So I’m SURE you’ve seen the doom and gloom forecasts posted all over. There is some level of hype for sure – but honestly? At the end of the day? They aren’t *too* far off. I agree with the idea of an active season overall because we’ve got La Nina settling in. In this article we will explain what La Nina is and how it could influence the pattern in many ways to bring conditions that could threaten the mainland US.
La Niña, a climate phenomenon characterized by cooler-than-average sea surface temperatures in the central and eastern equatorial Pacific Ocean, often leads to more active hurricane seasons in the Atlantic. This increased activity is due to a combination of atmospheric and oceanic conditions that La Niña influences, which are conducive to the development and intensification of tropical cyclones. Here is a detailed explanation of the factors involved:
1. Atmospheric Circulation Patterns
a. Walker Circulation
- La Niña strengthens the Walker Circulation, a pattern of atmospheric circulation in the tropics.
- This results in enhanced easterly trade winds over the tropical Atlantic.
- These strengthened trade winds reduce the vertical wind shear (the change in wind speed and direction with height), which is crucial for the formation and maintenance of hurricanes. Low vertical wind shear allows storms to grow and organize without being disrupted by upper-level winds.
b. Position of the Jet Stream
- During La Niña, the subtropical jet stream tends to shift northward.
- This northward shift reduces the wind shear in the tropical Atlantic, further creating favorable conditions for hurricane development.
2. Sea Surface Temperatures (SSTs)
- La Niña conditions are often associated with warmer-than-average sea surface temperatures in the Atlantic’s main development region (MDR), which spans the tropical Atlantic and the Caribbean Sea.
- Warm SSTs provide more energy and moisture to developing storms, fueling their growth and intensification.
3. Atmospheric Pressure Patterns
- La Niña typically leads to lower-than-average atmospheric pressure in the tropical Atlantic.
- Lower pressure at the surface enhances convection (upward motion of warm, moist air), which is essential for the formation of thunderstorms that can develop into tropical cyclones.
- These lower pressures create a more favorable environment for the genesis and strengthening of hurricanes.
4. Moisture and Humidity
- The enhanced easterly trade winds during La Niña can increase the transport of moist, unstable air from the tropical Atlantic into the hurricane formation regions.
- Higher humidity levels in the mid and lower troposphere (the lowest layer of the atmosphere) provide the necessary fuel for storm development and intensification.
5. Subsidence and Stability in the Pacific
- La Niña leads to increased subsidence (downward motion of air) and stable conditions in the central and eastern Pacific, reducing hurricane activity in that region.
- The reduced activity in the Pacific means there is less competition for atmospheric resources (e.g., moisture and favorable conditions), allowing more of these resources to be available in the Atlantic.
6. Increased Frequency of Tropical Waves
- La Niña conditions can lead to an increased frequency and strength of tropical waves (disturbances moving westward from Africa).
- These tropical waves are often the precursors to tropical storms and hurricanes in the Atlantic.
- With more and stronger tropical waves, the likelihood of these disturbances developing into significant tropical cyclones increases.
7. Steering Currents and Storm Tracks
- The changes in atmospheric circulation patterns associated with La Niña can alter the steering currents that guide the movement of tropical cyclones.
- During La Niña, these steering currents can direct more storms towards landmasses, particularly the Caribbean, Gulf of Mexico, and the southeastern United States, making them more noticeable and impactful.
La Niña creates a more conducive environment for hurricanes in the Atlantic through a combination of reduced vertical wind shear, warmer sea surface temperatures, lower atmospheric pressures, higher humidity levels, increased frequency of tropical waves, and favorable steering currents. These conditions collectively lead to an increased number and intensity of hurricanes, resulting in more active hurricane seasons.
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