A Coupled Air–Sea–Monsoon Oscillator for the Tropospheric Biennial Oscillation

Abstract

The cause of the tropospheric biennial oscillation (TBO) in a simple coupled ocean–atmosphere model is examined. The model is first reduced to a pair of coupled linear first-order differential equations, piecewise in time, for analysis. It is found that two ingredients are essential for the biennial oscillation in the model. The first ingredient is the amplification of SST perturbations in both the Indian Ocean and western Pacific in opposite directions during the northern autumn, winter, and spring seasons, reflecting a positive feedback process. The second ingredient is the decay and change of signs of the SST anomaly in the western Pacific during the northern summer, representing a negative feedback process. Under such a scenario, the simple model exhibits a regular biennial oscillation. Diagnosis of the model TBO reveals that the western Pacific SST and zonal wind anomalies have a lagged correlation at a timescale of 2–3 months, similar to observations. Such a phase lag results from both remote and local ocean–atmosphere–land interaction processes. The remote processes involve the large-scale east–west circulation associated with anomalous monsoon heating, whereas the local processes include the ocean horizontal and vertical advection and surface wind–evaporation–SST feedback. It is concluded that the phase lag between the SST and wind is a result rather than a cause of the TBO. Oscillatory and nonoscillatory regimes of the model’s solutions are obtained with the tuning of key parameters within realistic ranges. It is found that the model TBO is sensitive to both internal air–sea coupling coefficients and external basic-state parameters. With the slight change of these parameters, the model may undergo a bifurcation from a TBO regime to a chaotic regime or an annual oscillation regime—a possible scenario for the TBO irregularity. In particular, with a specification of interdecadal change of the basic-state wind, the model may undergo a continuous warming pattern in the eastern Pacific, resembling the prolonged El Niño condition in the early 1990s.