Labrador Sea. Dickson et al. attributed this largescale pattern to the NAO, in which Labrador Sea
Water formation is vigorous when the westerlies
take a northern path, and in which Eighteen
Degree Water formation is vigorous when the
westerlies shift towards the south and west.
In the North Pacific subtropical gyre, Bingham
et al. (1992) compared surface temperature structures of the western North Pacific in two pentads,
1938–42 and 1978–82, using hydrographic data
for the former and mostly XBT data for the latter.
They found that NPSTMW during the former
period was thicker, more uniform in temperature
and more confined geographically. They speculated that the greater thickness during the former
pentad resulted from stronger wintertime cooling, and that the difference in geographic extent
between the two pentads might be caused by
reduced advection of the Kuroshio, which was in
a large meander state for a larger fraction of the
former pentad.
In the mid-1970s, all environmental variables
for the North Pacific showed a shift into a state
dominated by a deep Aleutian Low (Nitta and
Yamada, 1989; Trenberth, 1990; Tanimoto et al.,
1993). Yasuda and Hanawa (1997) compared temperature fields for the decades before and after the
shift: 1966–75 and 1976–85. In the latter decade,
North Pacific Central Mode Water (NPCMW) was
widely distributed and its temperature was much
colder than before. In the western subtropical gyre,
in the North Pacific STMW (NPSTMW) formation
area, the water in the southwest was warmer in the
latter decade, while colder water occupied the eastern part. These decadal changes in temperature for
NPSTMW and NPCMW were attributed partly to
the enhancement of heat loss at the sea surface and
from enhanced Ekman heat divergence and partly
to the larger heat advection by the Kuroshio, which
was spun-up during the latter decade.
5.4.4.4 Comments on temporal variability of
mode waters
As mentioned in the beginning of Section 5.4.4,
the existence of substantial temporal variability on
various time scales is expected since mode water
formation reflects late winter conditions, which
have interannual and decadal variability. Variability
in circulation strength, which is related to windfield strength, also affects mode water properties,
for instance in the effect of a strengthened Kuroshio
on NPSTMW temperature, or the effect of the
Kuroshio large meander on NPSTMW distribution.
The variability of NASTMW and NPSTMW have
been documented to some degree, although the
mechanisms for their variations have not been fully
explored. Unfortunately at present data are lacking
to undertake such studies for the rest of the mode
waters. It is expected that the accumulation of monitoring data like XBT/XCTD measurements, among
others, will make the description of variability possible in the near future and that these results will
provide new insight to ocean climatic variations
and changes.
5.4.5 Summary
Mode waters are part of the continuous stratification of the upper oceans, where they are prominent because of their volumetric significance. Thus
they are likely to have a disproportionate influence
on surface water and surface temperature evolution, which are central to ocean–atmosphere
climate interactions. As a community, we have not
yet fully documented the properties and distribution of mode waters, which must be the first step.
This work is in progress by a number of investigators, including the authors and collaborators. Second, fluxes, formation rates and mechanisms are
far from elucidated, including for instance basic
questions about the reason for the existence of the
eastern subtropical mode waters, the role of eddies
in formation, and even the role of topography and
local mixing for some mode waters such as Subpolar Mode Water that are close to boundaries.
The authors hope that in the WOCE-AIMS
(Analysis, Interpretation, Modelling and Synthesis)
period, using various kinds of observations taken
during WOCE, including hydrography, XBTs, surface temperatures and fluxes, every mode water in
the world’s oceans will be fully described, with
major progress in understanding mechanisms for
formation and feedback of mode waters on the
atmosphere.
SECTION 5 FORMATION AND TRANSPORT OF WATER MASSES
386
Water formation is vigorous when the westerlies
take a northern path, and in which Eighteen
Degree Water formation is vigorous when the
westerlies shift towards the south and west.
In the North Pacific subtropical gyre, Bingham
et al. (1992) compared surface temperature structures of the western North Pacific in two pentads,
1938–42 and 1978–82, using hydrographic data
for the former and mostly XBT data for the latter.
They found that NPSTMW during the former
period was thicker, more uniform in temperature
and more confined geographically. They speculated that the greater thickness during the former
pentad resulted from stronger wintertime cooling, and that the difference in geographic extent
between the two pentads might be caused by
reduced advection of the Kuroshio, which was in
a large meander state for a larger fraction of the
former pentad.
In the mid-1970s, all environmental variables
for the North Pacific showed a shift into a state
dominated by a deep Aleutian Low (Nitta and
Yamada, 1989; Trenberth, 1990; Tanimoto et al.,
1993). Yasuda and Hanawa (1997) compared temperature fields for the decades before and after the
shift: 1966–75 and 1976–85. In the latter decade,
North Pacific Central Mode Water (NPCMW) was
widely distributed and its temperature was much
colder than before. In the western subtropical gyre,
in the North Pacific STMW (NPSTMW) formation
area, the water in the southwest was warmer in the
latter decade, while colder water occupied the eastern part. These decadal changes in temperature for
NPSTMW and NPCMW were attributed partly to
the enhancement of heat loss at the sea surface and
from enhanced Ekman heat divergence and partly
to the larger heat advection by the Kuroshio, which
was spun-up during the latter decade.
5.4.4.4 Comments on temporal variability of
mode waters
As mentioned in the beginning of Section 5.4.4,
the existence of substantial temporal variability on
various time scales is expected since mode water
formation reflects late winter conditions, which
have interannual and decadal variability. Variability
in circulation strength, which is related to windfield strength, also affects mode water properties,
for instance in the effect of a strengthened Kuroshio
on NPSTMW temperature, or the effect of the
Kuroshio large meander on NPSTMW distribution.
The variability of NASTMW and NPSTMW have
been documented to some degree, although the
mechanisms for their variations have not been fully
explored. Unfortunately at present data are lacking
to undertake such studies for the rest of the mode
waters. It is expected that the accumulation of monitoring data like XBT/XCTD measurements, among
others, will make the description of variability possible in the near future and that these results will
provide new insight to ocean climatic variations
and changes.
5.4.5 Summary
Mode waters are part of the continuous stratification of the upper oceans, where they are prominent because of their volumetric significance. Thus
they are likely to have a disproportionate influence
on surface water and surface temperature evolution, which are central to ocean–atmosphere
climate interactions. As a community, we have not
yet fully documented the properties and distribution of mode waters, which must be the first step.
This work is in progress by a number of investigators, including the authors and collaborators. Second, fluxes, formation rates and mechanisms are
far from elucidated, including for instance basic
questions about the reason for the existence of the
eastern subtropical mode waters, the role of eddies
in formation, and even the role of topography and
local mixing for some mode waters such as Subpolar Mode Water that are close to boundaries.
The authors hope that in the WOCE-AIMS
(Analysis, Interpretation, Modelling and Synthesis)
period, using various kinds of observations taken
during WOCE, including hydrography, XBTs, surface temperatures and fluxes, every mode water in
the world’s oceans will be fully described, with
major progress in understanding mechanisms for
formation and feedback of mode waters on the
atmosphere.
SECTION 5 FORMATION AND TRANSPORT OF WATER MASSES
386