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Decadal ocean water mass changes: Global observations and interpretation


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Helm, KP (2008) Decadal ocean water mass changes: Global observations and interpretation. PhD thesis, University of Tasmania.

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Using a combination of three quality controlled oceanographic datasets, changes in
temperature, salinity, and oxygen were interpolated to neutral density surfaces and
investigated on a global scale. The analysis was broken into two comparisons, where
759,713 profiles (1940-1988 with a mean year of 1970) and then 242,087 Argo profiles
(mean year of 2005), were objectively mapped to the locations of 38,463 full-depth
WOCE profiles (mean year of 1992).
The largest water property changes along neutral density surfaces (1970-1992) occurred
in the upper 1500 m, with a near-global cooling and freshening on density
surfaces around the salinity minimum in both hemispheres (AAIW, NPIW), warming
and salinity increases at the shallow salinity-maximum, freshening of NADW
in the North Atlantic and salinity increases further south, and coherent salinity increases
in CDWin the Southern Ocean for this period. Mid and high-latitude density
surfaces all deepened, while there was shoaling in the equatorial low-latitudes. The
pattern of water mass changes from 1992 to 2005 was generally consistent with the
1970-1992 observations on a global scale. However some regional decadal variability
in the South Indian and the North Atlantic Oceans occurred, with the cooling and
freshening pattern switching to warming and salinity increases on density surfaces
from 1992 to 2005. Oxygen concentration levels in the upper-1500 m have reduced
across all ocean basins at an average 1.82 μmol l−1 (1970-1992), with changes being
strongest in mode, intermediate, and deep water ventilation regions. Less than 15%
of the oxygen decreases can be attributed to temperature increases. The pattern of
oxygen change poleward of 25± is correlated with upper-ocean stratification increases
in both hemispheres.
Integrating changes along density layers from the equator to the surface outcrop
of the layer highlights the importance of high-latitude regions to global budgets,
with 117% of apparent heat flux increases, 116% of apparent freshwater flux increases,
and 94% of oxygen flux decreases occurring in density surfaces that outcrop
poleward of 40±. Statistically significant heat flux increases were evident at all latitudes,
but the largest apparent heat and freshwater flux increases occurred around
50±N and 55±S (1970-1992). Precipitation-minus-evaporation decreases of up to 10 mm yr−1 have occurred in the low latitudes. Using the pure warming, freshening,
and heave modes of subduction (Bindoff and McDougall, 1994), changes in salinity
appear to have had relatively little contribution to density and hence to steric
sea-level change. Instead pure warming and pure heave appear as the dominant subduction
scenarios with both explaining the observed density changes with equal skill.
Both of these subduction mechanisms are the basis for a proposed physical model
of a shift in the poleward boundaries of the subtropical gyres. The observed water
mass changes imply an enhanced global hydrological cycle, and increased surface
warming. Together these changes have lead to increased upper-ocean stratification,
consistent with a reduced renewal rate of all water masses. Our observations
compare well with the patterns of ocean change produced by model simulations of
increasing carbon dioxide.

Item Type: Thesis (PhD)
Keywords: oceanography, hydrological cycle, salinity, climate change, heat content, oxygen changes, water mass, density surfaces
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Copyright 2008 the Author

Date Deposited: 19 Aug 2011 01:41
Last Modified: 11 Mar 2016 05:53
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