GEOPHYSICAL RESEARCH LETTERS, VOL. 32, L20603, doi:10.1029/2005GL023948, 2005

Seasonal and interannual variability of temperature inversions in thesubarctic North Pacific
Hiromichi Ueno,1 Eitarou Oka,1 Toshio Suga,1,2 and Hiroji Onishi3
Received 29 June 2005; revised 8 September 2005; accepted 20 September 2005; published 25 October 2005.

[1] Hydrographic data from profiling floats obtained
during 2001 –2004 were analyzed to study seasonal andinterannual variability of temperature inversions (T-invs) inthe subarctic North Pacific (SNP). In the western SNP andBering Sea, the temperature minimum at the top of T-invs
outcropped and was renewed every winter, causing aseasonal cycle in the magnitude of T-invs, with themaximum at the end of winter. In the Gulf of Alaska inthe eastern SNP, the temperature minimum outcropped inwinters 2002 and 2004, but scarcely outcropped in winter2003. Consequently, the magnitude of the T-invs showedremarkable interannual variation; its monotonic decrease
through winter 2003 overwhelmed the seasonal cycle.
The year-to-year variation of the magnitude of the T-invsin each region of the SNP was consistent with andthereby attributable to that of the winter sea surface
temperature anomaly there. Citation: Ueno, H., E. Oka,
T. Suga, and H. Onishi (2005), Seasonal and interannual
variability of temperature inversions in the subarctic North
Pacific, Geophys. Res. Lett., 32, L20603, doi:10.1029/
2005GL023948.

1. Introduction[2] Temperature inversions (T-invs) are intrinsic featuresof the subarctic North Pacific (SNP) north of ca. 40°N,
associated with the strong permanent halocline [Dodimeadet al., 1963; Uda, 1963; Roden, 1964; Favorite et al., 1976;
Ueno and Yasuda, 2000, 2005; Miura et al., 2002]. They arethought to be formed mainly by wintertime surface cooling,preserving the memory of wintertime atmospheric conditions. After their formation, T-invs gradually weaken as theseason progresses, due to surface heating and verticalmixing. They often vanish during fall in the eastern SNP,
while they generally survive throughout the year in thewestern SNP [Ueno and Yasuda, 2000]. To better
understand air –sea interactions in the SNP, it is essential
to clarify the seasonal cycle and year-to-year variation ofT-invs.
[3] T-invs in the SNP have been examined mostly using
historical hydrographic data, which are quite limited inwinter. Therefore, the formation and seasonal evolution ofT-invs are not fully understood, and their interannual
variation remains unclear. The Argo Project [Argo Science
1

Institute of Observational Research for Global Change, Japan Agency
for Marine-Earth Science and Technology, Yokosuka, Kanagawa, Japan.
2
Also at Department of Geophysics, Graduate School of Science,Tohoku University, Sendai, Japan.
3
Division of Marine Bioresource and Environmental Science, GraduateSchool of Fisheries Science, Hokkaido University, Hakodate, Japan.
Copyright 2005 by the American Geophysical Union.
0094-8276/05/2005GL023948$05.00

Team, 2001], which began in 2000 and involved thedeployment of 3,000 profiling floats to monitor the temperature and salinity of the world’s oceans in 10-day cycles,
has been rapidly easing these limitations. Using Argo float
data, Wirts and Johnson [2005] investigated seasonal andinterannual upper ocean variability in the southeastern
Bering Sea, and Freeland and Cummins [2005] documented
the variation in the ocean circulation of the eastern SNP.
In this paper, we discuss the formation and seasonal/
interannual variation of T-invs in the entire SNP, using thefloat data from 2001 – 2004.

