Behavior of Midlatitude Decadal Oscillations in a Simple Atmosphere-Ocean System
Masahiro Watanabe and Masahide Kimoto
Center for Climate System Research, University of Tokyo
Toward a physical understanding of the decadal oscillation found in midlatitude sea surface temperatures (SSTs), a numerical study was carried out using a simple atmosphere-ocean coupled model. The ocean model consists of the linearized shallow-water, quasi-geostrophic equation and the mixed-layer temperature equation, while the atmospheric perturbations are expressed by steady equations which employ an empirical relationship between SST and surface wind anomalies. In the model, the empirical relation manifests the positive wind-evaporation feedback on decadal scales. The coupled system was integrated to investigate the behavior of numerical solutions depending on three parameters of the mean current strength (λc), dynamical and thermodynamic coupling coefficients (λt and λh, respectively).
For extreme cases of λc=0 or λt=0, oscillation occurs when λt or λc is sufficiently large, except for the case of λt=0 in the North Pacific. The geostrophic current anomaly induced by the long Rossby wave is responsible for the oscillation for λc=0 while the advection by the mean gyre is crucial for the oscillation for λt=0. Behavior of solutions in the parameter space shows that the advective effect is of importance as well as the wave adjustment in the North Atlantic. On the other hand, the oscillation in the North Pacific is nearly due to the wave dynamics although the gyre advection affects the period of oscillation.
Stochastic weather noise introduced into the system increases the variance of decadal oscillation if the noise is distinctly larger than the coupled anomalies. The amplification results from the resonance of the intrinsic mode with the noise, and the resonant oscillation is slightly shifted toward lower frequency. However, the stochastic noise with comparable magnitude with the coupled component rather suppresses the decadal variance. Although the noise forcing without coupling reddens the temperature spectrum, the decadal peak due to the resonance can be still dominant.
Effect of interdecadal variations in wind intensity and density stratification on ocean ecosystem in the North Pacific
Takashige Sugimoto*1 and Kazuaki Tadokoro*2
*1 Ocean Research Institute, Univ. of Tokyo
*2 Japan National Res. Inst. of Far Seas Fisheries
Influence of wind intensity and density stratification and their interannual-interdecadal variations on the spatial and temporal variations in the plankton biomass and fish population in the subtropical North Pacific are discussed. Biomass of the plankton decreased after the early 1980's in most of the places of the North Pacific, which related to the intensification of the wind mixing and density stratification.
