Warm sea water is what powers hurricanes. Usually, sea surface temperature (SST) in the Gulf of Mexico needs to exceed 29°C to intensify a hurricane, and every fraction of a degree above 29°C increases the chance — dramatically — of not just intensifying, but super-charging it, creating a “monster storm.”
Which makes one wonder … if a storm passes by, what are the odds the sea surface temperature (SST) will exceed 29°C? Or more? Have the odds changed over time? Of course SST isn’t the only factor at play, only fools say so, but only bigger fools deny its impact on tropical storms.
To learn more about the history of SST in the area visited by hurricane Ida during her journey from Cuba to New Orleans, I selected the region from longitude 92°W to 85°W, latitude 23°N to 30°N, and retrieved daily data for SST in that region from 1981 through Aug. 2 of this year, from the OI (optimal interpolation) v2 dataset:
There’s an obvious annual cycle (hotter in summer, colder in winter), and if we remove it to define anomaly we get this:
The red line is a trend estimate from least-squares regression, and indicates that SST has increased since 1981, by about 0.85°C. It’s overwhelmingly “statistically significant.”
For sea surface temperature and its impact on hurricanes, that amount of warming is HUGE. It’s easy to dismiss it as “quite small” with an offhand “less than 1°C.” But if you really know hurricanes, you know that “every fraction of a degree counts” is, if anything, an understatement. An increase of a “mere” 1°C is nothing like “quite small.”
Of course that’s anomaly, not temperature itself, and it’s for the entire year, not the summer months when temperatures are highest. I split the time span into four decades, from 1981 to 1991, 1991 to 2001, 2001 to 2011, and 2011 to 2021. Then I estimated the probability density function (pdf) during the summer months (Jun/Jul/Aug/Sep) for each decade by two methods: a histogram, and with a smoothed estimate (kernel smooth). I got this for the four decades:
It’s obvious; the chance of meeting or exceeding that 29°C threshhold has increased, particularly from the 1981-1991 decade to the 2011-2021 decade.
If we look at the survival function (1 minus the cumulative distribution function), not only does it give the probability of meeting or exceeding any given value, it’s straightforward to estimate uncertainty ranges for those probabilities (shaded regions surrounding solid lines):
Between the 1981-1991 decade, and 2011-2021, the chance of meeting (or exceeding) the 29°C threshhold went from about 42%, to over 70%. The chance of meeting (or exceeding) 29.5°C, offering much more energy to the storm, went from about 18% to just over 50%. As for the 30°C limit — big trouble in hurricane town — the odds go from a mere 3% chance to a whopping 21%, a 7-fold increase.
Next, I’ll look at SST in the Gulf of Mexico over a longer time span than just the past 40 years.
I chose the title for this post based on a quote from TV weather personality Al Roker,
“We are looking at the result of climate change … that’s what created this monster storm.”
Evidently, the comments by Mr. Roker and others annoy Cliff Mass, who feels compelled to lecture us about looking at real data.
Happy to oblige.
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