The mean seasonal cycle in sea level estimated from a data-constrained general circulation model

A near-global ocean state estimate over the period 1992-2004 is used to study the mean seasonal cycle in sea level ζ. The state estimate combines most available observations, including all the altimetric missions, with a general circulation model in an optimization procedure. The annual cycle tends to be larger than the semi-annual one, except in tropical regions. For global mean ζ, annual thermosteric and freshwater terms are nearly out-of-phase and lead to an annual cycle of only a few mm in amplitude. Regionally, surface wind stress and heat flux are the primary drivers for seasonal ζ variations in the tropics and midlatitudes, respectively, with both mechanisms playing a role at high latitudes. A substantial part of the annual ζ variability can be assigned to changes in thermosteric height in the upper 100 m in midlatitudes and 200 m in the tropics. Bottom pressure variability is larger at high latitudes, and also in some regions in the Southern Ocean and North Pacific. Apparent nonlinear rectification processes lead to a noticeable impact of submonthly forcing on the annual cycle in the western North Atlantic and North Pacific. Other features include the substantial ζ gradients associated with strong spatial variability in seasonal surface heat flux in some western boundary regions, the damping effects of surface heat flux on the seasonal cycle in the tropics, and the importance of wind driving and bottom pressure in shallow regions, which can cause differences in the seasonal cycle in some coastal and contiguous deep-ocean regions.