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Anticyclonic Eddies in the Alaskan Stream
HIROMICHI UENO,* HOWARD J. FREELAND,1 WILLIAM R. CRAWFORD,1 HIROJI ONISHI,#EITAROU OKA,*,@ KANAKO SATO,* AND TOSHIO SUGA*,&* Institute of Observational Research for Global Change, Japan Agency for Marine-Earth Science and Technology,
Yokosuka, Kanagawa, Japan
1 Institute of Ocean Sciences, Fisheries and Oceans Canada, Sidney, British Columbia, Canada# Division of Marine Bioresource and Environmental Science, Graduate School of Fisheries Science,
Hokkaido University, Hakodate, Japan@ Ocean Research Institute, University of Tokyo, Tokyo, Japan& Department of Geophysics, Graduate School of Science, Tohoku University, Sendai, Japan(Manuscript received 7 November 2007, in final form 21 October 2008)
ABSTRACT
Anticyclonic eddies propagating southwestward in the Alaskan Stream (AS) were investigated throughanalysis of altimetry data from satellite observations during 1992–2006 and hydrographic data from profiling
float observations during 2001–06. Fifteen long-lived eddies were identified and categorized based on theirarea of first appearance. Three eddies were present at the beginning of the satellite observations; anotherthree formed in the eastern Gulf of Alaska off Sitka, Alaska; and four were first detected at the head of the
Gulf of Alaska near Yakutat, Alaska. The other five eddies formed along the AS between 1578 and 1698W,
and were named AS eddies. While the eddies that formed in the Gulf of Alaska mainly decayed before
exiting the Gulf of Alaska, the AS eddies mostly crossed the 1808 meridian and reached the western subarctic
gyre. Four of five AS eddies formed under negative or weakly positive wind stress curls, which possibly
caused AS separation from the coast. Comparison of eddy propagation speeds in the AS with the bottom
slope showed that eddies propagated faster over steeper slopes, although eddy speeds were slower than thosepredicted by the topographic planetary wave dispersion relation. An AS eddy was observed by profiling floats
in the western subarctic gyre after it detached from the AS. Intermediate-layer water near the eddy center
had low potential vorticity compared with the surrounding water, suggesting that AS eddies provided thewestern subarctic gyre with water just south of the Aleutian Islands.

1. Introduction
The Alaskan Stream (AS; Fig. 1) is a subarctic westernboundary current in the North Pacific Ocean, flowing
southwestward from the head of the Gulf of Alaska(;1458W) along the Alaskan Peninsula and the AleutianIslands (Reed and Schumacher 1986). The AS serves as aconnection between the Alaskan gyre, the western subarctic gyre, and the Bering Sea gyre, playing an important role in the volume, heat, and freshwater transports
in the subarctic North Pacific (Onishi and Ohtani 1999).
The AS and the Alaska Current, a broad northwardcurrent flowing in the eastern Gulf of Alaska, are known
Corresponding author address: Hiromichi Ueno, Institute ofObservational Research for Global Change, Japan Agency for
Marine-Earth Science and Technology, 2-15 Natsushima-cho,
Yokosuka, Kanagawa 237-0061, Japan.
E-mail: uenohiro@jamstec.go.jp
DOI: 10.1175/2008JPO3948.1
Ó 2009 American Meteorological Society

to involve offshore meanders and anticyclonic eddies(both are referred to as eddies in this paper). In the Gulf
of Alaska, three types of eddies have been identified
based on their formation region. Haida eddies form in
winter off the west coast of the Queen Charlotte Islands(;538N) and move mostly westward to the central Gulf
of Alaska (Crawford and Whitney 1999; Crawford 2002).
Sitka eddies appear off Sitka, Alaska (;578N) (Tabata1982). They move mostly westward (Gower 1989;
Crawford 2002), while some of them move northwestward and become embedded in the AS (Crawford et al.
2000; identified as CCF2000 in Table 1). Yakutat eddies
form at the head of the Gulf of Alaska (1418–1448W)
near Yakutat, Alaska, and propagate southwestward in
the AS along the northwestern continental margin of
the Gulf of Alaska (Ladd et al. 2005b, 2007).
Previous studies reported that eddies in the Gulf of
Alaska had a significant impact on the heat, freshwater,

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935

FIG. 1. Schematic representation for the currents of the subarctic North Pacific. Bathymetric contours
are 200 m and every 1000 m, starting from 1000 m. Thick red lines are the 1000-m depth contours between
1758E and 1358W, and the line 28 south of it. The area between the two lines corresponds to the area
where the SLA was averaged in Fig. 3.

nutrient, and biota exchange between the shelf region
and the offshore region. Haida eddies and some Sitka
eddies, which propagate westward to the central Gulf of
Alaska, carry warm, fresh, and nutrient-rich coastal watercontaining biota in the shelf region to the central Gulf of
Alaska by trapping shelf water at the eddy center and by
advection in the outer ring of the eddy (Crawford 2002;
Mackas and Galbraith 2002; Whitney and Robert 2002;
Crawford et al. 2005). The deep-sea region of the eastern
subarctic North Pacific is a high-nutrient, low-chlorophyll(HNLC) environment (Martin et al. 1989; Boyd et al.
1998; Harrison et al. 1999; Wong et al. 2002), and theprimary production is limited by iron (Boyd et al. 2004).
Haida eddies supply the Alaskan gyre with iron-richcoastal waters (Johnson et al. 2005), and so they may playa crucial role in the productivity of the Alaskan gyre.
Yakutat eddies and some Sitka eddies propagating
southwestward in the AS in the northwestern Gulf of
Alaska also affect the water exchange between the shelfand offshore regions. Okkonen et al. (2003) indicatedthat these eddies altered the structure of the shelf breakfront, and in so doing, influenced the shelf-slope exchange of biota and water mass properties. Ladd et al.
(2005c) and Crawford et al. (2007) showed that Sitka
and Yakutat eddies influenced cross-shelf exchange bytrapping coastal water in their interiors and propagating
offshore, and also by interfering with the slope circulation and resulting in cross-shelf flow.
Some of the eddies in the Gulf of Alaska werereported to go out of the Gulf of Alaska and propagate
downstream in the AS along the Aleutian Islands.
Crawford et al. (2000) analyzed Ocean Topography Experiment (TOPEX)/Poseidon altimeter data from 1992

