JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 117, C08032, doi:10.1029/2011JC007506, 2012

Modification of a Kenai eddy along the Alaskan Stream
Hiromichi Ueno,1 Ichiro Yasuda,2 Sachihiko Itoh,2 Hiroji Onishi,1 Yutaka Hiroe,3
Toshio Suga,4,5 and Eitarou Oka2
Received 15 August 2011; revised 1 July 2012; accepted 5 July 2012; published 25 August 2012.

[1] A Kenai eddy was studied through analyses of satellite altimeter data and hydrographic
data from shipboard and Argo float observations. This eddy formed in December 2006
south of the Kenai Peninsula and propagated southwestward along the Alaskan Stream.
The eddy held horizontally uniform warm core water in January 2007. In late winter 2007,
this core water was cooled from the top and a subsurface temperature maximum wasformed around 26.5sq. Two years later in summer 2009, warm and low-dissolved-oxygen(low-DO) water characterized by a temperature maximum around 26.5sq was observed
again in the eddy core and was likely the remnant of original core water. At the same time,cold and high-DO water intrusions occurred in the eddy core, suggesting that strong
modification of core water was ongoing. After summer 2009, the core water was fullychanged through interaction with another eddy.
Citation: Ueno, H., I. Yasuda, S. Itoh, H. Onishi, Y. Hiroe, T. Suga, and E. Oka (2012), Modification of a Kenai eddy along the
Alaskan Stream, J. Geophys. Res., 117, C08032, doi:10.1029/2011JC007506.

1. Introduction[2] In the subarctic North Pacific, mesoscale eddies havesignificant impacts on the heat, freshwater, macro- andmicro-nutrient, and biota exchanges between shelf and offshore regions and thus play an important role in the productivity of the offshore region [Johnson et al., 2005; Whitney
et al., 2005; Crawford et al., 2007; Ueno et al., 2010;
Ikenoue et al., 2012]. For example, Haida, Sitka, Yakutat,
and Alaskan Stream eddies affect water exchanges between
the shelf and offshore regions by trapping coastal water intheir interiors and propagating offshore. These eddies alsointerfere with the slope circulation and result in cross-shelf
flow [Okkonen et al., 2003; Crawford, 2005; Ladd et al.,2005; Janout et al., 2009; Ueno et al., 2009].
[3] Rovegno et al. [2009] observed an anticyclonic eddy
southwest of Kodiak Island in September 2007 and reported
that this eddy contained warm core water with relatively
uniform temperature-salinity (TS) relations near the eddy
center. Their analysis of satellite altimeter data as well ashydrographic data indicated that this eddy originated near
the Kenai Peninsula and propagated southwestward along
the Alaskan Stream. They thus called it a Kenai eddy. The
1Faculty of Fisheries Science, Hokkaido University, Hakodate, Japan.
Atmosphere and Ocean Research Institute, University of Tokyo,
Kashiwa, Japan.
3
National Research Institute of Fisheries Science, Fisheries Research
Agency, Yokohama, Japan.
4
Graduate School of Science, Tohoku University, Sendai, Japan.
5Japan Agency for Marine-Earth Science and Technology, Yokosuka,
Japan.
2

Corresponding author: H. Ueno, Faculty of Fisheries Science, c/o
Graduate School of Environmental Science, Hokkaido University, N10 W5
Kita-ku, Sapporo, Hokkaido 060-0810, Japan. (ueno@fish.hokudai.ac.jp)©2012. American Geophysical Union. All Rights Reserved.
0148-0227/12/2011JC007506

formation region of Kenai eddies (150 W) is between theformation regions of Yakutat (143 W) and Alaskan Stream(160 W) eddies. A more recent study [Lippiatt et al., 2011]
investigated macro- and micro-nutrients in and around a
Kenai eddy based on the data obtained in September 2007
[Rovegno et al., 2009] and indicated that the Kenai eddy
drove biological productivity in the western Gulf of Alaska
via water exchange between the shelf region and the offshore
region.
[4] In summer 2009, we observed the same eddy as theone observed in September 2007 by Rovegno et al. [2009].
Observations were made on board the T/S Oshoro-maru andthe R/V Hakuho-maru and by two Argo floats deployednear the center of the eddy at different times. In this paper,
we discuss the propagation and modification of the Kenai
eddy through analysis of satellite altimeter data and hydrographic data obtained by the Oshoro-maru, Hakuho-maru,
and Argo floats.

