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 total time
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 ( ˆF
), 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.
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