CHEN Yanxu
Ocean ventilation at the mesoscale
18 juillet 2022
Composition du jury
Laurent BOPP (LMD, ENS): President of jury
Lynne TALLEY (Scripps, USCD): Reviewer
Xavier CARTON (LOPS, UBO): Reviewer
Rosemary MORROW (LEGOS, UPS): Examiner
George NURSER (NOC): Examiner
Nicholas KOLODZIEJCZYK (LOPS, UBO): Examiner
Sabrina SPEICH (LMD, ENS): Thesis directer
Résumé
Within the Earth’s climate system, the ocean is engaged as a huge
reservoir of important properties such as heat and carbon,
predominantly resulting from exchanges with the atmosphere on
timescales from hours to millennia. Such large volume of storage in
the ocean interior thus questions the mechanisms of water property
transport and distribution, leading to the concept of ocean
ventilation, a process that connects ocean surface waters with the
interior. Commonly associated with an increase in density of surface
waters, ventilation is typically interpreted as a downward transfer
of water masses due to stability and other fine-scale processes.
Understanding the dynamics and thermodynamics of water mass
formation, ventilation and dissipation, is therefore one of the key
scientific challenges confronting the entire climate community.
In this thesis, several processes related to ventilation have been
discussed and a specific attention has been given to the mesoscale
whose typical length is less than 100 km and timescale spans on the
order of a month. The largest proportion of mesoscale kinetic energy
is contained by coherent vortices, known as mesoscale eddies, which
are nearly geostrophic and can have the vertical extent down to the
thermocline. Aimed at a combination between the ventilation theory
and mesoscale dynamics, the first part of this thesis has been
devoted to a revisit to the theory of subduction at the bottom of
mixed layer that quantifies long-term (permanent) transport of
surface water masses into the main thermocline. Interpreted as a
transient state in the subduction process, mode waters are a
specific type of water mass homogeneous in properties (i.e.,
characterized by low potential vorticity) and residing between the
seasonal and main thermoclines.
Such transiency of mode waters is associated with their formation
mechanism largely due to surface buoyancy forcing that is
season-dependent. The second part of this thesis is thus related to
an algorithm development to detect more precisely than other
available methods the surface mixed layers and mode waters from
several profiling databases. By co-locating mode waters with
mesoscale eddies identified from the satellite altimetry, it is
possible to quantify 1) the percentage of mode waters carried by
eddies in an Eulerian sense, and 2) anomalies of temperature,
salinity and others transported within eddies in a Lagrangian
framework. Accordingly, a revisit to global mode water distribution
has been provided, in terms of their dynamics and thermodynamics at
the mesoscale. The South Atlantic Subtropical Mode Water has been
considered as a special example and brought into details in the last
chapter, since it not only forms according to the typical
baroclinity at the western boundary, but also develops due to a
large amount of inter-basin transport carried by anticyclonic
Agulhas Rings shedding from the Indian Ocean.
Apart from the thermohaline perspective of ocean circulation and
ventilation, i.e., surface convection and its significance on mode
water formation and renewal, this thesis also provides an assessment
on the wind-driven aspect and a combination of these two components.
In specific, we extended the Ekman dynamics to allow for an
influence from geostrophic motions and self-advection. A brief
discussion on diapycnal and more complex physics of ventilation at
the mesoscale is also presented.