The Ocean is Shaping Tropospheric Adjustments

Tuesday, 16 December 2014
Maria Rugenstein, Jan Sedlacek and Reto Knutti, ETH Swiss Federal Institute of Technology Zurich, Zurich, Switzerland
We analyze the ocean’s role in shaping the atmosphere’s fast response to radiative forcings with a 120-member ensemble of the coupled climate model CESM. Recently, it has been argued that the initial tropospheric adjustment to a forcing agent should be treated as part of the forcing and not as a feedback, as long as the surface temperature response is negligible. In most of these studies, the ocean is not considered to play an active role in the adjustment phase.

We show that on time scales of months to years after a forcing, the ocean heat uptake (OHU) strongly varies spatially. While the tropical Pacific initially loses heat, locations of high OHU shift poleward during the first three years, before becoming stationary. The ocean is passively and actively linked to the atmospheric fast responses via latent heat exchange, freshwater redistribution, and the rate and efficiency of local heat uptake. Globally integrated, the OHU efficiency is as high as the climate feedback parameter for more than a decade. Since the oceanic adjustment involves surface heat fluxes, differentiating between time scales of adjustment and feedbacks is even more convoluted than in the atmosphere.

We conclude that analysing the ocean’s role is necessary to quantify a forcing’s short time scale contribution to transient and equilibrium climate sensitivity because of (1) the oceans’ impact on the magnitude of the atmospheric adjustment and (2) the initially very high OHU efficiency.