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Hurricane season 20217/22/2023 ![]() This hurricane made its first landfall in Cuba on August 27 as a Category 1 storm. (Source: NASA)įollowing Henri came Hurricane Ida, the costliest U.S. Total rainfall from Henri between August 19 and 23. There were several active MJO phases this season, the first of which occupied much of May, and the next occurred in the first half of July.įigure 2. ![]() The Madden Julian Oscillation (MJO), described by NOAA as “a mode of sub-seasonal atmospheric variability that influences the location and strength of tropical precipitation”, is another factor. A marked increase in SAL activity covered much of the Atlantic basin following Hurricane Elsa and did much to suppress tropical cyclone development until mid-June, when the season’s first dramatic burst of hurricane activity occurred. These African waves account for 85% of all Atlantic major hurricanes and 60% of all named storms. The SAL contributes significantly to dry air, which inhibits tropical cyclone development over the Main Development Region (MDR), which is the area between the coast of Africa and Central America, so called because virtually all African waves originate in this region. The Saharan Air Layer (SAL)-a dry and dusty air mass that develops over the Sahara Desert-is periodically carried out over the Atlantic where it can impact cyclogenesis. Several sub-seasonal indices also affect tropical cyclone development. The AMO has been in a warm phase since the late 1990s, and studies have shown that during such warm phases heightened hurricane activity is to be expected. These tend to fluctuate between warm and cool phases every 40-60 years. The Atlantic Multidecadal Oscillation (AMO), a long running index of sea surface temperatures, provides a measure of the average surface temperatures across the North Atlantic. ![]() Pre-season forecasts accurately predicted both the neutral ENSO and above average sea surface temperatures. East Coast and across the entire sub-tropical North Atlantic. Sea surface temperatures in the North Atlantic were much warmer than average, particularly off the U.S. La Niña is associated with cooler sea surface temperatures (SSTs) in the eastern or central Pacific, which enhances convection across the Atlantic and creates favorable conditions for cyclogenesis. Part of what drove early hurricane activity in the Atlantic basin was a continuing but weakening La Niña-the cooler of ENSO’s phases. These are affected by El Niño/La Niña-Southern Oscillation (ENSO)-a periodic fluctuation of SSTs and air pressure in the equatorial Pacific Ocean. Several factors influence Atlantic hurricane activity, including sea surface temperatures, wind shear, and vertical instability. Elsa also became the first hurricane of the season a day later. Seasonal activity began in earnest when four storms formed in rapid succession-Bill, Claudette, Danny, and Elsa, the last of which became the earliest fifth named storm on record on July 1, surpassing last year’s record-setting Edouard, which formed on July 6. Because of this streak of pre-season storms the National Hurricane Center (NHC) issued the first of its Tropical Weather Outlooks on May 15 this year, two weeks earlier than in previous years. (Source: NASA, NHC) Climate Signals and the Early Part of the Seasonįor the seventh year in a row, the naming of the first storm occurred before the official start of the season on June 1 when Tropical Storm Ana formed in the Atlantic on May 23. Tracks of all tropical cyclones in the 2021 Atlantic hurricane season. For the second year in a row, and only for the third time ever, all 21 of the names allocated by the World Meteorological Organization for the season were used. In total, 21 named storms formed this year, the third highest total on record after 2005 (27) and 2020 (30). Although not quite as active as 2020’s, the 2021 Atlantic hurricane season was a very active one and records were set.
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