Intensifying La Niña portends an active hurricane season


TAMPA (WFLA) — Over the past few weeks, La Niña has strengthened. This is due to a thrust from the easterly trade winds which blow cool water from the eastern Pacific into the western Pacific along the equator. This reinvigorates the already present La Niña.

This is concerning because La Niña generally corresponds to active hurricane seasons in the Atlantic Ocean. The last two hurricane seasons have been marked by La Niña and both have been very active in the Atlantic Basin.

The hope and expectation was that La Niña would fade before summer, maybe even turn into El Niño. But that no longer seems likely.

La Niña is a cooling of the surface waters of the equatorial Pacific Ocean. This tends to weaken the subtropical Jetstream, making it dry in winter in the southeast, including the Tampa Bay area. This has been true this winter.

But when La Niña extends into summer, it can mean more active hurricane seasons. The same effect is at play in which weak winds aloft stretching from the Pacific to the Atlantic allow hurricanes to fester over the Atlantic.

The reverse is true for El Niño. Warm tropical Pacific waters power a stronger subtropical jet stream in winter, wetter conditions in Florida, and if it persists during hurricane season, stronger winds aloft can tear hurricanes above the Atlantic.

The recent increase in easterly trade winds has indeed intensified the current La Niña. The reason can be explained by the physical dynamics that control it.

When winds blow east to west across the equatorial Pacific, cold water is forced west across the basin. This pushes warm water further west towards Asia. Replace that cool water in the Eastern Pacific with more cool water from below. This is called upwelling.

The gust of easterly winds that have occurred in recent weeks have invigorated what appeared to be a weakening of La Niña. He is no longer weakening.

While there is one more chance it will weaken by summer, transitioning to a neutral event, it is now much less likely to become an El Niño. This will likely mean we won’t have the helping hand of El Niño to guide us through hurricane season.

It’s a shame because summers with El Niño tend to produce fewer hurricanes. In the image below, since 1995, El Niño years have averaged between four and five hurricanes. While summers that have La Niña, or a neutral event, tend to be much more active with an average of eight hurricanes.

The La Niña effect of weaker wind shear aloft and increased Atlantic activity may be intensified by warmer Atlantic Ocean waters. It is present at the moment and it will probably continue until the summer.

Hurricane season is still over two months away and a lot can change between now and then, but if past seasons are any indication, we should prepare for an active summer in the Atlantic Basin.

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