Warming of the Global Ocean: Spatial Structure and Water-Mass Trends

This study investigates the multidecadal warming and interannual-to-decadal heat content changes in the upper ocean (0–700 m), focusing on vertical and horizontal patterns of variability. These results support a nearly monotonic warming over much of the World Ocean, with a shift toward Southern Hemi...

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Bibliographic Details
Published in:Journal of climate 2016-07, Vol.29 (13), p.4949-4963
Main Authors: Häkkinen, Sirpa, Rhines, Peter B., Worthen, Denise L.
Format: Article
Language:English
Subjects:
Climate change
Content analysis
Datasets
Decomposition
Density
Enthalpy
Global warming
Gulf Stream
Heat
Heat content
Hydrographic data
Isopycnals
Marine
Missing data
North Atlantic Current
Ocean warming
Oceanic analysis
Oceans
Salinity
Shoaling
Sinking
Southern Hemisphere
Subduction
Surface temperature
Temperature
Temperature changes
Time
Trends
Upper ocean
Variability
Ventilation
Volume transport
Water masses
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Summary:This study investigates the multidecadal warming and interannual-to-decadal heat content changes in the upper ocean (0–700 m), focusing on vertical and horizontal patterns of variability. These results support a nearly monotonic warming over much of the World Ocean, with a shift toward Southern Hemisphere warming during the well-observed past decade. This is based on objectively analyzed gridded observational datasets and on a modeled state estimate. Besides the surface warming, a warming climate also has a subsurface effect manifesting as a strong deepening of the midthermocline isopycnals, which can be diagnosed directly from hydrographic data. This deepening appears to be a result of heat entering via subduction and spreading laterally from the high-latitude ventilation regions of subtropical mode waters. The basin-average multidecadal warming mainly expands the subtropical mode water volume, with weak changes in the temperature–salinity (θ–S) relationship (known as “spice” variability). However, the spice contribution to the heat content can be locally large, for example in Southern Hemisphere. Multidecadal isopycnal sinking has been strongest over the southern basins and weaker elsewhere with the exception of the Gulf Stream/North Atlantic Current/subtropical recirculation gyre. At interannual to decadal time scales, wind-driven sinking and shoaling of density surfaces still dominate ocean heat content changes, while the contribution from temperature changes along density surfaces tends to decrease as time scales shorten.
ISSN:0894-8755
1520-0442
DOI:10.1175/jcli-d-15-0607.1