2. Data and Method[5] We used temperature and salinity data obtained by the
T/S Oshoro-maru in June 2009 (Figure 1a) and temperature,salinity, and oxygen data obtained by R/V Hakuho-maru inAugust 2009 (Figure 1b). Observations by the Oshoro-maru
were performed with a conductivity-temperature-depth profiler (CTD) and an expendable CTD (XCTD), and observations by the Hakuho-maru were performed with a CTD and a
CTD-dissolved-oxygen sensor (CTDO). CTD observations
by the Hakuho-maru include observations using a movingvessel profiler (MVP), which obtains hydrographic data withfine horizontal resolution. Since there was a bias in MVP
salinity data, we corrected the data using ship-based CTDO
data, which were calibrated with IAPSO standard seawater.
DO data obtained by the CTDO on board the Hakuho-maru
were calibrated with bottle-sampled data. All CTD, CTDO,

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marginal seas, was removed from each weekly map of the
SLA to compensate for seasonal steric effects.
[8] Eddy tracking was performed using the Okubo-Weissparameter W [Okubo, 1970; Weiss, 1991] calculated from SLA
data assuming geostrophy. We defined the area of W < 2 
10 12 s 2 as an eddy area following Chelton et al. [2007] andtracked the eddy in the same manner as Henson and Thomas[2008] and Inatsu [2009]. To determine the relative locationsof Argo float observations in and around the eddy, we evaluated the radius of the eddy (Reddy) assuming a radially symmetrical eddy whose area corresponded to the eddy area. The
eddy centers were calculated as the geometrical mean of theeddy area. We defined the same three regions as used by Ueno
et al. [2010]: (1) inner-eddy (D < 0.5 Reddy), (2) outer-eddy(0.5 Reddy < D < Reddy), and (3) periphery of eddy (Reddy < D <2Reddy), where D is the distance from the eddy center. Notethat center locations estimated from altimeter data can give
errors of >50 km due to data resolution and eddy propagation[Ladd et al., 2005, 2007].

