Transition Zone

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The central North Pacifi c Transition Zone (hereafter

the Transition Zone) (Figure 137), is climatologically

positioned between the 32°N and 42°N latitudes.

Large-scale frontal systems that bound the Transition

Zone to the north and south play a key role in defi ning

the biology of the region. Many animals, including some

of the most populous species (e.g., fl ying squid, Pacifi c

pomfret, blue sharks, Pacifi c saury) that inhabit the

Transition Zone undergo extensive seasonal migrations

from summer feeding grounds at the subarctic fronts or

within subarctic waters to and from winter spawning

grounds in the subtropics.

Several commercial fi sheries currently operate in

Transition Zone waters, including Japanese and U.S.

vessels targeting tunas and billfi shes, Japanese and

U.S. troll vessels targeting tunas (notably albacore

tuna), and a distant water Japanese (and much lesser

extent U.S.) jig fi shery for squid. The now defunct

multinational Asian high-seas driftnet fi shing fl eets

targeting squid and tunas and billfi shes also operated

in Transition Zone waters until 1992. Information

gleened from the observer program tasked with

monitoring the driftnet fi sheries provided much of our

current understanding of the region’s nektonic faunal

composition.

140˚E 160˚E 180̊ 160˚W 140˚W 120˚W10˚N

20˚N

30˚N

40˚N

50˚N

60˚N

South Subtropical FrontSubtropical Front

Subtropical Frontal Zone (STFZ)Kuroshio

Transition Zone

(oceanography adapted from Roden (1991) and Seki et al. (2002))

Subarctic BoundarySubarctic Frontal Zone (SAFZ)

Subarctic Domain

Subtropical Domain

[Figure 137] Schematic representaiton of the North Pacifi c transition zone and its relation to the major features of the North Pacifi c ocean.

background

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The central Pacifi c region that includes most of the Transition

Zone generally exhibits a low-frequency oscillation between

anomalously cooler and warmer phases. Presently, the central

Pacifi c region appears to be in a warm phase with elevated

temperatures and increased sea level height (SLH); a rapid

phase shift from cooler, lower SLH was observed in 1998-99

(Figure 138). These temperature phases have been shown

to be in generally good coherence with indices from an

empirical orthogonal function (EOF) analysis on detrended

sea level height (SLH) data from the TOPEX/JASON satellites

and with the Pacifi c Decadal Oscillation (PDO) index.

1st EOF Monthly Time Series of AVISO Sea Surface Height Anomalies (11.6%)

Ecosystem and EnvironmentInvestigation

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1999-2002 Minus 1993-1997 MCSST

[Figure 138] The magnitude of the detrended variation in the fi rst mode of the EOF analysis of sea level height is illustrated in the top panel. The bottom panel presents the difference in SST (°C) anomalies of the period 1993-98 subtracted from the period 1999-2000.368

HydrographyTransition Zone waters are characteristically of both

subarctic and subtropical origin and are present in

proportions refl ecting lateral mixing along isopycnal

surfaces and distance from their source regions.369,370

Surface temperatures and salinities gradually increase from

north to south and the vertical distribution of properties

is highlighted by a deep salinity minimum of about

33.9 ± 0.02 that slopes gradually southward from about

400-450 m north (ca. 40°N) to about 550-600 m

at the southern bounds; the salinity minimum is

reportedly associated with a of 26.70±0.04 g·l-1.371,369,372

Additionally, there is no shallow temperature minimum,

high-stability layer (hydrostatic stabilities are lower here

than anywhere else in the open North Pacifi c), nor a well

developed halocline.

At the northern and southern boundaries of the Transition

Zone are the Subarctic (SAFZ) and Subtropical (STFZ)

Frontal Zones, respectively (Figure 137). These frontal

zones, stretching laterally across the central North Pacifi c,

are typically regions embedded with multiple individual

surface fronts that signify abrupt changes in thermohaline

properties with change in latitude. Climatologically, the SAFZ

is located at about 40° to 43°N latitudes, a region of strong

eastward wind stresses (i.e., the Westerlies). The STFZ, on

the other hand typically occupies the region around 28°N to

34°N latitudes, an area of minimum zonal wind stress and

just north of the area corresponding to the strongest net

evaporation gradients in the region dynamically infl uenced

by the Ekman transport convergence from the subtropical

easterly tradewinds.372,373,374 The dynamics and structure of

the Transition Zone and its frontal zones are thus principally

driven by the large scale convergent wind-driven currents.

