The transition zone pattern requires more analysis to be fit into this hypothetical
scheme. It stretches east–west across the North Pacific between about 34° and 43°N,
right along the main axis of the Oyashio–Kuroshio confluence and west wind drift,
which might be expected to push its contents east against North America. This space
between the cool subarctic and warm subtropical gyres is bounded in upper layers by
fronts in salinity and seasonal temperature, both increasing southward. It is also the
range of latitudes (an oscillating band of 6° to 8° latitude centered around 40°N) in
which there is no strong density stratification for part of the year. Subarctic waters to
the north have a persistent halocline at about 110 m that survives winter mixing, and
the subtropical gyre to the south has a near-surface thermocline right through the year
and a deep, permanent thermocline. There is formation of subtropical-mode water to
the western side of the subtropical gyre with mixing to 500–600 m, but it does not
cool the surface below ∼16°C. Just in the transition zone there is winter
destratification with potential for deep mixing and stronger cooling. This latitudinal
band is not narrow, except relative to the whole expanse of the Pacific. It occupies a
major swath of ocean. A modest list of zooplankton species live in this band,
including at least Nematoscelis difficilis, Thysanoessa gregaria, Euphausia gibboides,
Clausocalanus pergens, Eucalanus hyalinus, a subspecies (“type B”) of Limacina
helicina, and Pseudosagitta lyra. Several species of small nekton have similar
patterns. Distributions for the euphausiid species, and most of the others, do extend all
the way to Japan, but numbers consistently fall off in the west end compared to
longitudes east from 165°E.
(^) The puzzle of transition-zone patterns is solved in principle (Olson & Hood 1994;
Olson 2001), although a necessary quantification of the population budgets has not
been undertaken. East of the Emperor Seamounts at about 170°E the flow is sluggish.
The mean flow at 158°W is eastward with a mean speed of only a few cm s−1.
Moreover, the mesoscale eddy field has local velocities about the same, i.e. very slow.
Drifters drogued at 15 m show the water moving mostly east at a very stately pace.
This is true on both sides of the transition as well, with eddy velocities much higher
just south of the transition than in it or to the north. The mesoscale eddies extend way
down, to approximately 600 m, and thus include the entire vertical range of any
epipelagic zooplankton. Probably in this range of longitudes, the critical net loss is
due to eddy diffusion past the northern and southern boundaries of habitat conditions
to which the animals are adapted, and the drifter data suggest that those losses are
small. The net flow coming in from the west replaces losses to net flow out to the
east. So, sufficient births plus good survival readily sustain transition endemics along
their meridional band, despite the lateral advective losses.
(^) West of the Emperor Seamounts, the relations are different. Out to about 150°E, the
net flows in the Oyashio (c.41°40′N) and Kuroshio (34°N) push water through in
nearly coherent and unidirectional fashion, and quite fast, especially in the Kuroshio