3. Propagation and Modification of a Kenai Eddy:
Satellite Altimeter Data Analyses

Figure 1. Locations of CTD, XCTD, CTDO, and MVP
observations by the (a) T/S Oshoro-maru and (b) R/V
Hakuho-maru overlaid on satellite sea level anomaly dataaround the date of ship observation (contours).
and XCTD data were analyzed after 10-m box-averaging in
the vertical direction to remove small-scale fluctuations.
[6] In addition to Oshoro-maru and Hakuho-maru data,
we used temperature and salinity profiles recorded by Argo
floats [Argo Science Team, 2001] around the Alaskan
Stream from December 2006 to July 2011 (five Argo floats,
see Figure 2). The real-time quality-controlled float data
were downloaded from the FTP site of the Argo Global Data
Assembly Center. From these data, defective temperatureand salinity profiles, such as those with measurements
flagged as bad and those lacking intermediate layers forcertain depths, were eliminated following the procedures ofOka et al. [2007].
[7] We also used delayed-time maps of sea level anomalies (SLA) of a merged-altimeter satellite product distributed
at 7-day intervals by AVISO [Collecte Localisation
Satellites, 2011]. The spatial resolution of the SLA data
was 1/4  1/4 , and we used the data for the period
December 2006 to July 2011. The weekly spatial mean stateof the subarctic North Pacific north of 45 N, except for the[9] Using the Okubo-Weiss parameter, we tracked theeddy observed on board the Oshoro-maru and Hakuho-maru
in summer 2009 (Figure 3a). We found that the eddy formed
at 57.8 N, 148.2 W south of the Kenai Peninsula in the endof 2006. Therefore, we called the eddy Kenai 2006. Kenai2006 propagated southwestward along the Alaskan Stream
and was observed by Rovegno et al. [2009] around 155 W insummer 2007. The eddy split into two (Kenai 2006 andKenai 2006a) around 163 W in February 2009 (Figure 3)
before observations by the Oshoro-maru and Hakuho-maru.
Kenai 2006 propagated westward along the Alaskan Stream
and was absorbed by a large anticyclonic eddy (an Aleutian
eddy formed south of the Aleutian Islands [Rogachev et al.,2007]) around 175 E in July 2010. Meanwhile, Kenai 2006awas detached from the Alaskan Stream around 170 W in
January 2010. Just before and during the detachment, Kenai
2006a was located next to Kenai 2006, and some SLA
contours encircled both eddies (Figure 4), suggesting that
eddy-eddy interaction between Kenai 2006 and Kenai 2006ainduced detachment of Kenai 2006a from the Alaskan
Stream. After detachment, Kenai 2006a propagated southward away from the Aleutian Islands and decayed in June
2011. Hereafter, we focus on the main part of the eddy
(Kenai 2006).
[10] Figure 5 shows time series of longitude, westward
propagation speed, SLA, and area of Kenai 2006. Westward
propagation speed ranged from 0.8 to 5.0 km day 1 andwas 1.8 km day 1 on average, which is similar to the propagation speeds of the Sitka, Yakutat, and Alaskan Stream
eddies along the Alaskan Stream ranging from 0.5 to
7.0 km day 1 with an average of 2.0 km day 1 [Ueno et al.,2009]. Ueno et al. [2009] compared westward eddy propagation speeds in the Alaskan Stream with the bottom slope,
Alaskan Stream velocity, and SLA. They found that bottom
slope effects dominated, with faster propagation over steeper
slopes. Since Kenai 2006 propagated in a similar route andwith similar speed to eddies studied by Ueno et al. [2009],
bottom slope likely also accounted for the propagation speedof Kenai 2006. Figures 5b and 5c further show that Kenai

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Figure 2. Trajectories of Argo floats (WMO IDs: (a) 4900740, (b) 4900867, (c) 4900870, (d) 4900938,
and (e) 4900939) which observed inner-eddy (red circles) or outer-eddy (blue circles) areas of Kenai 2006
from December 2006 to July 2010. Observations located in the periphery of Kenai 2006 and outside theperiphery of Kenai 2006 are shown by green circles and orange dots, respectively. Large white circles
indicate the location of Kenai 2006 (see caption of Figure 3a for details) when its inner-eddy or outer-eddy
area was observed.
2006 stopped propagating around 155 W for three months(October–December 2007), and SLA suddenly increased in
February 2008 after this stop. This might be related to eddy
intensification due to low-salinity surface water outflow from
Shelikof Strait around 155 W [Reed et al., 1986, 1987;
Stabeno and Hermann, 1996; Stabeno et al., 2004] as discussed by Ueno et al. [2009].
[11] Figures 5c and 5d indicate that Kenai 2006 intensified in the first three months (SLA) and five months (area).
After that, SLA decreased with fluctuations to a minimum(20–25 cm) from November 2007 to January 2008. It thensuddenly increased in February 2008 as mentioned in theprevious paragraph. After January 2008, SLA was mostly
constant for a year, gradually decreased in 2009, increasedin the early half of 2010, and rapidly decreased in July
2010. SLA increase in the early half of 2010 occurred near170 W, where eddies sometimes intensified possibly due tooutflow of low potential vorticity water from the Bering Sea[Ueno et al., 2009]. On the other hand, eddy area was relatively high (15,000 km2) from June 2007 to September

