Climatologies and Variability from the WOCE High-Resolution
XBT Network in the Pacific
Bruce D. Cornuelle, Dean H. Roemmich, Janet Sprintall, Lynne Talley
(Scripps Institution of Oceanography, UC San Diego), Michele Morris
(NIWA), Rick Bailey (CSIRO)
The World Ocean Circulation Experiment (WOCE) repeated high-resolution
bathythermograph (HRXBT) network in the Pacific is used as the basis for
climatological anaylses. The network presently consists of 7 sections, spanning
the Pacific basin, which repeat the same Volunteer Observing Ship tracks
approximately every 3 months (See Figure 1). A technician or scientist is
accommodated on board the ship in order to sample around the clock using
an automatic XBT launcher. Precise XBT locations are controlled by GPS
Temperature sampling is to 800 m using Sippican Deep Blue XBT probes.
conductivity and temperature profiler (XCTD) profiles are taken by the observer
at key locations to supplement the historical T-S relations. Over 1300 XBT
profiles are obtained from each repeat of the network. Sampling begins and
ends near the shelf break in waters between 100 and 200 m depth. Probe spacing
is as large as 50 km in mid-ocean quiet areas, increasing near boundaries,
the Equator, islands, and other regions of enhanced variability to a maximum
of about 10 profiles per degree. The sampling is designed to capture most
of the variance associated with mesoscale features and boundary currents.
The first HRXBT sampling began in 1986, with new sections started in 1991,
1993, and 1997.
The HRXBT sections with at least 6 years of data are used to estimate the
seasonal march of temperature, heat content, and steric height. After removal
of the seasonal march, the residuals are smoothed in time to separate the
mesoscale variability from the interannual variability in the sections.
The residuals show many examples of persistent small-scale structure, including
horizontal variations in the annual cycle. Computing the covariances and
correlations between points on the sections show significant non-local and
small-scale features in both the correlations and the EOFs used to decompose
the horizontal variability. These statistics can be used to compare to model
simulations and as simple checks on surfacing forcing climatologies.
Figure 1: Tracks of all HRXBT sections to date.
Figure 2: Comparison of section-average temperature with section-average
transport. In the p50/p34 transect, the temperature has been corrected for
variability of the trackline using the meridional temperature gradient from
Levitus. Black is net geostrophic transport northward (left hand scale).
Red is average temperature (right hand scale). Note that the temperature
scale is inverted.
Figure 3 is the matrix plot.
px38 px37 px31 px06 px34 px50
Time mean, using all section repeats, estimated as part of the seasonal
to interannual time scales. The mean is estimated independently at every
point in the section, without smoothing. The estimator avoids bias due to
irregular distribution of cruises in time and fights crosstalk from the
seasonal march. Contour interval is 1 degree C.
Root-mean-square (RMS) variability around the time mean at every point in
the section. Contour interval is 0.25 degree C.
Annual Cycle Amplitude
Amplitude of annual cycle (12 month period) component. Contour interval
is 0.25 degree C.
Annual Cycle Phase
Phase of the annual cycle in months relative to January 1. A phase of 0
means that the annual cycle is at a maximum at January 1. A phase of 3 means
that the annual cycle is at a maximum at April 1. Contour interval is 1
Section average temperature, at the surface and averaged from the surface
to 200, 400, 600, and 800 m. All time axes are the same.
The time series figures on the far right of the matrix of plots (Figure
3) show the extent of the time coverage, and an indication of the large-scale
interannual variability on the section. The surface temperature has the
largest fluctuations, and may be aliased by the sampling, but the deeper
averages show a smooth progression through the years, with the longer sections
showing smoother variations than the shorter sections, as expected. There
is a sudden warming in the Tasman Box sections (PX31, PX06, PX34) which
may be related to the aftermath of the 1997-1998 El Nino, but the series
are not long enough to show multiple events.
Figure 2 shows a comparison of section-average temperature with section-average
transport. There is a similarity between the curves that suggests a
One explanation of this relationship is advection of the meridional temperature
gradient by the transport.
There is considerable detail in the sections; here only a few of the most
prominent points are discussed. For the mean temperature sections, the lines
including western boundary currents (PX37, PX31, PX34, and PX50) show
recirculation features. In PX50 the recirculation feature is the East Cape
Eddy, as documented by Roemmich and Sutton (1998). In PX06, a series of
minor fronts, at about 31.5 S, 27 S, and 21 S indicates filaments of the
currents associated with the Tasman Front (Morris, et al., 1995).
The RMS plots show a familiar pattern of strong surface variability, driven
by the surface heat flux, and subsurface thermocline variability, much of
which is presumably driven by the wind through Ekman pumping. There is also
deep variability associated with Western boundary currents, and PX50 shows
surprising deep variability in the center of the section which is not
with annual forcing. This may be affected by the variability of the track,
and does not show much annual cycle component. PX37 shows the lack of
in the warm pool, and the strong variability in the layers just below it.
The annual cycle amplitudes show a remarkable uniformity in depth of
of the heat-forced cycle, which appears to be about 75 m for almost all
regions. The deep variability in PX50 does therefore not project well on
the annual band, even though deep mixed layers are found there and in parts
of the other sections. The annual cycle is only one component of the seasonal
march, and the late spring mixed layers may project more strongly on the
higher harmonics of the annual cycle.
The comparison of the annual cycle to RMS for PX37 shows a minimum in the
annual cycle response just below the surface layer which is not anticipated
by the RMS plot. This can perhaps be rationalized as a nodal minimum due
to interference between the annual cycle of heating at the surface and of
wind forcing of the thermocline, but this minimum does not occur in the
other tropical sections, such as PX06 or PX31.
These figures are meant as a starting point for analysis of the dynamics
controlling the variability, and comparisons are underway with Pacific
Roemmich, D. and P. Sutton, 1998. The mean and variability of ocean circulation
past northern New Zealand; determining the representativeness of hydrographic
climatologies. Journal of Geophysical Research, 103,
Morris, M., D. Roemmich and B. Cornuelle, 1995. Observations of variability
in the South Pacific subtropical gyre. Journal of Physical Oceanography,