An annual cycle of open-ocean submesoscale dynamics in the Northeast Atlantic from observations

• Main Author: Yu, Xiaolong
• Format: Thesis
• Language: English
• Published: University of Southampton 2018
• Subjects: Northeast Atlantic
• Online Access: https://eprints.soton.ac.uk/421107/; https://eprints.soton.ac.uk/421107/1/Yu_Xiaolong_PhD_Thesis_May_18.pdf

This thesis presents an investigation of the annual cycle of the submesoscale dynamics from two nested clusters of meso- and submesoscale-resolving moorings, deployed in a typical mid-ocean area of the Northeast Atlantic. Vertical velocities inferred using the non-diffusive density equation are substantially stronger at submesoscales (horizontal scales of 1-10 km) than at mesoscales (horizontal scales of 10-100 km). Submesoscale vertical flows are found to drive significant upper-ocean restratificaiton in response to the enhancement of submesoscale lateral fronts in the presence of intense mesoscale frontogenesis, indicating that mesoscale frontogenesis is a regular precursor of the submesoscale turbulence in the study region. The integrated upper-ocean restratification induced by submesoscale flows over the annual cycle is comparable in magnitude to the net destratification driven by local atmospheric cooling. Further investigation of the upper-ocean potential vorticity budget and its relationship to the occurrence of submesoscale frontal instabilities reveals that wind forcing of fronts is centrally involved in symmetric and gravitational instabilities. In spite of persistent atmospheric cooling during wintertime, conditions favorable to symmetric instability are often observed when surface winds have a downfront (i.e., oriented in the direction of the geostrophic shear) component. The forced symmetric instability is dominant in the part of the mixed layer where potential vorticity takes the opposite sign to the Coriolis parameter, below a near-surface convective layer where gravitational instability dominates. Another common outcome of the interaction between winds and mixed-layer currents is the occurrence of near-inertial waves. The annual cycle of downward propagation of wind-generated near-inertial waves is examined from the mooring observations and a mixed-layer slab model. Near-inertial kinetic energy is found to dominate the internal wave band, and to radiate predominantly downward in a few strong ...