2009 and low (10,000 km2) after October 2009, and finallydecreased to 6000 km2 in July 2010.
[12] As suggested by Figure 5, the propagation speed,
SLA, and area of Kenai 2006 largely changed around theend of 2009 when Kenai 2006 was located next to Kenai
2006a and some SLA contours encircled both eddies. These
results suggest that eddy-eddy interaction occurred aroundthe end of 2009 and influenced the propagation speed, SLA,
and area of Kenai 2006. Changes in water mass in Kenai2006 during this period will be discussed in Section 4.3.

4. Modification of the Kenai Eddy: Hydrographic
Data Analyses
4.1. Observations in 2007: Argo Float Data Analysis[13] Two Argo floats (WMO ID: 4900740 and 4900867,
hereafter Argo floats (a) and (b), respectively) observed
Kenai 2006 in the first half of 2007 (Figures 2a and 2b).
Fortunately, Argo float (a) was located in the inner-eddy areaof Kenai 2006 on 1 January 2007 (red circles in Figures 2aand 6a), within a week of the first evidence of the eddy in

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Figure 3. (a) Trajectories of Kenai 2006 and Kenai 2006a. Colors represent SLA (cm) at the eddy andthe radius of each circle in the map mostly corresponds to an Okubo-Weiss radius (Reddy). Bathymetriccontours are depicted every 1000 m, with a thicker contour at 1000 m depth. (b) Longitude-time diagramof SLA along the northern boundary of the Pacific Ocean. SLA was averaged within 2 south of the1000 m depth contour. Arrows represent the rough location (Figure 3a) and date (Figure 3b) of eachobservation. R indicates Rovegno et al. [2009], O represents the Oshoro-maru, H represents the
Hakuho-maru, and Aa, Ab, Ac, Ad, and Ae are the Argo floats shown in Figures 2a, 2b, 2c, 2d, and2e, respectively.
altimeter data. This observation revealed that Kenai 2006 heldwater warmer than 6 C in the density range of 25.8–26.5sq
near the eddy center just after its formation (Figure 6a). Similar
TS profiles were observed in the outer-eddy area on 11 January 2007 (blue triangles in Figure 6a), suggesting that relatively uniform warm water was distributed in the eddy area asdefined using the Okubo-Weiss parameter. This water, which
would be the original core water that Kenai 2006 gained atformation, was warmer than the core water observed by
Rovegno et al. [2009] in September 2007, which was characterized by a subsurface temperature maximum (Tmax) of 5.5–
5.8 C at 26.4–26.6sq.

