Ocean Observations During Hurricanes by Air-Deployable Profiling Floats

We report on unexpected observations from Hurricane Ignacio (2015) and Irma (2017). The ocean response to Hurricane Irma was observed by CTD-equipped ALAMO floats at quarter-degree resolution in profiles to 300-m depth at 2-h intervals. Deployed from USAF 53^rd Weather Reconnaissance Squadron WC-130J Hurricane Hunter aircraft across the storm track 24 hours ahead of the center of this system, these high-resolution temperature and salinity observations captured the ocean’s response to the 160-knot winds beneath the eyewall of this category 5 system. Here, we analyze the variation in the ocean response across the tropical cyclone's track and examine the relationship between the responses of temperature and salinity to the passage of this major hurricane. In particular, the rapid profiling showed a strong downwelling current as the storm passed over or near the floats. This was then followed by the expected deepening of the mixed layer by shear-driven mixing and upwelling as predicted by theory. We discuss theoretical models of why this strong downwelling may occur, and examine numerical models of the processes.

The development of an air-deployable profiling float that can be launched from Hurricane Hunter aircraft now offers the opportunity to monitor the upper-ocean thermal structure over a time span of many months. These Argo-type profiling floats can be deployed in advance of, or during, a tropical cyclone from any aircraft equipped with an A-sized (AXBT) launch tube. The floats have the same dimensions as an AXBT and weigh about 8.5 kg. Upon deployment, the floats parachute to the surface, detach and automatically begin their programmed mission. The recorded temperature data is subsampled to 1-meter bins that are reported back via the Iridium satellite phone network, which is then automatically processed and posted to the GTS. The floats are also reprogrammable via the 2-way communication afforded by Iridium.