to 1998 and detected six long-lived eddies in the AS.
One of these eddies propagated from the Gulf of Alaska
to 1808, although its formation was not observed due todata limitations. Ladd et al. (2005b, 2007) demonstratedusing altimetry data during 2001–06 that a Sitka eddy
propagated along the AS and reached just east of 1808,with a lifetime exceeding 5 yr. Okkonen (1992) used
Geosat altimeter data in 1987 and 1988 to describe an
eddy in the AS that crossed the 1808 meridian and entered the western subarctic gyre, although the eddy wastraced only between 1758W and 1748E and its formationand decay were unclear because of data limitations.
Eddies in the AS were also seen in a worldwide map ofeddy trajectories from TOPEX/Poseidon and European
Remote Sensing Satellites-1 and -2 (ERS-1/2) altimeterdatasets in Chelton et al. (2007), but individual trajectories were not demonstrated in their analysis. Rogachev et al. (2007) investigated eddies in the western
subarctic gyre, which were mainly formed at 1708–1758E
along the Aleutian Islands, and indicated that they
supplied the East Kamchatka Current with heat.
Eddies in the AS along the Aleutian Islands could
have a significant impact on the heat, freshwater, nutrient, and biota exchange between the coastal area
south of the Aleutian Islands and the offshore region in
the western and central subarctic North Pacific, in a similar manner to the eddies in the Gulf of Alaska. In addition, eddies south of the Aleutian Islands were suggested to drive the flow between the North Pacific and
the Bering Sea (Okkonen 1996). However, comprehensive study of eddies in the AS along the AleutianIslands from their formation to decay has not been
performed using long-term data.

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In this study we use satellite altimeter data from
October 1992 to December 2006 to examine the formation, evolution, and termination of eddies in the AS
and those detached from the AS. We also use hydrographic data from Argo profiling floats to study the
structure of eddies in the AS. Our analysis of eddies,
based on 14 yr of altimetry observations as well as float
observations, would improve our understanding of theinfluence of eddies in the AS on the physical and biogeochemical fields in the western and central subarctic
North Pacific as well as in the Bering Sea.

2. Data and methods
We used delayed-time data of sea level anomaly(SLA) and absolute dynamic topography (ADT) produced by the Segment Sol Multimissions d’Altimétrie,d’Orbitographie et de Localisation Précise/Multimission
Altimeter Data Processing System (SSALTO/DUACS)
from TOPEX/Poseidon, the Geosat Follow-On (GFO),
Jason-1, ERS-1/2, and Environmental Satellite (Envisat)observations, distributed by the Archiving, Validation
and Interpretation of Satellite Oceanographic (AVISO)
data center of Collecte Localisation Satellites (CLS) (LeTraon et al. 2003; Pascual et al. 2006; AVISO 2007). The
temporal and spatial resolutions are 7 days and 0.258 3
0.258, respectively, and we analyzed the data from 14
October 1992 to 27 December 2006. The spatial meanstate of the subarctic North Pacific north of 458N, except
for the marginal seas, was removed from each weekly
map of the SLA to compensate for seasonal steric effects.
The effective temporal resolution is determined by therepeat times of satellites. The ERS and Envisat tracks
repeat every 35 days, the longest interval of all altimeter
satellites. These satellites provide the best spatial resolution.
We also used temperature and salinity profiles recorded by Argo floats (Argo Science Team 2001) in the
North Pacific north of 408N during 2001–06 to study thehydrographic structure of eddies in the AS. The realtime quality-controlled float data were downloaded
from the ftp site of Argo Global Data Assembly Center(information online at ftp://usgodae1.fnmoc.navy.mil/
pub/outgoing/argo and ftp://ftp.ifremer.fr/ifremer/argo).
From these data, defective temperature and salinity
profiles, such as those with measurements flagged as badand those lacking intermediate layers for certain depths,
were eliminated, following the procedures of Oka et al.
(2007).
Daily wind stress data and monthly evaporation andprecipitation (from the latent heat flux) data from the
National Centers for Environmental Prediction–National
Center for Atmospheric Research (NCEP–NCAR) re-

VOLUME 39

analysis (Kalnay et al. 1996) were also used. Wind stressdata were analyzed after applying a 30-day running mean
at each grid point. We also used bottom topography
data of 29 gridded elevations/bathymetry for the world(ETOPO2v2; National Geophysical Data Center 2006).
Anticyclonic eddies in the AS were detected as SLA
maxima, neglecting eddies whose maximum SLAs wereless than 5 cm. In this study, we concentrated on theeddies that first appeared east of 1808, persisted for atleast 6 months, and propagated southwestward to thewest of 1558W along the AS. The eddies are identified
based on the year of the first appearance (e.g., eddy
96a). When the main part of the eddy detaches from theAS, the detached eddy is called, for example, eddy 96a0
and the portion remaining in the AS is called, for example, eddy 96a1. The added number increases every
time a detachment occurs.
The weekly propagation speed along the 1000-m
depth contour for each eddy was evaluated from theeddy locations 4 weeks before and after, so that the
typical eddy propagation distance during each 8-weekperiod was long enough compared with the spatial resolution of SLA (0.258 3 0.258). The propagation speed
was evaluated only for eddies that were located within
28 south of the 1000-m depth contour (Fig. 1) between
1808 and 1458W, and not for those detached from theAS. As the result, 1084 eddy propagation speed data
points were obtained.
The sea bottom slope and surface AS velocity at eachweekly eddy location were evaluated as follows. We
averaged the original bottom topography data with aresolution of 29 3 29 in 0.18 3 0.18 grid boxes, and calculated the bottom slope (gradient) normal to 1000-m
depth contour for each grid with a horizontal scale of
50 km, neglecting bottom topography deeper than 5000 m.
The bottom slope for each weekly eddy location wasthen calculated as a 8-week average of bottom slope
along the eddy trajectory. The alongshore surface AS
velocity for each weekly eddy location was evaluated as
an average of the surface AS velocities at the longitudesof 2.58 west and 2.58 east of the eddy location to avoid
the influence of the eddy on the AS velocity. Surface AS
velocity at each longitude was evaluated as the highest
alongshore surface velocity within 28 south of the 1000-m
depth contour (Fig. 1), using ADT data and assuming
geostrophy.
We evaluated potential vorticity (PV) as r21fDsu/Dz,
where f is the Coriolis parameter, r is density, and Dsu/Dz
is vertical gradient of potential density. The Dz was set
to be 50 m. For the eddies whose PVs were evaluated,the magnitude of the relative vorticity was O(1025 s21)
at the surface, which was one order smaller than f.
Therefore, we neglected relative vorticity in this study.

