kick off convection (i.e. thunderstorms), low vertical shear in the troposphere, and a substantial amount of large-scale spin available (either through the monsoon trough or easterly waves).
While one would intuitively expect tropical cyclones to peak right at the time of maximum solar radiation (late June for the tropical Northern Hemisphere and late December for the tropical Southern Hemisphere), it takes several more weeks for the oceans to reach their warmest temperatures. The atmospheric circulation in the tropics also reaches its most pronounced (and favorable for tropical cyclones) at the same time. This time lag of the tropical ocean and atmospheric circulation is analogous to the daily cycle of surface air temperatures - they are warmest in mid-afternoon, yet the sun's incident radiation peaks at noon.
6. What determines the movement of tropical cyclones ?
Tropical cyclones can be thought of as being steered by the surrounding environmental flow throughout the depth of the troposphere (from the surface to about 12 km or 8 mi). Dr. Neil Frank, former director of the National Hurricane Center in Miami, used the analogy that the movement of hurricanes is like a leaf being steered by the currents in the stream, except that for a hurricane the stream has no set boundaries.
In the tropical latitudes (typically equatorward of 20°-25°N or S), tropical cyclones usually move toward the west with a slight poleward component. This is because there exists an axis of high pressure called the subtropical ridge that extends east-west poleward of the storm. On the equatorward side of the subtropical ridge, general easterly winds prevail. However, if the subtropical ridge is weak - often times due to a trough in the jet stream - the tropical cyclone may turn poleward and then recurve back toward the east. On the poleward side of the subtropical ridge, westerly winds prevail thus steering the tropical cyclone back to the east. These westerly winds are the same ones that typically bring extratropical cyclones with their cold and warm fronts from west to east.
Many times it is difficult to tell whether a trough will allow the tropical cyclone to recurve north or northeastward or whether the tropical cyclone will continue west or northwest.
For more non-technical information on the movement of tropical cyclones, see Pielke and Pielke's "Hurricanes: Their Nature and Impacts on Society". For a more detailed, technical summary on the controls on tropical cyclone motion, see Elsberry's chapter in Global Perspectives on Tropical Cyclones.
7. Why doesn't the South Atlantic Ocean experience tropical cyclones ?
In March, 2004 a hurricane did form in the South Atlantic Ocean and made landfall in Brazil but this still leaves the question of why hurricanes are so rare in the South Atlantic. Though many people might speculate that the sea surface temperatures are too cold, the primary reasons that the South Atlantic Ocean gets few tropical cyclones are that the tropospheric (near surface to 200mb) vertical wind shear is much too strong and there is typically no inter-tropical convergence zone (ITCZ) over the ocean (Gray 1968). Without an ITCZ to provide synoptic vorticity and convergence (i.e. large scale spin and thunderstorm activity) as well as having strong wind shear, it becomes very difficult to nearly impossible to have genesis of tropical cyclones.
The National Hurricane Center in Miami has documented the occurrence of a strong tropical depression/weak tropical storm that formed off the coast of Congo in mid-April 1991 (McAdie and Rappaport (1991)). This storm lasted about five days and drifted toward the west-southwest into the central South Atlantic. So far, there has not been a systematic study as to the conditions that accompanied this rare event.
Penn State University article on the South Atlantic hurricane
8. Does an active June and July mean the rest of the season will be busy too ?
Yes and No. The vast majority of Atlantic activity takes place during August-September-October, the climatological peak months of the hurricane season. The overall number of named storms (hurricanes) occurring in June and July (JJ) correlates at an insignificant r = +0.13 (+0.02) versus the whole season activity. In fact, there is a slight negative relationship between early season storms (hurricanes) versus late season - August through November - r = -0.28 (-0.35). Thus, the overall early season activity, be it very active or quite calm, has little bearing on the season as a whole. These correlations are based on the years 1944-1994.
However, as shown in (Goldenberg 2000), if one looks only at the June-July Atlantic tropical storms and hurricanes occurring south of 22°N and east of 77°W (the eastern portion of the Main Development Region [MDR] for Atlantic hurricanes), there is a strong association with activity for the remainder of the year. According to the data from 1944-1999, total overall Atlantic activity for years that had a tropical storm or hurricane form in this region during JJ have been at least average and often times above average. So it could be said that a JJ storm in this region is pretty much a "sufficient" (though not "necessary") condition for a year to produce at least average activity. (I.e., Not all years with average to above-average total overall activity have had a JJ storm in that region, but almost all years with that type of JJ storm produce average to above-average activity.) The formation of a storm in this region during June-July is taken into account when the August updates for the Bill Gray and NOAA seasonal forecasts are issued.
9. Why do hurricanes hit the East coast of the U.S., but never the West coast ?
Hurricanes form both in the Atlantic basin (i.e. the Atlantic Ocean, Gulf of Mexico and Caribbean Sea) and in the Northeast Pacific basin to the west of the U.S. However, the ones in the Northeast Pacific almost never hit the U.S., while the ones in the Atlantic basin strike the U.S. mainland just less than twice a year on average. There are two main reasons. The first is that hurricanes tend to move toward the west-northwest after they form in the tropical and subtropical latitudes. In the Atlantic, such a motion often brings the hurricane into the vicinity of the U.S. east coast. In the Northeast Pacific, a west-northwest track takes those hurricanes farther off-shore, well away from the U.S. west coast. In addition to the general track, a second factor is the difference in water temperatures along the U.S. east and west coasts. Along the U.S. east coast, the Gulf Stream provides a source of warm (> 80°F or 26.5°C) waters to help maintain the hurricane. However, along the U.S. west coast, the ocean temperatures rarely get above the lower 70s, even in the midst of summer. Such relatively cool temperatures are not energetic enough to sustain a hurricane's strength. So for the occasional Northeast Pacific hurricane that does track back toward the U.S. west coast, the cooler waters can quickly reduce the strength of the storm.
