Surging Ocean Temperatures in Gulf of Mexico and off Florida’s Coast Raise Alarms for Hurricane Season

As with much of the American south in recent weeks, Florida has been suffering from some of the hottest temperatures ever recorded.

During the week from July 3 – 9, Florida recorded scorching absolute temperatures. On July 5, Jacksonville reached a high of 95° F (35° C), as its hottest temperature in the last two weeks. Miami reached a high of 96° F (35.6° C) on July 7. Fort Myers reached its two-week high on July 5, with a peak of 95°F. The hottest place in the state last week was in Tampa, where temperatures in that city rose to 97° F (36° C).

Mean Florida temperature map for the week of July 3-9, 2023. (Map generated 7/10/2023 using provisional data.) Image: NOAA Southeast Regional Climate Center

Temperatures across the state have been running roughly 4-6° F higher than normal in recent weeks. The typical temperature for Jacksonville on July 5, for example, is 90° F. For Miami on July 7, when it reached its recent high, the temperature also averages 90° F. For Fort Myers, where the average typical temperatures for July 5 there are just 84° F, the temperature on July 5, 2023, was 11° F higher than normal.

Of more serious concern within the state last week were the heat index numbers recorded. The heat index is a measure of what the actual temperature would feel like if the relative humidity were just 40%, a relatively mild value. At higher humidities, the body has a harder time cooling itself because there is more moisture in the air. As a result, it feels hotter than the actual temperature measurement would suggest.

As an example, if the temperature outside were 90° F (32° C) and actual humidity were just 20 percentage points higher (60%) than the 40% used as a baseline, the “heat index” temperature would be 100 °F (38 °C).

The heat index temperature is calculated based on a person being exposed to the air temperatures noted while in the shade, out of the way of direct sunlight.

Why this is important is that as the heat index rises, the body causes the heart to pump blood faster into the capillaries just beneath the skin, to help cool the body by sweating more profusely. The increased strain on the entire body, even for otherwise healthy people in good physical condition, poses a more severe risk of heat stroke. At some temperature point, the heat index is high enough that the body cannot cool itself fast enough through its own natural cooling methods, even if one is sitting in the shade.

Though there is considerable debate as to the maximum heat index temperature one can survive, a 2020 study suggested exposures to a heat index as low as 95° F (35° C) will be difficult for most to manage for many hours at a time. Living in areas with a heat index of 110° F (43° C) for many hours would be considered extremely dangerous.

Last week in Florida, the heat index values were in that extremely dangerous category, pushed to extremes because of the higher-than-normal humidity coupled with the high temperatures.

Using the same example cities, last week Jacksonville peaked with a heat index of 107° F on July 5, with projections it will reach a heat index of 110° F sometime this week. Miami’s heat index climbed almost as high during its hottest day last week, reaching an equivalent of 105° F. Fort Myers, which on July 5, its hottest day last week, reached a humidity level of 70%, produced a heat index equivalent temperature of over 120° F.

Miami and Fort Myers are both projected to see their heat indices rise to over 105° F sometime this week.

All those temperatures are classified as being in the “dangerous” category. People should be kept inside and in cool places when the heat index grows that high.

While the heat index numbers were bad, a root cause behind why those numbers are at record highs is causing meteorologists to worry about more than just heat stroke. That root cause is the superheated nature of the waters surrounding Florida this particular year, both along the Atlantic coast and the Gulf of Mexico.

Over the past ten years of measurements, up until this year the hottest temperature ever recorded in July in the upper levels of the sea surrounding Florida, was recorded at Marco Island. That maximum was 86.5° F (31° C).

This year the temperatures off the Florida coast are hovering at 7° - 11° F higher than that. The warmest of those numbers were recorded at the tip of the Florida Keys. The temperatures are so high that many of them have values which are beyond the color range charts typically provided for graphically mapping the ocean surface temperature data.

One Florida meteorologist called those temperatures “downright shocking”.

The National Oceanic and Atmospheric Administration (NOAA) was a bit more scientific in its descriptions. It labeled the heat levels observed at the surface of the Gulf and both along Florida’s east and southern coasts as “severe”, or a category 3 on the 5-point marine heat wave scale, where 5 is the most extreme.

NOAA marine heatwave map for July 8, 2023, for the Gulf of Mexico and surrounding Florida. Note the marine heatwave indicators, ranging from 1 (no heatwave) to 5 (extreme heatwaves). Much of the region surrounding Florida is currently at levels 2 and 3. The hot ocean waters in all areas shown will cause hurricanes to intensify faster and to more dangerous levels than normal this year. Image: Colin McCarthy, via NOAA and Twitter

The ocean heating has already begun to do some of its damage to the marine ecosystems that live in the shallower waters close to Florida. The temperatures are sufficiently warm that NOAA’s Coral Reef Watch just measured an “Alert Level 1” notification for the region, a point at which mass coral reef death has already begun this year at record levels. Alert Level 1 is the second-highest level of warnings of this kind, a point at which NOAA says there will likely be “significant bleaching”.

The bleaching happens in waves. Within the first four weeks of this high a level of heat stress bleaching will start but may be somewhat recoverable. By eight weeks, which appears to be what is about to happen here, NOAA says there will be severe to widespread bleaching of the coral. All that will in turn bring with it mass death of marine life dependent on what lives in and around the reefs.

How will this affect hurricanes forming in the Atlantic and the Gulf? Forecasters warn that the intensity increase in hurricanes which we have seen in the last few years is going to be far worse within just weeks. Decades ago, a tropical storm might take three to four days to intensify to hurricane force levels. Within the last few years that shortened to sometimes a 48-hour period or less. With the currently much warmer waters in the oceans, hurricanes gain energy at a more rapid rate than a few years ago. They will tend to gain rather than lose energy the closer they get to shore, too. That will happen simply because of the presence of hotter ocean water closer to the coastland than normal. There is no longer a band of cooler currents close to where landfall might happen, and in its place now is a hot band that will further fuel growth of the windstorm.

Weather forecasters also point out that weaker wind patterns surrounding Florida are contributing to the hurricane intensification problem. Normally the winds are strong enough that they cause the ocean to churn, bringing cooler waters up from below. This year those winds are calmer, and the ocean is mostly just sitting there, stagnant. That too contributes to a much hotter ocean pattern.

Based on these changes, NOAA is currently working on updated hurricane forecasts for Florida and the Gulf. When the new ones come out, expect the total projected number of hurricanes to rise and the average intensity to be far stronger.