Download Temperature Inversions in the Subarctic North Pacific

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VOLUME 35
Temperature Inversions in the Subarctic North Pacific
HIROMICHI UENO
Institute of Observational Research for Global Change, Japan Agency for Marine-Earth Science and Technology, Yokosuka, Japan
ICHIRO YASUDA*
Department of Earth and Planetary Science, Graduate School of Science, The University of Tokyo, Tokyo, Japan
(Manuscript received 3 January 2005, in final form 1 June 2005)
ABSTRACT
Hydrographic data from the World Ocean Database 2001 and Argo profiling floats were analyzed to study
temperature inversions in the subarctic North Pacific Ocean. The frequency distribution of temperature
inversions [F(t-inv)] at a resolution of 1° (latitude) ⫻ 3° (longitude) was calculated. Temperature inversions
seldom occurred around 50°N in the eastern subarctic North Pacific but were more common in the northern
Gulf of Alaska and the southeastern subarctic North Pacific (42°–48°N, 140°–170°W). Large temperature
inversions occurred throughout the year in the western and central subarctic North Pacific (north of 42°N
and west of 180°) except near the Aleutian and Kuril Islands. Near those islands, F(t-inv) was characterized
by pronounced seasonal variations forced by surface heating/cooling and strong tidal mixing.
1. Introduction
Temperature inversions (i.e., temperature increases
with depth, where temperature minima and maxima
occur respectively at the upper and lower edges of the
inversion) occur in most of the subarctic North Pacific
Ocean (e.g., Uda 1963; Roden 1964; Dodimead et al.
1963; Favorite et al. 1976; Ueno and Yasuda 2000;
Miura et al. 2002; Belkin et al. 2002). The subarctic
North Pacific is defined as the area north of about 42°N
(Favorite et al. 1976); for locations see Fig. 1. The formation and circulation of waters in the subarctic North
Pacific that include temperature maxima at the inversion bottom are closely related to intermediate water
circulation in the subarctic North Pacific and the northern subtropical Pacific (Ueno and Yasuda 2003). Pre* Current affiliation: Ocean Research Institute, The University
of Tokyo, Tokyo, Japan.
Corresponding author address: Hiromichi Ueno, Institute of
Observational Research for Global Change, Japan Agency for
Marine-Earth Science and Technology, 2-15 Natsushima-cho, Yokosuka, Kanagawa, 237-0061, Japan.
E-mail: [email protected]
© 2005 American Meteorological Society
vious studies (e.g., Yasuda 2003) have suggested that
long-term variability of the intermediate water circulation is related to the climate in the North Pacific. Water
masses including temperature minima at the top of the
inversion are more directly related to the climate of the
North Pacific because such waters may form in the winter mixed layer (e.g., Uda 1963; Ueno and Yasuda 2000;
Miura et al. 2002). However, the distribution of waters
with temperature minima and maxima, and of temperature inversions, is poorly understood.
Makarov (1894) was the first to study temperature
inversions in the North Pacific. Subsequently, Uda
(1935) examined the distribution, formation, and circulation of minimum temperature water in the western
subarctic North Pacific. Temperature inversions in the
Pacific Ocean near Japan have been investigated by
Kawai (1955), Kuroda (1959, 1960), and Nagata (1967a,
b). Nagata (1968, 1979) described the frequency of temperature inversions in the Pacific Ocean near Japan and
also examined the number of inversion layers in a single
profile derived from mechanical bathythermograph
(MBT) data. Uda (1963), using data for the entire subarctic North Pacific, showed that temperature inversions occur over the entire region north of 40°–45°N. In
contrast, Roden (1964) showed no temperature inver-
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FIG. 1. Distribution of the number of stations used in this study in each 1° (latitude) ⫻ 3° (longitude) box superimposed
by place names. The CTD, OSD, XBT, and MBT data in WOD01 and data from Argo profiling floats in all seasons are
indicated. A blank means the box has fewer than five stations.
sions in the central part of the Alaskan gyre (48°–53°N,
145°–160°W); however, inversions did occur in the
northern and southern parts of the gyre. Of course, the
data available in 1964 were quite limited.
Recently, Ueno and Yasuda (2000) used a climatological hydrographic dataset [World Ocean Atlas 1994
(WOA94); Levitus and Boyer 1994; Levitus et al. 1994]
to examine the distribution and formation of temperature inversions over the entire subarctic North Pacific.
They found temperature inversions north of 45°N except around 50°N east of 170°W. More recently, Miura
et al. (2002) examined the distribution of minimum
temperature waters in summer with a fine-resolution
gridded dataset in which the half-amplitude scale of
filter response functions was much smaller than the
scale used in the WOA94. In contrast to Ueno and
Yasuda (2000), Miura et al. (2002) showed few temperature inversions along the Alaskan Stream and in
the northern Gulf of Alaska. Temperature inversions
did occur around 50°N, even in the area east of 170°W.
