Measurements of hurricane induced high-frequency currents

Abstract

Hurricanes are powerful, energetic storms that can be fueled by warm ocean waters, while simultaneously driving transport and mixing under their path. Wind-driven mixing is an important mechanism for generating internal waves, and hurricanes are capable of generating particularly high levels of mixing. The internal waves in turn allow diapycnal mixing in the ocean, accelerating heat transfer from the near surface to deeper waters. This plays a role in global thermohaline circulation, affecting heat transfer and therefore density properties throughout the oceans. However, while the importance of internal waves is well established, direct measurements of hurricane-generated internal waves over the shelf and slope regions are scarce. As a result, the mechanisms for the generation of these waves by storms are poorly understood. Here we examine the high frequency response and generation of internal waves by Hurricane Ivan as it travelled over the continental shelf edge and slope in the Gulf of Mexico. Velocity data were collected as part of the Naval Research Laboratory's Shelf Energetics and Exchange Dynamics (SEED) experiment. Moorings consisted of Trawl Resistant Bottom Mounts (TRBMs) in the form of a dome-shaped pod known as a Barny due to its barnacle-like shape. The Barnies housed ADCPs and wave/tide gauges, and during the hurricane were subject to extreme current conditions. In particular, over the shelf where water depths are 60 - 90m and surface waves reached significant wave heights of at least 20 m, bottom currents generated by these waves were over 2 m s <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> . Despite these extreme conditions, which set the nearby National Data Buoy Center (NDBC) buoy 42040 adrift, the Barnies proved themselves to be robust, and continued to measure water velocity and pressure both during and after the hurricane's presence in the region. This was the first test of the instrumentation setup under these extreme conditions, and their survival allowed a unique suite of measurements to be made which would not have otherwise been possible. Two distinct responses were observed over the shallow shelf edge (~90 m) and the deep slope (~500 - 1000 m). During the forcing stage of the hurricane over the shelf edge, internal wave motions were found to be three-dimensional, and after the passage of the hurricane velocity fluctuations became primarily horizontal and lasted about 3 days. Over the slope, inertial (f) and super-inertial waves with frequencies of 2f, 3f, 4f and higher were excited by the hurricane. These super-inertial waves persisted for 2-4 days while near-inertial waves lasted more than a week. The super-inertial fluctuations were found near bottom over the slope (500 - 1000 m) where kinetic energy levels were at least 25 times larger than the kinetic energy level during calm weather, indicating that turbulent dissipation rates and eddy diffusivities increased by two orders of magnitude. The storm-generated super-inertial motions have the potential to enhance mixing in the deeper part of the thermocline. The storm-generated super-inertial motions lead to mixing both along and across isopycnals, acting as a potential vector for warmer waters to reach the deep ocean.