The ocean fueled Hurricane Ida’s explosive growth

From snowfall in the Sierras to Hurricane Ida’s destructive winds and rains, the ocean touches us all. How? As winds sweep across the wild and warm ocean waters, it transfers heat and moisture to the atmosphere. This exchange influences our weather and climate. Picture a lake, early in the morning, with mist rising off the surface. This evaporation is only visible when the air is cool and moisture condenses into mist, but it is always there. Now imagine the vastness of the ocean and this massive transfer of moisture occurring on a global scale. The snowpack that fills the US West Coast reservoirs comes from moisture picked up by winter storms and atmospheric rivers over the Pacific. The rains that flooded New York City came from moisture picked up by Hurricane Ida in the Gulf of Mexico. Then there is also the heat and moisture that the ocean can transfer to the atmosphere, fueling massive storms and hurricanes, like Ida.

Hurricane Ida passed over hot ocean temperatures in the Gulf of Mexico which helped to almost double in size in 24 hours. The massive evaporation from the ocean was felt all the way in New York where there was record flooding from rainfall as the storm disintegrated. This figure shows ocean temperatures on August 27, Hurricane Ida’s track is the white line with its August 27 location shown by the thicker line. Image credit: https://doi.org/10.5281/zenodo.5483603.

Hurricane Ida swept into the Gulf of Mexico and almost doubled in strength in 26 hours — going from an 80 mph category 1 storm to a category 4 with winds of 150 mph. Winds sweep across the ocean around storms. The air these winds move absorbs heat and moisture from the ocean. As these warm and humid winds are drawn inward, they converge and are forced upwards, where the moisture condenses into clouds and rain. As this condensation occurs, it releases heat into the surrounding atmosphere making it more buoyant, moving upward, and feeding the strength of the storm as more air rushes in at the surface to fill-in the rising air. The energy released by condensation in a single day in an average hurricane is about 200 times the entire world’s electrical energy production capacity! Most of this energy is coming from the warm ocean waters in the Tropics. Some of this energy is expended to drop the central pressure of the hurricane further and strengthen it even more.

Storms can be disrupted, and some are, when winds aloft move at different horizontal speeds and basically tear off the top of the storm. With Ida, this didn’t happen. Sometimes dry air is sucked into the storm, cutting off the source of energy, but this didn’t happen either. Instead, the very warm ocean temperatures in the Gulf of Mexico provided a steady source of fuel for the storm.

Within the ocean, massive storms mix water vertically, bringing deep water to the surface. Usually, deep water is cold and can cause a storm to stop growing or stall out. Ida traveled over a massive warm ocean eddy, where the deeper waters were also warm, and continued to feed its growth. Warmer air can hold more moisture and even after Ida’s destructive winds began to dissipate, the impact of that moisture boost was seen by the torrential rains and flooding that it unleashed across the U.S.

Streamflows were much higher than normal under Hurricane Ida as it traveled from Louisiana to New York. Image from https://twitter.com/USGS/status/1433844762101043200.

Without this massive transfer of energy from the ocean to the atmosphere, Ida would have dissipated and disappeared without much impact. But it isn’t just these massive storms that the ocean influences. The El Niño-Southern Oscillation (ENSO) in the Tropical Pacific Ocean influences temperatures and precipitation globally. How do we know what these temperatures are and what the transfer of energy is?

Hurricane Ida drew heat and mixed the ocean. After the storm passed, ocean temperatures went from being ~1K warmer to ~1.5K cooler on September 3rd, a massive drop in such a short period, showing how much heat the ocean supplied to the storm. The cooling (green and blue) is larger on the right side of the storm track (white line) where the winds are most intense. Image credit: https://doi.org/10.5281/zenodo.5483603.

The ocean surface temperatures in the figure come from NASA’s Multi-scale Ultra-high Resolution (MUR) sea surface temperature analysis. MUR takes data from multiple satellites orbiting the globe and merges them together to create a daily map. These ocean temperature maps, along with other satellites and in situ observations are used to understand how the ocean influences our lives. While ocean surface temperature measurements tell one part of the story about how the ocean influences the atmosphere, there is still much to learn about how small or ephemeral features map to larger-scale changes in weather and climate. As we look to understand how the Earth works as a system, improved models and better data are only part of the answer. Working openly and sharing knowledge will help us advance faster, so we can better prepare for these extreme events that are impacting more of us.

Credits: C. Gentemann original text; A. Subramanian & S. Gille, M. Bourassa edits. Figure 1: Hall, Gentemann, Chin. Figure 2: USGS twitter. Figure 3. Hall, Gentemann, Chin.