| Atmospheric General Circulation Models (AGCMs) forced with sea surface temperature (SST) conditions have become a common tool to perform predictions beyond the two-weeks limit of atmospheric predictability. These models are usually integrated with prescribed SST anomalies (either forecasted or fixed initial observed anomalies), thus assuming a one-way interaction process in which the ocean always forces the atmospheric anomalies without any feedbacks. Hence, this approach is not optimal in situations when the atmosphere is the driver. In this study we use a dynamical rule based on the phase relationship between the ocean and the atmosphere to diagnose the forcing direction in locally coupled anomalies. Under this guidance, we assess the relative importance of ocean-driving versus atmosphere-driving over the globe and examine the implications of using ocean-driving scenarios on atmospheric simulation and prediction.; This guidance applied to the NCEP/NCAR Reanalysis data confirms that the atmosphere tends to drive the ocean in the extratropics, whereas the reverse is true in the tropics. It is also found, for the first time, that the longer-lasting extratropical and tropical anomalies tend to have, respectively, atmosphere-driving and ocean-driving phase relationships. A similar procedure applied to data from a one-way interaction model, with the same model and same SST used for the Reanalysis but without data assimilation, produces fewer (more) long lasting anomalies in the extratropics (tropics) than observed. This indicates that ignoring the atmosphere's feedback effect on the ocean leads to erroneous shortening (lengthening) of atmospheric anomalies in the extratropics (tropics).; The introduction of a “coupling” index, which accounts for both fraction area and average strength of the forcing direction in a region, allows an a priori assessment of the skill of simulation and prediction in one-way interaction model scenarios. We show that prescribing observed SST is detrimental to the simulation and prediction of extratropical anomalies that are predominantly in atmosphere-driving phase relationship. Results of an analysis of the growth of unstable modes using the “breeding” technique in low-order coupled models are included. We found very reliable rules for prediction of chaotic systems and a method to separate fast and slow modes in coupled chaotic systems. |
