Estimating trends of Atlantic meridional overturning circulation from long-term hydrographic data and model simulations |
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Authors: | Gerrit Lohmann Helmuth Haak Johann H Jungclaus |
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Institution: | (1) Alfred Wegener Institute for Polar and Marine Research, Bussestr. 24, 27570 Bremerhaven, Germany;(2) Max-Planck Institute for Meteorology, Bundesstr. 53, 20146 Hamburg, Germany |
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Abstract: | The ocean meridional overturning circulation (MOC) plays a central role for the climate in the Atlantic realm. Since scenarios
for future climate change indicate a significant reduction of the MOC under global warming, an assessment of variations and
trends of the real MOC is important. Using observations at ocean weather ship (OWS) stations and along oceanic sections, we
examine the hydrographic information that can be used to determine MOC trends via its signature in water mass properties obtained
from model simulations with the climate model ECHAM5/MPI-OM. We show that temperature trends at mid-latitudes provide useful
indirect measure of large-scale changes of deep circulation: A mid-depth warming is related to MOC weakening and a cooling
to MOC strengthening. Based on our model experiments, we argue that a continuation of measurements at key OWS sites may contribute
to a timely detection of a possible future MOC slowdown and to separate the signal from interannual-to-multidecadal MOC variability.
The simulations suggest that the subsurface hydrographic information related to MOC has a lower variability than the MOC trend
measured directly. Based on our model and the available long-term hydrographic data, we estimate non-significant MOC trends
for the last 80 years. For the twenty-first century, however, the model simulations predict a significant MOC decline and
accompanied mid-depth warming trend. |
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Keywords: | Ocean circulation Ocean weather ship stations Atlantic meridional circulation Temperature trends |
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