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3. Resultsa. Formation, propagation, and termination of eddies
in the Alaskan Stream
During the analysis period, 15 long-lived eddies wereobserved in the Alaska Current and the AS (Fig. 2,
Table 1). These eddies formed along the AS south of the
Alaskan Peninsula and the Aleutian Islands as well as
along the Alaska Current in the northeastern Gulf of
Alaska, and propagated downstream along the AS.
During the propagation, some eddies left the AS, andsome separated into two or three eddies.
Eddies 92a, 92b, and 92c already existed on 14 October 1992, the beginning of the altimetry data, so theirformation regions could not be identified. They propagated downstream in the AS to ;1808 (eddies 92a and92c) and to ;1708W (eddy 92b).
Eddies 96d, 00a, and 04a were first observed in thenortheastern Gulf of Alaska off Sitka, and were considered to be Sitka eddies. They propagated northwestward after their formation, became embedded in
the AS, and propagated southwestward along the AS.
Eddy 96d detached from the AS around 1528W andmoved southwestward while decaying. After the detachment, a small eddy was left in the AS, but it soon
vanished. Eddy 00a propagated to ;1808 along the AS,
while detaching an isolated eddy around 1718W. The
detached eddy moved southwestward and decayed at
1758W. Eddy 04a propagated to ;1578W, still existing at
the end of our analysis period.
Eddies 95a, 96c, 99c, and 03a were first detected at thehead of the Gulf of Alaska near Yakutat (Yakutat eddies),and propagated southwestward along the AS. Eddy 95a
detached a small isolated eddy near 1568W; both thedetached and remaining eddies decayed rapidly. Eddy
96c stopped propagating and weakened near 1578W. It
then propagated eastward to 1558W, was absorbed by
the AS, and vanished. Eddy 99c decayed near 1608W.
Eddy 03a detached an isolated eddy close to 1558W; theeddy left in the AS still existed with an SLA higher than
40 cm at the end of the analysis period, while the detached eddy stayed near 1558W and decayed there. All
isolated eddies detached from Sitka and Yakutat eddies
in the AS moved less than 58 in longitude after theirdetachment, persisting for 1–16 months.
Eddies 96a, 96b, 99a, 99b, and 02a appeared along the
AS between 1578 and 1698W. The formation region ofthese eddies lies to the west of regions where Haida,
Sitka, and Yakutat eddies form. We call these eddies
Alaskan Stream (AS) eddies, following Okkonen et al.
(2001) who detected eddy formation near 1608W in anumerical model. Crawford et al. (2000) first detected
an AS eddy, eddy 96b (eddy 5 in their paper), in the937

altimeter data, but did not discuss its lifetime behavior
due to data limitations. In this study we detected five AS
eddies and examined them from their formation to decay, revealing their behavior over their full lifetimes.
Eddy 96b first appeared at 1588W south of the Alaskan
Peninsula and moved southwestward along the AS while
increasing its maximum SLA up to 67 cm. It detachedfrom the AS near 1708W and moved southwestward to
468N, 1798E, a little beyond the 1808 meridian. Eddy 99b
also formed south of Alaskan Peninsula but decayed in;10 months at 1698W. Eddy 96a formed at 1638W, theeastern edge of the Aleutian Islands, propagated westward to 1758E, and detached an isolated eddy at;1708W. Eddy 99a was first detected at 1658W south of
Unimak Pass and moved westward while detaching two
isolated eddies at ;1708W and ;1808. The eddy left in
the AS separated from the AS around 1758E. All threeeddies born from eddy 99a (99a0, eastern; 99a1, middle;
99a2, western) moved southwestward away from the AS.
Eddies 99a1 and 99a2 propagated deeply into the western subarctic gyre west of 1808. Eddy 02a first appeared
at 1698W, moved westward along the AS, left the AS at;1788W, and propagated southwestward to 488N, 1758E
in the western subarctic gyre.

b. Behavioral aspects of eddies in the Alaskan Stream
Five of 15 long-lived eddies in the AS moved westward beyond 1808. One of these eddies (eddy 92c), which
was present in the beginning of satellite altimetry observation, passed the 1808 meridian by just 18 along the
AS. The remaining four eddies (eddies 96a, 96b, 99a,and 02a) that crossed 1808 and reached the western
subarctic gyre were all AS eddies. This might be related
to the formation region of AS eddies (1578–1698W).
East of the region, eddies in the AS (Sitka and Yakutat
eddies) likely decayed (Fig. 2, Table 1).
All 15 long-lived eddies in the AS are seen in Fig. 3,
the longitude–time diagram of the SLA along the
Alaska Current and the AS. In addition to eddies shownin Fig. 2 (white dots), westward-propagating SLA maxima with a lifetime shorter than ;1 yr were frequently
observed, particularly between 1408 and 1508W, once toseveral times every year. Many of them disappeared in
the Gulf of Alaska before reaching 1558W.
SLAs along the Alaska Current and the AS werelower during 1999–2004 than 1992–98 and 2005–06 (Fig.
3). During 1999–2003, fewer eddies were observed in
the AS, although strong eddylike features were seen in
the Gulf of Alaska near 1458W (Fig. 3), as indicated by
Ladd (2007). Therefore, the fewer eddies in the AS
might be due to relatively strong eddy decay (or weak
eddy enhancement) in the western Gulf of Alaska during
the period. Henson and Thomas (2008) indicated that

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FIG. 2. Trajectories of long-lived eddies in the AS. Colors represent the SLA (cm) at the eddy
center. Bathymetric contours are the same as in Fig. 1. The figures located just below or above theeddy trajectory indicate the month and year when the eddy was observed.

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TABLE 1. Features of 15 long-lived eddies in the AS.

