Open Access Repository

Assimilation of altimetry data in a high-resolution limited-area primitive equation model of the Southern Ocean

Downloads

Downloads per month over past year

Woolf, Andrew (2002) Assimilation of altimetry data in a high-resolution limited-area primitive equation model of the Southern Ocean. PhD thesis, University of Tasmania.

[img]
Preview
PDF (Whole thesis)
whole_WoolfAndr...pdf | Download (35MB)
Available under University of Tasmania Standard License.

| Preview

Abstract

An eddy-permitting (44 km at 50°S) numerical model of the open ocean south
of Australia is combined with satellite altimetry data in an assimilation study of
Southern Ocean dynamics. The domain chosen is from 110 to 190° East and 30
to 70° South, encompassing both the Southeast Indian and Macquarie Ridge complexes.
This region is thought to play an important role in the momentum balance
of the Antarctic Circumpolar Current (ACC) and meridional heat and freshwater
exchanges.
The Hamburg Ocean Primitive Equation (HOPE) model (Wolff et al. 1997) is
an implicit free-surface primitive-equation model discretised on an Arakawa E-grid.
It is modified to operate in an open boundary configuration. For the barotropic
mode, the open boundary condition is based on characteristic variables and extends
the condition proposed by Flather (1976). The boundary condition preserves water
volume while allowing the transmission of shallow-water gravity waves, an important
feature with sequential data assimilation. Volume transports normal to the
boundaries are required. Across the ACC, these are determined adaptively through
a thermal wind calculation. On the northern boundary, prescribed transports model
the Leeuwin Current and East Australia Current. For the baroclinic velocities, a
modified Orlanski-type (Orlanski 1976) radiation condition is applied on the eastern
boundary, while a zero-gradient condition is applied on the northern boundary.
Because of the staggered grid in HOPE, the baroclinic velocities on the western
boundary can be calculated prognostically as in the interior. For tracers a combination
of relaxation towards climatology for inflow and upstream advection on outflow
is applied along all the open boundaries. The model undergoes considerable drift
in a forty-year spinup run. This results from a lack of sea-ice in the model, and inadequate surface forcing and deep-water formation.
The reduced-order optimal interpolation System for Ocean Forecasting and
Analysis (SOFA) of De Mey (1999) is integrated with HOPE to perform the assimilation.
The order-reduction applies a transformation in the vertical between observed
sea-level anomalies and the internal density structure, preserving both water-mass
properties and potential vorticity on isopycnals (Cooper and Haines 1996). Such a
model of water-column variability is consistent with that observed in repeat hydrographic
sections of the WOCE SR3 line through the model domain. A dynamical
analysis of the vertical projection scheme in an assimilation context shows that it
excites primarily barotropic topographic Rossby waves.
One year of data from the TOPEX/POSEIDON satellite mission is used in
a series of trial assimilation runs. The assimilation system produces reasonable
analyses of sea-level anomaly, and improved estimates of meridional eddy heatflux.
A comparison with a WOCE hydrographic section succesfully captures a significant
eddy feature, but at a reduced level. A trial is made of a modification to the vertical
projection scheme that includes a degree of barotropic variability. This modification
provides small but significant improvements to statistics of forecast performance and
patterns of sea-level variability.

Item Type: Thesis (PhD)
Keywords: Ocean temperature, Climatic changes, Mathematical models
Copyright Holders: The Author
Copyright Information:

Copyright 2002 the Author - The University is continuing to endeavour to trace the copyright owner(s) and in the meantime this item has been reproduced here in good faith. We would be pleased to hear from the copyright owner(s).

Additional Information:

Thesis (Ph.D.)--University of Tasmania, 2002. Includes bibliographical references

Date Deposited: 04 Feb 2015 23:28
Last Modified: 22 Aug 2016 01:32
Item Statistics: View statistics for this item

Actions (login required)

Item Control Page Item Control Page
TOP