Shifting to El Niño ENSO Phase Could Influence North Atlantic Ocean Hurricane Activity
The status of the El Niño Southern Oscillation (ENSO) phase in the Pacific Ocean dominated the conversation prior to the 2023 hurricane season. At the time, many asked if conditions in the Pacific Ocean would shift from La Niña to El Niño and whether that would subdue hurricane activity in the Atlantic Ocean. The answer to that question may soon reveal itself.
As of June 8, the National Oceanic and Atmospheric Administration (NOAA) Climate Prediction Center stated that El Niño conditions are now present in the Pacific and are expected to strengthen over the course of this year.
But what is ENSO, and what does it mean to shift from La Niña to El Niño conditions? How do sea surface temperatures (SSTs) and weather patterns in the Pacific affect hurricane activity in the Atlantic? The answers may be more complex (and surprising) than they seem at first glance.
Meteorologists based the 2023 hurricane season outlooks on the return of ENSO neutral and El Niño conditions this summer and fall, which historically created conditions that impeded storm development in the Atlantic.
Seasonal outlooks indicate that hurricane activity in the North Atlantic may be reduced due to the ENSO shifting away from a La Niña phase. But that is not the full story. SSTs are not only above the long-term historical average, but they are warm enough to provide fuel for hurricane formation in the main development regions across the Atlantic.
What Is ENSO?
ENSO is an SST pattern in the Eastern and Central Pacific that is influenced by trade winds.
Typically, the trade winds in the Pacific blow from east to west along the equator and push warm surface water from the coast of South America toward Asia. The speed and direction of these winds are dictated by the atmospheric pressure gradient across the Pacific. When trade winds weaken, or even reverse direction, warm ocean water moves eastward toward South America. When the east-to-west trade winds are stronger than normal, they push more warm ocean water toward Asia. This causes strong upwelling to occur, bringing deeper colder waters toward the surface to replace the warm near-surface water that has been pushed west.
This bimodal pattern in atmospheric pressure and SST is known as the ENSO. ENSO phases can be one of the following:
- El Niño
- La Niña
- Neutral Phase
Determining which phase is currently active requires a calculation of the normalized difference in atmospheric pressure between Tahiti and Darwin, Australia.
During an El Niño event, the lower-pressure gradient between Darwin and Tahiti weakens the easterly trade winds, and warm water is pushed back east. The opposite is true for a La Niña event. When the pressure gradient increases, the trade winds strengthen, and more warm water is pushed west toward Asia, bringing deep colder waters to the surface.
A Connected System
Changes in ocean temperatures have a substantial influence on both local weather patterns and global meteorological conditions.
For example, during an El Niño event, warm SSTs in the eastern Pacific combined with weakened trade winds typically reduce rainfall and increase temperatures in Australia, which leads to higher brushfire risk. In the western Pacific and coastal South America, an El Niño event brings warmer temperatures, more moisture and reduced vertical wind shear, all of which are conducive to tropical cyclone formation.
But how do ocean temperature patterns in the Pacific affect hurricane activity in the Atlantic?
What Does This Mean for Hurricane Season 2023?
Since late summer 2020, the ENSO cycle has been in a strong La Niña phase. These conditions have aided hurricane formation in the Atlantic over the past several years.
However, El Niño conditions have returned to the Pacific, according to recent Oceanic Niño Index (ONI) values. Most ensemble model runs indicate a high probability of strong El Niño conditions by the end of the fall or winter of 2023.
Meteorologists based the 2023 hurricane season outlooks on the return of ENSO neutral and El Niño conditions this summer and fall, which historically created conditions that impeded storm development in the Atlantic. However, there is still some uncertainty around El Niño’s strength and how its development will ultimately unfold by the fall.
But ENSO is not the whole story. If it was, the season outlooks would anticipate a much milder hurricane season, rather than a slightly below-average or near-normal season. That is because ESNO is not the sole climatological pattern influencing hurricane development in the Atlantic.
The Atlantic Multidecadal Variability (also known as the Atlantic Multidecadal Oscillation) is another large-scale ocean pattern. The Atlantic Multidecadal Variability (AMV) operates on much longer time scales (20-40 years) relative to ENSO (2-7 years). The AMV, like ENSO, has warm and cool phases and is responsible for SSTs in the Atlantic. A warm AMV is characterized by warm Atlantic SSTs and increased thunderstorm activity. Additionally, warm SSTs provide the fuel for hurricane development.
The AMV is currently in a warm phase and has been since 1995. SSTs in the Atlantic are already warmer than the historical average. As of early June 2023, SSTs off the coast of Africa, where North Atlantic hurricanes can begin their life cycle, are more than 3°C higher than the historical average. SSTs are also elevated across the Atlantic Main Development Region, where a large percentage of major hurricanes are known to strengthen.
The result is that during the 2023 hurricane season, competing factors may influence the formation of storms. An ENSO-neutral or El Niño phase may hamper hurricane development in the Atlantic by increasing atmospheric stability and vertical wind shear, but thanks to the warm AMV phase, the increased ocean temperatures could offset those limiting factors.
Putting It All Together
The return of El Niño conditions in the Pacific could be great news for those living in the Atlantic hurricane-prone regions of the United States.
However, warmer-than-average SSTs may prove to be enough to counterbalance the effects of the El Niño ENSO phase. As it stands, there is still uncertainty around how strong El Niño will be and whether the warm AMV phase will prove strong enough to influence hurricane development in the North Atlantic.
The 2023 hurricane season outlooks have indicated that an ENSO neutral phase or El Niño event may reduce the number of named storms, hurricanes and major hurricanes in the Atlantic this year — but only slightly when compared with the historical averages.
Those residing in counties subject to hurricane damage in the United States should not ignore the risk just because ENSO is shifting from La Niña to El Niño. Remember, in terms of the number of storms, 2022 was an average year but included Hurricane Ian, which transformed the hurricane season into a catastrophic one. So, instead of rolling the dice, people should prepare themselves and their communities for the consequences of these inevitable storms
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