Mechanisms of Subantarctic Mode Water re-emergence in a hybrid-coordinate global GCM


Zuo, Hao (2009) Mechanisms of Subantarctic Mode Water re-emergence in a hybrid-coordinate global GCM. University of Southampton, School of Ocean and Earth Science, Doctoral Thesis , 223pp.

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Description/Abstract

This study seeks to investigate the global circulation and re-emergence of Subantarctic
Mode Water (SAMW), which is thought to be important in providing
the source of nutrients to drive biological production over large proportions of
the world ocean. For this purpose, the HYbrid isopycnic-cartesian Coordinate
Ocean general circulation Model (HYCOM) is congured to simulate the global
ocean circulation for timescales of up to centuries. After development and validation
of the model, a control run of 120 years is undertaken in order to reach
a near-equilibrium. A tracer is then injected into the SAMW formation regions
and used to track the global circulation of SAMW for a further 150 years. The
upwelling regions in which SAMW is returned to the upper-ocean mixed layer
are identied, and the importance of dierent mechanisms of SAMW/nutrient
re-emergence north of the Southern Ocean is assessed. It is found that for the
global ocean north of 30S, entrainment driven by surface buoyancy loss and/or
wind stirring is the most important mechanism for SAMW re-emergence in the
model (accounting for 33% of the total SAMW upwelling). Substantial regional
variation in upwelling mechanisms exists though. Regridding mixing driven by
the Ekman pumping eect and shear-induced mixing dominate SAMW upwelling
in the equatorial oceans. Induction at isopycnal outcrops is critical to
SAMW upwelling in the high-latitude North Atlantic.
It is found that SAMW has little in
uence in the North Pacic when compared
with the North Atlantic and the dierent circulation and upwelling patterns
of SAMW in the Northern Hemisphere are investigated in the model. The
conned penetration distance of SAMW to the North Pacic is found to be associated
with the relatively lighter density levels that the SAMW core resides
at in the Pacic Ocean, and the dierent vertical extensions of the Equatorial
Under Current (EUC) and Kuroshio Current (KC) there. The mechanisms of
nutrient supply into the North Atlantic Ocean in association with the SAMW
has been studied in more detail with a set of perturbation experiments. Intercomparison
shows that the relatively high nutrient concentrations and primary
production in the upper thermocline isopycnals and surface within the Gulf
Stream (GS) are likely to be associated with the imported nutrients through
isopycnal transport, and diapycnal mixing need not to be invoked to explain the
Gulf Stream's high nutrient concentrations. Based on the tracer experiment,
a new nutrient circulation and upwelling model is proposed for the North Atlantic
Ocean, which comprises four steps following the seasonal cycle of the
thermocline.

Item Type: Thesis (Doctoral)
Subjects: G Geography. Anthropology. Recreation > GC Oceanography
Divisions: University Structure - Pre August 2011 > School of Ocean & Earth Science (SOC/SOES)
ePrint ID: 145063
Date Deposited: 15 Apr 2010 14:48
Last Modified: 27 Mar 2014 19:06
URI: http://eprints.soton.ac.uk/id/eprint/145063

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