Climate Factors
Which climate factors influence the Atlantic hurricane activity?
Hurricanes require high sea surface temperatures, mid-level moisture and low vertical wind shear (which refers to the change in wind direction and speed as the altitude changes) for their formation. Furthermore, certain large-scale atmospheric and oceanic phenomena (such as El Niño-Southern Oscillation) can alter the environment over the tropical Atlantic through so-called teleconnections and influence hurricane formation.
Several climate factors have been documented to impact Atlantic hurricanes, and some are discussed below in more detail.
North Atlantic Ocean Temperature
A warmer-than-normal tropical North Atlantic Ocean creates more favorable hurricane conditions. These warm temperatures enhance the latent and sensible heat fluxes that fuel tropical cyclones, as well as drive lower pressures and reduced low-level trade winds, which also feed back to a more conducive environment for hurricanes.
The warming of the tropical North Atlantic Ocean should also be measured relative to warming in other areas. The relative sea surface temperature is frequently used, which measures the difference between sea surface temperatures in the tropical North Atlantic Ocean and the rest of the tropics. This index is important because it dictates where anomalous rising and sinking motion is likely to occur. When the tropical North Atlantic Ocean is abnormally warmer than the remainder of the tropics, anomalous rising motion is likely in this area, favoring enhanced tropical cyclone activity.
Atlantic Meridional Mode
An index that is often utilized to assess the favorability of the tropical North Atlantic Ocean to hurricane formation is the Atlantic Meridional Mode (AMM; Vimont and Kossin, 2007). The AMM assesses both the thermodynamic and dynamic state of the tropical North Atlantic Ocean through analysis of low-level winds, as well as sea surface temperatures. A positive phase of the AMM is associated with a warmer tropical North Atlantic relative to the tropical South Atlantic, as well as with reduced trade winds in the tropical North Atlantic. Positive phases of the AMM are generally characterized by above-normal Atlantic hurricane activity.
El Niño-Southern Oscillation
At interannual timescales, El Niño-Southern Oscillation (ENSO) is considered a primary driver of Atlantic tropical cyclone variability. ENSO is a climate fluctuation driven by changes in ocean temperature that occurs in the tropical Pacific Ocean, with cycles of warm El Niño and cool La Niña episodes. El Niño suppresses Atlantic hurricane activity, while La Niña increases Atlantic hurricane activity.
Above average conditions in the central and eastern Pacific during El Niño shift the convective activity in the tropical Pacific eastward, and modify the Walker circulation in the tropics. The influence of ENSO on Atlantic tropical cyclone activity is well documented and is understood to occur mainly through local changes in vertical wind shear: during El Niño (La Niña) conditions, the eastward (westward) shift in convection in the tropical Pacific leads to anomalous upper-level westerlies (easterlies) over the Atlantic, which then increases (decreases) the vertical wind shear, thus decreasing (increasing) tropical cyclone activity.
Sahel Precipitation
Hurricane formation over the tropical Atlantic is affected by changes in convective precipitation over the Sahel region. These precipitation changes can impact zonal winds in the upper-troposphere and in turn modulate vertical wind shear over the main development region (i.e. region in the tropical Atlantic Ocean expanding roughly from the western coast of Africa to the western Caribbean Sea, where hurricanes are more likely to form).
Changes in the nature of the African Easterly Waves might also play a role. Years with higher Sahel rainfall tend to be associated with more active hurricane seasons. The influence of the Sahel precipitation is mostly felt when the thermodynamic conditions over the tropical Atlantic are not conducive to hurricane formation.
Saharan Dust
Dust outbreaks from western Africa impact tropical cyclone activity by reducing Atlantic tropical sea surface temperatures ahead of the hurricane season. Episodes of dust outbreak are also associated with extremely dry air coming from the Sahara Desert, another factor detrimental to cyclone formation.
