The 2012 Atlantic hurricane season is expected to be near or slightly above the long-term average with 12 named storms, 6 hurricanes, and 3 major hurricanes. Significant factors contributing to this decrease in tropical cyclone activity, compared to previous years, include the dissipation of La Niña in the equatorial Pacific Ocean, above normal Sea Level Pressures, rainfall patterns in the Atlantic and above normal Sea Surface Temperatures.
Figure 1. My predicted numbers compared to other forecasting agencies.
- Entire U.S. coastline – 51% (average for last century is 52%)
- U.S. East Coast – 23% (average for last century is 31%)
- U.S. Gulf Coast – 41% (average for last century is 30%)
- Expected near average major hurricane landfall risk in the Caribbean Sea
The BasicsEL NIÑO-SOUTHERN OSCILLATION/VERTICAL WIND SHEAR
- El Niño-Southern Oscillation & Vertical Wind Shear Relationship
- Rainfall Patterns over West Africa & Tropical Atlantic Relationship
- North Atlantic Oscillation & Mean Sea Level Pressure Relationship
- Sea Surface Temperatures
El Nino-Southern Oscillation, or ENSO, is a quasiperiodic climate pattern that occurs across the tropical Pacific Ocean roughly every five year. During El Niño, high Sea Surface Temperatures across the East Pacific generates intense thunderstorm activity. As a result, outflow produced by those thunderstorms enhances westerly wind shear across the Gulf of Mexico, Caribbean Sea, and Western Atlantic. This provides generally favorable conditions for tropical cyclogenesis in the Pacific. Oppositely, during Neutrals and weak to moderate La Niña, above average Sea Surface Temperatures in the Atlantic generate intense thunderstorm activity that produces significant outflow aloft, providing high vertical wind shear across the Pacific. This provides generally favorable conditions for tropical cyclogenesis in the Atlantic.RAINFALL PATTERNS OVER WEST SAHEL/TROPICAL ATLANTIC
West Africa is the birthplace for most Atlantic tropical cyclones. In addition, it is also the origin of the West African Dust outbreaks known as the Sahara Air Layer. Wetter than normal conditions across the Sahel argue for wetter and more convective tropical waves, and also prevent major SAL outbreaks during the season. Drier than normal conditions produce hotter tropical waves that weaken as they move into the East Atlantic due to an unfavorable temperature gradient. This also means many major SAL outbreaks are likely during the season.
A wetter atmosphere across the Atlantic is typically indicative of Madden-Julian Oscillation, or MJO, in its upward phase. During this phase, tropical cyclogenesis is enhanced as instability remains high and convection is encouraged. A drier atmosphere is typically indicative of a downward phase of the MJO, which reduces vertical instability and suppresses convection. Uncommonly, tropical cyclones do form during the downward phase of the MJO.NORTH ATLANTIC OSCILLATION/MEAN SEA LEVEL PRESSURE
There are two phases of the North Atlantic Oscillation: positive and negative. A positive North Atlantic Oscillation (NAO) implies a stronger subtropical ridge in the Atlantic and deeper than normal Icelandic low. The negative NAO phase indicates a weaker than normal subtropical ridge and shallower than normal Icelandic low. The positive phase of the NAO reduces Sea Surface Temperatures in the Atlantic due to increased evaporational cooling, while the negative phase of the NAO increases Sea Surface Temperature due to decreased evaporational cooling.
The negative NAO typically enhances tropical cyclone activity in the Atlantic due to weaker trade winds, while a positive NAO typically suppresses tropical cyclone activity due to stronger than normal trade winds that can not only act to shear the system, but prevent the disturbance from attaining a well-defined center.
The positive NAO correlates with above average Mean Sea Level Pressure (MSLP) which is unfavorable for tropical cyclogenesis, while a negative NAO correlates with below average MSLP which enhances tropical cyclogenesis.SEA SURFACE TEMPERATURES
Sea Surface Temperatures have an enormous effect on the peak intensities of tropical cyclones and usually determine whether a system is tropical or subtropical. Values below 26 °C aren’t typically warm enough for tropical cyclones, and cyclones typically develop as subtropical cyclones. Values above that are conducive for tropical cyclone development.Expected in 2012Analogue years
In order to retrieve analogue years for a particular hurricane season, I compare different historical ENSO cycles that resembled weather patterns similar to that of the winter and early spring of 2012. Using that technique, I came up with quite a few analogue years: 1951, 1957, 1965, 2002, and to a lesser extent, 2004. The average of all aforementioned hurricane seasons give us 12.75 named storms, 5.6 hurricanes, and 3.2 hurricanes. I have taken the lower values of all these numbers due to the uncertainty pertaining to whether or not El Niño will surface during the season.Summary
In short, the motto for this hurricane season is that there will be less named storms overall, but there is an increased chance for USA landfalls. An early start to the hurricane season appears likely, and an early end to the hurricane season is plausible, especially if El Niño surfaces in the equatorial Pacific. I am expecting 12 named storms, of which 6 will strengthen into hurricanes, and of which 3 will intensify into major hurricanes, or hurricanes with maximum sustained winds greater than 111 mph.
Thanks for reading,
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