“It's tough to make predictions, especially about the future” HO fail all WEVER… HO fail all WEVER… “You can see a lot by looking” Yogi Berra High Pressure at Top is created by latent heat of condensation in the developing thunderstorms. That vertical pressure gradient will then continue to draw in more air from the surface. SST threshold of 26 C is needed to first activate this.

Formation by Organized Convection One theory explains that hurricane formation requires cold air above an organized mass of thunderstorms, where the release of latent heat warms the upper troposphere, creates high pressure aloft, which pushes air outward and causes a low to deepen at the surface. Air moving toward this low intensifies the cycle. Figure 16.5A Figure 16.5B

Formation by Heat Engine Another theory of hurricane development proposes that a heat engine cycle, fueled by warm moist input air and the release of heat when it converts to cool dry air. Differences in the input and output temperatures determine the amount of work on the ocean and winds that is performed.

Hurricane Stages of Development The initial components of a hurricane may form as a tropical disturbance, grow into a tropical depression when winds exceed 20 knots, become a tropical storm when winds exceed 35 knots, and finally then qualify as a hurricane when winds exceed 64 knots. Figure 16.6

Tropical cyclone structure Hurricane characteristicsHurricane characteristics Sustained winds greater than 120 km/hr (74 mph)Sustained winds greater than 120 km/hr (74 mph) Average diameter = 600 km (350 mi)Average diameter = 600 km (350 mi) Central pressure = ~ 950 mb to 870 mbCentral pressure = ~ 950 mb to 870 mb Maintaining the coherent outflow is key to rate of hurricane pressure evolution

Outflow at top driven by internal pressure increase due to latent heat addition to the column

Subject to Dust Suppression

11 Tropical cyclone genesis areas and related storm tracks

Conditions in Active Seasons This inter-related set of conditions has strong links to tropical climate factors. CPC has an extensive monitoring program to assess and predict these conditions and the associated climate factors.

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Better Visualization

Cat 3 heat map

– feature absent

Strong Primary Peak (sep 10) – but also an Oct 18 secondary peak

Driven by thunderstorms in Caribbean

The Anomalous Path of Sandy

Erratic Paths of Hurricanes

August – October SSTs: Ten Most Active – Ten Least Active Atlantic Hurricane Seasons Since 1950

Current SST Pattern – Mid-April 2015 April SST Pattern Correlated with Seasonal Atlantic ACE

b – La Niña a – El Niño

El Niño suppresses hurricane activity by increasing the vertical wind shear. Strong wind shear can prevent hurricanes from forming, and can also kill an existing hurricane. El Niño and La Niña El Niño More Shear Increased Wind Shear, Fewer Hurricanes Warm, Wet La Niña favors more hurricane activity by decreasing the vertical wind shear. La Niña Less Shear Decreased Wind Shear, More Hurricanes Cool, Dry El Niño and La Niña reflect large year-to-year changes in tropical Pacific Ocean temperatures.

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This climate pattern lasts years, and produces key ingredients of a high- activity era for Atlantic hurricanes: warmer waters, reduced wind shear, and favorable winds that strengthen cloud systems coming from Africa. Drier Warmer Stronger West African Monsoon Tropical Multi-Decadal Signal Associated with Atlantic High-Activity Era

ACE index measures overall season strength by accounting for the combined number, intensity and duration of tropical storms and hurricanes. ACE=Sum of squares of maximum sustained surface wind speed (measured 6-hourly) for all named storms while at least tropical storm strength.

THC (or AMO) STRONG THC (or AMO) WEAK

12 3 H H Courtesy of John Marshall (MIT)

Annual Correlation ( ) between SST and Meter Averaged Salinity (50-60°N, 50-10°W)

Linear Trend in Annual Sea Surface Temperature ( )

There is some recent evidence that overall Atlantic hurricane activity may have increased since in the 1950s and 60s in association with increasing sea surface temperatures… Source: Kerry Emanuel, J. Climate (2007). PDI is proportional to the time integral of the cube of the surface wind speeds accumulated across all storms over their entire life cycles.

Late 21st Century projections: increased vertical wind shear may lead to fewer Atlantic hurricanes “storm-friendly”“storm-hostile” Average of 18 models, Jun-Nov Source: Vecchi and Soden, Geophys. Res. Lett., (2007)