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Figure 4. SLA (cm) distributions in (a) 7 October 2009, (b) 28 October 2009, (c) 18 November 2009,(d) 9 December 2009, (e) 30 December 2009, (f) 20 January 2010, and (g) 10 February 2010. The contour
interval is 2 cm.
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Figure 5. Time series of Kenai 2006 (a) longitude ( W), (b) westward propagation speed (black), southward propagation speed (blue) and propagation speed (red) (km day 1), (c) SLA (cm) averaged in the
0.5  0.5 area around the geometrical mean of the eddy area, and (d) area (103 km2). Triangles locatedat the top represent the rough location of each observation (see caption of Figure 3 for the meaning of Aa, etc.)[14] Kenai 2006 was not observed by Argo floats fromFebruary to April 2007. The observation restarted on 1 May
2007 around 177 E (Figure 2a) by Argo float (a) and indicated that a strong temperature inversion formed between
25.9sq and 26.5sq in the outer-eddy area (blue triangles withplots of 4.0 C at 25.9sq and >5.5 C at 26.5sq in Figure 6b).
The Tmin density of 25.9sq corresponded to the climatological winter-mixed-layer density in the northwestern Gulf
of Alaska [Suga et al., 2004], indicating that the strong
temperature inversion was formed by surface cooling in late
winter 2007. The TS relations observed by the Argo floats in
May–July 2007 in the eddy area (blue triangles in Figure 6b)
were almost the same as those observed in the eddy core in
September 2007 by Rovegno et al. [2009]. Between October2007 and August 2009, Argo floats did not observe Kenai2006 and thus float data to describe the modification of theeddy were unavailable.
[15] Properties in the periphery of Kenai 2006 in May–July
2007 varied from those similar to eddy core water (dots withTmax warmer than 5.5 C in Figure 6b) to those similar tobasin water (dots with Tmax colder than 5.5 C in Figure 6b).
This is probably because of uncertainty in eddy location
determination from satellite altimeter data [Ladd et al., 2005,2007]. The trajectories of Argo floats (a) and (b) show a kindof eddy-like feature (Figures 2a and 2b). However, it wasdifficult to estimate eddy location from these trajectories
because their resolutions were low compared with the eddy
scale. The parking depth (1000 dbar) of Argo floats (a) and(b) might also make it difficult to estimate the eddy locationfrom the trajectory data.
4.2. Observations in Summer 2009
4.2.1. Oshoro-maru Data Analysis[16] Figure 7a shows an east-west potential temperaturesection of Kenai 2006 obtained by the Oshoro-maru in June2009 (see Figure 1a). At 168.25 W (red open circle in
Figure 7), a subsurface temperature maximum (Tmax) of
5.4 C was observed at 170 m depth and 26.5sq. This Tmax
water was similar to that in the eddy core in September 2007
(5.5–5.8 C at 26.4–26.6sq) [Rovegno et al., 2009], suggesting that the Oshoro-maru cruise captured the Kenai 2006 corewater at 168.25 W.
[17] At 168.25 W, a strong temperature inversion wasobserved between 25.7sq (3.6 C) and 26.5sq (5.4 C). The
subsurface temperature minimum (Tmin) of 3.6 C at 25.7sq
was colder than that in the eddy core in September 2007
(about 4.9 C) [Rovegno et al., 2009]. Tmin was formed by
winter surface cooling in the northern Gulf of Alaska [Uenoand Yasuda, 2000; Ueno et al., 2005], suggesting that

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C08032[19] TS relations at stations east of 168 W, which weresimilar to one another, were between those of the core water(red open circles, 168.25 W) and the basin water (black open
squares, 169.8 W) and were similar to the TS relation of thetransitional water indicated by Rovegno et al. [2009], withwater properties between those of the Kenai core water andthe basin water. Therefore, we considered the water east of168 W to be the transitional water. The TS relation of thetransitional water was similar to that of the Alaskan Stream
water [e.g., Favorite et al., 1976, Figure 48], suggesting thatthe waters east of 168 W were influenced by the Alaskan
Stream flowing along the southern rim of Kenai 2006. In
addition, water east of 168 W could be influenced by thesmall cyclonic eddy (around 51.5 N and 166 W) indicatedby the altimeter contours in Figure 1a.
4.2.2. Hakuho-maru Data Analysis[20] The Hakuho-maru observed Kenai 2006 in moredetail using CTD, CTDO, and MVP. Along the observation