Eddy name
92a
92b
92c
95a
96a
96b
96c
96d
99a

99b
99c
00a
02a
03a

04a

Day and locationof first appearance

Last day andlocation of observation

Formation
area

Previous studies

Not identified
Not identified
Not identified
18 Jan 1995
(59.258N, 144.258W)
13 Mar 1996
(53.758N, 163.008W)
13 Mar 1996
(54.758N, 157.758W)
23 Oct 1996
(58.758N, 145.008W)
24 Jan 1996
(56.258N, 136.008W)
3 Mar 1999
(53.758N, 164.508W)

15 Sep 1993(51.008N, 180.008E)
21 Jul 1993(51.758N, 170.508W)
30 Aug 1995(51.008N, 179.258E)[0] 3 Apr 1996(52.508N, 154.258W)[1] 20 Mar 1996(53.508N, 160.008W)[0] 6 Aug 1997(48.508N, 171.758W);[1] 22 Oct 1997(50.008N, 175.008E)
5 Jul 2000(45.508N, 178.508E)

N/A
N/A
N/A
YakutatEddy 1 in CCF2000
Eddy 2 in CCF2000
Eddy 3 in CCF2000
Eddy 4 in CCF2000

AS

N/A

AS

Eddy 5 in CCF2000

10 Dec 1997(53.758N, 155.258W)

YakutatN/A[0] 25 Nov 1998(53.008N, 153.508W)[1] 7 Jan 1998(55.508N, 154.758W)[0] 13 Dec 2000(47.508N, 174.258W);[1] 20 Nov 2002(44.008N, 174.008E);[2] 25 Feb 2004(47.758N, 170.258E)
9 Feb 2000(52.008N, 169.008W)

Sitka

Eddy 6 in CCF2000

AS

N/A

AS

N/A

2 May 2001(54.008N, 161.008W)

Yakutat[0] 5 Jul 2006(48.258N, 174.508W);[1] 8 Jun 2005(51.008N, 179.008W)
9 Jun 2004(47.508N, 175.008E)

Sitka

Eddy A in Okkonen
et al. (2003)
2001 eddy in
Ladd et al. (2007)
N/A

21 Apr 1999
(55.008N, 156.758W)
10 Mar 1999
(59.258N, 143.008W)
29 Nov 2000
(56.008N, 134.508W)
16 Jan 2002
(52.508N, 168.508W)
8 Jan 2003
(59.008N, 143.008W)
22 Dec 2004
(57.258N, 137.008W)AS[0] 22 Feb 2006(52.508N, 156.258W);[1] 27 Dec 2006(51.508N, 169.508W)

Yakutat

27 Dec 2006(54.258N, 157.258W)

Sitka

the area occupied by eddies was small during 1999–2002
in the AS between 142.58 and 1608W because the alongcoast wind component was weak and thus unfavorable
for downwelling along the eastern and northern coastsof the Gulf of Alaska during this period.
Occasionally, eddies appear to stop propagating for awhile and amplify. Eddies 92c, 95a, 96c, 99c, 00a, and03a stayed around 1558W for several months and amplified; three of them (eddies 95a, 96c, and 03a) detached from the AS (Figs. 2 and 3). Eddies 92c, 96a, 96b,
99a, and 00a stayed near 1708W for several months,
intensified, and finally detached from the AS (except foreddy 92c). These behaviors might be related to thewater exchange between the AS and the shelf/strait
regions. This topic will be discussed in section 4b.

c. The structure of eddies in the Alaskan Stream
We further investigate the hydrographic structure ofeddies in the AS using Argo profiling float data. In the
Gulf of Alaska, hydrographic surveys across eddies in
the AS have been conducted (Okkonen et al. 2003; Ladd
et al. 2005b, 2007). Therefore, we restrict our analysis tothe area west of ;1608W, where few ship-based studies

Yakutat eddy (Ladd et al.
2005b), 2003 eddy
(Ladd et al. 2007)
N/A

have sampled. In this longitude range, three eddies (00a,02a, and 03a) were sampled by Argo floats. For eacheddy, we composited all Argo float data observed near
the eddy in time (, 3 days) and in space (, 220 km)(Figs. 4–6). Since seasonal variation is large near the sea
surface, we here discuss the subsurface layer below 100-m
depth.
Eddy 00a (Sitka eddy) was observed between 1598
and 1648W from May to December 2003 by an Argo
float while it was propagating westward in the AS(Fig. 4). At depths greater than 250 m, isopycnals,
isotherms, and isohalines lowered toward the centerof the eddy, similarly to Sitka eddies previously observed in the eastern Gulf of Alaska (e.g., Tabata1982). The layer lowering was significant within ;70 km
from the center, suggesting that this eddy had a diameter of ;140 km. The temperature–salinity (T–S) diagram shows two types of subsurface water with temperatures of 48–58C and 58–68C at 26.0–26.8 su (Fig. 4d).
The former colder water, observed more than 80 km
away from the eddy center, is considered to be theRidge Domain Water in the Alaskan gyre (Musgrave
et al. 1992) located outside of the eddy. Meanwhile, the940

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FIG. 3. Longitude–time diagram of the SLA along the northern boundary of the Pacific
Ocean. The SLA was averaged within 28 south of the 1000-m depth contour (see Fig. 1). White
dots indicate the locations of eddies shown in Fig. 2 excluding eddies more than 28 south of the
1000-m depth contour.

latter warmer water, located in the eddy (, 70 km), wassimilar to the Alaska Current Water, which was observed in the high velocity core of the AS (Musgrave
et al. 1992).
At depths between 150 and 250 m, water at the eddy
center is colder than the water 25–50 km away from theeddy center. This was also true on isopycnals between
25.8 and 26.4 su (Fig. 4d), in contrast with Tabata (1982),
who indicated that waters at the center of Sitka eddies inthe eastern Gulf of Alaska were isopycnally warmer
than those away from the center in a density range of
25.6–26.6 su. Two profiles closest to the eddy center (redAPRIL 2009

