Glenn, S. M., T. N. Miles, G. N. Seroka, Y. Xu, R. K. Forney, F. Yu, H. Roarty, O. Schofield, and J. Kohut (2016), Stratified coastal ocean interactions with tropical cyclones, Nature Communications, 7, 10887, doi:10.1038/ncomms10887.
Hurricane Irene, 2011
On August 25th, 2011, Governor Chris Christie declared a state of emergency in New Jersey. Hurricane Irene was tracking northeastward along the east coast aiming to make landfall on the Jersey shore with predicted wind speeds of close to 60 mph (Fig. 1). 1,500 National Guard troops were deployed to the state. 6,000 gas and electric workers stood by for possible power outages. Several southern towns faced mandatory evacuations, and residents of low-lying areas along the Hudson River were advised to voluntarily evacuate.
By the time Irene made landfall in Little Egg Inlet in southeastern New Jersey, its maximum wind speed had died down to about 40 mph. Several inland towns along rivers experienced record-setting flooding, but the southern counties were lucky. They were not nearly as damaged as predicted. Overall, Irene was not as intense as expected, and many of the preparations were unnecessary. We knew exactly where Irene was going to be, but overestimated the trouble it was going to cause.
Hurricanes and how we forecast them
Atmosphere-ocean models have gotten pretty good at predicting hurricane tracks. Significant improvements over the past twenty years have helped save many lives by focusing evacuation warnings, but predictions of hurricane intensity are still somewhat inaccurate. To make better models, we need more observational data about what actually goes on inside a hurricane.
Hurricanes form over warm tropical water (Fig. 2). The warm water evaporates and brings energy in the form of heat into the atmosphere. All this warm air leaving the surface creates an area of low pressure. Air from the surrounding areas higher pressure rushes towards it and begins to spin counter-clockwise because of the Coriolis force. As long as there is warm water to evaporate, the process continues. Once hurricanes make landfall, they weaken quickly since they no longer have a warm body of water acting as fuel.
But Hurricane Irene weakened before it made landfall. The models failed to predict this weakening, partially because they did not have entirely accurate information. In particular, they needed accurate sea surface temperature directly under the storm, but the clouds created by the storm itself make it impossible for satellites to see through to the ocean surface. To get around this, models usually assume that the water temperature stays the same. But we know this isn’t true, because satellite images from directly after Hurricane Irene passed showed that sea surface temperatures were 5-11°C cooler than before the storm arrived!
In order to find out what’s really going on beneath a storm, and to get better data to improve our models, we need to measure temperatures from the ocean surface itself. Traditionally, this kind of information has been hard to get because we can’t just send a boat out into the eye of a storm. We can, however, send robots.
Heading out into the storm
Autonomous underwater vehicles called gliders are designed to slowly dive and climb from the ocean surface down to the bottom and back again, measuring properties like temperature, salinity, and suspended particles (like the sediment that gets kicked up during storms) (Fig. 3). Gliders are battery-powered and can stay out in the water for weeks or months. Because they spend most of their time underwater, they don’t get sloshed around quite as much as a boat would in a storm. They only need to surface occasionally to use satellite connections to check their location and heading, send data, and receive new instructions from onshore operators.
Researchers from Rutgers University sent a glider about 100 km offshore of the coast of New Jersey ahead of the predicted track of Hurricane Irene. Before, during, and after the hurricane passed, the glider took continuous profiles of the water column. Even before the eye of the hurricane passed the glider, it observed significant cooling at the ocean surface and deepening of the mixed layer (the part of the ocean that stays warm in the summer) by several meters (Fig. 4).
This ahead-of-eye cooling is important for the energetics of the hurricane. Once sea surface temperatures drop below air temperatures, the transfer of energy in the whole system changes. Heat no longer moves from the ocean to the atmosphere, fueling the storm. It reverses, taking energy away from the storm, thus reducing its intensity. At the location of the glider, after most of the ocean cooling had occurred, the eye of the hurricane was still 200 km away! This means that for the majority of the time that the storm was moving over that patch of water, it was encountering water that was already cooled and taking energy away from the storm.
What caused the cooling?
Hurricane Irene, like all tropical cyclones in the northern hemisphere, spun counter-clockwise, with winds on the leading edge pushing surface currents toward the coast. With the coast acting as a wall, the water had nowhere to go when it reached land but down. This set up a down-welling circulation that caused water near the seafloor to move away from the coast as water at the surface continued moving toward the coast (Fig. 5).
The two layers of water moving over each other in opposite directions created a lot of friction at the interface. Water at the interface became agitated which led to a lot of vertical mixing. Cold water from below mixed with the warm surface water and the strong stratification got weaker, deepening and cooling the mixed layer.
Hurricane Irene was not a special case. The researchers went back to study all the summertime hurricanes that passed over the Mid Atlantic Bight continental shelf over the last 30 years. They found significant cooling ahead of the eye in every one of the tropical cyclones that came during the summer months. It appears that same cooling and weakening processes occurred during all these storms.
There is hope for the forecast models
In an effort to recreate what happened with Hurricane Irene, the researchers ran over a hundred simulations to see how the hurricane trajectory, wind speed, and air pressure responded if they changed the sea surface temperature. They checked what would happen if the water temperature got a little cooler, a lot cooler, a littler warmer, a lot warmer, or stayed the same. In every case that resulted in a similar track to Irene’s, the pre-landfall weakening of the hurricane was recreated only when the ocean temperature cooled ahead of the eye. So, not only did ahead-of-eye cooling happen in all the past hurricanes the team studied, but this type of cooling is the only way to weaken a hurricane before it hits land.
This is a significant breakthrough for forecast models, because it means we can more accurately predict hurricane intensity. It’s important that we keep working to improve hurricane forecast models so that people have faith in the predications. Residents of vulnerable towns need to trust evacuation warnings, but towns also need to know when not to worry and avoid spending money preparing for storm surges and high winds that never come. With robots at the ground level, we can give the atmosphere-ocean modelers exactly the information they need to make more accurate predictions.
I’m interested in how physical processes occurring in different parts of the ocean affect local ecosystems and climate. For my PhD research at Rutgers University (New Brunswick, NJ), I am studying the circulation and pathways of heat transport in the waters of the West Antarctic Peninsula continental shelf, one of the fastest warming regions of the planet. When I’m not thinking about the ocean, I do a lot of swim-bike-running and compete very uncompetitively on the Rutgers Triathlon team.