The SAFZ extends from about 40°N to 43°N latitudes and

separates the cold (<8°C), low salinity (33.0) subarctic

water from the transitional region and subtropical waters

to the south. The limits of the SAFZ are defi ned by the

surface outcroppings of the 33.0 isohaline to the north

(the southern bound of unadulterated subarctic water) and

the 33.8 isohaline to the south. These isohalines generally

form the bottom and top of the halocline in the Subarctic

Domain. The southern limit of the SAFZ, historically referred

to as the Subarctic Front375 or the Subarctic Boundary376 can

also be characterized by the vanishing of large temperature

inversions that might exist in the subarctic halocline and

by the rapid increase in the depth of the 9° and 10°C

isotherms.370 Since horizontal temperature and salinity

gradients in the surface layer exist in near balance, fronts

at the SAFZ are almost totally density compensated (i.e.,

only weak density gradients exist) and baroclinic fl ow

dynamics do not play a major role in defi ning the region’s

oceanographic patterns.369,372,375

Status and Trends

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Considerably different characteristics are found associated

with the STFZ. The STFZ is a broad, generally zonal band

between about 28°N and 34°N latitudes.377 At the surface,

cooler, lower salinity water from the north, converges

with and sinks below warmer, saltier subtropical water

found to the south resulting in a broad geographic region

of physical gradients. Accordingly, multiple, large-scale,

individual, semi-permanent fronts form, the most prominent

climatologically located at 32°-34°N and at 28°N-30°N

latitudes.

These particular fronts, nominally referred to as the

Subtropical Front and the South Subtropical Front,

respectively,377 are typically associated with the meridional

bounds of the frontal zone.373,378 Roden374,370 describes the

STFZ as the latitudinal belt that divides cooler (<18°C), lower

saline (<34.8) surface water from warm (>18.5°C), higher-

saline (>35.1) water. Considerable interannual variability

in the latitudinal position and intensity is apparently

characteristic of the STFZ fronts, as evidenced in annual

springtime surveys conducted along the 172°W (1996-97)

and 158°W (1998-2000) longitudes (Figure 137).

The fronts of the STFZ differ from those at the SAFZ in other

respects. First, the STFZ fronts tend to exhibit moderate

to signifi cant density gradients in the upper 100m in

response to the incomplete balance that exists between the

meridional temperature and salinity gradients. The resultant

baroclinic fl ow fi eld has signifi cant infl uence on the three

dimensional mesoscale dynamics and is thus instrumental to

the formation of the frontal structure observed on synoptic

time scales.

Explicitly, the subtropical fronts are characterized by strong,

meandering west to east current fl ow patterns, with pervasive

mesoscale cyclonic activity to the north and anticyclonic

rotating features to the south of the strongest streamlines.

Unlike subarctic fronts, surface temperature (and thus

density) fronts at the STFZ are not present all year round.

Intense insolation during summer obliterates horizontal

temperature gradients and inhibits the manifestation of

the STFZ temperature fronts in months outside the cooler

periods from late fall through winter to spring (Figure 140).

Surface salinity fronts, however, are present at both the

SAFZ and STFZ throughout the year.

[Figure 139] Comparative vertical sections of (left) temperature (°C) and (right) chloropigment (mg m-3) from annual surveys 1996-2000; 1996-97 surveys conducted along 172°W and 1998-2000 along 158°W longitudes.

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ChemistryConcentrations of nutrients (e.g., nitrate, phosphate, and

silicate) in the Transition Zone generally refl ect the gradient

between the high levels found in the Subarctic and low

levels in the Subtropics. During the summer, the stratifi ed,

oligotrophic waters of the Transition Zone are generally

thought to be N-limited.379

PlanktonWhile plankton dynamics in the Transition Zone have been

poorly studied when compared to the adjacent subarctic

and subtropical waters to the north and south, respectively,

it’s evident that the Transition Zone is heavily infl uenced

by high gradient regions of horizontal oceanographic

variability; i.e., large-scale frontal systems (biological as

well as physical) and mesoscale dynamic features (such as

meanders, eddies, and jets) where productivity is enhanced

and so much of the ecosystem energy is mobilized.

Phytoplankton With limited information available from

direct observations, satellite remote sensing of surface chl-

a densities provides our best insight into the phytoplankton

dynamics of the Transition Zone. In the subtropical central

Pacifi c, surface chl-a concentrations are generally <0.15

mg·m-3 and northward towards the subarctic are >0.25

mg·m-3.

