Waters across the Atlantic’s tropical belt — extending from the coast of Africa through the Caribbean — are hotter now than in any other late May on record, with over 90% of the area’s sea surface engulfed in record or near-record warmth. The extent of marine heat has never been greater heading into a hurricane season, outpacing by wide margins the previous late May record-holder in 2005, a year remembered for one of the most active and destructive hurricane seasons in modern history.

Although record-setting sea surface temperatures alone don’t guarantee a busy hurricane season, they do strongly influence it, especially when the abnormal warmth coincides with the tropical belt known as the Main Development Region, or MDR, the area where 85% of Category 3, 4, and 5 hurricanes form. When considered alongside a developing La Niña — the periodic cooling of the equatorial Pacific that reduces storm-busting Atlantic wind shear — the unprecedented ocean heat is driving up seasonal hurricane outlooks higher than ever before.

Colorado State University — the group that pioneered seasonal hurricane forecasts in the 1980s — issued its most aggressive April forecast last month in almost 30 years of doing such preseason outlooks. NOAA, the parent agency of the National Weather Service, will release its first 2024 hurricane season outlook May 23, and expectations are for similarly bullish numbers.

What’s behind the record-breaking Atlantic heat?

The exceptional warm-up of the North Atlantic began in earnest last spring and continued through the 2023 hurricane season. Despite one of the strongest El Niño events on record, which would typically deter hurricane activity, the Atlantic churned out 20 named storms last season, the fourth-highest number since 1950, according to NOAA. Forecasters largely credited the record-warm tropical Atlantic for counteracting the heavy hand of El Niño.

Although a rapid transition from La Niña to El Niño last year along with human-caused global warming remain the primary factors behind the Atlantic’s ongoing hot streak, they don’t fully explain the abrupt jump into uncharted territory. A significant reduction in global sulfate emissions since 2020 from new shipping regulations and an increase in stratospheric water vapor from an explosive South Pacific volcanic eruption in 2022 are also likely contributors. However, the jury’s still out on these players, and so far both appear to have only fractional effects on the recent temperature spike.

That leaves scientists closely monitoring the progress of El Niño in the eastern Pacific, which is already beginning a transition to La Niña. In theory, the transition out of a strong El Niño should begin to cool the Atlantic to levels more in line with the current trajectory of global warming. So far, this hasn’t happened.

Locally, the Bermuda or Azores High in the Atlantic continues to remain much weaker than average. This semi-permanent area of high pressure controls the east-to-west flowing trade winds across the Atlantic tropical belt. With a weaker subtropical high, the Atlantic trade winds slow, which reduces ocean mixing that cools surface waters. This feedback loop further warms the Atlantic across its primary hurricane breeding ground.

What do record-hot ocean temperatures mean for the Caribbean?

The epicenter of the Atlantic heat wave is the Caribbean, where waters are averaging 84.7 degrees Fahrenheit for the week, a weekly temperature never seen before August, and already reaching well above the 1991-2020 average annual peak that typically isn’t hit until September.

The Caribbean is home to some of the strongest Atlantic hurricanes ever recorded. Half of the 40 Category 5 Atlantic hurricanes since 1851 formed in the Caribbean, including Hurricane Wilma in October 2005, the most intense Atlantic hurricane ever observed.

But strong winds aren’t the only threat when sea surface temperatures rise. When water temperatures warm from 80 to 85 degrees Fahrenheit, studies show tropical rainfall — one of the deadliest calling cards of tropical storms and hurricanes — increases by a factor of five. This amplifies the concern for flooding rains for those islands and land areas bordering the Caribbean, especially early in the hurricane season when waters are usually in the low 80s.

European model forecast for the probability of above average (green hues) or below average (brown hues) rainfall for the peak months of the Atlantic hurricane season (August-September-October). The red box shows the Caribbean, with forecast rainfall in the top 20% of the 24-year climatology, a consequence of record-breaking sea surface temperatures. Credit: ECMWF

Additionally, if the anomalous warmth persists deeper into the hurricane season as forecast models suggest, it raises the specter of rapid intensification for hurricanes that do form. Once sea surface temperatures exceed 83 or 84 degrees Fahrenheit, fully-formed hurricanes are more likely to undergo periods of rapid intensification, especially when wind shear isn’t strong. A peer-reviewed study published last fall also found the pace at which tropical storms and hurricanes are strengthening is quickening in the Caribbean compared to the past, with a bullseye in the western Caribbean.

As absolute temperatures warm into the summer and early fall, these departures above the average will become increasingly problematic for the marine environment as well. 2023 saw some of the worst coral bleaching on record in the region due to the extreme heat stress across the Gulf of Mexico and Caribbean. The lengthening duration of heat stress — NOAA confirmed in April the fourth global coral bleaching event is already underway — could once again threaten the survival and recovery of corals in 2024.

Map series comparing the coral bleaching so far in 2024 (bottom map) to previous global events in 1998 (topmost map), 2010, and 2014-2017. Each map shows the location and intensity of coral heat stress during the peak of the events. According to NOAA, none of the previous global bleaching events showed as severe and widespread heat stress throughout the Atlantic and the ongoing bleaching event. Over the past year, 99.7% of Atlantic tropical reef areas experienced bleaching-level heat stress. Credit: NOAA

South Florida already feeling the heat

Last week, most of South Florida bathed in heat not typically felt until the height of summer. Last Wednesday, Miami’s high hit 96 degrees Fahrenheit, with another daily record reached on Sunday. The daily average temperature in Miami was a remarkable 89 degrees on Sunday, an average temperature never observed before late June in Miami.

The heat index in Miami hit 112 degrees Fahrenheit over the weekend, shattering the old heat index record for May by a full six degrees Fahrenheit. Since 1948, on only three days ever in Miami — each in July or August — has the heat index hit 112 degrees or higher, according to a heat index climatology maintained by University of Miami scientist Brian McNoldy.

The startling South Florida heat is partly the byproduct of a staunch high-pressure dome aloft — extending from Mexico through the Florida Straits — which has stifled afternoon Florida thunderstorms. But perhaps the bigger contributor to the extreme early heat has been the moisture influx from southerly flow over the record-warm waters of the Caribbean and nearby Gulf Stream. The dew point temperature — an indicator of how moisture-ridden the air is — was nearly 80 degrees in Miami on Sunday afternoon when air temperatures were approaching the mid-90s. This uncomfortable dew point at the apex of the day not only sent heat indexes into the lower 110s but kept nighttime lows from dipping below 82 degrees Fahrenheit.

Unless surrounding waters begin to cool down, South Florida may be in store for another long, hot summer.

Will the record Atlantic warmth subside before the hurricane season peak?

All signs from seasonal forecast models point to a continued tropical Atlantic heat wave through the summer. Both the North American Multi-Model Ensemble forecast system and the European Centre’s long-range forecast system predict high odds of much warmer-than-average waters from the Gulf of Mexico and Caribbean through the Atlantic’s Main Development Region for the months ahead.

September 2024 forecast for sea surface temperature departures from average (anomalies) from the North American Multi-Model Ensemble (NMME) forecast system. Seasonal forecasts largely show a continuation of much warmer-than-average waters through the peak of the 2024 hurricane season. Credit: TropicalTidbits.com.

The extreme heat building in the waters of the Atlantic — both at the surface and below — doesn’t bode well for the health of the marine ecosystem, nor will it lessen the health hazards for those living nearby and exposed to relentless “feels-like” temperatures.

Whether hurricane season lives up to this year’s hype is to be seen, but with 90% of the tropical belt immersed in record or near record warm waters, the Atlantic powder keg awaits its first spark.