Miura et al. (2002) excluded profiles with small temperature inversions or profiles without minimum temperature waters around 26.6 ␴␪ in the mapping of minimum temperature water distributions; Ueno and Yasuda (2000) included such profiles.
Ueno and Yasuda (2000) and Miura et al. (2002)
both based their results on horizontally smoothed climatological datasets, and smoothing may have distorted their results. Analyses of individual hydrographic data are required to describe the distribution of
temperature inversions. Kobayashi (2004) analyzed individual hydrographic data and showed that tempera-
ture inversions occur throughout the subarctic North
Pacific, in contrast to results in Roden (1964), Ueno
and Yasuda (2000), and Miura et al. (2002). However,
Kobayashi (2004) did not detail the strength or seasonal
variations of the temperature inversions. In addition,
previous studies have not evaluated the frequency of
temperature inversions over the entire subarctic North
Pacific; such information would help describe the distribution and formation of temperature inversions
more comprehensively.
This paper uses analyses of individual hydrographic
datasets to reexamine the distribution of temperature
inversions. This approach circumvents problems that
arise from the use of climatological hydrographic data.
Section 3 discusses the frequency and distribution of
temperature inversions in the subarctic North Pacific.
Temperature inversion formation is discussed in section 4.
This paper will examine temperature inversions as
defined in section 2b for the entire subarctic North Pacific except for those having small magnitude or small
vertical scale and categorize them based on their properties. A clear, simple definition of temperature inversion was adopted in this study (see section 2b and Fig.
2). This study did not focus on temperature inversions
with a particular property or in a particular region, in
contrast to some previous studies, for example, Miura
et al. (2002) who studied temperature inversions with
minimum temperatures around 26.6 ␴␪ in the Bering
Sea. Comparison between the results of this study and
previous studies is therefore essential and is presented
in sections 3 and 4.
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FIG. 2. Observed vertical potential temperature profiles at (a)
58.93°N and 168.52°E on 18 Aug 1991 and (b) 39.03°N and
147.55°E on 13 Dec 1992. Black lines represent the profiles to
which a 10-m running mean was applied (RM10). The ⌬T, ⌬D,
minimum temperature (T MIN ), and maximum temperature
(TMAX), which are defined in the text, were calculated from profiles shown by the black lines. Red lines are the profiles to which
a 100-m running mean was applied (RM100).
2. Data and methods
a. Data
Temperature profiles observed by Argo profiling
floats (Argo Science Team 2001) and individual temperature profiles of the World Ocean Database 2001
(WOD01) (Conkright et al. 2002), which includes extensive historical hydrographic data, were used to examine the frequency of temperature inversions [hereinafter F(t-inv); Fig. 1]. Temperature profiles were derived from high-resolution conductivity–temperature–
depth (CTD) profiles, bottle or low-resolution CTD
(OSD), expendable bathythermograph (XBT), and
MBT temperature data from WOD01 and CTD data
from Argo floats (from January 2001 to August 2004).
Density and salinity at the depth of the maximum and
minimum temperatures were evaluated with CTD and
VOLUME 35
OSD data when both temperature and salinity data
were available. Temperature inversions were examined
using accepted (WOD quality flag: 0) or good (Argo
quality flag: 1) data at the observed depth.
The area of analysis was north of 30°N in the North
Pacific and included the entire subarctic Pacific (see
Favorite et al. 1976). Stations were ignored if the deepest observations were shallower than 500 m. This is
because the maximum temperature at the bottom of a
temperature inversion occurred mostly at depths shallower than 500 m in the subarctic North Pacific except
in and around the Sea of Okhotsk (Kobayashi 2004).
Our preliminary analysis based on the CTD data with
depths exceeding 2000 m and the definition of temperature inversion used in this study also showed the same
result as Kobayashi (2004) [more than 99% of temperature maxima occurred at depths shallower than 500 m
in the area of 40°–60°N, 170°E–140°W (⬃95% in 40°–
60°N, 160°–170°E; ⬃80% in 40°–50°N east of 140°W
and 150°–160°E excluding the Sea of Okhotsk; ⬃2% in
the Sea of Okhotsk)]. In addition, in the Kuroshio Extension region and Mixed Water Region (30°–40°N
west of 170°), about one-half of the temperature
maxima are at depths deeper than 500 m. Therefore,
reanalyses were conducted using stations with deepest
observations at depths below 1000 m in and around the
Sea of Okhotsk and in the Kuroshio Extension region
and the Mixed Water Region.