2. Data and Method[4] We used temperature and salinity profiles recorded by
Argo floats (http://www.argo.net/) in the North Pacific northof 30°N during 2001 –2004. These profiles typically had amaximum depth of 2000 db (2000  104Pa) and a verticalresolution of 5 – 25 db at depths shallower than 1000 db and50– 100 db at deeper than 1000 db. We used only thoseprofiles reaching deeper than 500 db and having more than
20 vertical levels between 0 and 500 db, since most T-invs
in the SNP occur above 500 db [Kobayashi, 2004]. We used
15,211 profiles, although the data were limited in the firsthalf of 2001.
[5] For each profile, the difference (DT) between thetemperature maximum (T-max) at the bottom of the T-inv
and the minimum (T-min) at the top of the T-inv wasdetermined following Ueno and Yasuda [2005]. T-min
was considered to outcrop when the difference between
the T-min density and sea surface density was smaller than
0.03 kg m 3 . This density criterion, when used forindividual profiles, detected the mixed layer bottom
accurately, compared to the previously used larger values,
such as 0.125 kg m 3 [de Boyer Montégut et al., 2004].
Note that an 0.03 kg m 3 density difference criterion willallow profiles with as much as an 0.3°C temperature
difference between surface temperature and the T-min to
be classified as ventilated in these regions. The density at
the sampling depth nearest 10 db was regarded as the sea
surface density because the floats cannot conduct measurements at depths shallower than several decibars. We also
used climatological hydrographic data from the World
Ocean Atlas 2001 (WOA) [Boyer et al., 2002; Stephens etal., 2002]. For the sea surface temperature anomaly (SSTA:deviation from the monthly climatology), we used NOAA
optimum interpolation SSTA data [Reynolds et al., 2002].

3. Results[6] The Argo data detected T-invs with DT larger than
0.1°C existing ubiquitously in the SNP (Figure 1). DT

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Figure 2. Time series of DT (thick solid line) andtemperatures at T-min (dotted line) and T-max (dashedline) averaged in the area (a) 160– 180°E, 45– 60°N except
the Bering Sea, (b) east of 180° in the Bering Sea, and(c) 120– 160°W, 45– 60°N except the Bering Sea (seeFigure 1a for locations), calculated from Argo float data.
Vertical bars for DT represent the standard error. The
profiles without a T-inv were excluded from thecalculations. The thin solid line shows the time seriesof DT, calculated from the WOA for the area deeper than500 db.
increased to the west and north, and was especially large inthe western SNP and Bering Sea. The T-invs in the Argo data
extended much more widely than that in the WOA. For
example, in the Argo data, DT larger than 0.1°C was observedin the eastern SNP east of 160°W year-round, where T-invs
seldom occurred during summer in the WOA. In the Mixed
Water Region east of Japan (ca. 140 –160°E and 33– 45°N)
between the Oyashio Front and Kuroshio Extension Front[Kawai, 1972], DT larger than 1.0°C was often recordedin the Argo data, but T-invs did not occur in the WOA.
These differences probably resulted from the strong
spatial and temporal smoothing used in the WOA.
[7] In the western and central SNP and in the Bering Sea,
T-min generally outcropped in late winter (Figures 1a, 1c,L20603

and 1e). In these areas, T-min is thought to be formed
locally in the winter mixed layer. In contrast, T-min almostdid not outcrop in the Mixed Water Region, where T-min
was formed not in the local winter mixed layer, but in thesubsurface layer, probably due to lateral intrusion of cold
freshwater originating from the Oyashio region [e.g.,Nagata, 1979]. In the Gulf of Alaska (east of 160°W andnorth of 52°N) in the northeastern SNP, T-min outcropped
in winter 2002 and partially outcropped in winter 2004, but
scarcely outcropped except in the coastal area during winter2003. In the southeastern SNP (east of 160°W and 45–48°N), it outcropped only in limited areas in winter 2004,
and never in winters 2002 and 2003, although DT larger
than 0.5°C was observed constantly. This supports Uenoand Yasuda’s [2000] hypothesis that T-invs in this area aremaintained by subsurface advection of the T-min water fromthe western and central SNP.
[8] To examine the quantitative seasonal and interannual
variation of DT and its relation to those of the T-min outcropand SSTA, DT and the temperatures at T-min and T-max
were averaged for three regions, the western SNP, eastern
Bering Sea, and eastern SNP (Figure 2), where the float
density was relatively large during the analysis period(Figure 1). In the western SNP where T-min outcropped
every winter, DT had a significant maximum every late
winter (Figure 2a). In this area, the variation of the T-max
temperature was small except around the end of 2002,
and the fluctuation in DT was determined primarily bythat of the T-min temperature, which decreased every
winter due to the T-min outcrop. The winter DT (T-mintemperature) was relatively large (cold) in 2002 and 2003
and small (warm) in 2004. This is consistent with theinterannual variation in the winter SSTA for the area,
which was 0.8 to 0.2°C in 2002, 0.5 to 0.0°C in2003, and 0.2 to 0.0°C in 2004.
[9] In the eastern Bering Sea, where T-min also
outcropped every winter, DT showed seasonality similar
to that in the western SNP (Figure 2b). In this area, theT-max temperature fluctuation was also small, except
around the end of 2003, and the DT fluctuation wasmostly determined by the T-min temperature variation.
The winter DT was large in 2002, small in 2003, andintermediate in 2004, which corresponded to the year-toyear variation in the winter SSTA: 1.0 to 0.0°C in2002, 0.2 to 1.5° in 2003, and 0.0 to 1.0°C in 2004. Thisrelation between the T-min temperature and SSTA in theeastern Bering Sea was also reported by Wirts andJohnson [2005].
[10] DT in the eastern SNP was much larger than thatfrom the WOA, and represented remarkable interannualvariation, in contrast to the western SNP and eastern Bering
Sea (Figure 2c). It increased from November 2001 to March2002, decreased from March 2002 to December 2003, andincreased again in winter 2004. The monotonic decrease inDT through winter 2003 corresponds to the fact that T-min

