350
Chapter 11: The Pacific Ocean
into the California Current system. Nor is it the 50- to 150-m-deep halocline layer in
the Alaska Gyre of Uda (1963), and, finally, it is not the Transition Zone of Dodimead
et al. (1967), who apply the term to a zone to the north of the Subarctic Front. Here,
it is used in the same sense as by the PICES oceanographers, who locate it climatically
between 42
N and 32
N, or between the Subarctic and Subtropical Frontal Zones (SAFZ
and STFZ, respectively): I shall use these same terms for the poleward limit of the NPPF
province and the equatorward limit of the NPST province.
Defining Characteristics of Regional Oceanography
The Transition Zone includes the area of meandering flow across the ocean that originates
at the confluence of Kuroshio and Oyashio currents. It extends south to the Subtropical
Convergence of Roden (1975) and north to the Subarctic Boundary that are, respectively,
the SAFZ and STFZ. The warm and cold flows maintain their integrity, despite much
meandering in the frontal zones, until they diverge on approaching the American continent to enter the eastern boundary current and the subpolar gyre, respectively. Both
NPPF and NPST lie below sufficient wind stress of the winter westerlies that significant
mixing occurs at this season, but more especially in the NPPF province. The NPST is
exposed to weaker wind stress and so winter mixing is more moderate.
The extreme southern limit of the NPST province, at the subtropical front or STFZ,
lies below the convergence between the westerly winds of the temperate zone and the
subtropical easterlies or trades. Roden (1970) reminds us, however, that although climatic
maps of wind stress suggest that the westerlies blow persistently toward the east, instantaneous maps reveal a series of depressions propagating toward the east at the crests of
long planetary waves, of which two or three usually occupy the air mass over the North
Pacific. Now that satellite images are commonplace, we are constantly reminded that
instantaneous images reveal the extent to which climatic means are misleading. Nor is the
STFZ itself as simple as portrayed in textbook diagrams; it is bounded by two meandering
fronts between which surface water is characteristically 180–185
C, and between which
there is a strong salinity gradient to water having salinity >351%, characteristic of the
subtropical gyre. Statistical analysis of the locations of these thermal fronts, as observed
by satellite sensors, suggests that they may be stronger and more numerous in El Niño
years than at other times.
Near-surface Ekman transport in the ocean beneath the zones of westerlies and easterlies exhibits convergent flow at the STFZ (Roden, 1975). Here, low-salinity northern
water descends below warmer and saltier subtropical water, so that individual fronts
within the STFZ tend to exhibit strong salinity gradients at the surface. Consequently the
STFZ is, in winter, both a temperature and salinity frontal zone, although, in summer,
it includes only haline fronts: for this reason, the surfacing of the 35.1% isohaline is a
useful indication of its location at all seasons.
The Transition Zone is energetic, and although density gradients across it are small,
baroclinic shear is large, associated with the formation of eddies on the scale of 100–
1000 km that are especially frequent in the western half of the ocean. The Transition
Zone also differs from the subarctic and subtropical water bodies to north and south in
the relatively low stability of the water column. To the north, the subarctic domain has
a permanent shallow halocline (see PSAG) that is extremely resistant to winter mixing,
whereas to the south in the tropical domain (see NPTG) the combined effects of the
thermocline and halocline also confer relatively high stability (Roden, 1970, 1975). We
can expect that these factors will have biological consequences distinguishing this province
from those adjacent to it, poleward and equatorward.
Chapter 11: The Pacific Ocean
into the California Current system. Nor is it the 50- to 150-m-deep halocline layer in
the Alaska Gyre of Uda (1963), and, finally, it is not the Transition Zone of Dodimead
et al. (1967), who apply the term to a zone to the north of the Subarctic Front. Here,
it is used in the same sense as by the PICES oceanographers, who locate it climatically
between 42
N and 32
N, or between the Subarctic and Subtropical Frontal Zones (SAFZ
and STFZ, respectively): I shall use these same terms for the poleward limit of the NPPF
province and the equatorward limit of the NPST province.
Defining Characteristics of Regional Oceanography
The Transition Zone includes the area of meandering flow across the ocean that originates
at the confluence of Kuroshio and Oyashio currents. It extends south to the Subtropical
Convergence of Roden (1975) and north to the Subarctic Boundary that are, respectively,
the SAFZ and STFZ. The warm and cold flows maintain their integrity, despite much
meandering in the frontal zones, until they diverge on approaching the American continent to enter the eastern boundary current and the subpolar gyre, respectively. Both
NPPF and NPST lie below sufficient wind stress of the winter westerlies that significant
mixing occurs at this season, but more especially in the NPPF province. The NPST is
exposed to weaker wind stress and so winter mixing is more moderate.
The extreme southern limit of the NPST province, at the subtropical front or STFZ,
lies below the convergence between the westerly winds of the temperate zone and the
subtropical easterlies or trades. Roden (1970) reminds us, however, that although climatic
maps of wind stress suggest that the westerlies blow persistently toward the east, instantaneous maps reveal a series of depressions propagating toward the east at the crests of
long planetary waves, of which two or three usually occupy the air mass over the North
Pacific. Now that satellite images are commonplace, we are constantly reminded that
instantaneous images reveal the extent to which climatic means are misleading. Nor is the
STFZ itself as simple as portrayed in textbook diagrams; it is bounded by two meandering
fronts between which surface water is characteristically 180–185
C, and between which
there is a strong salinity gradient to water having salinity >351%, characteristic of the
subtropical gyre. Statistical analysis of the locations of these thermal fronts, as observed
by satellite sensors, suggests that they may be stronger and more numerous in El Niño
years than at other times.
Near-surface Ekman transport in the ocean beneath the zones of westerlies and easterlies exhibits convergent flow at the STFZ (Roden, 1975). Here, low-salinity northern
water descends below warmer and saltier subtropical water, so that individual fronts
within the STFZ tend to exhibit strong salinity gradients at the surface. Consequently the
STFZ is, in winter, both a temperature and salinity frontal zone, although, in summer,
it includes only haline fronts: for this reason, the surfacing of the 35.1% isohaline is a
useful indication of its location at all seasons.
The Transition Zone is energetic, and although density gradients across it are small,
baroclinic shear is large, associated with the formation of eddies on the scale of 100–
1000 km that are especially frequent in the western half of the ocean. The Transition
Zone also differs from the subarctic and subtropical water bodies to north and south in
the relatively low stability of the water column. To the north, the subarctic domain has
a permanent shallow halocline (see PSAG) that is extremely resistant to winter mixing,
whereas to the south in the tropical domain (see NPTG) the combined effects of the
thermocline and halocline also confer relatively high stability (Roden, 1970, 1975). We
can expect that these factors will have biological consequences distinguishing this province
from those adjacent to it, poleward and equatorward.