Multiple Satellite Missions Confirming the Theory of Seasonal Oceanic Variability in the Northern North Pacific
Using ocean-bottom-pressure (OBP) data from the Gravity Recovery and Climate Experiment (GRACE) mission, sea-surface-height (SSH) data from satellite radar altimeters, wind data from QuikSCAT, and a derived steric-sea-level (SSL) product from Kuo (2006), we have investigated the large-scale seasonal variations of the North Pacific. It is shown that the strong annual cycle of OBP variability in the northern North Pacific can be explained by the wind stress curl and the annual SSH variability is mostly due to steric changes. This confirms the classic theory of Gill and Niiler (1973); that is, (1) the seasonal OBP variability is the barotropic response to changes in the wind stress, resulting from a small residual in mass divergence from the balance between Ekman and Sverdrup transport, and (2) the SSH variability is largely produced by expansion and contraction of the water column due to changing fluxes of heat and water across the surface. However, in the semiannual frequency, the steric effect becomes insignificant, leaving both OBP and SSH to vary with the wind stress. This is because the Ekman pumping creates a low/high-pressure center in the upper ocean, which projects onto the barotropic mode and leads to a negative/positive OBP and SSH anomaly. In addition, it is found the bottom pressure torque leads the density change and the Ekman pumping velocity term by 3.4 months, suggesting a transition period is needed for the sea level to fully respond to the wind forcing.
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