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941

FIG. 4. Composite cross sections of (a) potential density, (b) potential temperature, and (c) salinity, and (d) a T–S diagram for eddy 00abased on Argo float observations from 15 May (159.258W) to 10 Dec 2003 (163.58W). Colored squares with white outline in (a)–(c)represent water properties observed by Argo floats, and background colors and contours denote properties horizontally box averaged
with a width of 44 km. Horizontal axes in (a)–(c) are the radial distance from the eddy center, which is positive (negative) when an Argo
float is located east (west) of the eddy center. Positions of eddy centers are determined from satellite SLA data. Color in (d) represents
the distance from the eddy center. Contour intervals are (a) 0.1 kg m23, (b) 0.258C below 78C and 0.58C above 78C, and (c) 0.1 psu.

circles with a temperature of ;5.58C between 26.0 and26.6 su) have a weak temperature minimum between
26.1 and 26.2 su. The subsurface temperature minimum
observed in the Gulf of Alaska in 2003 is considered to

be formed in the mixed layer in the 2001/02 winter andto survive for more than 1 yr (Ueno et al. 2005, 2007).
Eddy 00a was located at the head of the Gulf of Alaska
in the 2001/02 winter (Fig. 2). This suggests that this942

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FIG. 5. Same as in Fig. 4, but for eddy 03a from 11 Jan (1648W) to 27 Dec 2006 (169.58W).

eddy transported water formed in the 2001/02 wintermixed layer at the head of the Gulf of Alaska to the areaof 1598–1648W where it was sampled by an Argo float.
Eddy 03a (Yakutat eddy) was observed at 1648–1708W
in 2006 by three Argo floats while propagating southwestward in the AS (Fig. 5). At depths greater than 250 m,
isopycnals, isotherms, and isohalines lowered to theeddy center within ;100 km from the center. Their
depths were almost constant in the area farther than
150 km, suggesting that the edge of the eddy was locatedbetween 100 and 150 km away from the eddy center. Thisis consistent with the T–S diagram, which indicates that
T–S relations of subsurface water in the area farther than
150 km were similar to those of Ridge Domain Water.

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943

FIG. 6. Same as in Fig. 4, but for eddy 02a from 25 Dec 2002 (179.58W) to 29 Oct 2003 (176.258E). The potential temperature contour
interval is 0.18C below 58C and 0.58C above 58C.

Prior to our float observations, hydrographic observations across this eddy were performed 4 times: in May
2003 (eddy center at ;1468W), September 2003 (;1478W),
May 2004 (;1528W), and October 2004 (eddy centers at;1548 and ;1568W) (Ladd et al. 2005b, 2007). In May
2003, the water at the eddy center was isopycnallywarmer than the surrounding waters in the density

range of 25.4–26.2 su (Ladd et al. 2007). This difference
gradually decreased with time, although water at theeddy center was still isopycnally warmer than the surrounding waters in October 2004 (Ladd et al. 2007).
Figure 5 shows that the T–S relations near the eddy
center (radius , 25 km) are almost indistinguishable
from other observations off the center, indicating that

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the original water properties at the eddy center werelost by 2006 when Argo floats sampled the eddy.
Eddy 02a (an AS eddy), which formed at 1698W southof the Aleutian Islands in January 2002 and moved to
the western subarctic gyre, was observed by three Argo
floats between 1768E and 1808 from December 2002 to
October 2003 (Fig. 6), although all float observations
were conducted in the area more than 50 km away fromthe eddy center. Unlike eddies 00a and 03a, eddy 02a
was an isolated eddy detached from the AS when observed by the Argo floats. Near the eddy center, thiseddy shows a weak depression of isopycnals and isohalines at depths greater than 200 m, and a weak depression of isotherms at depths greater than 500 m. It is
hard to determine the diameter of this eddy.
Since eddy 02a was observed in the western subarctic
gyre after it was detached from the AS, we investigatedthis eddy from the viewpoint of water transport from the
AS region to the western subarctic gyre. For this purpose, PV was used as a conservative property as in
Yasuda et al. (2000) because T–S relations changed little
across the eddy. The thickness of the intermediate layer
at 26.5–26.8 su decreases outward from the eddy center;
the PVs at the 26.8su isopycnal surface are 1.9, 2.6, and
4.7 3 10210 m21 s21 in the areas where the distancesfrom the center are less than 100 km, between 100 and
150 km, and 150–220 km (probably out of the eddy),
respectively. The PV near the center of eddy 02a waslower than the climatological PV at the same location[. 3.5 3 10210 m21 s21; Macdonald et al. (2001); Sugaet al. (2004)], but similar to the climatological PV justsouth of the Aleutian Islands at 1808 [, 2.0 3 10210 m21 s21
at 26.8su; Fig. 3e in Onishi (2001)] and the PVs near thecenters of eddies 00a and 03a observed along the AS(# 2.0 3 10210 m21 s21). These PV values suggest that
eddy 02a supplies the deep-sea region of the western
subarctic gyre with intermediate water just south of the
Aleutian Islands characterized by low PV. This low PV
supply might affect the pycnocline structure and thus
the upwelling in the western subarctic gyre, which is a
key part of the intermediate-water circulation in thesubarctic North Pacific (Ueno and Yasuda 2003).

4. Discussiona. Water exchange among the Aleutian Islands
region, the western and central subarctic North
Pacific, and the Bering Sea
Okkonen et al. (2003) and Ladd et al. (2005c) indicated that eddies in the AS in the Gulf of Alaska have asignificant impact on the heat, freshwater, nutrient (including iron), and biota exchange between the shelf re-

