4.1.1 Background
Through many centuries mariners have observed
ocean surface currents by noting how their vessels
drift, and cartographers have prepared maps on
the basis of these ship-drift reports. Instrumental
observations of ocean currents on a global basis
began about 115 years ago during the voyage of
HMS Challenger. She left Portsmouth, England, on
21 December 1872, on an ocean expedition that
was to navigate the globe in three and a half years.
The measurements of the state of the ocean at 354
stations already included ocean current observations with drifting buoys (Thompson, 1877).
Aboard were drifters consisting of 44 feet crossshaped canvas sea anchors, attached with various
lengths of cod line to 1 foot5 feet spindle-shaped
iron watch buoys that were used as surface floats
(Fig. 4.1.1). The drifters were deployed from a skiff
when the Challenger was firmly anchored at an
ocean station, with the bottom dredge over the fantail and the skiff lowered over the side. The sea
anchors, or drogues, were at various depths, sometimes as deep as 600 fathoms. At 1- to 2-h intervals,
the displacements of the watch buoys were obtained
by triangulation from the anchored Challenger.
The scientists aboard the Challenger interpreted
the displacement of the watch buoy, divided by the
time between observations, as an ocean current
at the depth of the sea anchor. They admitted that
they did not know how to correct these observations for the effects of the forces of the winds on the
watch buoy or the relative water motion between
the buoy and the drogues on the cod line. Additionally, they could not obtain data in rough weather.
The instrumental technique of observing ocean
currents by following drifters in the open sea captured the imagination of oceanographers and has
held it to this day. With the advent in the early
1970s of a cost-effective technique of locating
radio transmitters from the TIROS weather satellites, scores of drifting objects could be followed as
they moved across the ocean surface of the entire
globe (Garrett, 1980). But the problem of interpreting the drift of objects on the ocean surface as
a current at some depth was a problem that challenged oceanographers for a hundred years after
the Challenger. Significant action for its solution
was taken in the early 1980s when scientists began
planning the World Climate Research Programme
(WCRP). They recognized that large arrays of
satellite-tracked drifters could be used economically to observe the entire global ocean nearsurface circulation, provided the following technical problems were solved (McWilliams and
Masterson, 1982).
The water-following capability of drifters had
to be determined so that a rational hydrodynamic
design could be developed. Over a dozen designs
of drifters were in use, but no field-verified engineering formulae were available for choosing the
shapes or relative sizes of the surface float, the
tether line or the drogue. Could the effects of wind
and vertical shear on the slip of drogues through
water be measured and corrected? A calibration at
sea of the water-following capability of drifting
buoys was needed, and that was the first task.
The existing drifters were large, unwieldy and
costly to deploy in large numbers. For a century
4.1
The World Ocean Surface Circulation
Peter Niiler
193
OCEAN CIRCULATION AND CLIMATE
Copyright © 2001 Academic Press
ISBN 0-12-641351-7
All rights of reproduction in any form reserved
CHAPTER
Through many centuries mariners have observed
ocean surface currents by noting how their vessels
drift, and cartographers have prepared maps on
the basis of these ship-drift reports. Instrumental
observations of ocean currents on a global basis
began about 115 years ago during the voyage of
HMS Challenger. She left Portsmouth, England, on
21 December 1872, on an ocean expedition that
was to navigate the globe in three and a half years.
The measurements of the state of the ocean at 354
stations already included ocean current observations with drifting buoys (Thompson, 1877).
Aboard were drifters consisting of 44 feet crossshaped canvas sea anchors, attached with various
lengths of cod line to 1 foot5 feet spindle-shaped
iron watch buoys that were used as surface floats
(Fig. 4.1.1). The drifters were deployed from a skiff
when the Challenger was firmly anchored at an
ocean station, with the bottom dredge over the fantail and the skiff lowered over the side. The sea
anchors, or drogues, were at various depths, sometimes as deep as 600 fathoms. At 1- to 2-h intervals,
the displacements of the watch buoys were obtained
by triangulation from the anchored Challenger.
The scientists aboard the Challenger interpreted
the displacement of the watch buoy, divided by the
time between observations, as an ocean current
at the depth of the sea anchor. They admitted that
they did not know how to correct these observations for the effects of the forces of the winds on the
watch buoy or the relative water motion between
the buoy and the drogues on the cod line. Additionally, they could not obtain data in rough weather.
The instrumental technique of observing ocean
currents by following drifters in the open sea captured the imagination of oceanographers and has
held it to this day. With the advent in the early
1970s of a cost-effective technique of locating
radio transmitters from the TIROS weather satellites, scores of drifting objects could be followed as
they moved across the ocean surface of the entire
globe (Garrett, 1980). But the problem of interpreting the drift of objects on the ocean surface as
a current at some depth was a problem that challenged oceanographers for a hundred years after
the Challenger. Significant action for its solution
was taken in the early 1980s when scientists began
planning the World Climate Research Programme
(WCRP). They recognized that large arrays of
satellite-tracked drifters could be used economically to observe the entire global ocean nearsurface circulation, provided the following technical problems were solved (McWilliams and
Masterson, 1982).
The water-following capability of drifters had
to be determined so that a rational hydrodynamic
design could be developed. Over a dozen designs
of drifters were in use, but no field-verified engineering formulae were available for choosing the
shapes or relative sizes of the surface float, the
tether line or the drogue. Could the effects of wind
and vertical shear on the slip of drogues through
water be measured and corrected? A calibration at
sea of the water-following capability of drifting
buoys was needed, and that was the first task.
The existing drifters were large, unwieldy and
costly to deploy in large numbers. For a century
4.1
The World Ocean Surface Circulation
Peter Niiler
193
OCEAN CIRCULATION AND CLIMATE
Copyright © 2001 Academic Press
ISBN 0-12-641351-7
All rights of reproduction in any form reserved
CHAPTER