Themain development region (MDR) is the area of warm water in theAtlantic Ocean stretching from the west coast ofnorthern Africa to the east coast ofCentral America and theGulf Coast of the United States. Manytropical cyclones form within this area. Record-breakingsea surface temperatures in the main development region are on average hotter than any time on record.[1][2]
Tropical cyclone formation requires several factors, including: highhumidity, lowwind shear, and sufficiently warmsea surface temperatures. Regions of Earth's oceans with the required conditions are generally found between the latitudes of 8° and 20° from theEquator.[3] An ocean temperature of at least 26.5 °C (79.7 °F) is normally considered the minimum to maintain atropical cyclone. If water temperatures are lower, a system will most likely weaken. Conversely, higher water temperatures can enable a system to undergorapid intensification.[4]
In the Atlantic, the area between 10°N and 20°N spawns the most hurricanes in a given season because of the warmer temperatures. Hurricanes do not form outside this range because nearer to the equator theCoriolis effect is not strong enough to create the tight circulation needed, and farther north the temperatures are too cool.[5] The waters are only at the necessary temperatures from July until mid-October. In the Atlantic this is the height of theseason.
Since hurricanes rely on sea surface temperature, sometimes an initially active season becomes quiet later. This is because the hurricanes are so strong that they churn the waters and bring colder waters up from the deep. This creates an area of the sea the size of the hurricane, which has cooler waters, which can be 5–10 °C (9.0–18.0 °F) lower than before the hurricane. When a new hurricane moves over the cooler waters they have no fuel to continue to thrive, so they weaken or dissipate.[6]
According to anAzores High hypothesis of geographer Kam-biu Liu, an anti-phase pattern is expected to exist between theGulf of Mexico coast and theNorth American Atlantic coast. During the quiescent periods (3000–1400 BC, and 1000 AD to present), a more northeasterly position of the Azores High would result in more hurricanes being steered towards the Atlantic coast. During the hyperactive period (1400 BC to 1000 AD), more hurricanes were steered towards the Gulf coast as the Azores High was shifted to a more southwesterly position near the Caribbean.[7][8] Such a displacement of the Azores High is consistent with paleoclimatic evidence that shows an abrupt onset of a drier climate inHaiti around 3200 years ago,[9] and a change towards more humid conditions in theGreat Plains during the late-Holocene as more moisture was pumped up theMississippi Valley through the Gulf coast. Preliminary data from the northern Atlantic coast seem to support the Azores High hypothesis. A 3,000-year proxy record from a coastal lake inCape Cod suggests that hurricane activity has increased significantly during the past 500–1,000 years, just as the Gulf coast was amid a quiescent period of the last millennium.[10]
