海洋中有多少水？

The calculation of variations（变更） in the sea level is relatively simple. It is by far more complicated to then determine the change in the water mass. A team of geodesists（测地学） and oceanographers（海洋学家） from the University of Bonn, as well as from the GFZ German Research Centre for Geosciences and the Alfred-Wegener Institute for Polar and Marine Sciences, two centres of the Helmholtz Association, have now, for the first time succeeded in doing this. The researchers were able to observe short-term fluctuations（起伏现象） in the spatial distribution（空间分布） of the ocean water masses. Their results are, amongst others, important for improved climate models. In order to determine the ocean volume in a certain region, one only needs to know, in addition to the topography（地形，地势） of the seabed, the height of the sea level. For this purpose, researchers have long been resorting to gauging stations（水位站） and satellite altimetric（高度计） procedures. The ocean mass depends, however, not only on the volume, but also on the temperature and on the salt content. Water expands when heated. Warm water, thus, weighs less than the same quantity of cold water.

For the calculation of the ocean mass it is, therefore, necessary to know the temperature and salt content profiles. However, this is not easy to quantify（定量，量化）. "For our study we, therefore, combined different procedures so as to be able to judge changes in mass", explains Professor Dr. Juergen Kusche. The geodesist from Bonn is co-author of a scientific paper, which has just been published in the Journal of Geophysical Research.

On the one hand the researchers used data from the German-American satellite mission GRACE where the distance between two satellites (popularly known as Tom and Jerry as one chases the other in the same orbit ) are measured exactly to thousandths of millimetres. The larger the ocean mass at a certain point of the Earth, the stronger the gravitational strength. This influences the flight altitude of the satellites and thus the distance from each other. The gravitation（引力） and, hence（今后，从此）, the mass distribution can be calculated from the change in distance between the two satellites.