03/31/2023
From Brian Norcross:
HURRICANE SEASON PREVIEW: Early thoughts on Hurricane Season 2023 and why spring forecasts are tough
A big transition is underway in the Pacific Ocean, which will affect the forecasts for the upcoming hurricane season. The large and persistent area of cooler-than-normal water along the equator south of Hawaii is fading away. And the warming trend is expected to continue into hurricane season and beyond.
Check out this sequence of images from NOAA that shows the ocean-water temperatures in the Pacific over the last 30 days. You can see the erosion of the big blue blob, which delineates the belt of cooler-than-normal water that has dominated the middle of the ocean.
https://coralreefwatch.noaa.gov/data/5km/v3.1/current/animation/gif/ssta_animation_30day_pacific_930x580.gif
You can also see a warm blob forming on the right side of the images. That warm water is close to the South American coast, but many of the latest computer predictions indicate it will expand to the west into the previously cold area over the next few months.
The Pacific Ocean is so big that water temperatures there affect weather patterns worldwide. When the large belt of equatorial water is warm, the air above the ocean is heated, and it rises. The air rising en masse alters the jet stream's path, which drives where storms, cold air, and heat waves are more likely to occur.
In addition, that extra rising air blows from the Pacific across Central America and can create an atmospheric pattern over the tropical Atlantic that is more hostile to hurricane development than the pattern we've seen for the past few years.
When the ocean water in that central-Pacific equatorial zone is moderately warmer than average, an El Niño condition is said to exist. Significantly warmer water in the key equatorial belt typically correlates with limited hurricane development. That’s called a strong El Niño. Conversely, colder than normal water in that zone is called a La Niña.
The Pacific has been in a cold La Niña mode for the last three years. It's unusual, though not unprecedented, for the ocean to get stuck in one mode for that amount of time. But now it looks like the La Niña phase will finally end, and a warmer regime is likely this year.
We don't yet know, however, how warm the critical belt of water in the Pacific will be during the hurricane season. For an El Niño to be official, the water temperature across that zone has to reach about a degree Fahrenheit above normal. Significant hurricane-limiting effects typically don't kick in until the average temperature reaches 2 or 3 degrees above average.
Seasons when the water is near or slightly warmer than average can go either way. Other atmospheric factors are more likely to control the busyness of the hurricane season in years with "neutral" conditions in the Pacific.
Because the warm-water pattern is just beginning to develop, there's no way to know whether a neutral condition or a hurricane-suppressing strong El Niño will develop by this summer. The available long-range computer forecast models show that both are reasonable outcomes.
The only apparent certainty is that a hurricane-enhancing La Niña is unlikely.
This is not an uncommon situation. Predicting the El Niño/La Niña status during the August to October peak of the hurricane season is always iffy in the spring. Scientists call it the "spring predictability barrier." This time of year, the winds and the ocean surges that trigger pools of warm or cold water to form are so subtle that a small impulse can thwart or enhance a trend toward one outcome or the other.
By June, the status is typically better established, and forecasts become more reliable.
If we end up with a weak El Niño or neutral conditions – in other words, temperatures in the key Pacific zone are slightly warm or close to average – anything can happen. Other factors in the atmosphere/ocean system come into play.
The notorious hurricane seasons of 2004 and 2005, which produced multiple storms that affected Florida and caused devastation in that state and elsewhere, were weak El Niño and neutral years, respectively.
Since the array of possibilities for this hurricane season is forecast to range from neutral to a strong El Niño, making a seasonal forecast is dicey. The odds seem to favor some measure of a storm-limiting El Niño effect, but the degree is highly uncertain. And there could be no effect at all. As a result, I expect seasonal forecasts to be at or slightly above the long-term average.
The status of El Niño or La Niña is the biggest factor that affects the number of systems that strengthen into tropical storms and hurricanes in a normal season. So, in general, getting the forecast for the water temperatures in that critical belt in the Pacific is essential to predicting how busy the hurricane season will be. This year's range of possibilities is larger than it has been lately when La Niña conditions were forecast with reasonable certainty.
The other factor we look at in the spring is the water temperature in the tropical Atlantic. The water there is somewhat warmer than average this year, which would enhance storm development if there is no negative effect from El Niño-generated upper-level winds.
There are always exceptions to these rules, of course. Last year, a moderate La Niña condition prevailed, which would normally increase the number of intense storms. But due to two or three different mechanisms, dry air penetrated the tropics, shutting down storm activity for the month of August. In September, however, the tropics came roaring back in line with the La Niña enhancement.
And famously, 1992 was a strong El Niño year, meaning fewer storms would typically form. And indeed, only 7 storms were named. But the first one was Andrew, which found a pocket of supportive atmosphere and blew up into a monster that hit South Florida.
The first takeaway from all this is that spring hurricane-season forecasts are always iffy because of the intrinsic uncertainty in predicting what the status of El Niño or La Niña will be in August, September, and October. It's an interesting scientific exercise to try to figure it out, but Mother Nature throws up barriers to accurately predicting the ocean-temperature pattern in the Pacific until closer to the beginning of the Atlantic hurricane season in June.
In the end, the key point for everyone along the hurricane coast to remember is that the forecast is irrelevant to whether preparation is required. Hurricane Andrew is usually held up as a prominent example of a disastrous hurricane in a season with very few storms. But there are other examples in the historical record as well.
Living along the Gulf and Atlantic coast means living with hurricanes. There is nothing to do but be prepared.
