Coupled North Atlantic Subdecadal Variability In Cmip5 Models

The interaction between the atmosphere, specifically the North Atlantic Oscillation, and the North Atlantic Ocean circulation on subdecadal time scale is analyzed in a subset of models participating in the Coupled Model Intercomparison Project phase 5. From preindustrial control runs of at least 500-year length, we derive anomaly patterns in the atmospheric and ocean circulation and of air-sea heat exchange. All models simulate a distinct dipolar oceanic overturning anomaly at the subdecadal time scale, with centers at 30 degrees N and 55 degrees N. The dipolar overturning anomaly goes along with marked anomalies in the North Atlantic sea surface temperature and gyre circulation. Lag-regression analyses demonstrate, with relatively small ensemble spread, how the atmosphere and the ocean circulation interact. The dipolar anomalies in the overturning are forced by North Atlantic Oscillation-related wind stress curl anomalies. Anomalous surface heat fluxes in concert with anomalous vertical motions drive a meridional dipolar heat content anomaly in the upper ocean, and it is this dipolar heat content anomaly which carries the coupled system from one phase of the subdecadal cycle to the other by reversing the tendencies in the overturning circulation. The coupled subdecadal variability derived from the Coupled Model Intercomparison Project phase 5 models is characterized by three elements: a wind-driven part steering the dipolar overturning anomaly, surface heat flux anomalies that support a heat buildup in the subpolar gyre region, and the heat storage memory which is instrumental in the phase reversal of the North Atlantic Oscillation.Plain Language Summary The climate variability of the North Atlantic region is largely determined by interactions between atmosphere and ocean. The North Atlantic Oscillation in the atmosphere and variations in the oceanic meridional heat transport are key phenomena in the Atlantic sector and responsible for large fractions of variability. In the ocean, the meridional heat transport is closely related to the Atlantic meridional overturning circulation, and to the subtropical and subpolar gyres. We derive the interaction between the atmosphere and ocean, on the subdecadal time scale in a subset of 12 climate models. Here we find a distinct dipolar overturning anomaly with centers at 30 degrees N and 55 degrees N and a period between six and eight years. Marked anomalies in North Atlantic sea surface temperature and gyre circulation go along with the dipolar overturning variability. Anomalous wind fields in concert with anomalous vertical motions in the ocean changes the heat content pushed the coupled system from one North Atlantic Oscillation phase to the other. The subdecadal variability is characterized by three elements: a wind-driven part steering the overturning, surface heat flux anomalies supporting an oceanic heat buildup, and the heat storage memory which is instrumental in the phase reversal of the North Atlantic Oscillation.