Temporal and Spatial Variability of Global Upper Ocean Freshwater Content

Abstract

Observational salinity data from the Global Temperature and Salinity Profile Program (GTSPP) are used to identify temporal and spatial variability of the upper ocean freshwater content (FWC). First, the optimal spectral decomposition (OSD) is used to build up monthly synoptic temperature and salinity data set from January 1990 to December 2009 on 1o×1o grids and the same 33 vertical levels as the World Ocean Atlas 2005. Then, the monthly varying upper layer FWC fields (F) with four different layer thicknesses (surface to 50, 100, 300, and 700 m depths) are obtained. Second, a composite analysis is conducted to obtain the totaltime mean FWC field ( F ) and the mean annual FWC variability ( F %), which is found an order of magnitude smaller than F . Third, an empirical orthogonal function (EOF) method is conducted on the residue data ( Fˆ ), deviating from F + F % , in order to obtain non-annual variability of the FWC fields. The first two EOF modes account for 63.6-76.4% and 6.7-6.8% of the variance for the four layer thicknesses, representing near-global scale (EOF-1, dominant mode) and northern-southern alternative (EOF-2, minor mode) freshening/salinization events. The near-global scale freshening/salinization has a decadal trend from salinization before 2002 to freshening after 2002. The northern-southern alternative freshening/salinization does not have evident trend.