Abstract Author: Feng He / Zhengyu Liu
Abstract Title: The Nature of the Decadal Variability of Surface Climate Over the North Atlantic Ocean -------- Coupled Ocean-Atmosphere-Sea Ice Interaction Organized by Damped Ocean Mode
Abstract: The nature of the observed 11~14-year decadal variability of the surface climate over the North Atlantic Ocean is investigated using the Fast Ocean Atmosphere Model (FOAM) and its data-atmosphere configuration. A 14~16-year damped ocean mode (DOM), characterized by the decadal variability of Atlantic Meridional Overturning Circulation (AMOC) and southward-moving Western Boundary Low Frequency Waves (WBLFW), is found to be able to organize coupled atmosphere–ocean–sea ice interaction (CAOSII) in the North Atlantic and produce the decadal variability of the surface climate in this region. In the North Atlantic, the coupled simulation produces a faithful simulation of the observed decadal variability in terms of the pattern and periodicity of the main variability of sea surface temperature (SST), surface air temperature (SAT) and sea level pressure (SLP). Moreover, the coupled simulation also bears the observed dipole pattern of dynamic topography in West North Atlantic and the Great Salinity Anomaly (GSA)-like decadal salinity signature in the subpolar gyre. These connections between the coupled simulation and observations suggest COASII organized by DOM is a plausible explanation for the observed decadal variability in this region.
In the coupled simulation, two physical processes, namely SAT-convection feedback and sea ice-convection feedback, were found to be critical for the decadal variability of surface climate in this region. The SAT-convection feedback is the local effect of SAT on oceanic convections in the Labrador Sea. Under cold episodes of surface climate in the North Atlantic, DOM is pushed to its "positive" mode when the cold air induces stronger convective activity in the Labrador Sea. The stronger convections produces more vigorous AMOC and the trough of WBLFW in the subpolar gyre. The clockwise circulation associated with the trough of WBLFW induces more heat transport into the subpolar North Atlantic and generates warmer SST and SAT. This gives rise to the warm episodes of surface climate in the North Atlantic and a new cycle starts. The sea ice-convection feedback is the effect of salinity anomaly on oceanic convections in Labrador Sea due to nonlocal sea ice melting/growing. The aforementioned warmer SAT induces more sea ice melting and results in low sea surface salinity (SSS) in the northern subpolar gyre, especially in Irminger Sea. After the low salinity is transported into Labrador Sea, it suppresses local convective activity and acts jointly with SAT-convection feedback to switch DOM into its "negative" mode.
DOM was uncovered in the stochastic atmospheric simulation and found to be responsible for the decadal time scale in the coupled simulations. The stochastic atmospheric simulation also reproduces the southward propagation of WBLWF and the decadal variability of AMOC, but fails maintaining the decadal variability of ocean temperature and salinity in the North Atlantic both at surface and subsurface. This discrepancy between the coupled and stochastic atmosphere simulation underscores the critical role of COASII in simulating the decadal variability of surface climate in the North Atlantic. In the coupled simulation, the atmospheric response to the subpolar SST substantially enhances the variability of DOM on decadal time scales: when the subpolar SST is warmer, the warm-ridge dynamic response of the atmosphere provides the positive feedback by inducing northward Ekman transport to further increase the warming of SST; the local SAT-convection feedback and the remote sea ice-convection feedback act jointly to provide the delayed negative feedback in COASII. In fact, given the insurmountable damping effect of the stochastic atmosphere, it is impossible to sustain the decadal variability of surface climate without any atmospheric response. Therefore it is COASII organized by DOM that generates the decadal variability of the surface climate in the North Atlantic Ocean.