172
Joseph L. Reid
extension into and beneath the less dense but more saline surface waters of the central
and eastern Pacific. The characteristics of the vertical minimum could be created and
maintained over much of the subarctic gyre, where the surface salinity is very low.
In contrast, Talley (1991) has proposed that most of the characteristics of the
water of the vertical minimum can originate in the Okhotsk Sea, directly from the
surface, and that open-ocean mixing in the far north is less important. You et al. (2000)
still hold for the downward mixing in both the Okhotsk Sea and the Gulf of Alaska.
It remains an interesting problem.
In the Atlantic Ocean, Tsuchiya (1989) has traced the Antarctic Intermediate
Water across the equator and along the western boundary and around the anticyclonic
gyre up to 45
◦ N.
There are similar subsurface minima in the South Pacific, South Atlantic, and
South Indian oceans. The characteristics and density of the water at these salinity
minima are found in the surface layer of the circumpolar flow, which is apparently
the source of the subsurface salinity minima of lower latitudes (Taft, 1963).
GEOPOTENTIAL ANOMALY
One of the changes that have occurred during the last fifty years is that the mapping of
geopotential anomaly, or dynamic height, to obtain relative or adjusted geostrophic
flow, has become common for the Indian and Pacific oceans and may be beginning
in the Atlantic.
Geopotential anomaly, or dynamic topography of the ocean was first proposed
early in the twentieth century, but it was used by only a few investigators before 1950.
This was partly because of the problem of a reference velocity.
I can imagine that in the early days of the field, after geostrophy had been introduced for studies of the atmospheric flow, someone, perhaps Bjerknes or Sandstrom,
gave a lecture to introduce the geostrophic approximation to studies of the ocean. He
would have explained that geostrophy could give only the vertical shear—the vertical
difference in speed between two depths. To get the actual speed one needs something
else—the flow measured or assumed at some depth. If, for example, it seemed that
the flow changed sign at some depth, that would define a flow of zero, or a level of
no motion.
There is really no reason to suppose that there is a surface of no motion everywhere, or that it is level or even continuous, but that such a distinguished scientist has
used the phrase “level of no motion” as an example (even without arguing for it) gave
it some standing. It has been used extensively in calculations of horizontal transports
across the many zonal lines of stations in the Atlantic.
The relative geostrophic flow—the flow at some pressure with respect to a
deeper pressure (500/2000 decibars for example)—has become more common. Over
most of the ocean the flow is strongest near the surface and decreases rapidly below,
and its direction may change. Using some depth as a reference for a shallow flow may
Joseph L. Reid
extension into and beneath the less dense but more saline surface waters of the central
and eastern Pacific. The characteristics of the vertical minimum could be created and
maintained over much of the subarctic gyre, where the surface salinity is very low.
In contrast, Talley (1991) has proposed that most of the characteristics of the
water of the vertical minimum can originate in the Okhotsk Sea, directly from the
surface, and that open-ocean mixing in the far north is less important. You et al. (2000)
still hold for the downward mixing in both the Okhotsk Sea and the Gulf of Alaska.
It remains an interesting problem.
In the Atlantic Ocean, Tsuchiya (1989) has traced the Antarctic Intermediate
Water across the equator and along the western boundary and around the anticyclonic
gyre up to 45
◦ N.
There are similar subsurface minima in the South Pacific, South Atlantic, and
South Indian oceans. The characteristics and density of the water at these salinity
minima are found in the surface layer of the circumpolar flow, which is apparently
the source of the subsurface salinity minima of lower latitudes (Taft, 1963).
GEOPOTENTIAL ANOMALY
One of the changes that have occurred during the last fifty years is that the mapping of
geopotential anomaly, or dynamic height, to obtain relative or adjusted geostrophic
flow, has become common for the Indian and Pacific oceans and may be beginning
in the Atlantic.
Geopotential anomaly, or dynamic topography of the ocean was first proposed
early in the twentieth century, but it was used by only a few investigators before 1950.
This was partly because of the problem of a reference velocity.
I can imagine that in the early days of the field, after geostrophy had been introduced for studies of the atmospheric flow, someone, perhaps Bjerknes or Sandstrom,
gave a lecture to introduce the geostrophic approximation to studies of the ocean. He
would have explained that geostrophy could give only the vertical shear—the vertical
difference in speed between two depths. To get the actual speed one needs something
else—the flow measured or assumed at some depth. If, for example, it seemed that
the flow changed sign at some depth, that would define a flow of zero, or a level of
no motion.
There is really no reason to suppose that there is a surface of no motion everywhere, or that it is level or even continuous, but that such a distinguished scientist has
used the phrase “level of no motion” as an example (even without arguing for it) gave
it some standing. It has been used extensively in calculations of horizontal transports
across the many zonal lines of stations in the Atlantic.
The relative geostrophic flow—the flow at some pressure with respect to a
deeper pressure (500/2000 decibars for example)—has become more common. Over
most of the ocean the flow is strongest near the surface and decreases rapidly below,
and its direction may change. Using some depth as a reference for a shallow flow may