Temperature inversions were calculated at stations
with observations at more than 10 levels between 10
and 500 m. The results did not change much if stations
with more than 25 levels were used. Data at depths
shallower than 10-m depth were removed. A 10-m running mean was applied to each profile to preclude detection of temperature inversions with very small vertical scale (less than 10 m). This study focused on temperature inversions with vertical scales larger than 10
m. In practice, this running mean was applied to highresolution CTD and XBT profiles, reducing their practical resolution of about 1 m to a resolution similar to
that of MBT (i.e., about 10 m). A vertical resolution of
10 m is somewhat higher than the vertical resolution of
the OSD (several tens of meters between depths of 10
and 500 m); the results did not change much if lowresolution OSD data were ignored.
This study included temperature data at 164 641 stations (CTD: 21 733; OSD: 71 182; XBT: 49 695; MBT:
9266; Argo: 12 765). There were 33 027 stations in winter [January, February, and March (JFM)], 47 010 in
spring [April, May, and June (AMJ)], 50 272 in summer
[July, August, and September (JAS)], and 34 332 in autumn [October, November, and December (OND)].
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There were 98 601 stations with both temperature and
salinity data (CTD: 20 348; OSD: 65 488; Argo: 12 765)
(winter: 19 011, spring: 29 787, summer: 31 700, autumn:
18 103).
b. Methods
At each observation station, the potential temperature inversion ⌬T and the depth and potential temperature of the associated minimum (TMIN) and maximum
(TMAX) temperature were evaluated (Fig. 2). In the
northwestern subarctic North Pacific, TMIN and TMAX
waters typically exist in subsurface and intermediate
layers, respectively (Fig. 2a), but that is not necessarily
the case in the other regions (e.g., Fig. 2b). A clear and
simple definition of ⌬T was adopted, as described below, to facilitate a comprehensive study of the horizontal distribution of the temperature inversion.
When a profile included a minimum temperature
that was colder and shallower than the maximum temperature (a temperature inversion), ⌬T was defined as
the difference between the maximum and the minimum. The black line in Fig. 2a shows an example of a
profile with an inversion. If multiple temperature inversions existed in a profile, ⌬T was defined using the
largest temperature difference between all maxima and
minima, given that the minimum was colder and shallower than the maximum (black line in Fig. 2b). Here,
TMIN (TMAX) was the minimum temperature (maximum) located at the upper (lower) edge of ⌬T as defined above. In the present study, the surface TMIN and
bottom TMAX, which could accompany temperature inversions, were considered. The thickness between the
depths of TMAX and TMIN is defined as ⌬D.
In practice, inversions were identified using the following method. At a station with N observed temperature levels, the differences T(N ) ⫺ T(N ⫺ 1), T(N) ⫺
T(N ⫺ 2), . . . , T(N ) ⫺ T(1), T(N ⫺ 1) ⫺ T(N ⫺ 2),
T(N ⫺ 1) ⫺ T(N ⫺ 3), . . . , T(N ⫺ 1) ⫺ T(1), . . . , T(3)
⫺ T(2), T(3) ⫺ T(1), and T(2) ⫺ T(1) were calculated
where T(n) is the potential temperature at level n; T(1)
is at the top and T(N ) is at the bottom. If all T(m) ⫺
⫺T(n) were negative, where m and n are integers of
N ⱖ m ⬎ n ⱖ 1, no temperature inversion existed in the
profile. When only one combination [T(m) ⫺ T(n)] was
positive, T(m) ⫺ T(n) was defined as ⌬T, and T(m) and
T(n) were defined as TMAX and TMIN, respectively. If a
number of combinations were positive, the largest
[T(m) ⫺ T(n)] was defined as ⌬T, and T(m) and T(n)
were TMAX and TMIN, respectively.
The frequency of temperature inversions F(t-inv) in
each 3° (longitude) ⫻ 1° (latitude) box was computed
as the number of stations with temperature inversions
TABLE 1. The 90% confidence intervals of F(t-inv) for the case
of EF ⫽ 95%, 75%, 50%, 25%, and 5%, where EF is the estimated F(t-inv). The intervals are estimated assuming 50, 20, and
5 stations are available to evaluate EF (M ⫽ 50, 20, and 5).
EF
EF
EF
EF
EF
⫽
⫽
⫽
⫽
⫽
95%
75%
50%
25%
5%
M ⫽ 50
M ⫽ 20
M⫽5
89.8%–100.0%
64.6%–85.3%
38.0%–62.0%
14.7%–35.4%
0.0%–10.2%
86.6%–100.0%
58.3%–91.7%
30.7%–69.2%
8.3%–41.7%
0.0%–13.4%
75.3%–100.0%
34.0%–100.0%
4.9%–95.1%
0.0%–66.0%
0.0%–24.7%
divided by the number of stations observed in the box.
