Northern constraints on the Atlantic thermohaline circulation
Tor Eldevik (1) and Jan Even Ø. Nilsen (2)
(1) Geophysical institute, University of Bergen, Bergen, Norway (tor.eldevik@gfi.uib.no), (2) Nansen Environmental and
Remote Sensing Center, Bergen, Norway (even@nersc.no)
The Atlantic Ocean’s thermohaline circulation is an important modulator of global climate. Its northern limb
extends through the Nordic Seas to the cold Arctic, a region that appears to be particularly sensitive to climate
change. We present an analytical model, rooted in observations and corroborated by a multi-centennial climate
simulation, that constrains the strength and variability of the Arctic/Atlantic thermohaline circulation. The model
suggests, maybe surprisingly, that the total circulation is relatively insensitive to anomalous freshwater input or
storage; it mainly reflects changes in the northern heat budget. Freshwater anomalies are predominantly balanced
by the circulation’s partition into an estuarine and an overturning branch. We find in particular that recent variability
derived from observed Atlantic hydrography concurs with available current measurements, and that present
Arctic sea ice retreat and freshwater input supports a long-term strengthening of the Arctic/Atlantic thermohaline
circulation. Our study thus provides a simple framework for constraining the thermohaline circulation’s response
to observed or estimated past and future change in the northern seas.