Chaotic Behaviors in the Response of a Quasigeostrophic Oceanic Double Gyre to Seasonal External Forcing

Abstract

In an oceanic double-gyre system, nonlinear oscillations of the ocean under seasonally changing external forcing are investigated using a 1.5-layer quasigeostrophic model and a simple model related to energy balance of the oceanic double gyre. In the experiments, the variable parameter is the amplitude of external seasonal forcing and the Reynolds number is fixed as 39, at which periodic shedding of inertial subgyres occurs. The authors found that entrainment (at 2 times the period of the forcing) and intermittency (on-off type), phenomena that are often seen in nonlinear systems, emerge with increasing amplitude of the forcing. They seem to be related to the generation mechanism and characteristics of long-term (from interannual to decadal) variations in the strong current region of subtropical gyres such as the Kuroshio and its extension region.

Low-frequency variability of the separated western boundary current in response to a seasonal wind stress in a 2.5-layer model with outcropping

“…This solution equilibrated to a stable period orbit with a period of 8 months. Shimokawa and Matsuura (2010) similarly showed with a time-dependent forcing but a quasi-geostrophic model that at least two different regimes were possible. Here, no parameters are modified.…”

“…Usually, the forcing fields are stationary in these approaches. To our knowledge, the influence of a time-varying forcing has only been investigated by Shimokawa and Matsuura (2010). Using a 1.5-layer quasi-geostrophic model, they showed that synchronization appeared between an intrinsic frequency of the system and half the frequency of the forcing; on the other hand, intermittency (irregular variations) emerged with an increasing amplitude of the forcing.…”