Seasonal variations of F(t-inv) were evaluated with 3°
⫻ 3° boxes because of data limitations. Furthermore,
⌬T, ⌬D, potential temperature and depth at TMIN and
TMAX were averaged in each 3° ⫻ 1° box; potential
density and salinity at TMIN and TMAX were averaged in
each 3° ⫻ 3° box. All averages used data from stations
with temperature inversions. Temperature inversions
with ⌬T smaller than 0.1°C were ignored. This temperature difference is near the temperature accuracy of
MBT data (about 0.1°C; Nagata 1979), which had the
lowest accuracy of all temperature data used in this
study.
The statistical significance of F(t-inv) was determined
from the confidence interval for F(t-inv) between EF ⫺
ta(␷)[EF(1 ⫺ EF)/M]1/2 and EF ⫹ ta(␷)[EF(1 ⫺ EF)/
M]1/2, where EF is the estimated F(t-inv), M is the number of stations used to evaluate EF, and ta(␷) is the ␷%
point of a Student’s t distribution. The confidence interval of F(t-inv) expands as M decreases or as EF approaches to 50% as shown in Table 1. When we drew
the map of F(t-inv), we blanked out the area where
90% confidence interval is wider than EF ⫾ 25%; that
is, ta(90)[EF(1 ⫺ EF)/M]1/2 ⬎ 25%. For example, F(t-inv)
maps in boxes with stations fewer than five were set to
blank.
3. Results
This section examines the frequency of temperature
inversions F(t-inv) and the properties of TMIN and
TMAX waters. The formation of temperature inversions
is discussed in section 4.
a. Frequency of temperature inversions, F(t-inv)
Temperature inversions are common in the subarctic
North Pacific (Fig. 3a); F(⌬T ⬎ 0.1°C) exceeds 75%
except around 50°N east of 160°W, in the area east of
140°W and south of 54°N, and along the Aleutian Islands; F(⌬T ⬎ 0.1°C) generally decreases to the east,
and the distribution [e.g., F(t-inv) is small around 50°N
east of 160°W] is similar if weak temperature inversions
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FIG. 3. Frequencies of temperature inversions (a) exceeding 0.1°C, i.e., F(⌬T ⬎ 0.1°C) (%); (b) F(⌬T ⬎ 0.3°C) (%); (c) F(⌬T ⬎ 0.5°C)
(%); and (d) F(⌬T ⬎ 1.0°C) (%) for all seasons in each 3° ⫻ 1° box. These values were calculated with profiles smoothed by the 10-m
running mean (RM10: the black lines in Fig. 2). A blank means the area where 90% confidence interval is wider than EF ⫾25%. Values
were computed from stations shown in Fig. 1.
are ignored (Figs. 3b–d). Figures 3c and 3d also show a
frequency minimum along the Kuril Islands.
Temperature inversions frequently occur [e.g., F(⌬T
⬎ 0.1°C) ⬎ 75%] in the southeastern subarctic North
Pacific (42°–48°N, 180°–140°W) and in the northern
Gulf of Alaska [140°–170°W, north of 51°N (near
170°W) ⫺ 54°N (near 140°W)]. Even large temperature
inversions (⌬T ⬎ 0.5°C) have a frequency exceeding
50%. In contrast, large temperature inversions seldom
occur [e.g., F(⌬T ⬎ 0.5°C) ⬍ 25%] around 50°N east of
160°W. These results are consistent with results found
by Roden (1964) and Ueno and Yasuda (2000).
Temperature inversions ⬎0.1°C are almost always
present (Fig. 3a) in the Bering Sea, Sea of Okhotsk, and
western subarctic gyre (WSAG), defined as the open
North Pacific north of 42°N and west of 180°. Large
temperature inversions (e.g., ⌬T ⬎ 0.5° or 1.0°C) are
frequently observed in the western part of the WSAG
and in the Sea of Okhotsk (Figs. 3c,d). Because the
estimated TMAX depth around the Kuril Islands exceeds 500 m (Kobayashi 2004), F(t-inv) was reevaluated using stations with deepest observations exceeding
1000 m. Results were similar to those in Fig. 3. Large
temperature inversions are also common in the Bering
Sea, especially the western Bering Sea. An area of large
F(t-inv) extends east of Japan southward to the Kuroshio Extension [e.g., an area of F(⌬T ⬎ 0.1°C) ⬎ 50%
extends to 36°N, west of 150°E], consistent with findings by Nagata (1979). There, results also do not change
if using stations with deepest observations exceeding
1000 m.
The distribution of temperature inversions in Fig. 3
[e.g., the distribution of F(⌬T ⬎ 0.1°C) ⬎ 75% in Fig.
