Master´s thesis defense by Turid Lakså – Niels Bohr Institute - University of Copenhagen

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Master´s thesis defense by Turid Lakså

Title: Inertial resonance in the ocean mixed layer

Abstract: Near-inertial waves affect the climate through mixing in the upper oceans and thereby the sea surface temperature. Changes in sea surface temperature lead to changes in the energy absorption in the oceans. Cold oceans contain less energy than warm oceans, hence cold oceans absorb more CO2. It is therefore important to understand and parameterize near-inertial waves. To accurately simulate sea surface temperatures in climate models is important, since it can help produce more reliable estimates of CO2 uptake in the future. 

The performance of the Climate Earth System Model (CESM) is examined by comparing winds, currents and temperatures with mooring data from the Pilot Research Moored Array in the Tropical Atlantic (PIRATA) project at two mooring sites located at 4N 23W and 12N 23W. The annual mean near-inertial velocity is determined using two different methods. One method determines the near-inertial variance of the near-inertial spectral band. The near-inertial velocity is found at 4N 23W to be 13.74 cm/s for the model and 12.14 cm/s for the observations, at 12N 23W the near-inertial velocities are found to be 11.06 cm/s for the model and 16.54 cm/s for the observations. The other method uses a Butterworth bandpass filter in the near-inertial frequency band, ranging between 0.7-1.3f_I with f_I as the local inertial frequency, filtering the current time series. The near-inertial velocities of this method are found to be 10.37 cm/s for the model and 9.97 cm/s for the observations at 4N 23W, and at 12N 23W they are found to be 8.54 cm/s for the model and 12.92 cm/s for the observations. 

A case study of Hurricane Helene examines the ocean response to a hurricane and how well it is resolved in the model compared to mooring data from the mooring site at 12N 23W. The observational temperature data show a clear indication of mixing during the passage of Hurricane Helene. This is recognized in the model as a deepening of the boundary layer depth, but this is not convincing since the deepening does not stand alone as a notable event. Looking over annual time series, it is found that, the model performs well since the estimates of the near-inertial velocities are consistent with previously published modeling and observational efforts. However, looking at a single storm event the model performs poorly in resolving the winds.