Mode Waters And The North Pacific Subtropical Countercurrent: Its’ Decadal Variability And The Response To Global Warming

Mode waters and the North Pacific subtropical countercurerent (STCC): itsdecadal variability and the response to global warming are examined based on thecoupled general circulation models from the Coupled Model Intercomparison ProjectPhase5(CMIP5). Decadal variability of the STCC is examined in a300-yr controlsimulation by the Geophysical Fluid Dynamics Laboratory coupled model(GFDL_CM2.1). Response of the STCC and its variability to global warming isexamined in the CMIP coupled models that are forced by increasing greenhouse gasconcentrations. Discrepancy of the mode water and STCC simulation in climatemodels is also investigated by comparing ocean general circulation models whereeddies are either parameterized or resolved, and observations. Some new results arefound as follows:1. On decadal timescales in the central subtropical gyre (170E-130W,15-35N), the dominant mode of sea surface height variability is characterized by thestrengthening and weakening of the STCC due to variations in mode-water ventilation.In oceanic and coupled models, northeast-slanted bands often appear in anomalies oftemperature and circulation at and beneath the surface. Our results show that suchslanted bands are characteristic of changes in mode-water ventilation.2. Compared with the present climate, the upper ocean is more stratified, andthe MLD shoals in warmer climate. Due to the reduced subduction rate and a decreasein sea surface density, mode waters form on lighter isopycnals with reduced thickness.Advected southward, the weakened mode waters decelerate the STCC，resulting inbanded structures in sea surface warming. The STCC mode decays as CO2concentrations double in the twenty-first century, owing both to weakened modewaters in the mean state and to reducedvariagbility in mode waters.3. The full evolution of the North Pacific mode water and STCC in globalwarming is accessed over300years following the RCP4.5. The mode water and STCC first show a sharp weakening trend when the radiative forcing increases, butthen reverse to a slow strengthening trend of smaller magnitude after the radiativeforcing is stablized.4. There are large discrepancies in mode water subduction and dissipationpatterns between eddy resolving and non-eddy resolving models. The eddysubduction significantly enhances the total subduction rate. After subduction, the lowPV water decay rapidly along the mean trajectory of the mode water cores, due to thestrong dissipation effects by eddies.