3a] resembles the distribution shown by Ueno and Yasuda (2000), but there are differences along the Aleutian and Kuril Islands. Figure 3 shows relatively low
F(t-inv) along the Aleutian and Kuril Islands where
Ueno and Yasuda (2000) showed temperature inversions. Ueno and Yasuda (2000) used a heavily
smoothed climatological dataset (WOA94). In contrast,
Miura et al. (2002) used a dataset with a relatively small
smoothing scale and detected few temperature inversions near the Aleutian and Kuril Islands. In addition,
their climatological dataset did not permit smoothing
across the Aleutian Islands or across the Kuril Islands.
Temperature inversions occur over the subarctic
North Pacific (the area north of about 42°N) with frequencies that vary from region to region. In this study,
F(⌬T ⬎ 0.1°C) ranges from 25% to 100%. These results
are consistent with results in Uda (1963) and Kobayashi
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FIG. 4. Distribution of (a) F(⌬T ⬎ 0.1°C) (%), (c) F(⌬T ⬎ 0.5°C) and (e) F(⌬T ⬎ 1.0°C) evaluated with profiles smoothed by the
100-m running mean (RM100: the red lines in Fig. 2), and differences (b) between F(⌬T ⬎ 0.1°C) of RM10 and F(⌬T ⬎ 0.1°C) of
RM100, (d) between F(⌬T ⬎ 0.5°C) of RM10 and F(⌬T ⬎ 0.5°C) of RM100, and (f) between F(⌬T ⬎ 1.0°C) of RM10 and F(⌬T ⬎
1.0°C) of RM100. Contours of (b), (d), and (e) were superimposed on (a), (c), and (e), respectively. Data used are shown in Fig. 1. A
blank means the area where 90% confidence interval is wider than EF ⫾ 25%.
(2004), who both showed temperature inversions across
the entire subarctic North Pacific.
A 100-m vertical running mean, rather than a 10-m
running mean as discussed above, was applied to the
temperature profiles to focus on the distributions of
temperature inversion with large vertical scale (⬎100
m). Temperature inversions with large vertical scale occur over the northwestern subarctic North Pacific (e.g.,
Uda 1963) as shown in Fig. 2a. In contrast, temperature
inversions with smaller vertical scale (10–50 m as in Fig.
2b) are observed in some areas (e.g., east of Japan;
Nagata 1979).
Figure 4a shows F(⌬T ⬎ 0.1°C) for temperature profiles to which the 100-m running mean was applied. In
the WSAG, the Bering Sea, and the Sea of Okhotsk,
F(⌬T ⬎ 0.1°C) still exceeds 95%. This is similar to the
10-m running-mean case (Fig. 3a) and suggests that the
vertical scale of temperature inversions is large (at least
⬎ 100 m) there. In the other regions except the area
F(⌬T ⬎ 0.1°C) of 10-m running mean was less than 5%,
F(⌬T ⬎ 0.1°C) of 100-m running mean was smaller than
that of 10-m running mean; F(⌬T ⬎ 0.1°C) strongly
decreased, from ⬃80% to ⬃20% in the area of 140°–
160°W and 45°–50°N, suggesting that small verticalscale temperature inversions are predominant there. In
the area south of 40°N except the area of 33°–40°N,
140°–155°E, , F(⌬T ⬎ 0.1°C) of 100-m running mean is
less than 5%; that is, temperature inversions with large
vertical scale do not occur there. The southern limit of
temperature inversions with large vertical scale roughly
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FIG. 5. Distributions averaged in each 3° ⫻ 1° box of (a) temperature difference, ⌬T (°C); (b) thickness ⌬D (m) between TMIN and
TMAX waters; (c) TMIN temperature (°C); (d) TMIN depth (m); (e) TMAX temperature (°C); and (f) TMAX depth (m) superimposed by
contours of F(⌬T ⬎ 0.1°C) (%) (see Fig. 3a). These values were calculated with profiles smoothed by the 10-m running mean. Data used
are shown in Fig. 1. Average values were not estimated (blanked out) in areas where temperature inversions seldom occur [F(⌬T ⬎
0.1°C) ⬍ 5%] and in the area of insufficient data (fewer than five stations with temperature inversions).
corresponds to the southern limit of temperature inversions in Uda (1963).
Here F(⌬T ⬎ 0.5°C) and F(⌬T ⬎ 1.0°C) of 100-m
running mean also decreases when compared with those
of 10-m running mean. However, even after 100-m running mean, F(⌬T ⬎ 1.0°C) still exceeds 95% in the area
off Kamchatka Peninsula and in the Sea of Okhotsk,
suggesting robust temperature inversions exist there.
b. Water properties at the TMIN and the TMAX
Distributions of ⌬T, ⌬D, and water properties at
TMIN and TMAX averaged in each 1°(latitude) ⫻ 3 °(lon-
gitude) (or 3° ⫻ 3°) box are now presented. The averaged ⌬T is large in the western subarctic North Pacific,
especially in the Sea of Okhotsk and east of Kamchatka
(Fig. 5a), where large temperature inversions frequently occur (e.g., Fig. 3d). Average potential temperature in TMIN and TMAX waters are coldest in the
Sea of Okhotsk and increase to the east and south. The
largest ⌬D occurs in the Sea of Okhotsk (Fig. 5b). The
⌬D value decreases to the east to less than 100 m east
of 170°W. Because TMIN water depths are nearly uniform throughout the subarctic (Fig. 5d), as noted in
Ueno and Yasuda (2000) and Miura et al. (2002), the
⌬D distribution matches the TMAX depth distribution.
