Mean flow and interannual variability in the Pacific Ocean

Two different four-layer, semi-spectral atmospheric models are coupled to the same mixed-layer dynamic ocean model. The first model (MI) is a full primitive equation model with a zonally symmetric basic state that varies with time, representing the annual cycle. The second model (MII) is linearized about a zero zonally symmetric basic state. MII is the model type used by most coupled ocean atmosphere systems that successfully produce ENSO-like oscillations in the Pacific Ocean. The model geography consists of land representing the African and Indian continents, the Indian Ocean and the Pacific Ocean. The Indian and Pacific Oceans are separated by a thin solid wall and the two oceans communicate with each other only through the atmosphere which is cyclic. Both the models are forced with an annual monsoonal heating function over the land and a parameterized heating over the ocean. The atmospheric winds force the oceans through the wind stress. Heating from the ocean forces the atmosphere through latent heating.;The Indian Ocean response in both of these coupled models is similar as it is closely tied to the annual cycle of the forcing. The response of the Pacific Ocean is very different in the two models. Only MII shows low frequency, large amplitude variations of ENSO character. With MI, the system is locked in the cold phase of the ENSO cycle with no evidence of Pacific Ocean interannual variability. When the characteristics of MII are analyzed in detail, it is found that the basic state is given by the atmospheric wave number zero solution. The primary effect of the zonally symmetric component is to accumulate mass along the western Pacific Ocean through mass transport. The ocean responds to the increased mass through eastward propagating equatorial Kelvin waves. The Kelvin depresses the thermocline increasing the Sea Surface Temperature (SST) in the east. The atmospheric convection moves eastward in response to the SST. It is concluded that for coupled models to exhibit interannual variability in the Pacific Ocean, a basic state which has a meridional convergence into the heat source with sufficient strength is necessary. Within this system, strong dissipation near the surface is necessary. In fact, using the more realistic atmospheric model MI with strong dissipation also produces interannual variability.