transition zone - home - pices - north pacific marine ... central north c transition pacifi zone...
TRANSCRIPT
Transition Zone
[201]
O
cea
n a
nd
Cli
ma
te C
ha
ng
es
86587_p201_210.indd 20186587_p201_210.indd 201 12/30/04 4:52:17 PM12/30/04 4:52:17 PM
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
[202]
O
cea
n a
nd
Clim
ate
Ch
an
ge
s
86587_p201_210.indd 20286587_p201_210.indd 202 12/30/04 4:52:17 PM12/30/04 4:52:17 PM
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
-4
-3
-2
-1
0
1
2
3
4
1992199319941995199619971998199920002001200220032004
-10
0
10
20
30
40
50
60
140 160 180 200 220 240 260
0
0
0.25
0.25
0.5
0.75
0.75
1
11.25
-1
-1-0.75
-0.75
-0.5
-0.5
-0.5
-0.25
-0.25
0
0
-2.0
-1.5
-1.0
-0.5
0.0
0.5
1.0
1.5
2.0
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
Tra
siti
on
Zo
ne
[203]
86587_p201_210.indd 20386587_p201_210.indd 203 1/12/05 10:58:57 PM1/12/05 10:58:57 PM
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.
141415
1515
1516
1616
17
18
1920 2122
2223 23
131
14 14 1415151616
17
18
192021
2223
13
1414
15
16
16
17
17 17
18
18 181818
1919
1919
20
21
22
1314
15 151516 16
1617
1718
19
1
20
22
13 13 1314
14
15 15 15
16
17
1
1819
202021
22
0.20.2
0.2 0.2 0.
0.2
0.2
0.2
0.4 0.40.4 0.40.4 0.40.4 0.4
0.4
0.40.40.60.6 0.6 0.60.6
0.6
[204]
O
cea
n a
nd
Clim
ate
Ch
an
ge
s
86587_p201_210.indd 20486587_p201_210.indd 204 1/19/05 11:06:52 PM1/19/05 11:06:52 PM
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).
Tra
siti
on
Zo
ne
[205]
86587_p201_210.indd 20586587_p201_210.indd 205 12/30/04 4:52:19 PM12/30/04 4:52:19 PM
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
160E
160E
180
180
160W
160W
140W
140W
120W
120W
100W
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]
O
cea
n a
nd
Clim
ate
Ch
an
ge
s
86587_p201_210.indd 20686587_p201_210.indd 206 12/30/04 4:52:20 PM12/30/04 4:52:20 PM
155̊ E, early and late June
32.5
35.0
37.5
40.0
42.5
45.0
47.5
50.0
75 80 85 90 95 100Year
= 1000= 500= 100
170˚E, mid-July
32.5
35.0
37.5
40.0
42.5
45.0
47.5
50.0
75 80 85 90 95 100Year
175˚E, late July
32.5
35.0
37.5
40.0
42.5
45.0
47.5
50.0
75 80 85 90 95 100Year
_>
[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.
Tra
siti
on
Zo
ne
[207]
86587_p201_210.indd 20786587_p201_210.indd 207 12/30/04 4:52:21 PM12/30/04 4:52:21 PM
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.
[208]
86587_p201_210.indd 20886587_p201_210.indd 208 12/30/04 4:52:22 PM12/30/04 4:52:22 PM
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: [email protected]
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
Tra
siti
on
Zo
ne
[209]
86587_p201_210.indd 20986587_p201_210.indd 209 12/30/04 4:52:23 PM12/30/04 4:52:23 PM