The role of the annual cycle in coupled ocean-atmosphere variability in the tropical Atlantic Ocean.

摘要

Many previous studies point to a connection between the annual cycle and interannual variability in the tropical Atlantic Ocean. Evidence for this relationship in observations will be reviewed. To investigate the importance of the annual cycle in modulating tropical Atlantic variability (TAV) as well as its associated coupled feedback and forcing mechanisms, a set of controlled experiments is conducted using a global coupled ocean-atmosphere general circulation model (GCM) in which the climatological annual cycle is modified. The two component models are the Center for Ocean-Land-Atmosphere's (COLA) atmospheric; GCM and the Poseidon ocean GCM. An anomaly coupling strategy was developed to improve the model simulated annual cycle and mean sea surface temperature (SST), which is critical to the experiments. Experiments include a control run in which the annual cycle is present and several fixed annual cycle runs in which the model is forced to remain in perpetual annual, spring, or summer conditions corresponding to the observed annual cycle. Results reveal that the patterns of TAV are present even in the absence of the annual cycle; thus, the structure of the patterns is not dependent on the annual cycle. The mean ocean-atmosphere state is the determining factor for the structure of this variability. The role the annual cycle plays is to modulate this variability. There are three ways in which this happens: seasonal background states can promote the growth of particular variability, forcing mechanisms and remote influences are active during certain seasons, and in some cases the progression from season to season is necessary for variability to occur. Examples from the research for each of these cases will be presented.