A numerical investigation of decadal variability in the Pacific Ocean

An ocean general circulation model is used with a data assimilation procedure to investigate oceanic processes contributing to exchanges between the tropical and extratropical Pacific Ocean on decadal timescales, and the relative important of remote and locally forced tropical variability. Model experiments are conducted in which observed temperature data are assimilated into different regions of the Pacific basin and climatological or observational forcing fields are used at the model surface in such a way that each model run isolates an element of remote or local forcing in the tropics or midlatitudes.; Three interrelated processes are found by which the tropical thermal structure is modulated by low-frequency variability in the midlatitudes. First, temperature anomalies from the midlatitudes can be subducted into the ventilated thermocline and advected to the subtropics along isopycnal surfaces. Second, thermal anomalies in the central Pacific can be carried to the western boundary as Rossby waves, equatorward as coastally trapped Kelvin waves, and eastward as equatorially trapped Kelvin waves. Finally, as a result of oceanic adjustment to changes in wind stress curl, the strength of the subtropical gyre is altered, affecting mass transports along the western boundary, and causing changes in the equatorial circulation and thermal structure.; Decadal variability in the tropical Pacific is forced both locally and remotely. An attempt is made to quantify the relative importance of a locally forced response governed by a Sverdrup balance between the zonal winds and the local pressure gradient, and a remotely forced response representing an oceanic adjustment caused by midlatitude variability. The local response leads the remote response by approximately 5 years, and accounts for approximately 27% of the tropical low-frequency variability while the remote response explains only 17%.; From an oceanic viewpoint, tropical variability is found to have only a small impact on decadal variability in the North Pacific. Interannual variability in the tropics is communicated to the midlatitudes by the northward propagation of coastally trapped Kelvin waves and the westward propagation of Rossby waves, though the resultant changes in the midlatitudes account only for a small fraction of the total variance in the midlatitude ocean.