G) Climatology

G1) When is the hurricane season for each basin?
G2) How does El Niño-Southern Oscillation affect tropical cyclone activity around the globe?
G3) How might global warming change hurricane intensity, frequency, and rainfall ?
G4) Why do tropical cyclones occur primarily in the summer and autumn?
G5) hat determines the movement of tropical cyclones?
G6) Why doesn't the South Atlantic Ocean experience tropical cyclones?
G7) How much lightning occurs in tropical cyclones?

Sujet G1) When is the hurricane season for each basin?

Contributed by Neal Dorst and Anne-Claire Fontan.

• The Southwest Indian Ocean cyclone season is officially from 15 November to 30 April. These dates were selected to encompass 9 of the 10 tropical cyclones.
  • 80% of storms and cyclones exist between December and March
  • more than 50% of the activity occurs in January and February
  • more than 50% of the cyclones occurs in January and February, when ocean sea temperatures are warm
  • but once in a few years there may be a storm or a depression occurring "out of season"

• 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 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. June 1st has been the traditional start of the Atlantic hurricane season for decades. However, the end date has been slowly shifted outward, from October 31st to November 15th until its current date of November 30th. 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 - see Subject D1) 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.

• 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. NHC's official dates for this basin are from May 15th to November 30th.

• The Northwest Pacific basin has tropical cyclones occurring all year round regularly. There is no official definition of typhoon season for this reason. There is a distinct minimum in February and the first half of March, and 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 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.

Subject G2) How does El Niño-Southern Oscillation affect tropical cyclone activity around the globe?

Contributed by Chris Landsea and Anne-Claire Fontan

El Niño/Southern Oscillation (ENSO)
During El Niño 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 Niña 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 Niño (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 Niña (cold ENSO) events. See papers by Nicholls (1979),, Revell and Goulter (1986), Dong (1988), Nicholls (1992).

The western portion of the Northeast Pacific basin (140°W to the dateline) has been suggested to experience more tropical cyclone genesis during the El Niño year and more tropical cyclones tracking into the sub-region in the year following an El Niño (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 160E to the dateline during El Niño events. The opposite occurred during La Niña events. Again there is also the tendency for the tropical cyclones to also form closer to the equator during El Niño 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.
However for the southern part of the Indian Ocean, it seems the El Niño events are associated to increased cyclone activity on the western portion of the basin and the La Niña events on the eastern portion of the basin.

Subject G3) How might global warming change hurricane intensity, frequency, and rainfall ?

Contributed by Chris Landsea

In November 2006 the global community of tropical cyclone researchers and forecasters as met at the 6^th International Workshop on Tropical Cyclones of the World Meteorological Organization in San Jose, Costa Rica. They released a statement on the links between anthropogenic (human-induced) climate change and tropical cyclones, including hurricanes and typhoons. The following is a summary of their report.

1. There have been a number of recent high-impact tropical cyclone events around the globe. These include 10 landfalling tropical cyclones in Japan in 2004, five tropical cyclones affecting the Cook Islands in a five-week period in 2005, Cyclone Gafilo in Madagascar in 2004, Cyclone Larry in Australia in 2006, Typhoon Saomai in China in 2006, and the extremely active 2004 and 2005 Atlantic tropical cyclone seasons - including the catastrophic socio-economic impact of Hurricane Katrina.
2. Some recent scientific articles have reported a large increase in tropical cyclone energy, numbers, and wind-speeds in some regions during the last few decades in association with warmer sea surface temperatures. Other studies report that changes in observational techniques and instrumentation are responsible for these increases.

1. Though there is evidence both for and against the existence of a detectable anthropogenic signal in the tropical cyclone climate record to date, no firm conclusion can be made on this point.
2. No individual tropical cyclone can be directly attributed to climate change.
3. The recent increase in societal impact from tropical cyclones has largely been caused by rising concentrations of population and infrastructure in coastal regions.
4. Tropical cyclone wind-speed monitoring has changed dramatically over the last few decades, leading to difficulties in determining accurate trends.
5. There is an observed multi-decadal variability of tropical cyclones in some regions whose causes, whether natural, anthropogenic or a combination, are currently being debated. This variability makes detecting any long-term trends in tropical cyclone activity difficult.
6. It is likely that some increase in tropical cyclone peak wind-speed and rainfall will occur if the climate continues to warm. Model studies and theory project a 3-5% increase in wind-speed per degree Celsius increase of tropical sea surface temperatures.
7. There is an inconsistency between the small changes in wind-speed projected by theory and modeling versus large changes reported by some observational studies.
8. Although recent climate model simulations project a decrease or no change in global tropical cyclone numbers in a warmer climate, there is low confidence in this projection. In addition, it is unknown how tropical cyclone tracks or areas of impact will change in the future.
9. Large regional variations exist in methods used to monitor tropical cyclones. Also, most regions have no measurements by instrumented aircraft. These significant limitations will continue to make detection of trends difficult.
10. If the projected rise in sea level due to global warming occurs, then the vulnerability to tropical cyclone storm surge flooding would increase.

A PDF version of the official report is available here.

Subject G4) Why do tropical cyclones occur primarily in the summer and autumn?

Contributed by Chris Landsea

As described in Subject G1, the primary time of year for getting 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 fa vorable 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.

Subject G5) What determines the movement of tropical cyclones?

Contributed by Chris Landsea

Tropical cyclones - to a first approximation - 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 U.S. National Hurricane Center, 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 back out to sea (for those folks on the eastern edges of continents) or whether the tropical cyclone will continue straight ahead and make landfall.

For more non-technical information on the movement of tropical "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".

Subject G6) Why doesn't the South Atlantic Ocean experience tropical cyclones?

Contributed by Chris Landsea

What never ?? Well, hardly ever.

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.

In addition, the US OAR 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 write up on the South Atlantic hurricane.

Subject G7) How much lightning occurs in tropical cyclones?

Contributed by Chris Landsea

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).