Significance of non‐rotational wind stress in the seasonal variation of continental torque

The physical mechanism by which seasonally varying atmospheric wind stress exerted on the sea surface is communicated to the solid earth as oceanic pressure torque (continental torque) and bottom frictional torque is investigated with a linear shallow-water numerical model of barotropic oceans. The model has a realistic land–ocean distribution and is driven by a seasonally varying climatic wind stress. A novel way to decompose the wind stress into rotational and non-rotational components is devised. The rotational component drives ocean circulations as classical theories of wind-driven circulations demonstrate. The non-rotational component does not produce ocean circulations within the framework of a barotropic shallow-water model, but balances with the pressure gradient force due to surface displacement in the steady state. Based on this decomposition, it is shown that most of the continental torque which plays a major role in producing the seasonal variation of length of day (LOD) is caused by the non-rotational component of the wind stress. Both continental torque due to the wind-driven circulation produced by the rotational component of the wind stress and the bottom frictional torque are of minor importance