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FIG. 6. Distributions of (a) TMIN density (␴␪), (b) TMIN salinity (practical salinity units, psu), (c) TMAX density (␴␪), and (d) TMAX
salinity (psu) averaged in each 3° ⫻ 3° box. These values were calculated with profiles smoothed by the 10-m running mean. Only
stations with both temperature and salinity were used. Average values were not computed in areas where temperature inversions
seldom occur [F(⌬T ⬎ 0.1°C) ⬍ 5%] and in areas of insufficient data (fewer than five stations with temperature inversion). Contours
of F(⌬T ⬎ 0.1°C) (%) (see Fig. 3a) are superimposed.
That is, the TMAX depth is greatest in the Sea of
Okhotsk, and the TMAX depth decreases to the east.
The ⌬D distribution is also consistent with Fig. 4b; the
area of small ⌬D (e.g., ⬍100 m) in Fig. 5b roughly
corresponds to the area where an abrupt decrease of
F(⌬T ⬎ 0.1°C) (e.g., ⬎25%) occurs when the 100-m
running mean is applied to the temperature profiles.
East of Japan (32°–42°N, 140°–170°E), both TMIN and
TMAX waters exist in the deep layer. Properties in
Fig. 5 were reevaluated using stations with deepest observations exceeding 1000 m: it was found ⌬D and
TMAX depth were more than 800 m in the sea of
Okhotsk. In other areas, properties in Fig. 5 did not
change much.
Figure 6 shows potential density and salinity at TMIN
and TMAX depths averaged in each 3° ⫻ 3° box. The
TMIN density increases westward. The heaviest TMIN
water occurs near the Kuril Islands and east of Japan
(Fig. 6a). The TMIN water freshens to the north, and the
freshest TMIN water is in the northern Gulf of Alaska
(Fig. 6b). The TMAX water is heaviest and saltiest in the
Sea of Okhotsk. The TMAX density and salinity decrease toward the east (Figs. 6c,d), as noted by Ueno
and Yasuda (2000). When properties were reevaluated
using stations with deepest observations exceeding 1000
m, TMAX density and salinity were more than 27.2 ␴␪
and 34.2 in the Sea of Okhotsk; others did not change
much.
c. Seasonal variation of temperature
inversions
Temperature inversions vary with the season. Large
seasonal variations of F(t-inv) occur in the northern
Gulf of Alaska (Fig. 7). In winter, large F(⌬T ⬎ 0.1°C)
and F(⌬T ⬎ 0.5°C) are widespread. The area gradually
decreases as the seasons progress, and big temperature
inversions (⌬T ⬎ 0.5°C) in autumn are rare. The mean
⌬T also decreases in the northern Gulf of Alaska as the
seasons progress (Fig. 8). Both TMIN and TMAX temperatures are cold in winter and warm in autumn and
the seasonal variability of TMIN temperature is larger
than that of TMAX temperature (Fig. 8), whereas ⌬T
decreases from winter to autumn. In the southeastern
subarctic North Pacific (42°–48°N, 140°–175°W), seasonal variations in temperature inversions are relatively
small. Strong temperature inversions with ⌬T ⬎ 0.5°C
seldom occur at any time of the year around 50°N east
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VOLUME 35
FIG. 7. Distributions of (left) F(⌬T ⬎ 0.1°C)% and (right) F(⌬T ⬎ 0.5°C)% in (a) winter, (b) spring, (c) summer, and (d) autumn
in each 3° ⫻ 3° box. These values were calculated with profiles smoothed by the 10-m running mean. Blank regions are for boxes where
90% confidence interval is wider than EF ⫾ 25%.
of 160°W. Seasonal variations of temperature inversions east of 180° are similar to those shown in Ueno
and Yasuda (2000).
In the WSAG, the area of large F(t-inv) and ⌬T is
largest in winter and shrinks thereafter (Figs. 7 and 8).
