Direct Wave Measurements Captured by Subsurface EM-APEX Floats in Tropical Ocean

Abstract:

Surface wind stress modulated by surface waves can force ocean current thereby the shear instability in the upper ocean. The Stokes drift velocity of surface waves can also induce Langmuir circulation near the sea surface. The sea surface temperature (SST) cooled by the enhanced turbulent mixing will then affect the weather systems in the tropical ocean, such as tropical cyclones and Madden-Julian oscillations, by reducing the air-sea heat fluxes and moisture. In situ measurements on surface waves are thus crucial for improving the model forecast on the air-sea interaction. Here, we propose a new method for directly measuring surface waves on the Electromagnetic Autonomous Profiling Explorer (EM-APEX) floats, though these floats have been used in the upper ocean studies for more than 10 years. Compared with the previous method of data processing, the horizontal voltage measurements are in the first time decoded and interpreted to derive the horizontal velocity of surface waves. The measurements taken by the accelerometer on the floats are also decoded to derive the vertical acceleration of surface waves. The measurements of surface waves' horizontal and vertical motion can then be used for estimating surface wave directional spectrum. Preliminary results of surface waves using the float measurements before the passage of Typhoon Megi 2010 will be presented and compared with the model results in the WAVEWATCH III. Because the microstructure sensors can also be mounted onto the floats, future studies can use EM-APEX floats to explore the roles of surface waves in the turbulent mixing for changing the upper ocean structure directly. Institute of Oceanography in National Taiwan University plans to conduct a field experiment between Taiwan and Guam in the summer of 2021. More analysis will be finished after the experiment, such as validating float measured surface waves using wave measurements taken by other platforms.