VOLUME 39

gion and the offshore region by 1) trapping coastal waterat the eddy center and 2) changing the slope circulation.
We here found that some eddies detached themselvesfrom the AS south of the Aleutian Islands and propagated southwestward, likely exchanging water between
the relatively shallow region just around the AleutianIslands (the Aleutian Islands region) and the offshore
region by the mechanism (1) in the central and western
subarctic North Pacific. Indeed, eddy 02a was observed
to carry low-PV water into the western subarctic gyre.
Ten eddies propagating westward in the AS along the
Aleutian Islands during 1992–2006 (Fig. 2) might also
cause water exchange by the mechanism (2) throughdisturbing circulation south of the Aleutian Islands.
In addition to the water exchange between the
Aleutian Islands region and the western and central
subarctic North Pacific, eddies in the AS could affect thewater exchange between the North Pacific and the Bering
Sea. Okkonen (1996) indicated that an eddy in the
Alaskan Stream south of the Amchitka Pass (;1808)
drove flow through the pass. Therefore, the six eddies
that passed near the Amchitka Pass (see Fig. 2) might
influence the water exchange between the North Pacificand the Bering Sea.
How much heat and freshwater are transported by aneddy that detached from the AS and entered the western subarctic gyre? Based on the limited hydrographic
observations available from Argo floats (section 3c), we
roughly estimated the heat and freshwater transports
assuming that an eddy with averaged T–S profiles ofeddies 00a and 03a followed the same trajectory as eddy
02a, and finally changed to the climatological T–S profiles at the location where eddy 02a decayed.
First, we averaged 17 T–S profiles within 50 km fromthe center of eddies 00a and 03a (Figs. 4b,c and 5b,c) at
each depth between 100 and 1000 m, weighting them by
the distance from the eddy center. The top 100-m layer
was not used for calculation to avoid the influence of
seasonal variations. Assuming that the water column of
the averaged T–S profile was transported to 47.58N and175.58E, where eddy 02a decayed, and changed to theclimatological annual-mean water column from the World
Ocean Atlas 2005 (Locarnini et al. 2006; Antonov et al.
2006), the heat and freshwater released along the trajectory from the AS region to the western subarctic gyre
were estimated to be 3.4 3 1019 J and 5.0 3 1010 m3,
respectively. These values are almost the same as theheat and freshwater transports of 3 3 1019 J and 5 3 1010 m3
released to the deep-sea region of the Gulf of Alaska bya typical Haida eddy (Crawford 2005).
Assuming that an AS eddy releases its heat and salt in
the 58 3 58 area of the western subarctic gyre in a year,
its heat and freshwater fluxes are evaluated as 5.4 W m22

APRIL 2009

UENO ET AL.

and 0.26 m yr21, respectively. These are comparable
to NCEP–NCAR surface net heat and freshwater fluxes(10–20 W m22 and 0.5–1.0 m yr21 into the ocean) along
the trajectory of eddy 02a in the western subarctic
gyre. Since AS eddies did not enter the western subarctic gyre every year, they might affect the interannual
variation of temperature and salinity in the western
subarctic gyre.

b. Formation and evolution mechanisms of eddies
in the AS
Thomson (1972) investigated the AS analytically via
steady, barotropic frictional theory and indicated thatthe frictional boundary layer (the AS) would separate
from the coast when the planetary vorticity variation
could not balance the diffusion of negative vorticity
from the boundary as the result of the changing zonal
orientation of the boundary. Based on his theory, separation of the AS more likely occurs where AS flows
nearly zonally and when the wind stress curl over the AS
is negative. Alaskan Stream eddies formed at 1578–
1698W, where the angle between the latitude lines andthe shelf break was small. Therefore, we investigated
the wind stress curl along the AS to study the formation
mechanism of AS eddies.
Figure 7 shows the 30-day running-mean wind stresscurl along the AS between two red contours in Fig. 1 at
1758E–1358W from October 1993 to December 2006,
superimposed by the location of the eddies. The standard deviation of the wind stress curl (curlt) along the
AS at 1578–1698W before the running mean was 4.1 31027 N m23, whose contribution to the vorticity balance(curlt /rh) in the AS is about 20% of that of the planetary vorticity advection (bv sinu), assuming that thewater density (r) is 103 kg m23, the water depth (h) is
5000 m, the latitudinal change of the Coriolis parameter(b) is 1.4 3 10211 m21 s23, the speed of AS (v) is 10 cm s21,
and the angle between the AS and latitude lines (u) is
208. Therefore, the wind stress curl could play a key rolein the separation of the AS and thus the formation of
AS eddies.
The AS eddies 96a, 96b, 99a, and 99b formed during
or just after the negative or weakly positive wind stresscurl event (Fig. 7), suggesting that wind stress curl
contributed to the formation of these eddies. In addition, ADT data suggest that the AS was relatively weakin 1996 and 1999. Therefore, the contribution of thewind stress curl to the vorticity balance in the AS might
be relatively large in those years. On the other hand,eddy 02a formed in the period of strong positive windstress curl, which likely stabilized the AS. Some otherformation mechanisms might contribute to the formation of the eddy.

945

We next discuss the evolution mechanism of theeddies near 1558 and 1708W, as described in section 3b.
Eddies 92c, 95a, 96c, 99c, 00a, and 03a were intensified
around 1558W, which is close to the exit of the sea valley(southwest of Shelikof Strait; see Fig. 1). From this sea
valley, low-salinity and low-density coastal water outflowed to the open ocean near the sea surface (Reedet al. 1986, 1987; Stabeno and Hermann 1996; Stabeno
et al. 2004). Such outflow of low-density water from asea strait (sea valley in this case) to the open ocean
could form an anticyclonic eddy. Di Lorenzo et al.
(2005) indicated that warm and freshwater outflows
from Hecate Strait east of the Queen Charlotte Islands
generated small patches of buoyant water in the area
west of the islands. The difference in density between
the core of the patches and the ambient water intensified and sustained the anticyclonic circulation of thesepatches, which then merged to generate a larger eddy,
the Haida eddy (Di Lorenzo et al. 2005). Based on theireddy formation and intensification mechanism, lowsalinity surface water outflow around 1558W indicatedby previous studies might intensify eddies propagating
southwestward along the AS. For instance, some surface
drifting buoys coming out of the sea valley were trappedby an eddylike feature in the AS (Stabeno and Hermann
1996; Stabeno et al. 2004).
The longitude of 1708W is located just west of
Samalga Pass, which is the easternmost Aleutian passwith a sill depth greater than 100 m. Water properties
just south of the Aleutian Islands changed westward
beyond the pass due to the southward flow from theBering Sea through the passes west of Samalga (Favorite
1974; Ladd et al. 2005a). At the Amukta Pass around
1728W, for example, southward flow of 0.4 6 0.2 Sv(1 Sv [ 106 m3 s21) was observed on the western side of
the pass (Reed and Stabeno 1997). Waters in the Bering
Sea and the passes along the Aleutian Islands arecharacterized by low PV compared with those in the
North Pacific (e.g., Suga et al. 2004), suggesting that
low-PV water outflows from the Bering Sea to the south.
A theoretical study indicated that such low-PV outflow
from a sea strait formed an anticyclonic eddy when theoutflow exceeded a critical volume (Kubokawa 1991).
Thus, low potential vorticity outflows from the Bering
Sea likely intensify eddies in the AS around 1708W. Thissituation is similar to that along the Kuril Islands discussed by Yasuda et al. (2000). They suggested that
Kuroshio warm-core rings that had propagated northeastward were arrested near the Bussol’ Strait along the
Kuril Islands and were amplified with the supply of lowPV water from the Okhotsk Sea. Furthermore, eddies
seem to detach preferentially at both longitudes of 1558
and 1708W, as described in section 3b. This might be946