Figure 6. TS relations in (a) January 2007 and (b) May–
July 2007 obtained by Argo floats located in the inner-eddy(red circles), outer-eddy (blue triangles), and periphery of
the eddy (green dots).
surface cooling in the winters of 2008 and 2009 cooled subsurface Tmin and strengthened the temperature inversion.
This result is consistent with findings that the climatologicalwinter-mixed-layer density and temperature around theobservation areas of Rovegno et al. [2009] and the Oshoromaru were 26.0–26.2sq and 3–4 C, respectively, and weredenser and colder to the west. On the other hand, the Tmax
water at 168.25 W was similar to that in September 2007 as
mentioned above. This indicates that winter surface cooling
hardly modified the Kenai 2006 core water characterized
by Tmax.
[18] The Oshoro-maru cruise also observed a hydrographic
structure that was different from that found by Rovegno et al.
[2009]. At 169 W (blue open triangles in Figure 7), temperature decreased by 1.5 C from 26.5 to 26.7sq, forming aTmin around 26.7sq. Since the Tmin formed by winter surface cooling was located at 26.1sq, we propose that theobserved hydrography was created by other mechanisms,
such as lateral intrusion of cold water. Since TS relations at
169 and 169.8 W were similar at the densities of cold water
intrusion, we speculate that this cold water originated fromthe basin water outside the eddy (e.g., water at 169.8 W,
black open squares, with cold Tmin of 3.1 C at 26.7sq). The
basin water is probably influenced by waters in the Western
Subarctic Gyre [see, e.g., Ueno and Yasuda, 2005].

Figure 7. (a) Potential temperature distribution ( C, colors)
superimposed by the density distribution (sq, contours) and(b) TS relations along the east-west section of the Oshoromaru cruise (Figure 1a). Colored squares with black outlines
in Figure 7a represent potential temperature observed by
XCTD, and background colors and contours in Figure 7adenote potential temperature horizontally box-averaged witha width of 0.8 in longitude. Symbols drawn at the top of
Figure 7a represent the location of the profile drawn in
Figure 7b.

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C08032even though the horizontal distance between the two casts
was just 1.5 km. Whereas a Tmax layer of 5.2–5.4 C wasobserved in the shallow cast (open red circles), a Tmin layerof 4.6–4.7 C was observed in the deep cast (solid red circles). Such non-uniformity in water properties near the eddy
center differs from the observation in 2007, when the TS
relation was relatively uniform among eight casts near theeddy center [Rovegno et al., 2009]. The eddy core water
likely lost its uniformity between 2007 and 2009.
[22] A strong Tmin layer was also observed by the
Hakuho-maru and Oshoro-maru at the intermediate depth
where winter surface cooling was unlikely to have formed theTmin layer. At the 51.5 N shallow cast (red open circles in
Figure 8), temperature decreased from 5.2 C (26.60sq and
220 m) to 4.6 C (26.66sq and 250 m) and increased to 4.9 C(26.72sq and 270 m), suggesting lateral intrusion of cold
water. A strong Tmin layer was also observed at the 51 N
shallow and deep casts (blue open and solid triangles) aroundthe 26.6sq isopycnal surface. The temperature and salinity of
the Tmin water were similar to those at 50.5 N (black open
and solid squares), whose TS relation resembled that oftransitional water discussed in Section 4.2.1. This finding
suggests that the transitional water influenced by the Alaskan
Stream modified the eddy core through lateral intrusions.

Figure 8. (a) Potential temperature distribution ( C, colors)
superimposed by the density distribution (sq, contours) and(b) TS relations along the Hakuho-maru cruise (Figure 1b).
Colored squares with black outlines in Figure 8a represent
potential temperature observed by CTD/CTDO/MVP, andbackground colors and contours in Figure 8a denote potential temperature horizontally box-averaged with a width of
0.4 in latitude. Symbols at the top of Figure 8a represent
the location of the profile in Figure 8b; solid and open symbols correspond to deep and shallow CTDO casts,
respectively.
line, Tmax water warmer than 5 C, which corresponded tothe core water of Kenai 2006, was observed in the wide areaof 51 –52 N (Figure 8). This is probably because the
Hakuho-maru observation line was closer than the Oshoromaru observation line to the center of Kenai 2006 (Figure 1).
In the eddy core area of 51 –52 N, waters warmer than 5 C
and colder than 4.5 C were patchily observed, suggesting
the modification of core water as described below.
[21] During the Hakuho-maru cruise, CTDO observation
was conducted twice at an observation station, with a deep
cast (0–2000 dbar: solid symbols in Figures 8 and 9) and ashallow cast (0–600 dbar: open symbols in Figures 8 and 9),
to obtain vertically high-resolution bottle-sampled data. TS
relations of deep and shallow casts at each station were
mostly similar, but those at 51.5 N (red open and solid circles) were significantly different at densities of 26.4–26.6sq

Figure 9. Same as Figure 8 but for the (a) dissolved oxygen distribution (ml l 1) and (b) dissolved oxygen profiles
in density coordinates.

