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

Abstract

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 resonant wind events throughout the year. Near-inertial waves are mostly
enhanced during wintertime when submesoscale flows are also active, suggesting that
these energetic waves may interact with submesoscale fronts and lead to elevated
dissipation.

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Identifiers

ORCID for Alberto Naveira Garabato: orcid.org/0000-0001-6071-605X

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