5.4.1 Ventilation and mode water
generation
‘Mode Water’ is the name given to a layer of
nearly vertically homogeneous water found over a
relatively large geographical area. Mode waters
usually occur within or near the top of the permanent pycnocline, and hence are apparent through
the contrast in stratification with the pycnocline
waters. Therefore, in a volumetric census in,
for instance, a temperature–salinity diagram, this
homogeneity in comparison with surrounding
higher stratification produces a maximum inventory. Masuzawa (1969) first introduced the term
‘Subtropical Mode Water’ (STMW) in reference to
the thick layer of temperature 16–18°C in the
northwestern North Pacific subtropical gyre, on
the southern side of the Kuroshio Extension.
This STMW is the counterpart of the previously
identified Eighteen Degree Water associated with
the Gulf Stream Extension in the North Atlantic
(Worthington, 1959). The terminology ‘Mode
Water’ was extended to the thick near-surface layer
north of the Subantarctic Front by McCartney
(1977), who identified and mapped the properties of the Subantarctic Mode Water (SAMW).
McCartney and Talley (1982) then applied the
term ‘Subpolar Mode Water’ (SPMW) to the thick
near-surface mixed layers in the North Atlantic’s
subpolar gyre. The term ‘Mode Water’ now is
nearly ubiquitous for describing any thick, broadly
distributed, near-surface layer. To distinguish it
from other mode waters, the Subtropical Mode
Water described by Masuzawa (1969, 1972) is now
usually referred to as North Pacific Subtropical
Mode Water (NPSTMW) and the North Atlantic’s
Eighteen Degree Water is sometimes called North
Atlantic Subtropical Mode Water (NASTMW).
Mode waters have been identified in every
ocean basin, always on the warm side of a current
or front. Subtropical mode waters are associated
with every separated western boundary current of
subtropical gyres. Mode waters have been identified recently in the northeastern portions of several
subtropical gyres – this identification is extended
herein to all basins. In the southern hemisphere,
the Subantarctic Front is the southern boundary of
the subtropical gyres. Because isopycnals plunge so
rapidly towards the north across the front, a very
thick mode water is found to the north of the
front.
Mode waters are generally distributed below
the surface far beyond their formation areas. Ventilation of the ocean interior occurs when fluid is
subducted, or pushed down, from the ocean surface. The initial view of how this occurs dates
back to Iselin (1939), who proposed that water
would be pushed downwards along sloping isopycnals from the base of the Ekman layer by the
wind-induced vertical Ekman pumping velocity.
Stommel (1979) recognized that only the fluid
leaving the deep winter mixed layer irreversibly
enters the permanent pycnocline (the ‘mixed-layer
demon’ hypothesis), and this biases the temperature and salinity properties of the main thermocline toward those of the deep winter mixed layer.
Consequently, it is actually the flow through the
base of the winter mixed layer (rather than the
Ekman layer) that ventilates the underlying ocean,
5.4
Mode Waters
Kimio Hanawa and Lynne D.Talley
373
OCEAN CIRCULATION AND CLIMATE
Copyright © 2001 Academic Press
ISBN 0-12-641351-7
All rights of reproduction in any form reserved
CHAPTER
generation
‘Mode Water’ is the name given to a layer of
nearly vertically homogeneous water found over a
relatively large geographical area. Mode waters
usually occur within or near the top of the permanent pycnocline, and hence are apparent through
the contrast in stratification with the pycnocline
waters. Therefore, in a volumetric census in,
for instance, a temperature–salinity diagram, this
homogeneity in comparison with surrounding
higher stratification produces a maximum inventory. Masuzawa (1969) first introduced the term
‘Subtropical Mode Water’ (STMW) in reference to
the thick layer of temperature 16–18°C in the
northwestern North Pacific subtropical gyre, on
the southern side of the Kuroshio Extension.
This STMW is the counterpart of the previously
identified Eighteen Degree Water associated with
the Gulf Stream Extension in the North Atlantic
(Worthington, 1959). The terminology ‘Mode
Water’ was extended to the thick near-surface layer
north of the Subantarctic Front by McCartney
(1977), who identified and mapped the properties of the Subantarctic Mode Water (SAMW).
McCartney and Talley (1982) then applied the
term ‘Subpolar Mode Water’ (SPMW) to the thick
near-surface mixed layers in the North Atlantic’s
subpolar gyre. The term ‘Mode Water’ now is
nearly ubiquitous for describing any thick, broadly
distributed, near-surface layer. To distinguish it
from other mode waters, the Subtropical Mode
Water described by Masuzawa (1969, 1972) is now
usually referred to as North Pacific Subtropical
Mode Water (NPSTMW) and the North Atlantic’s
Eighteen Degree Water is sometimes called North
Atlantic Subtropical Mode Water (NASTMW).
Mode waters have been identified in every
ocean basin, always on the warm side of a current
or front. Subtropical mode waters are associated
with every separated western boundary current of
subtropical gyres. Mode waters have been identified recently in the northeastern portions of several
subtropical gyres – this identification is extended
herein to all basins. In the southern hemisphere,
the Subantarctic Front is the southern boundary of
the subtropical gyres. Because isopycnals plunge so
rapidly towards the north across the front, a very
thick mode water is found to the north of the
front.
Mode waters are generally distributed below
the surface far beyond their formation areas. Ventilation of the ocean interior occurs when fluid is
subducted, or pushed down, from the ocean surface. The initial view of how this occurs dates
back to Iselin (1939), who proposed that water
would be pushed downwards along sloping isopycnals from the base of the Ekman layer by the
wind-induced vertical Ekman pumping velocity.
Stommel (1979) recognized that only the fluid
leaving the deep winter mixed layer irreversibly
enters the permanent pycnocline (the ‘mixed-layer
demon’ hypothesis), and this biases the temperature and salinity properties of the main thermocline toward those of the deep winter mixed layer.
Consequently, it is actually the flow through the
base of the winter mixed layer (rather than the
Ekman layer) that ventilates the underlying ocean,
5.4
Mode Waters
Kimio Hanawa and Lynne D.Talley
373
OCEAN CIRCULATION AND CLIMATE
Copyright © 2001 Academic Press
ISBN 0-12-641351-7
All rights of reproduction in any form reserved
CHAPTER