Figure 1. Maps of the magnitude of the T-invs during February – March and August – September 2002 – 2004,
constructed from Argo float data. The symbol color denotes the magnitude of the T-invs: red, DT > 1.0°C; green,0.5°C < DT < 1.0°C; and blue, 0.1°C < DT < 0.5°C. Stars (circles) represent T-invs whose T-min outcrops (does notoutcrop). Black dots represent profiles with DT < 0.1°C, including those without a T-inv. Red, green, and blue contours
represent isolines of DT = 1.0°, 0.5°, and 0.1°C, respectively, calculated from WOA. Black lines in the top panel
indicate the areas analyzed in Figures 2a, 2b, and 2c.
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there scarcely outcropped during that period (Figure 1c).
The interannual variation of DT was consistent with that of
the winter SSTA in the region, which was 1.0 to 0.2°C
in 2002 and 0.0 to 1.5°C in 2003. In contrast to the western
SNP and eastern Bering Sea, the DT variation did not
simply correspond to the T-min temperature variation. This
will be discussed in the next section.

4. Discussion[11] The temperatures at T-min and T-max in the eastern
SNP showed a conspicuous short-term (less than one year)
fluctuation, in contrast to a relatively modest variation in DT(Figure 2c). This short-term fluctuation probably arose
because each of the temperatures at T-min and T-max
showed strong spatial variation in the area, ranging between
4 and more than 10°C (not shown), associated withpredominant fronts and eddies there [e.g., Tabata, 1982].
The profiling float density was not sufficiently high tosmooth out this large spatial variation when the T-min andT-max temperatures were averaged in the area. Particularly,
the float distribution was strongly biased to the east in 2001,
causing extremely warm T-min and T-max temperaturesduring that period. Nevertheless, since a high (low) T-min
temperature tended to be associated with a high (low) T-max
temperature for each profile (not shown), DT varied
spatially much less than each of the T-min and T-max
temperatures, resulting in the modest temporal variation inthe area-averaged DT. In this context, DT likely gives amore robust measure of the regional impact of wintertime
surface cooling when it is examined or monitored using theArgo float array, which is intended for large-scale monitoring and does not resolve mesoscale variability itself.
[12] It has been proposed that the SST in the northeast
Pacific is related to El Niño/La Niña events, and is high during
El Niño events and low during La Niña episodes [e.g.,Schwing et al., 2002]. Therefore, the warm SST and small
DT in the northeastern SNP in winter 2003 were likely caused,
at least in part, by the 2002/2003 El Niño event. The results ofour study will facilitate further research on the impact ofclimate variation on the upper layer structure in the SNP.
[13] Acknowledgments. The Argo float data used in this study werecollected and made freely available by the International Argo Project andthe national programs that contribute to it (http://www.argo.ucsd.edu, http://argo.jcommops.org). Argo is a pilot program of the Global Ocean
Observing System. The authors thank the members of the JAMSTEC Argo
group for helpful comments and fruitful discussions. We also thank two
anonymous reviewers for helpful, constructive comments.

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E. Oka, H. Ueno, and T. Suga, Institute of Observational Research for
Global Change, Japan Agency for Marine-Earth Science and Technology,
2-15 Natsushima-cho, Yokosuka, Kanagawa 237-0061, Japan. (uenohiro@jamstec.go.jp)
H. Onishi, Division of Marine Bioresource and Environmental Science,
Graduate School of Fisheries Science, Hokkaido University, Hakodate 0418611, Japan.

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