Large-scale, low-frequency variability in wind-driven ocean gyres

The authors investigate the spontaneous occurrence of large-scale, low-freq uency variability of steadily forced, two-gyre, wind-driven circulations. T he model dynamics is quasigeostrophic, the density stratification is repres ented in 1.5- and 2-layer approximations, and the wind stress pattern is ei ther asymmetric or symmetric about the midbasin. The authors show that more generic variability arises when the forcing is strongly asymmetric, the Re ynolds number is relatively large, and the baroclinic instability mechanism is active. The variability is explored for a wide range of values for the viscosity coefficient, that is, the Reynolds number. The regimes include st eady circulation, periodic and quasiperiodic fluctuations near the beginnin g of the bifurcation tree, and chaotic circulations characterized by a broa dband spectrum. Both the primary and secondary bifurcation modes and the sp atiotemporal patterns within certain frequency bands in the chaotic regime are analyzed with an EOF decomposition combined with the time filtering. In the symmetric case the 1.5-layer flow develops anomalously low-frequency fluctuations with a very non-Gaussian distribution. The baroclinic instabi lity that arises in a 2-layer flow tends to weaken and regularize somewhat the low-frequency variability, but it still has the character of infrequent transitions between distinct pyre patterns. The variability of the circula tion forced by asymmetric wind differs substantially from the symmetric for cing case. In 2-layer solutions the power at low frequencies progressively increases with the Reynolds number. The dominant low-frequency variability is associated with changes in the position and shape of the eastward jet an d its associated western-basin recirculation zone. This variability occurs smoothly in time, albeit irregularly with a broadband spectrum.