August is typically a transition month for the Atlantic hurricane season. While the peak of the season is in September, tropical cyclone formation starts migrating to the “Cape Verde” origin region off the coast of Africa in August. At the time of writing, the National Hurricane Center was keeping an eye on an area of low pressure over the central tropical Atlantic Ocean. Forecasters give the storm a 20-40 percent chance of further development over the next 5 days. The United States is still recovering from the 2018 Atlantic hurricane season. Hurricane Michael, a rare Category 5 storm, rapidly intensified before making landfall in the panhandle of Florida. Communities in Florida, Alabama, and Georgia were devastated, and the rapid intensification was surprising to many people in the path of the storm. Intensity forecasts continue to lag track forecasting (see link for a discussion in Forbes), but a new robotic underwater glider system could move the needle on improving hurricane forecasts.
Hurricane formation requires many ingredients. One of the most important factors is warm water. According to the National Oceanic and Atmospheric Administration (NOAA), water that is “at least 79 degrees F (26.5 degrees C) over a depth of 50 meters powers the storm.” While it is common to see discussion of sea surface temperatures (SSTs) during hurricane season, the Oceanic Heat Content (OHC) is actually more important. A study published in 2018 revealed that the OHC associated with Hurricane Harvey was at record levels. According to an American Geophysical Union (AGU) press release:
“As hurricanes move over the ocean, their strong winds strafe the sea surface, making it easier for water to evaporate. The process of evaporation also requires energy from heat, and the warmer the temperatures are in the upper ocean and at the ocean surface, the more energy is available….So much heat was available in the upper layer of the ocean that, as the surface temperature was cooled from the storm (Harvey), heat from below welled up, rewarming the surface waters and continuing to feed the storm.”
The near surface temperatures were roughly 86 degrees F before the passage of Harvey and remained well above 80 degrees F after the storm passed. These findings suggest that knowledge of subsurface ocean temperatures are important for assessing hurricane intensity. New technology being tested by the University of Georgia (UGA) Skidaway Institute is promising. A university-issued media release points out:
“….autonomous underwater vehicles, also known as gliders, can collect valuable information. Gliders are torpedo-shaped crafts that can be packed with sensors and sent on underwater missions to collect oceanographic data. The gliders measure temperature and salinity, among other parameters, as they profile up and down in the water. Equipped with satellite phones, the gliders surface periodically to transmit their recorded data during missions that can last from weeks to months.”
As a meteorologist, I see data collected by the gliders as analogous to information from weather balloons in the atmosphere or dropsondes dispensed from airplanes within hurricanes. Catherine Edwards is a researcher and colleague at the UGA Skidaway Institute of Oceanography. She said, “satellite data provides a nice picture of where the surface ocean is warm, but the subsurface temperature field remains hidden.”
These robotic gliders are not just unproven, academic toys. They were evaluated during Hurricane Florence (2018). The UGA-based researchers and colleagues deployed two gliders and found according to Edwards “almost a 14-degree Celsius (approximately 25 degrees Fahrenheit) error that the glider corrects in the model. The ocean models that did not include glider data missed critical aspects of vertical temperature structure.. A third glider deployed in conjunction with researchers at the University of South Florida helped resolve a portion of the warm Gulf Stream ocean current, which can be a significant source of Ocean Heat Content for landfalling hurricanes.
As the 2019 hurricane season ramps up, new funding from NOAA will allow the researchers to continue evaluating the efficacy of the glider systems. Advances in hurricane intensity forecasts will only be realized when internal heat and ocean-atmosphere exchange processes associated with the hurricane are resolved. These robotic gliders offer another important piece to a complex puzzle.