New precision gravity data from ESA’s GOCE satellite revealed that the sea is lower heading north along the east coast of North America. Winds, currents, tides and different temperatures cause seawater to pile up in some regions and dip in others, so in fact, the height of sea is not the same everywhere on our planet.
Geodesists and oceanographers have disagreed about whether the sea is higher or lower heading north along east coast of North America. Just until recently, geodesists thought that the height of the sea increased with latitude along the Atlantic coast from Florida to Canada and now new GOCE measurements backed up oceanographers theories. Oceanographers were aware of the influence that the Gulf Stream would have on the height of the sea along the coast.
Most modern computer models of ocean circulation suggest that sea level falls travelling north, especially along the Florida coast. As this major current then sweeps away from North Carolina, coastal sea level should be essentially flat thereafter. This contrasts with the Pacific coast where there is no significant slope with latitude. The height of Earth's oceans changes by about 150 meters (almost 500 feet) from the north Indian Ocean (off the south coast of India) and western Pacific Ocean (off of New Guinea).
Until recently, conventional levelling, carrying surveying instruments thousands of kilometres and combining measurements in the national surveying datum that were assumed to be level reference surfaces, were used to determine conditions of sea height. Back from the 1920s, conclusions were based on traditional methods that connect values of mean sea level from tide gauge measurements.
It is quiet difficult to determine relative heights of the sea, especially near the coast. Waves, tides, ocean currents, ocean eddies and temperature of the upper ocean water are main factors to contribute the sea surface heights. Except for gravity, each changes over days, months, or a few years. Of these remaining factors, basin-wide ocean circulation is the most stable - it does not change significantly over periods of days or months.
GOCE maps variations in Earth’s gravity with extreme detail, resulting with a unique model of the ‘geoid’, which is essentially a virtual surface where water does not flow from one point to another. The new geoid and in situ gravity measurements have been used as a reference to establish levelling heights. Combining the GOCE geoid and GPS heights at tide gauges provides indirect means of calculating sea heights by levelling along coastlines. The new results prove conclusively that sea level decreases going north along the North American Atlantic coastline, in agreement with the ocean models. The findings are important for establishing a common height reference system between the US, Canada and Mexico.
“Data from GOCE will lead to a unified global height system so that we can consistently study sea-level change apparent in tide gauge and satellite altimetry data.” Reiner Rummel from the Technical University of Munich
Through ESA’s Support to Science Element programme, scientists from the National Oceanography Centre Liverpool in the UK, the Technical University of Munich in Germany and Newcastle University in the UK have developed a new method that largely uses GOCE data to determine a reference level surface. This work complements that of Dalhousie University in Canada and other oceanographic research groups also making use of new geoid information.
Featured image credit: JPL/NASA