Large-Scale, Low-Frequency Variability in Wind-Driven Ocean Gyres

Abstract

The authors investigate the spontaneous occurrence of large-scale, low-frequency variability of steadily forced, two-gyre, wind-driven circulations. The model dynamics is quasigeostrophic, the density stratification is represented in 1.5- and 2-layer approximations, and the wind stress pattern is either asymmetric or symmetric about the midbasin. The authors show that more generic variability arises when the forcing is strongly asymmetric, the Reynolds 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 steady circulation, periodic and quasiperiodic fluctuations near the beginning of the bifurcation tree, and chaotic circulations characterized by a broadband spectrum. Both the primary and secondary bifurcation modes and the spatiotemporal 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 instability 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 gyre patterns. The variability of the circulation forced by asymmetric wind differs substantially from the symmetric forcing 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 and its associated western-basin recirculation zone. This variability occurs smoothly in time, albeit irregularly with a broadband spectrum.