Seasonal variations of temperature inversions in the
Bering Sea are relatively small. However, pronounced
seasonal variations occur near the Aleutian Islands (including south of the islands); F(t-inv) and ⌬T decrease
dramatically from winter to autumn. The TMIN and
TMAX temperatures increase between winter and autumn (Fig. 8). The TMIN temperature increases more
UENO AND YASUDA
FIG. 8. Distributions of ⌬T, TMIN temperature, and TMAX temperature averaged in each 3° ⫻ 3° box in (a) winter, (b) spring, (c) summer, and (d) autumn superimposed by contours
of F(⌬T⬎ 0.1°C) (%). These values were calculated with profiles smoothed by the 10-m running mean. Average values were not estimated (blanked out) in areas where temperature
inversions seldom occur [F(⌬T ⬎ 0.1°C) ⬍ 5%] and in the areas with fewer than five stations with temperature inversions.
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than TMAX waters; thus, ⌬T decreases from winter to
autumn. Similar warming occurs near the Kuril Islands.
4. Summaries and discussion
Analyses of individual hydrographic stations from
the World Ocean Database 2001 and from Argo profiling floats were used to describe temperature inversions
in the subarctic North Pacific. The frequency of temperature inversions F(t-inv) in each 1°(latitude) ⫻
3°(longitude) (or 3° ⫻ 3°) grid was computed to yield a
distribution of temperature inversions throughout the
subarctic North Pacific.
The horizontal distribution of temperature inversions
derived in this study is similar to the distribution in
Ueno and Yasuda (2000), who used heavily smoothed
climatological hydrographic data (WOA94). Maps of
temperature inversions in the present study show that
large temperature inversions seldom occur around
50°N east of 160°W, but inversions do occur in the
northern Gulf of Alaska and in the southeastern subarctic North Pacific as indicated by Ueno and Yasuda
(2000). In contrast, although Ueno and Yasuda (2000)
showed temperature inversions along the Aleutian and
Kuril islands, this study and the fine-resolution climatological hydrographic dataset of Miura et al. (2002)
show few large temperature inversions in that region.
In addition, a pronounced seasonal variation in temperature inversions is apparent along the Aleutian and
Kuril Islands. The seasonal variation is probably owing
to strong tidal mixing, as discussed below.
The subarctic North Pacific can be subdivided into
eight regions, based on Figs. 3–8. Formation mechanisms for temperature inversions are discussed for each
region using results from the previous section and from
previous studies.
1) Temperature inversions show large seasonal variability in the northern Gulf of Alaska: F(⌬T ⬎
0.5°C) exceeds 75% in winter and is less than 50% in
autumn (Fig. 7). This variability can be linked to the
seasonal cycle of heating and cooling from the surface, as suggested by Uda (1963) and Ueno and Yasuda (2000). However, other effects cannot be neglected because temperature inversions with small
vertical scale are common (Figs. 4 and 5b). Very
deep vertical mixing in a local area (e.g., Van Scoy et
al. 1991; Musgrave et al. 1992) and anticyclonic
baroclinic eddies (e.g., Tabata 1982; Onishi et al.
2000) may play important roles in the formation of
temperature inversions in this region. In addition,
the effect of warm water advection in the Alaska
Current and the Alaskan Stream cannot be neglected.
VOLUME 35
2) Large temperature inversions are rare throughout
the year around 50°N east of 160°W (Figs. 3 and 7).
Three phenomena could explain the dearth of temperature inversions in this region. First, winter surface cooling is weak (e.g., Oberhuber 1988; da Silva
et al. 1994), as noted by Ueno and Yasuda (2000).
Second, intermediate water in this region [called the
ridge domain in Favorite et al. (1976)] is relatively
cold in comparison with surrounding regions. The
winter mixed layer in this region is therefore rarely
colder than the intermediate water. Third, advection
of cold subsurface water (TMIN water advection; e.g.,
Suga et al. 2003) from the west might be weak because the area is near the center of the Alaskan
Gyre.
3) Large temperature inversions occur nearly yearround in the southeastern subarctic North Pacific
(42°–48°N, 140°–175°W; Figs. 3 and 7). Ueno and
Yasuda (2000) suggested that this zonally elongated
region of large F(t-inv) and ⌬T reflects a subsurface
intrusion of low-temperature and low-salinity water,
which outcropped during the previous winter in the
western Pacific over warm and saline water transported from east of Japan. As shown in Fig. 5b, the
temperature inversion thickness in this region is the
smallest in the subarctic north Pacific, except for the
area of low F(t-inv) [e.g., the area of F(⌬T ⬎ 0.1°C)
⬍ 50%]. This may reflect the formation of temperature inversions by differential advection rather than
in the winter mixed layer.
4) Large temperature inversions do not occur as frequently along the Aleutian Islands as in adjacent
areas to the north and south (Fig. 3), perhaps because strong tidal mixing eliminates TMIN along the
Aleutians (Roden 1995, 1998; Miura et al. 2002).
Temperature inversions show a marked seasonal
variation along the Aleutian Islands (Fig. 7 and 8).
