1999 Volume 77 Issue 6 Pages 1185-1197
A hydrologic excitation of ten-yearly polar motion appearing in the combined five-daily polar motion data of International Polar Motion Service (IPMS) and SPACE94 during 1962-1995 is discussed. The discussion is based on time-dependent exponential decay models of land water storage supplied by precipitation, in which the National Oceanic and Atmospheric Administration (NOAA) monthly gridded precipitation anomaly data for 5°×5° is used. The decay parameter of the land water storage τd is determined when square of the difference between the observed ten-yearly polar motion and calculated one becomes minimum. Amplitude in the calculated polar motion can explain about 36% of that in observed ten-yearly polar motion when τd takes 3.7 months. The behavior of the calculated polar motion is similar to the observed one though the phase of the former leads that of the latter by about one year. When we do not consider the small phase difference, about 52% of amplitude in the observed polar motion can be explained when τd takes 5.1 months, suggesting a globally averaged decay time of the land water storage. The seesaw-like changes of precipitation that excite efficiently the ten-yearly polar motion are also confirmed between the North American continent and the western region of the Eurasian continent. These facts suggest a conceptual model of the feedback system on decadal hydrologic cycle centered in the North Atlantic Ocean, in which the system consists of atmospheric variations connected with the North Atlantic Oscillation, seesaw-like sea level changes, and precipitation changes in the North American and Eurasian continents.