OCEAN RESPONSE TO TROPICAL CYCLONES: INSIGHTS FROM LAGRANGIAN DATA ASSIMILATION OF SURFACE CURRENTS

Abstract

Tropical cyclones (TCs) inject substantial near-inertial energy through wind forcing, in the form of downward propagating gravity waves, deepening the mixed-layer, eroding stratification, and resulting in surface cooling. These ocean responses critically influence TC intensity and track through the resulting changes in air-sea feedback, making determination of accurate ocean initial conditions essential for TC forecasting. The "traditional" approach to constructing those ocean initial conditions through data assimilation of temperature and salinity observations is insufficient to limit the error growth in the sea surface current field under TC forcing, leading to spurious near-inertial energy and deviations from geostrophic balance. This spurious near-inertial energy can propagate through the mixed layer and into the thermocline where it will degrade the ocean part of a coupled forecast. The initial work, covered in Chapter two (see also Dong et al., 2023), describes work using surface drifters from the Grand Lagrangian Deployment experiment in the Gulf of Mexico as constraints within the Lagrangian extension of the Local Ensemble Transform Kalman Filter. This work shows promise in reducing errors in the ocean initial state. In Chapters three and four we extend this work to consider the ocean response to TCs in two other TC-prone regions in the Northern Indian Ocean: the weakly stratified Arabian Sea, where the traditional approach to assimilation tends to overestimate the vertical penetration of near-inertial energy, and the strongly haline-stratified Bay of Bengal, where accurate phasing of vertical oscillations of the thermocline is difficult to achieve. For both of these regions, we find that assimilating additional Lagrangian drifter trajectory information helps the assimilation system to avoid overestimation of near-inertial wind-work by reducing spurious dynamics imbalances in the ocean state estimate. In these chapters Observing System Simulation Experiments are carried out using wind forcing modeled on two historical TCs. The analysis of the experiments begins with an examination of the work done on the ocean by the winds and that energy is then traced as it propagates down into the ocean. This energy source is intermittent but also primarily in the near-inertial (close to 48-hour periods) frequency band. Energy in this frequency band has the property that it is able to propagate vertically efficiently in the form of near-inertial internal waves. The vertical scale of these near-inertial internal waves varies inversely with stratification, thus concentrating vertical shear at the base of the mixed layer and within the thermocline and enhancing the mixing potential. In contrast, energy introduced at sub-inertial frequencies cannot propagate as free waves and thus is less effective in penetrating the upper ocean. The results of these experiments demonstrate that accurate surface current representation is essential for capturing the coupled dynamics of wind-work, energy propagation by near-inertial gravity waves, and the resulting impact on ocean stratification. By improving the estimation of near-inertial energy and its pathways, and thus reducing spurious initialization errors, Lagrangian data assimilation together with information from surface drifters improves both the active (immediate) response of the ocean to a TC and also the changes in stratification that can lead to repeated TCs following similar paths.