Strong warming of TMIN waters from spring to autumn forces this variability. The seasonal cycle of
air–sea fluxes causing heating and cooling in the surface layer influences the TMIN layer because of
strong tidal mixing, enhancing the heat exchange
between surface and subsurface layers. Heat transport by the warm Alaskan Stream could also warm
waters around the Aleutian Islands and eliminate
inversions.
5) Miura et al. (2002, 2003) detailed the formation of
temperature inversions in the Bering Sea. Relatively
warm water in the Alaskan Stream enters the Bering
Sea across the Aleutian Islands and cold TMIN water
is formed in the winter mixed layer as water moves
cyclonically in the Bering Sea. Figures 3 and 5 agree
with this scenario; F(t-inv) and ⌬T are larger and
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UENO AND YASUDA
TMIN waters are colder in the western Bering Sea
than in the southeastern Bering Sea. However,
Roden (1995) noted that horizontal advection of
cold subsurface water also has an important role in
temperature inversion formation in the Bering Sea.
6) The Sea of Okhotsk experiences the coldest winter
mixed layer (e.g., Miura et al. 2002) and also holds
the coldest TMIN waters in the North Pacific (Fig. 5).
Consequently, F(t-inv) and ⌬T are very large in the
Sea of Okhotsk, despite the presence of the coldest
TMAX waters (Figs. 3 and 5). Deep vertical mixing
extends to 27.5 ␴␪ because of sea ice formation over
the northwestern continental shelf in winter and
tidal mixing around the Kuril Islands (Kitani 1973;
Alfultis and Martin 1987; Talley 1991; Talley and
Nagata 1995; Yasuda 1997; Watanabe and Wakatsuchi 1998; Itoh et al. 2003). Such deep vertical mixing helps form cold and deep TMAX water.
7) The F(t-inv) and mean ⌬T values along the Kuril
Islands are relatively small in comparison with those
in the Sea of Okhotsk and in the WSAG (Figs. 3 and
5). As in the Aleutians, strong tidal mixing near the
Kuril Islands may be responsible for the small values. Warming of TMIN and TMAX waters occurs from
winter (or spring) to autumn. The TMIN waters
warm more rapidly than TMAX waters, so ⌬T and
F(t-inv) decrease as the seasons progress. The TMAX
waters in the East Kamchatka Current are warmer
than TMAX water along the Kuril Islands and in the
Sea of Okhotsk (Fig. 5). The prominent seasonal
variability in temperature inversions along the Kuril
Islands is influenced, therefore, by strong tidal mixing as well as heating from the surface in summer
and intermediate heat transport by the warm East
Kamchatka Current.
8) The coldest TMIN waters in the WSAG are in the
East Kamchatka Current. This is also where the
largest ⌬T occurs (Fig. 5). Miura et al. (2002) noted
that very cold TMIN waters are formed in the winter
mixed layer in the Bering Sea and are advected into
the Pacific Ocean by the East Kamchatka Current.
Suga et al. (2003) suggested that TMIN waters north
of about 46°N along 180°E originated in the winter
mixed layer in the Bering Sea. The results in the
present study for TMIN waters (Figs. 5 and 6) are
consistent with their suggestions (Miura et al. 2002;
Suga et al. 2003). However, vertical and horizontal
mixing in the WSAG may modify TMIN waters. Future studies will help describe these mixing processes. On the other hand, Figs. 5 and 6 suggest that
TMAX waters are strongly modified near the Kuril
Islands, because TMAX waters east of the Kurils are
colder, saltier, and thus heavier than TMAX waters at
the northwestern edge of the Bering Sea.
Despite strong mixing and cooling along the Aleutian
and Kuril Islands, in the Bering Sea and in the Sea of
Okhotsk, an intermediate temperature maximum
(TMAX) persists throughout the western subarctic
North Pacific. Ueno and Yasuda (2000, 2001, 2003) discussed the maintenance of heat in the intermediate
TMAX water and suggested that intermediate heat
transport from the south helps to maintain intermediate
TMAX water in the subarctic North Pacific. Further
studies will clarify how the heat of the intermediate
TMAX water is maintained in each area.
Acknowledgments. The authors thank T. Suga of
IORGC JAMSTEC/Tohoku University and E. Oka
and T. Kobayashi of IORGC JAMSTEC for helpful
comments and fruitful discussions. We also thank L.
Talley, editor of this manuscript, and anonymous reviewers. Their comments were very helpful and improved this paper. The data observed by Argo profiling
floats were collected and made available by the International Argo Project and the national programs that
contribute to it (online at www.argo.ucsd.edu and argo.
jcommops.org). Argo is a pilot program of the Global
Ocean Observing System. This work was supported by
a Grant-in-Aid for Scientific Research from the Ministry of Education, Culture, Sports, Science and Technology and “The Argo Project—Advanced Ocean Observing System” as one of the Millennium Projects of the
Japanese Government.
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