Temporal variation of 137Cs water column inventory in the North Pacific since the 1960s

The temporal variation of water column inventories of Cs-137 in the North Pacific since the 1960s was examined based on the analysis of the Cs-137 profiles in HAM database. Cs-137 in seawater in the North Pacific have originated mainly from global fallout from atmospheric nuclear weapons tests, which occurred in the early 1960s. In the 1960s, both the meridional distribution of Cs-137 inventory in the North Pacific and that of fallout on land stations showed mid-latitude maximum. The region with higher deposition at land stations, however, was more northern than the latitudes where the Cs-137 inventory in the North Pacific showed a maximum. The difference of the latitude where maximum Cs-137 deposition/inventory was observed reflects the difference of the geographical distribution of the precipitation amount in the Pacific and Atlantic Oceans followed by the different warm current systems in each ocean. A good positive relation between Cs-137 inventory and annual precipitation amount was discovered in the ocean stations at the middle latitude in the North Pacific. The horizontal distribution of Cs-137 inventories at the middle latitudes in the North Pacific is characterized as west-high and east-low in the early 1960s, which was basically controlled by the distribution of annual precipitation amount. Eastward advection, then, modified it to be less difference in 1966-1967 after the highest deposition periods in 1963-1965. In the 1970s and 1980s, increases of the Cs-137 inventory at the lower latitude of 10-20 deg. N are found. Surface and subsurface southward transports are considered as the source of this increasing Cs-137 inventory. (C) 2003 Elsevier Science Ltd. All rights reserved.