0 5 10 15 20 25

60˚N

50̊ N

40˚N

30˚N

20˚N

160̊ E 170̊ E 180 170̊ W 160̊ W 130̊ W140̊ W150̊ W

SST (̊C)

Subarctic Frontal Zone

Subtropical Frontal Zone

Transition Zone

(A)

20̊ N

30̊ N

40̊ N

50̊ N

160̊ E 170̊ E 180 170̊ W 160̊ W 150̊ W 140̊ W 130̊ W

60̊ N

5 10 15 20 25 30

Subarctic Frontal Zone

North Pacific Transition Zone

SST (̊C)

(B)

[Figure 140] False color image of basinwide sea surface temperature (°C) structure for (A) late winter-early spring, e.g., March-April 1992 and (B) summer, e.g., August 1991. The satellite SST data are weekly averaged 18 km gridded daytime multichannel SST (MCSST) from archived Tiros-N/NOAA Advanced Very High Resolution Radiometer (AVHRR).

Between these regions of high/low surface chlorophyll lies

a sharp basin-wide surface chl-a front, the Transition Zone

Chlorophyll Front (TZCF). The TZCF seasonally oscillates

north to south about 1000 km with a latitudinal minimum in

January-February and maximum in July-August (Figure 141).

This biological front also exhibits considerable interannual

variations in position, meandering, and gradient strengths.

For example, prior to 1999, the southern minimum of the

TZCF resided below 30°N, reaching the waters close to and

including the NW end of the Hawaiian Archipelago while

positionally constrained to a southern minimum at about

30°N in the following years (1999 to 2003) (Figure 141).

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140E

140E

160E

160E

180

180

160W

160W

140W

140W

120W

120W

100W

100W

10N 10N

20N 20N

30N 30N

40N 40N

50N 50N

60N 60N

0.000 0.075 0.100 0.125 0.150 0.200 0.250 1.000 35.000Chlorophyll a (mg m-3 )

140E

140E

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180

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160W

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140W

120W

120W

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100W

10N 10N

20N 20N

30N 30N

40N 40N

50N 50N

60N 60N

A. February 1998

B. August 1998

25

30 N

35 N

40 N

45o

o

o

o

o

o o oo

N

160 E 180 160 W 140 W 120 W

1997199819992000200120022003

LaysanPearl/Hermes

[Figure 141] Surface chlorophyll density (mg·m-3) estimated from SeaWiFS ocean color for (A) February and (B) August in the North Pacifi c and (C) the southern minimum of the Transition Zone Chlorophyll Front (TZCF) defi ned as the 0.2 mg·m-3 contour for 1997-2003.380

Large scale oceanic thermohaline fronts in the Transition

Zone, particularly at the STFZ, and associated high gradient

mesoscale features often give rise to localized regions of

higher productivity, leading to aggregation and development

of a forage base. In stratifi ed, oligotrophic waters such as

found during the summer, recycling of nutrients between

the grazers and phytoplankton typically maintains primary

production at uniform low levels. Transient episodes of

upwelled nutrient-rich water from mesoscale events,

particularly strong cyclonic eddies and meanders, have

been shown to induce ‘new’ production, thus providing a

mechanism to shorten the trophic pathway and facilitate

energy transfer.

Zooplankton Very limited information is available

regarding the status of macro-zooplankton or micronekton

(which includes juvenile forms of Subarctic and Transitional

nektonic and micronektonic species that undergo extensive

latitudinal/longitudinal migrations to preferred feeding

and reproductive habitats as well as adult and young

mesopelagic animals that compose the sonic scattering

layer (SSL) in the region). A PICES Working Group will soon

report on status of the micronekton resources in the North

Pacifi c, including the Transition Zone.

Fish and InvertebratesMuch of our present knowledge of the fi sh and commercial

invertebrates in the Transition Zone can be attributed to

surveys conducted by Hokkaido University and by a spatially

extensive, temporally intensive, but short-lived monitoring

by scientifi c observers of the high-seas commercial driftnet

fi sheries for fl ying squid, Ommastrephes bartramii, and for

tunas and billfi shes during 1990-91.282

1995 1996 1997 1998 1999 2000 2001 2002Year

0

5000

10000

15000

20000

25000

Catc

h (t

)

0

1

2

3

Catch per vessel per day

[Figure 142] Catch and catch per vessel per day of fl ying squid (Ommastrephes bartrami) by Japanese jig fi shery381

Commercial fi sheries currently known to operate in the

Transition Zone include the Japanese and U.S. longline fi shery

for tunas and billfi shes, the U.S. troll fi shery for albacore tuna

(Thunnus alalunga), and the Japanese (and to a lesser extent,

U.S.) jig fi shery for fl ying squid. Recently, hundreds of Chinese

squid jigging vessels began operating in the Transition Zone,

however catch information for this fi shery is currently not

available. For the Japanese central North Pacifi c squid jigging

fi shery, CPUE has declined in 2000 and has remained low

(Figure 142). The status of the tuna fi sheries are described

elsewhere in this volume in a chapter prepared by the Inter-

American Tropical Tuna Commission. [206]