JOURNAL OF PHYSICAL OCEANOGRAPHY

VOLUME 39

FIG. 7. Longitude–time diagram of the wind stress curl averaged within 28 south of the 1000-m
depth contour (see Fig. 1). The black dots are the same as the white dots in Fig. 3. Locations ofAS eddy formation are encircled by ellipses.

related to the intensification of eddies at these longitudes.

c. Eddy propagation speed in the AS
Finally, we discuss the propagation speeds of eddies in
the AS, using eddy trajectories shown in Fig. 2. The
southwestward alongshore eddy propagation speed ranged

from 20.5 to 7.0 km day21, and was 2.0 km day21 on
average. Since Okkonen et al. (2003) indicated that
faster eddies tended to propagate closer to the shelf along
the AS in the Gulf of Alaska, we first investigate the relation between the topographic slope and the eddy speed.
In Fig. 8a, the principal ellipse of variance (see Freeland
et al. 1975) is elongated in the upper-right to bottom-left

APRIL 2009

UENO ET AL.

direction, and the principal axis is represented by y 5
0.09x 1 0.37, where y is the eddy speed (km day21) andx is the bottom slope (m km21), indicating that a fastereddy propagation speed is associated with a steeper bottom slope. The correlation coefficient is 0.58. This suggeststhat eddies along the AS are affected by the topographic
beta term. It is important to note that the southwestward eddy speeds in Fig. 8 were the 8-week-averaged
speeds, and thus it was hard to determine the equivalent
number of degrees of freedom.
The dispersion relation of the topographic planetary
wave is independent of stratification when the wavelength is long compared to the internal Rossby radius ofdeformation, assuming zero planetary beta and constantstratification (LeBlond and Mysak 1978, section 20).
The wavelengths of eddies in the AS are O(100 km)(Crawford et al. 2000), which is long compared with theinternal Rossby radii of deformation in the AS region of
10–20 km (Emery et al. 1984; Chelton et al. 1998).
Therefore, from the dispersion relation of a topographic
planetary wave (LeBlond and Mysak 1978), the alongshore phase speed of a topographic planetary wave,
CTPW, is written as
CTPW 5 2 f

Hh 1,
H k2 1 l 2(1)

where f is the Coriolis parameter; H is the water depth;
Hh is the bottom slope normal to the 1000-m depth
contour; and k and l are the wavenumbers along andnormal to the 1000-m depth contour, respectively.
Substituting the wavelength of 100 km, the water depthof 5000 m, and the Coriolis parameter at 558N, we obtained the relation between the bottom slope (m km21)
and the topographic wave speed (km day21) of CTPW 5
0.26 Hh. The slope of 0.26 is about 3 times as large (fast)
as the slope of principal axis in Fig. 8a, suggesting thatsome other mechanisms slow down the eddy propagation speed. As discussed in sections 3b and 4b, eddies
stopped propagating for a while and amplified at somelongitudes, which would indicate that nonlinear effect ofeddy intensification was dominant during the period.
This nonlinear effect might be one of the mechanisms
involved in slowing down the eddy speed.
Higher eddy propagation speed was associated withlower SLAs at the eddy center, with a correlation coefficient of 20.31 (Fig. 8b). This is consistent with Fig. 3,
which shows that eddies were intensified while theyslowed down. Figure 3 also implies that eddies with high
SLA tend to occupy a wide longitudinal range, suggesting that large eddies propagate slowly. This contradicts the dispersion relation of topographic planetary
waves, where the phase speed is independent of SLA
and increases with eddy size [see Eq. (1)]. It is thus

947

suggested that nonlinear effects play an important rolein the eddy propagation along the AS, possibly reducing
the propagation speed as demonstrated above.
We further examined the relations between the eddy
speed and the surface AS velocity, although the AS
velocity calculated from ADT data had relatively large
errors due to the closeness to the shelf break (Ducet
et al. 2000). Figure 8c shows that the mean eddy speed(2.0 km day21) is much slower than mean surface AS
velocity of 13.2 km day21 (15.3 cm s21). The mean eddy
speed is also much slower than the mean AS velocity ofO(10 km day21) at depths shallower than 1000 m across1808 (Onishi 2001), indicating that eddies are not justadvected by the AS. Furthermore, eddy speed is hardly
correlated to the AS surface velocity (correlation coefficient 5 0.19). This could be mainly attributed to theeffects of an AS velocity change to the direction normal
to the coast, which changes the background PV gradient. Okkonen (1993) indicated that the effects of thisbackground PV gradient on the eddy propagation speed
and the effects of advection due to the backgroundcurrent field could cancel one another for the Bering
Slope Current. He also indicated that the ratio of theformer effects to the latter is represented by L2/l2,
where L is the width of the current and l is the wavelength of the eddy. Since L and l are almost the sameorder in the AS, the two effects can cancel out, possibly
resulting in the eddy propagation speed being muchslower than the AS velocity and the low correlation
between the eddy speed and the AS velocity. Baroclinic
mean flow of the AS, which also changes the background PV gradient (Killworth et al. 1997), might contribute to the eddy speed as well.
The eddy propagation speed over a flat bottom is next
discussed briefly. From the principal axis in Fig. 8a, thebest-fit along-coast propagation speed of eddies over aflat bottom ocean was estimated to be 0.37 km day21.
This value is almost the same as the propagation speed
predicted at the latitude of the AS for a linear, nondispersive planetary wave of 0.1–0.4 km day21 from theinternal Rossby radius of deformation in the AS regionof 10–20 km (Emery et al. 1984; Chelton et al. 1998).
We finally discuss the seasonal and interannual variations of the eddy speed, SLA, and the AS velocity (Fig.
8d), averaging values in Figs. 8b and 8c in month andyear. The eddy speed is high in winter and spring andlow in summer and fall. The AS velocity is also high in
winter and low in summer as indicated by Strub andJames (2002), who analyzed SLA data combined withtide gauge data and indicated that the AS surface velocity took a minimum in summer in coincidence with aweak Aleutian low. The correlation coefficient between
the eddy speed and the AS speed is 0.68. The AS might

