Ocean surface topography

Ocean surface topography or sea surface topography, also called ocean dynamic topography, are highs and lows on the ocean surface, similar to the hills and valleys of Earth's land surface depicted on a topographic map. These variations are expressed in terms of average sea surface height (SSH) relative to Earth's geoid. The main purpose of measuring ocean surface topography is to understand the large-scale ocean circulation. Unaveraged or instantaneous sea surface height (SSH) is most obviously affected by the tidal forces of the Moon and the Sun acting on Earth. Over longer timescales, SSH is influenced by ocean circulation. Typically, SSH anomalies resulting from these forces differ from the mean by less than ± at the global scale. Other influences include temperature, salinity, tides, waves, and the loading of atmospheric pressure. The slowest and largest variations are due to changes in Earth's gravitational field (geoid) due to the rearrangement of continents, formation of sea mounts and other redistribution of rock. Since the Earth's gravitational field is relatively stable on decadal to centennial timescales, ocean circulation plays a more significant role in the observed variation of SSH. Across the seasonal cycle changes in patterns of warming, cooling and surface wind forcing affect circulation and influence SSH. Variations in SSH can be measured by satellite altimetry (e.g. TOPEX/Poseidon) and used to determine sea level rise and properties such as ocean heat storage. Ocean surface topography is used to map ocean currents, which move around the ocean's "hills" and "valleys" in predictable ways. A clockwise sense of rotation is found around "hills" in the northern hemisphere and "valleys" in the southern hemisphere. This is because of the Coriolis effect. Conversely, a counterclockwise sense of rotation is found around "valleys" in the northern hemisphere and "hills" in the southern hemisphere. Ocean surface topography is also used to understand how the ocean moves heat around the globe, a critical component of Earth's climate, and for monitoring changes in global sea level.