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C08032the other CTDO casts at densities of 26.4–26.6sq. This lowDO layer corresponds to the Tmax layer, which suggests thatwarm and low-DO original core water was carried from southof the Kenai Peninsula to south of the Aleutian Islands along
the Alaskan Stream for two and a half years.
[24] In addition to the transport of original core water,
modification of the core water was also indicated by the DO
data. At the 51.5 N shallow cast (red open circle) and the51.0 N shallow and deep casts (blue triangles), high-DO
water intrusion was observed at densities of 26.65sq and26.6sq, respectively. The DO value of this high-DO intrusion
was almost the same as the DO values at 50.5 N, indicatingthat water surrounding the eddy core intruded to the eddy
core. This intrusion corresponds to the cold Tmin water
intrusion noted in the previous paragraph, supporting the
hypothesis obtained by TS relation analysis that water in thesouthern rim of the eddy influenced by the Alaskan Stream
modified the eddy core through lateral intrusions.
4.2.3. Argo Data Analysis[25] In August 2009, we deployed two Argo floats (WMO
ID: 4900938, Argo float (d), and 4900939, Argo float (e),see Figures 2d and 2e, respectively) from the Hakuho-maru.
In addition, Argo float (a) also observed the outer-eddy areaof Kenai 2006 in September 2009 (Figure 2a). These three
Argo floats obtained many profiles in Kenai 2006 during
August–September 2009 (Figure 10a). During this period,
two types of profiles were observed simultaneously in the
inner-eddy area: profiles with Tmax warmer than 5 C andthose with Tmin colder than 5 C around 26.5sq (red circles
in Figure 10a). Such non-uniform TS profiles near the eddy
center correspond to the results from the Oshoro-maru andHakuho-maru observations that lateral intrusion of cold andhigh-DO water occurred actively in Kenai 2006.

Figure 10. Same as Figure 6 but for (a) August–September
2009, (b) October 2009–January 2010, and (c) June–July
2010.
[23] Kenai 2006 core water was also detected by dissolvedoxygen (DO) data (Figure 9). Previous research found that
DO in the coastal region of the Gulf of Alaska, where Kenai2006 formed, was lower than that in the deep-sea regionof the subarctic North Pacific [Whitney et al., 2007].
We examined the DO data obtained by the Hakuho-maru(Figure 9) and found that DO values obtained at the 51.5 N
shallow cast (red open circle) and the 51.0 N shallow anddeep casts (blue triangles) were lower than values obtained at

4.3. Observations After September 2009: Argo Float
Data Analysis[26] Observations by Argo floats from October 2009 to
January 2010 show a relatively uniform TS relation with5 C water at densities of 26.0–26.7sq in Kenai 2006. This
TS relation without marked Tmin and Tmax was completely
different from the relations observed in Kenai 2006 before
October 2009. Altimeter data suggested that eddy-eddy
interaction occurred between Kenai 2006 and Kenai 2006aduring this period (Figure 4). Therefore, the water mass in
Kenai 2006 might have been fully changed by waterexchange or mixing due to eddy-eddy interaction around theend of 2009.
[27] In June–July 2010, when Kenai 2006 became weak
and just before Kenai 2006 was absorbed by an Aleutian
eddy, Argo float (a) obtained hydrographic data in Kenai2006 (Figures 2a and 10c). Water in Kenai 2006 in June–July
2010 was colder than that observed from October 2009 to
January 2010 at densities between 26.0sq and 27.0sq(Figures 10b and 10c). Winter surface cooling would accountfor cooling at densities shallower than 26.4sq (the climatological winter mixed layer density in this area) [Suga et al.,2004]. Cooling at densities deeper than 26.4sq suggests
strong mixing or water exchange in Kenai 2006 due to eddy
decay during this period. However, it is also possible that thiscooling was a result of errors in the determination of eddy

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location from altimeter data, since only three profiles wereobtained in Kenai 2006 during this period.