Recently (Chenoweth and Landsea 2005), it was re-discovered that a hurricane struck San Diego, California on October 2, 1858. Unprecedented damage was done in the city and was described as the severest gale ever felt to that date nor has it been matched or exceeded in severity since. The hurricane force winds at San Diego are the first and only documented instance of winds of this strength from a tropical cyclone in the recorded history of the state. While climate records are incomplete, 1858 may have been an El Nino year, which would have allowed the hurricane to maintain intensity as it moved north along warmer than usual waters. Today if a Category 1 hurricane made a direct landfall in either San Diego or Los Angeles, damage from such a storm would likely be on the order of a few to several hundred million dollars. The re-discovery of this storm is relevant to climate change issues and the insurance/emergency management communities risk assessment of rare and extreme events in the region.
10. How much lightning occurs in tropical cyclones ?
Surprisingly, not much lightning occurs in the inner core (within about 100 km or 60 mi) of the tropical cyclone center. Only around a dozen or less cloud-to-ground strikes per hour occur around the eyewall of the storm, in strong contrast to an overland mid-latitude mesoscale convective complex which may be observed to have lightning flash rates of greater than 1000 per hour maintained for several hours.
Hurricane Andrew's eyewall had less than 10 strikes per hour from the time it was over the Bahamas until after it made landfall along Louisiana, with several hours with no cloud-to-ground lightning at all (Molinari et al. 1994). However, lightning can be more common in the outer cores of the storms (beyond around 100 km or 60 mi) with flash rates on the order of 100s per hour.
This lack of inner core lightning is due to the relative weak nature of the eyewall thunderstorms. Because of the lack of surface heating over the ocean ocean and the "warm core" nature of the tropical cyclones, there is less buoyancy available to support the updrafts. Weaker updrafts lack the super-cooled water (e.g. water with a temperature less than 0°C or 32°F) that is crucial in charging up a thunderstorm by the interaction of ice crystals in the presence of liquid water (Black and Hallett 1986). The more common outer core lightning occurs in conjunction with the presence of convectively-active rainbands (Samsury and Orville 1994).
One of the exciting possibilities that recent lightning studies have suggested is that changes in the inner core strikes - though the number of strikes is usually quite low - may provide a useful forecast tool for intensification of tropical cyclones. (Black (1975) suggested that bursts of inner core convection which are accompanied by increases in electrical activity may indicate that the tropical cyclone will soon commence a deepening in intensity. Analyses of Hurricanes Diana (1984), Florence (1988) and Andrew (1992), as well as an unnamed tropical storm in 1987 indicate that this is often true (Lyons and Keen 1994 and Molinari et al. 1994).
11. What is my chance of being struck by a tropical storm or hurricane ?
In the Pacific:
The following return periods have been suggestd for tropical cyclones wind speeds within 250 miles of Honolulu, close enough that watches would likely be issued for some portion of the state of Hawaii:
|Maximum wind speed (kt)
||1949-1995 (Full Data set)
||1970-1995 (Satellite Era)
Chu, P.S., and J.X. Wang, 1998: Modeling Return Periods of Tropical Cyclone Intensities n the Vicinity of Hawaii. Journal of Applied Meteorology, 37, 951-960.
In the Atlantic Basin
The figure here shows for any particular location what the chance is that a tropical storm or hurricane will affect the area sometime during the whole June to November hurricane season. We utilized the years 1944 to 1999 in the analysis and counted hits when a storm or hurricane was within about 100 miles (165 km). This figure is created by Todd Kimberlain.
For example, people living in New Orleans, Louisiana have about a 40% chance (the green-orange color) per year of experiencing a strike by a tropical storm or hurricane. For the U.S., the locations that have the highest chances are the following: Miami, Florida - 48% chance; Cape Hatteras, North Carolina - 48% chance; and San Juan, Puerto Rico - 42% chance.
For any particular location the chance that a hurricane will directly affect the area sometime during the whole June to November hurricane season is shown here. We utilized the years 1944 to 1999 in the analysis and counted hits when a hurricane was within about 60 miles (110 km). This figure is created by Todd Kimberlain. (For example, the chance for Miami, Florida is about 16%.)
For any particular location what the chance is that a major hurricane (Category 3, 4 or 5) will directly affect the area sometime during the whole June to November hurricane season is shown here. We utilized the years 1944 to 1999 in the analysis and counted hits when a hurricane was within about 30 miles (50 km). This figure is created by Todd Kimberlain. (For example, the chance for Miami, Florida is about 4%.)
Many folks are concerned about the possible impacts that a hurricane could have on their vacation. If so, please check with your hotel, cruise company, etc. to find out how they inform their guests when a hurricane is coming, what actions they plan and what refund policies they have (if any). Keep in mind that a direct hit by a major hurricane is an extremely rare event.
12. What is my chance of having a tropical storm or hurricane strike in the Atlantic Basin each month ?