948

JOURNAL OF PHYSICAL OCEANOGRAPHY

7

6

6

5

5

4

4

3

3

2

2

1

1

0

0
CC: 0.58

CC: -0.31

-1

-1
-20

0

20
40
Bottom slope (m/km)

60(c) AS-velocity/Eddy-speed relations

0

10 20 30 40 50 60 70 80
Sea Level Anomaly at the eddy center (cm)90(d) Seasonal & Interannual variations

7

6
5

6

4
3

5
Eddy speed (km/day)

Eddy speed (km/day)(b) Eddy-SLA/Eddy-speed relations

7

2
1

4

0
1

3

2

3

4

5

6

7

8

9

10

11

12
6

2

5
4

1

3
2

0
CC: 0.19

1

-1
10

20
AS velocity (cm/s)

30

1992

1994 1996 1998 2000 2002 2004
Month (upper panel) & Year (lower panel)

0
2006

Eddy speed (km/day) AS velocity (10cm/s) SLA (10cm)

Eddy speed (km/day)(a) Bottom-slope/Eddy-speed relations

VOLUME 39

FIG. 8. Scatterplot of (a) bottom slope normal to the 1000-m depth contour (Fig. 1), (b) the SLA at
the eddy center, (c) the surface AS velocity along the 1000-m depth contour, and the eddy speed alongthe 1000-m depth contour, superimposed by principal ellipses of the variance. (d) (top) Seasonal and(bottom) interannual variation of eddy speed along the 1000-m depth contour [black line with error(one standard deviation) bars], surface AS velocity along 1000-m depth contour (red lines), and SLA at
the eddy center (blue lines). Thin red (blue) lines denote the AS velocity (SLA) 61 standard deviation.

accelerate/decelerate the eddy propagation on a seasonal basis, although it is difficult to discuss the causality
from Fig. 8d. On the other hand, the eddy SLA is almost
constant throughout the year compared with its originalvariability (Fig. 8b), and has no relation with the eddy
speed (correlation coefficient is 20.08).
No significant relation was detected between interannual variations in eddy speed, AS velocity, and SLA (Fig.
8d, bottom panel). The correlation coefficients between
the eddy speed and SLA, and eddy speed and AS velocity, were 20.38 and 0.28, respectively. This is probably because interannual variations in eddy speed and
SLA depend on the characteristics (e.g., trajectory) of
each eddy, as suggested by Henson and Thomas (2008),

who investigated the relation between interannual variations in a number of individual eddies, SLAs, and eddy
speeds along the AS between 142.58 and 1608W.

5. Conclusions
We investigated anticyclonic eddies in the Alaskan
Stream (AS) using sea level anomaly (SLA) data during
1992–2006 and hydrographic data obtained by Argo
float observations during 2001–06. Fifteen long-lived
eddies were identified and categorized based on theirarea of first appearance.
Eddies 92a, 92b, and 92c were present from the beginning of the altimetry observations, and propagatedAPRIL 2009

949

UENO ET AL.

downstream along the AS without detachment from theAS. Eddies 96d, 00a, and 04a were first observed along
the northeastern Gulf of Alaska off Sitka (Sitka eddies),
propagating westward along the Alaska Current and theAS. Eddy 00a was observed by Argo floats around
1608W and water at the eddy center was isopycnallycolder than the water away from the center between
25.8 and 26.4 su, in contrast to Sitka eddies in theeastern Gulf of Alaska observed in the previous studies.
Eddies 95a, 96c, 99c, and 03a formed at the head of the
Gulf of Alaska near Yakutat (Yakutat eddies) andpropagated westward along the AS. Argo float observations of eddy 03a indicated that the original water
properties at the eddy center were lost by the time of the
Argo float observations. All Sitka and Yakutat eddies
stayed east of 1808. Some of them detached from theAS, but all of them moved less than 58 in longitude after
their detachment.
Eddies 96a, 96b, 99a, 99b, and 02a formed along the
AS between 1578 and 1698W and we call these eddies
Alaskan Stream eddies. These eddies, except eddy 99b,
propagated beyond 1808 and reached the western subarctic gyre. The AS eddies, except eddy 02a, formed
when the wind stress curl was either negative or weakly
positive, likely causing AS separation. Eddy 02a wasobserved by Argo floats in the western subarctic gyreafter it detached from the AS. The observations suggested that the eddy provided the western subarctic gyrewith low-PV intermediate water, as well as heat andfreshwater, in the area just south of the Aleutian Islands.
Comparisons of westward eddy propagation speeds in
the AS with bottom slope, AS velocity, and SLA
showed that the bottom slope effects dominated, withfaster propagation over steeper slopes. The eddy propagation speed was slower than that predicted by the
topographic planetary wave dispersion relation. This isprobably because of nonlinear effects due to, for example, eddy intensification associated with water input
from the Shelikof Strait sea valley and the Bering Sea.
Acknowledgments. The authors thank two anonymous reviewers and L. D. Talley, the editor of thismanuscript. Their comments were very helpful andimproved this paper. Most of this work was done while
H. Ueno was staying at the Institute of Ocean Sciences(IOS) as a visiting scientist. The altimeter products were
produced by SSALTO/DUACS and distributed by
AVISO with support from CNES. The Argo float data
used in this study were collected and made freely
available by the International Argo Project and thenational programs that contribute to it (information
online at http://www.argo.ucsd.edu and http://argo.
jcommops.org). Argo is a pilot program of the Global

Ocean Observing System. H. Ueno was partially supported by the Japanese Ministry of Education, Culture,
Sports, Science and Technology [KAKENHI, Grant-inAid for Young Scientists (B) 20740277].
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