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155̊ E, early and late June

32.5

35.0

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45.0

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50.0

75 80 85 90 95 100Year

= 1000= 500= 100

170˚E, mid-July

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175˚E, late July

32.5

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_>

[Figure 143] Catch per set of Pacifi c pomfret (Brama japonica) by size (green: fi sh ≥350 mm FL: yellow: fi sh <350 mm FL) and by latitude (°N) for the 1981-96 T/S Hokusei Maru research gillnet time series along 155°E (left), 170°E (center), and 175°E (right) longitudes.

Marine Birds and MammalsSeabirds Populations of Laysan albatross, Phoebastria

immutabilis, and blackfooted albatross, P. nigripes,

populations breed in the Hawaiian Archipelago and forage

in the North Pacifi c Transition Zone. A comparison of annual

reproductive success (proportion of chicks fl edged per

egg laid) of each species at French Frigate Shoals in the

northwestern Hawaiian Islands indicates strong coherence

between the two species that is especially evident during

two years of very low reproductive success (1984 and

1999). Interestingly, both years of low reproductive success

occurred about one year after major ENSO events. Other

seabird species such as red-footed boobies, Sula sula; red-

tailed tropicbirds, Phaethon rubricauda; and black noddies,

Anous tenuirostris, also exhibited very low reproductive

success in 1998-99, but not in 1984.382

1975 1985 1995 2005Year

0.00.10.20.30.40.50.60.70.80.91.0

Repr

oduc

tive

Suc

cess

LaysanBlackfooted

[Figure 144] Temporal trends in reproductive success of blackfooted and Laysan albatrosses monitored at French Frigate Shoals in the northwestwern Hawaiian Islands, 1980-2001.383

Mammals A status of marine mammals for the North

Pacifi c including the Transition Zone has been compiled

elsewhere.305

Turtles Two species of sea turtles are common occupants

of Transition Zone waters: loggerhead turtles, Caretta

caretta, and leatherback turtles, Dermochelys coriacea.

Loggerheads in particular appear to exploit the TZCF as a

migration pathway and as forage habitat.384 Interactions of

loggerhead turtles with commercial surface longline fi sheries

targeting swordfi sh in the southern Transition Zone have led

to a closure of the U.S. fi shery. While deep longline sets

continue to be set in the region for tuna, turtle interactions

have been dramatically reduced.

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How does variability in the position, strength, and

behavior of the TZCF impact the Transition ecosystem?

Some current evidence suggests that the degree of

meandering of the TZCF is a manifestation of enhanced

physical convergence and divergence and facilitates

trophic transfer at this specialized habitat. Field work

designated to examine the TZCF ecosystem dynamics

including community faunal composition, secondary

production, etc. are needed.

What is the impact of climate variability on key oceanic

habitats (e.g., fronts and frontal systems). Variations

in latitudinal position, degree of meandering,

intensifi cation of the currents fl ow fi eld, and coupled

biological responses to the environment associated

with changing regimes need addressing.

Biological time series are most diffi cult to come by.

The Hokkaido University/Japan Fisheries Agency

meridional hydrographic-research gillnet survey time

series should be closely examined for possible use in

assessing the status of the Transition Zone ecosystem.

Cursory examination of Pacifi c pomfret catches appears

to capture the transition from the more productive

regime brought about by the 1977-88 North Pacifi c

climate event characterized by an intensifi cation of the

Aleutian Low Pressure System385 to the less productive

period since and hypothesized return to pre-1976

levels (Figure 143). Support for the time series needs

to be considered.

issuesWhile the TZCF is readily monitored by satellite remote sensing of ocean

color, very little in situ information has been gathered to advance our

understanding of the ecosystem dynamics associated with this feature.

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AuthorshipMichael P. Seki

U.S. National Marine Fisheries Service

NOAA, Pacifi c Islands Fisheries Science Center

2570 Dole Street

Honolulu, HI

U.S.A. 96822-2396

E-mail: Michael.Seki@noaa.gov

ContributorsElizabeth N. Flint

U.S. Fish and Wildlife Service /Hawaiian & Pacifi c Islands

NWR Complex, Honolulu, USA

Evan Howell

NOAA/Fisheries, Honolulu, USA

Taro Ichii

Fisheries Research Agency, Shimizu, Japan

Jeffrey J. Polovina

NOAA/Fisheries, Honolulu, USA

Akihiko Yatsu

Fisheries Research Agency, Yokohama, Japan

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