The 2024
hurricane season wasn’t just busy. It was somewhat extraordinary.
In 2024, 18 named storms formed, with 11 becoming hurricanes, and five strengthening into major [Cat 3+] hurricanes.
An average season produces 14 named storms, 7 hurricanes, and 3 major hurricanes.
So, simply based on previous activity numbers, the 2025
hurricane season will likely be at, or above, average.
Right now, the warm phase of the ENSO cycle, known as El Niño, has just ended, and temperatures in the Pacific Ocean are steadily declining, with the onset of the La Niña phase.*
Scientists declare a La Niña when the average ocean temperature [Oceanic Niño Index, ONI], in the central and eastern equatorial
Pacific Ocean, drops by at least 0.5 °C, for at least three consecutive months. The greater the temperature difference, the stronger the weather patterns that ensue.
However, a transition to ENSO-neutral is likely [60% chance according to the National Weather Service's Climate Prediction Center/CPC], during March-May of 2025.
During a La Niña, the eastern Pacific hurricane season tends to be quiet, while the Atlantic season ramps up; with more tropical storms, and hurricanes, and more major hurricanes, as well.
The hurricane activity, in a given season, often reflects a combination of the multi-decadal signals [Atlantic Multi-Decadal Oscillation, AMO]
, and ENSO [El Niño-N-La Niña].
During an Atlantic AMO high-activity era, El Niño typically results in a near-normal season, and La Niña produces an above-normal season.
During an Atlantic AMO low-activity era, El Niño typically results in a below-normal season, and La Niña results in a near-normal season.
Similarly for the central and eastern Pacific basins, the combination of a low-activity era and El Niño often produces a near-normal season, while La Niña produces a below-normal season. For a Pacific high-activity era, El Niño often produces an above-normal season, while La Niña produces a near-normal season.
Simply put, El Niño favours stronger hurricane activity in the central and eastern Pacific basins, and suppresses it in the Atlantic basin.
Conversely, La Niña suppresses hurricane activity in the central and eastern Pacific basins, and enhances it in the Atlantic basin.
These impacts are primarily caused by changes in the vertical wind shear, which refers to the change in wind speed and direction between roughly 5,000-35,000 ft. above the ground. Strong vertical wind shear can rip a developing hurricane apart, or even prevent it from forming.
During La Niña, the area of tropical convection, and its Hadley circulation, is retracted westward, to the western Pacific and
Indonesia, and the equatorial Walker circulation is enhanced. Convection is typically absent, across the eastern half of the equatorial Pacific.
In the upper atmosphere, these conditions produce an amplified trough, over the subtropical Pacific, in the area north of the suppressed convection, and a downstream ridge over the
Caribbean Sea, and western tropical Atlantic.
Over the central and eastern subtropical Pacific, the enhanced trough is associated with stronger upper-level winds, and stronger vertical wind shear, which suppress hurricane activity.
Over the Atlantic basin, the anomalous upper-level ridge is associated with weaker upper- and lower- level winds, both of which reduce the vertical wind shear, and increased hurricane activity.
La Niña also favours increased Atlantic hurricane activity, by decreasing the amount of sinking motion, and During La Niña, the area of tropical convection, and its Hadley circulation, is retracted westward to the western Pacific, and
Indonesia, and the equatorial Walker circulation is enhanced. Convection is typically absent across the eastern half of the equatorial Pacific.
In the upper atmosphere, these conditions produce an amplified trough over the subtropical Pacific, in the area north of the suppressed convection, and a downstream ridge over the
Caribbean Sea, and western tropical Atlantic.
Over the central and eastern subtropical Pacific, the enhanced trough is associated with stronger upper-level winds, and stronger vertical wind shear, which suppress hurricane activity.
Over the Atlantic basin, the anomalous upper-level ridge is associated with weaker upper- and lower- level winds, both of which reduce the vertical wind shear, and increased hurricane activity.
La Niña also favors increased Atlantic hurricane activity, by decreasing the amount of sinking motion, and decreasing the atmospheric stability.
Another prominent climate factor to influence Atlantic hurricane activity is the Atlantic Multi-Decadal Oscillation [AMO].
The warm phase of the AMO is associated with high-activity eras, for Atlantic hurricanes.
Conversely, the cold phase of the AMO is associated with low-activity eras.
The warm phase of the AMO reflects warmer SSTs, across the Atlantic hurricane Main Development Region [MDR].
A key atmospheric feature of this pattern is a stronger West African monsoon, which produces a westward extension of the upper-level easterly winds [near 35,000 ft], along with weaker easterly
trade winds, in the lower atmosphere [near 5,000 ft].
This wind pattern is very conducive to increased Atlantic hurricane activity, partly because it results in weaker vertical wind shear. The weaker
trade winds also contribute to a more conducive structure [increased cyclonic shear] of the mid-level [near 10,000 ft] African Easterly Jet [AEJ], favouring hurricane development, from tropical cloud systems [easterly waves], moving westward from
Africa. At the same time, these wind patterns are associated with a more northward push, into the MDR, of deep tropical moisture, and unstable air, each of which also favours stronger hurricanes.
* “ENSO: Recent Evolution, Current Status and Predictions” ~ NOAA-NCEP (Jan 19, 2025) ➥
https://www.cpc.ncep.noaa.gov/produc...-fcsts-web.pdf
And ➥ https://www.cpc.ncep.noaa.gov/produc...y/ensodisc.pdf
Quote:
“Summary:
La Niña conditions are present.
Equatorial sea surface temperatures (SSTs) are below-average in the central
and east-central Pacific Ocean.
La Niña conditions are expected to persist through February-April 2025 (59%
chance), with a transition to ENSO-neutral likely during March-May 2025 (60% chance)...”
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