Total change in the depth (m) of the O2 =
60 mmol/kg surface on the CalCOFI survey grid over the
period 1984–2006. Stations with significant linear regressions
(p < 0.05) are marked black.
seen within the Bight (Station 93.30; Figure 2, Table 1),
where source waters within the California Undercurrent
have seen a significant DO decline (Bograd et al., manuscript
in preparation). Recent observations have shown a
thickening of the OML in the equatorial Pacific, which
could lead to reduced oxygen supply to the CCS [Stramma
et al., 2008]. And at all depths below 200 m, offshore areas
affected by intrusions of Subtropical Gyre waters have seen
the largest absolute and relative DO declines (Figure 2,
Table 1). Experiments with ROMS model hindcasts for the
period 1950–2004 have been shown to capture the physical-
biological dynamics in the California Current [Di
Lorenzo et al., 2008]. Further analysis of these experiments
(not shown) suggest that there has been increased advection
of Subtropical Waters (characterized by high temperature,
low DO) into the SCB since 1984.
[13] It is important to note that the observed DO declines
off California could have been forced locally, through
thermodynamic or biological processes, or remotely (e.g.,
a large-scale shoaling of the pycnocline and OML), and
subsequently advected to the region. We cannot distinguish
between locally-induced DO changes and the advection of
remotely-altered water masses, although both processes are
probably important. Quantification of the relative impact of
advection, stratification, and local changes in production/
respiration on the DO trends will require carefully planned
measurements as well as dedicated coupled physical-biological
modeling experiments.