Energetics and Kinematics of Inertial Oscillations in the Central Northern GOM

Abstract

Understanding the physics of generation, propagation, and dissipation of inertial currents is important from a variety of aspects. For the Gulf of Mexico, one such aspect is that these oscillations represent an uncertainty in the measurements and forecasting of the longer-period currents, such as those due to the Loop Current (LC) and meso-scale eddies. The Industry has a practice of applying an ‘uplift’ to estimates of current velocity to account for the effect of tidal and inertial currents in cases when observations or model estimates do not resolve the high-frequency current variability. The value of the ‘uplift’ is assumed to be proportional to the intensity of the low-frequency flow. Our analysis aims at testing whether this assumption is valid by providing a detailed description of the space-time variability, including seasonal changes, of inertial oscillations in the central northern Gulf of Mexico. From the analysis of long-term current profile observations and drifter data we found that, on average, near-inertial oscillations have higher amplitudes outside of the areas of strong low-frequency currents associated with a Loop Current Eddy (LCE). Within the upper 200m of the water column, periods characterized by the downward energy propagation dominate. In the layer below 200m, near-inertial waves propagate upward and downward, and the wave trains cannot be traced to a single source of energy. This suggests near-inertial waves within the main part of the water column are of ‘global’ rather than of ‘local’ origin. For most near-inertial wave generation events through wind forcing, the downward energy propagation could not be traced for any extended period of time and no deeper than approximately 200-m depth. The rate of downward energy propagation in the upper pycnocline is on the order of 10-12 m/day. For the near-inertial currents, the first two Empirical Orthogonal Functions (EOF) contribute only 40% into the total current variability for the period of LCE presence and 52% for the period of benign current conditions. The mode shapes vary within a wide range that, most likely, reflects a random distribution of mode shapes that depend on the lateral geometry of the forcing, mixed layer depth, and stratification.