Long-term regional sea level changes due to variations in water mass density during the period 1981-2007

Abstract

[1] Using a gridded ocean temperature and salinity field based on observations, the regional distribution of sea level changes during the last few decades was investigated. We calculated the baroclinic sea level change and decomposed it into vertical modes of pressure perturbation for internal waves, considering the vertical structure of the baroclinic pressure change. The first mode is associated with vertical displacement of the main pycnocline, which is generally a dynamical response to wind forcing, as demonstrated by previous studies using shallow water models. Regional sea level variations associated with this mode have large magnitudes, especially in the tropics, where, as shown by previous studies, interannual variability is relatively large compared to sea level changes. Other factors affecting sea level include changes in water mass density and horizontal movement of water masses. These are characterized as second and higher modes, and their combined effect on the amplitude of regional sea level changes is comparable with that of the first mode, especially in subtropical gyre regions. In the North Pacific, the combined second and higher order modes give rise to significant positive sea level trends. These long-term sea level trends are induced by steric contributions resulting from warming and freshening of the subtropical mode waters. This result suggests that changes in water mass properties such as temperature and salinity are also important for replicating local sea level change, especially in the subtropical gyre region.