Before we address the AAIW variability seen in the Argo
floats in detail it is necessary to understand the features of
the underlying climatological mean field seen in the WOA
data. AAIW in the South Pacific Ocean is supplied from
two different sources with slightly different minimum salinities
(Tomczak and Godfrey, 2003). The two AAIW varieties
meet in the New Zealand region, which leads to water movement
at 700–1100m depth across strong salinity gradients
(Fig. 5) in the western South Pacific.
In contrast, the eastern South Pacific is characterized by
the spreading of a single AAIW variety from its formation
region near southern Chile to the tropics. In the south this
creates a strong meridional salinity minimum gradient and a
“shadow zone” in the tropics, where the salinity minimum is
not always well defined.
Another reason for elevated standard deviations in the
WOA-derived time series could be the data distribution in the
World Ocean Data Base (WOD01: Conkright et al., 2001).
Salinity data are far less numerous than temperature data.
Large differences in the number of observations in neighbouring
cells, or different bias towards different seasons, can
potentially lead to differently weighted means that can result
in unrealistic salinity gradients. Strong seasonal affects at
the depth of AAIW are unlikely, but data density decreases
rapidly with depth, and means from sparse data can produce
distortions of the field.
To identify possible effects of varying data density the
WOA data distribution for the 1000m depth level in the
South Pacific was investigated. Figure 6 shows some