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distributed by Aviso with support from Cnes. The Argo float data used in thisstudy were collected and made freely available by the International Argo Project and the national programs that contribute to it (http://www.argo.ucsd.edu,
http://argo.jcommops.org). This work was supported by JSPS KAKENHI
grants 22740303 and 20221002.

5. Summary[28] We investigated the propagation and modification of a
Kenai eddy (Kenai 2006) through analysis of satellite altimeter data and hydrographic data obtained by the Oshoromaru, Hakuho-maru, and Argo floats. Kenai 2006 formed in
the area south of the Kenai Peninsula in December 2006 andpropagated southwestward along the Alaskan Stream, splitting into two around 163 W in February 2009. The main partpropagated westward along the Alaskan Stream and finally
was absorbed by an Aleutian eddy around 175 E in July
2010. The other part was detached from the Alaskan Stream
around 170 W in January 2009, propagated southward awayfrom the Aleutian Islands, and decayed in June 2011. The
two eddies were located close to one another around the endof 2009, and the occurrence of eddy-eddy interaction was
suggested during the period.
[29] Kenai 2006 held horizontally uniform core water
warmer than 6 C near the eddy center in January 2007, justafter its formation. This core water was cooled by surface
cooling in late winter 2007, and strong temperature inversion
was formed around 26.0–26.5sq. This cooling in late winter2007 did not break the uniformity of the eddy core, as demonstrated by Rovegno et al. [2009]. Two years later, in June–
September 2009, Kenai 2006 carried warm and low-DO
water characterized by Tmax of around 26.5sq in the eddy
core in the area south of the Aleutian Islands. This warm andlow-DO water was suggested to be the remnant of the original eddy-core water obtained at the eddy formation area.
Cold and high-DO water intrusions into the eddy core werealso observed, which suggest that strong modification of theeddy-core water was ongoing in summer 2009. Around theend of 2009, Kenai 2006 core water was fully changed,
probably because of eddy-eddy interaction between Kenai2006 and Kenai 2006a.
[30] Although water mass modification of a Kenai eddy
was investigated for the first time in this study, water masschanges in other eddies formed in the Gulf of Alaska havebeen studied. Chierici et al. [2005] studied changes in water
properties of a Haida eddy for a year and indicated that the TS
relation of the eddy showed little temporal change in waters
below 100 m in the eddy center during its first year. Ladd
et al. [2007] investigated water property change in a Sitka
eddy for one and one half years and indicated that heat andsalinity anomalies became less than half in a year and lessthan one third in a year and a half. Although a cold-water
intrusion was not observed in the Haida eddy [Chierici et al.,2005], fluctuation in the TS relation, which was similar tothat in Kenai 2006, was observed in the Sitka eddy [Laddet al., 2007], suggesting that Sitka core water was modified
by mechanisms similar to those in Kenai 2006. However,a more comprehensive study is necessary for detailed understanding of core water modifications in eddies formed in
the Gulf of Alaska.
[31] Acknowledgments. The authors thank two anonymous reviewers
for constructive and helpful comments. The authors also thank Atsushi Tsuda,
Jun Nishioka, Yutaka W. Watanabe and Masaru Inatsu for their supports.
The authors also thank officers and crew of T/S Oshoro-maru and R/V
Hakuho-maru. The altimeter products were produced by Ssalto/Duacs andReferences
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