Response of the thermohaline circulation to cold climates

Abstract

[1] A coupled atmosphere-ocean-sea ice-land surface-ice sheet model of intermediate complexity, the so-called McGill Paleoclimate Model, is employed to study the response of the thermohaline circulation (THC) to various global climate coolings, which are realized by increasing the present-day planetary emissivity to various values. Generally, it is found that the response of the THC to global cooling is nonlinear: For a slightly cold climate the THC in the North Atlantic and the Pacific upwelling become intensified. For a very cold climate the THC in the North Atlantic may be weakened or even collapsed. The associated Pacific upwelling for a very cold climate also becomes weak when the THC is weakened, and intermediate deep water may form in the Pacific when the THC is collapsed. Some support for this nonlinear response is found in recent paleoceanographic data. The reduced atmospheric poleward moisture transport due to the global cooling is mainly responsible for the intensification of the THC in the North Atlantic for a slightly cold climate. For a very cold climate the global cooling may lead to a decrease of the meridional surface density gradient and an increase of the vertical density difference (lower layer density minus upper layer density) in the deep water formation region, which can weaken or shut down the THC. It is the temperature-dependent part of the density differences that is mainly responsible for the weakening or shutting down of the THC. The potential influence of surface temperature changes must be taken into account for a full understanding of the role of the THC in the climate system.

Oceanographic dynamics and the end of the last interglacial in the subpolar North Atlantic

“…This allowed the warm and salty waters from the STG to penetrate to higher latitudes in the northeast Atlantic and into the Nordic seas (19,64), which in turn contributed to the intensification of northern AMOC during glacial inception, as proposed by previous model and data inferences (17,26,27). Paradoxically, despite the northward heat transport of this inflow, the intensified deep overturning at this time may have contributed to the regional and thus global glaciation by providing a moisture source for incipient glaciers (17,19,65). The evidence presented here confirms the persistence of millennial variability during MIS 5e (14,40,59,66) and indicates that these oscillations were superimposed on a longterm cooling trend associated with declining northern summer insolation throughout the last interglacial and leading into the glacial inception in the North Atlantic region.…”

Climate model simulation of anthropogenic influence on greenhouse-induced climate change (early agriculture to modern): the role of ocean feedbacks

“…13 Hypothesized early-anthropogenic CO 2 anomaly during the Holocene is the difference between the observed Dome C values (red circles) and the blue-line projection of natural trends. Direct anthropogenic emissions explain ∼25% of the total anomaly (9-10 ppm), with the rest ascribed to ocean feedbacks: decreased CO 2 solubility in a warmer ocean (5-9 ppm), and contributions from other consequences of anomalous warmth in the Southern Ocean (see text) Stouffer 1980;Wang et al 2002). However, we assume that these differences are likely to be small in our case.…”

The Plio-Pleistocene development of Atlantic deep-water circulation and its influence on climate trends

“…At high mean ocean temperatures, high latitude cooling is more effective at densifying surface waters and initiating deepconvection (De Boer et al, 2007), which may explain the widespread ventilation of the deep-ocean during the Pliocene. As the climate cooled, reduced atmospheric northward moisture transport and an increased equator-to-pole temperature gradient result in intensified deep-water production, particularly in the North Atlantic where surface water salinity is greater (De Boer et al, 2007;Wang et al, 2002). This allowed the long-term (>10 5 year) increase in AMOC after~2.4 Ma, providing a regionally important negative feedback to global cooling.…”