IN-SITU OBSERVATIONS
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spatial gradients, and diffusion bias if high float concentrations spread
preferentially in one direction (e.g. near a coast), see Figure 3. The spatial
and temporal resolution, as implemented in ARGO, is coarse. Floats are
expendable so can not be post-calibrated, thus a sensor drift is difficult to
detect or correct.
Further readings: Davis (1991), Davis et al (2001), ARGO website.
2.2 Surface drifters
Description:
Surface drifters are cheap and light-weight platforms that passively
follow the horizontal flow at the surface via a drogue/sail at usually 15m
depth. The drogue is connected to a small surface float which carries the
satellite transmitter and other electronics. All of them measure SST and
many also air pressure.
Application:
While profiling float data have the strongest synergy with satellite
altimetry measurements, sea surface temperature (SST) observations from
space are best complemented by surface drifters. The chapter by I. Robinson
in this volume explains in detail the difficulty in defining and observing the
different types of SST. Surface drifters are an important resource for
collecting such data.
A ten year long data set now also seems to allow an estimate of the mean
geostrophic surface circulation, after subtracting the Ekman component,
which can also serve to determine absolute SSH and thus the geoid. There is
a global operational drifter program under way, which maintains on the
order of 1000 drifters in the ocean.
Strengths and weaknesses:
The strengths and weaknesses of drifters are similar to the ones of floats
(section 2.1), but are restricted to the surface. In addition they normally do
not measure salinity, yet.
Further readings: Niiler et al (1995), Niiler et al (2003), Global drifter
center website.
2.3 Ship sections
Where better horizontal resolution is desired than achievable with floats
or drifters, especially on regular transects across ocean basins or across
boundary currents, ship sections are currently the best way to obtain this.
197
spatial gradients, and diffusion bias if high float concentrations spread
preferentially in one direction (e.g. near a coast), see Figure 3. The spatial
and temporal resolution, as implemented in ARGO, is coarse. Floats are
expendable so can not be post-calibrated, thus a sensor drift is difficult to
detect or correct.
Further readings: Davis (1991), Davis et al (2001), ARGO website.
2.2 Surface drifters
Description:
Surface drifters are cheap and light-weight platforms that passively
follow the horizontal flow at the surface via a drogue/sail at usually 15m
depth. The drogue is connected to a small surface float which carries the
satellite transmitter and other electronics. All of them measure SST and
many also air pressure.
Application:
While profiling float data have the strongest synergy with satellite
altimetry measurements, sea surface temperature (SST) observations from
space are best complemented by surface drifters. The chapter by I. Robinson
in this volume explains in detail the difficulty in defining and observing the
different types of SST. Surface drifters are an important resource for
collecting such data.
A ten year long data set now also seems to allow an estimate of the mean
geostrophic surface circulation, after subtracting the Ekman component,
which can also serve to determine absolute SSH and thus the geoid. There is
a global operational drifter program under way, which maintains on the
order of 1000 drifters in the ocean.
Strengths and weaknesses:
The strengths and weaknesses of drifters are similar to the ones of floats
(section 2.1), but are restricted to the surface. In addition they normally do
not measure salinity, yet.
Further readings: Niiler et al (1995), Niiler et al (2003), Global drifter
center website.
2.3 Ship sections
Where better horizontal resolution is desired than achievable with floats
or drifters, especially on regular transects across ocean basins or across
boundary currents, ship sections are currently the best way to obtain this.