Robustness of the Heat Released from the Atlantic Meridional Overturning Cell to the Atmosphere

Climatologists have been paying much attention to the question of how the climate of the northern hemisphere would change if the Atlantic Meridional Overturning Cell (AMOC) were to slow down. Since less heat would be released to the atmosphere, it has been suggested that a slow down of the AMOC would cause the northern hemisphere to cool. There are large international observational programs focusing on the question of how much the AMOC will slow down and how much the atmosphere will cool. Our research uses a sequence of nonlinear analytical models to show that the northward heat transport by the AMOC is very large compared to the heat released from the AMOC to the atmosphere and due to the non-linearity of the system even changes of 50% in the AMOC transport hardly change the atmospheric temperature. Our most realistic atmospheric convection model involves a warm oceanic current losing heat to an otherwise motionless and colder atmosphere. As a result of this heat exchange, the atmosphere convects and the generated air flow ultimately penetrates horizontally into the surrounding air in a way similar to the airflow in the familiar heat-island problem. The heat exchange near the air-sea interface follows the familiar bulk formulae and the compressible, convective atmosphere penetrates into a stably stratified atmosphere with a constant lapse rate (outside the convective region). This research illuminates the critical importance of the four key aspects of the heat exchange process: (i) the non-linear dependence of the atmospheric mass transport on the ocean mass transport, (ii) the dependence of the atmospheric transport on the ocean heat capacity and the large heat capacity ratio of water to air (~4) (iii) the proportionality of the heat-flux to the temperature difference between the ocean and the air (for our simplest conceptual models where saturation specific humidity, Bowen ratio, relative humidity are assumed to be constant), and (iv) the fact that the ocean is warmer (by 10-20 degrees) than the air above it. These four aspects of the problem combine in the non-linear system and lead the system to a saturation state where even significant changes in the AMOC have almost no effect on both the ocean/air heat-flux and the resulting outgoing atmospheric temperature. In the hypothetical limit of infinitesimally large specific heat capacity of water, there is no change in the atmospheric transport or the temperatures of the ocean and the atmosphere, regardless of the reduction in the AMOC transport.