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| Titel |
Did hydrographic sampling capture global and regional deep ocean heat content trends accurately between 1990-2010? |
| VerfasserIn |
Freya Garry, Elaine McDonagh, Adam Blaker, Chris Roberts, Damien Desbruyères, Brian King |
| Konferenz |
EGU General Assembly 2017
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| Medientyp |
Artikel
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| Sprache |
en
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 19 (2017) |
| Datensatznummer |
250144539
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| Publikation (Nr.) |
 EGU/EGU2017-8377.pdf |
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| Zusammenfassung |
| Estimates of heat content change in the deep oceans (below 2000 m) over the last thirty
years are obtained from temperature measurements made by hydrographic survey
ships. Cruises occupy the same tracks across an ocean basin approximately every 5+
years. Measurements may not be sufficiently frequent in time or space to allow
accurate evaluation of total ocean heat content (OHC) and its rate of change. It is
widely thought that additional deep ocean sampling will also aid understanding
of the mechanisms for OHC change on annual to decadal timescales, including
how OHC varies regionally under natural and anthropogenically forced climate
change.
Here a 0.25˚ ocean model is used to investigate the magnitude of uncertainties and
biases that exist in estimates of deep ocean temperature change from hydrographic sections
due to their infrequent timing and sparse spatial distribution during 1990 - 2010. Biases in the
observational data may be due to lack of spatial coverage (not enough sections covering the
basin), lack of data between occupations (typically 5-10 years apart) and due to occupations
not closely spanning the time period of interest. Between 1990 - 2010, the modelled biases
globally are comparatively small in the abyssal ocean below 3500 m although regionally
certain biases in heat flux into the 4000 - 6000 m layer can be up to 0.05 Wm−2. Biases in
the heat flux into the deep 2000 - 4000 m layer due to either temporal or spatial
sampling uncertainties are typically much larger and can be over 0.1 Wm−2 across an
ocean.
Overall, 82% of the warming trend below 2000 m is captured by observational-style
sampling in the model. However, at 2500 m (too deep for additional temperature information
to be inferred from upper ocean Argo) less than two thirds of the magnitude of the global
warming trend is obtained, and regionally large biases exist in the Atlantic, Southern and
Indian Oceans, highlighting the need for widespread improved deep ocean temperature
sampling. In addition to bias due to infrequent sampling, moving the timings of occupations
by a few months generates relatively large uncertainty due to intra-annual variability in deep
ocean model temperature, further strengthening the case for high temporal frequency
observations in the deep ocean (as could be achieved using deep ocean autonomous float
technologies). Biases due to different uncertainties can have opposing signs and differ in
relative importance both regionally and with depth revealing the importance of reducing all
uncertainties (both spatial and temporal) simultaneously in future deep ocean observing
design. |
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