Low‐frequency internal oceanic variability under seasonal forcing

Abstract

A series of numerical experiments are conducted using the Bryan‐Cox ocean general circulation model to investigate the potential existence of low‐frequency variability of the thermohaline circulation under seasonal forcing. Experiments are performed with different combinations of a seasonal cycle being present or not on the restoring temperature, the surface freshwater flux fields (mixed boundary conditions), and the surface wind forcing. Despite the presence of forcing on the dominant seasonal time scale, it is found that the system may oscillate at the decadal period or longer. The decadal variability is excited by changes in the net surface density flux which are due to the advection of temperature and salinity anomalies in the model domain. The magnitude of the seasonal cycle also plays an important role in determining the time scale of variability. Violent overturning events may occur on the century time scale under seasonal forcing. The magnitudes of the flushes are reduced compared with those found in similar experiments without the presence of a seasonal cycle.