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GEOPHYSICAL RESEARCH LETTERS, VOL. 34, L05603, doi:10.1029/2006GL028715, 2007

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ArticleFormation and variation of temperature inversions in the eastern
subarctic North Pacific
Hiromichi Ueno,1 Eitarou Oka,1,2 Toshio Suga,1,3 Hiroji Onishi,4 and Dean Roemmich5
Received 7 November 2006; revised 10 January 2007; accepted 31 January 2007; published 3 March 2007.

[1] Hydrographic data from profiling float observations for2001 – 2005 and from expendable bathythermographobservations for 1993– 2005 were analyzed to study theformation and variation of temperature inversions(T-inversions) in the eastern subarctic North Pacific (SNP).
The formation and variation of T-inversions differedsignificantly between the northern and southern regions of
the eastern SNP. In the northern region, the temperature
minimum (T-min) at the top of T-inversions outcropped to thesea surface and was cooled in the mixed layer nearly every
winter. This process caused a seasonal cycle in the magnitude
of T-inversions (DT), with a maximum in winter. In thesouthern region, the winter T-min outcropped relatively
infrequently and the DT did not exhibit a significant
maximum in winter during most years. The T-min in thesouthern region was likely to outcrop upstream near the dateline roughly one year earlier and was then advected to thesouthern region. Citation: Ueno, H., E. Oka, T. Suga, H. Onishi,
and D. Roemmich (2007), Formation and variation of temperature
inversions in the eastern subarctic North Pacific, Geophys. Res.
Lett., 34, L05603, doi:10.1029/2006GL028715.

1. Introduction[2] Temperature inversions (T-inversions), lying between
a subsurface temperature minimum (T-min) and a temperature maximum (T-max) underlying the minimum [e.g.,
Ueno and Yasuda, 2000, Figure 3], are widely distributed
over the subarctic North Pacific (SNP) north of ca. 40°N
[e.g., Dodimead et al., 1963; Uda, 1963; Roden, 1964;
Favorite et al., 1976]. Because of the presence of a strong,permanent halocline in this region, the winter mixed layer
can be colder than the underlying, more saline layer, thus
forming a T-inversion. In early spring, the upper portion of
the winter mixed layer begins to warm, leaving the bottom
portion as a T-min. As the season progresses, the T-min is
gradually eroded and weakened by further surface heating
and vertical mixing.
[3] Using temperature and salinity data from 2001 to 2004
obtained using Argo profiling floats [Argo Science Team,2001], Ueno et al. [2005] studied seasonal and interannual
variability of T-inversions in the SNP, focusing on three large
1Institute of Observational Research for Global Change, Japan Agency
for Marine-Earth Science and Technology, Yokosuka, Japan.
2
Ocean Research Institute, University of Tokyo, Tokyo, Japan.
3
Department of Geophysics, Graduate School of Science, Tohoku
University, Sendai, Japan.
4
Division of Marine Bioresource and Environmental Science, Graduate
School of Fisheries Science, Hokkaido University, Hakodate, Japan.
5
Scripps Institution of Oceanography, La Jolla, California, USA.

Copyright 2007 by the American Geophysical Union.
0094-8276/07/2006GL028715$05.00

regions where the float distribution density was relatively
high. Each year in the western SNP and Bering Sea, the wintermixed layer became colder than the T-min formed during theprevious winter. That is, the T-min outcropped to the sea
surface and was cooled in the mixed layer every winter,causing seasonal cycles in T-min and in the magnitude of
the T-inversion (DT; calculated as the temperature difference
between T-min and T-max). Specifically, the T-min was cold
in winter and warm in fall and the DT was large in winter andsmall in fall. This seasonal cycling of DT occurred because
the T-max temperature was relatively constant. In the eastern
SNP (defined as 45°– 60°N, 120° – 160°W, excluding theBering Sea), the T-min outcropped in winter 2002 and 2004
but scarcely outcropped in winter 2003, causing a monotonic
decrease in DT from March 2002 to December 2003. These
data suggest high interannual variability of T-inversions in theeastern SNP.
[4] Ueno et al. [2005] broadly examined T-inversion
variability in the eastern SNP. However, a previous study
using climatological hydrographic data [Ueno and Yasuda,2000] indicated that the existence and formation mechanismof T-inversions varied with location within the eastern SNP.
They found that T-inversions existed north of 52°N andsouth of 48°N, but did not exist between 48°N and 52°N.
This was confirmed by an analysis of individual temperature profiles, which indicated that the occurrence frequency
of T-inversions was high north of 52°N and between 42°N
and 48°N but low between 48°N and 52°N. In addition,T-min outcropped in winter north of 52°N, but did not
outcrop south of 48°N. Thus, T-inversions were formed
locally in the northern region but not in the southern region[Ueno and Yasuda, 2000]. A numerical particle-tracking
experiment by Ueno and Yasuda [2000] suggested that theT-inversions in the southern region were formed during theprevious winter in the western area near the date line andadvected to the southern region by the North Pacific Current.
[5] Despite these regional differences in the T-inversion
structure, our previous study [Ueno et al., 2005] did not detailthe formation and variation of T-inversions in the eastern SNP.
In the present study, we reexamined these aspects of
T-inversions in more detail using updated Argo float data
through 2005. These data include more profiles than used in
our previous study, particularly for the southern region of theeastern SNP. We also analyzed temperature data from repeated
expendable bathythermograph (XBT) observations between
Alaska and Hawaii for 1993–2005 to characterize long-term
variability of T-inversions in the eastern SNP.

2. Data and Methods[6] We used temperature and salinity profiles recorded by
Argo floats in the North Pacific north of 35°N and east of

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tology) data [Reynolds et al., 2002], and NCEP/NCAR
reanalysis surface heat flux data [Kistler et al., 2001].

3. Results

Figure 1. Distribution of XBT (squares) and Argo float(dots) observations in the eastern subarctic North Pacific(SNP). Red (blue) squares/dots represent observation points
where T-min outcropped (did not outcrop) during January –March. Lines indicate Regions A, B, and B’.

180° during 2001 – 2005 (Figure 1), although data were
extremely limited for the first half of 2001. Data were
downloaded from the website of the Argo Global Data
Assembly Center (http://www.argo.ucsd.edu, http://argo.
jcommops.org) and quality-controlled as outlined by Oka
et al. [2007]. We omitted profiles shallower than 500 db(500  104 Pa) and those having less than 20 verticallevels between 0 and 500 db, as outlined by Ueno et al.
[2005]. The total number of profiles selected was 9089,
among which 1641 and 585 existed in Regions A and B,
respectively.
[7] We also used temperature profiles along line PX38
between Alaska and Hawaii for 1993 – 2005 (Figure 1)
obtained from the high resolution XBT program of the
Scripps Institution of Oceanography [e.g., Douglass et al.,2006]. The along-track resolution off and near the coasts
was 30– 50 km and 10 km, respectively. We used profiles
averaged vertically over 10-m intervals, except for values at
0 – 10 m depth. Line PX38 was observed twice annually in
1993, 1994, 2004, and 2005, and 3 – 4 times a year from1995 to 2003. There was a winter (January– March) observation every year with the exception of 1993.
[8] For each profile, we determined the temperature
minimum (T-min) and maximum (T-max) at the top andbottom of the T-inversion, as well as the temperature
difference between T-min and T-max (DT), following Ueno
and Yasuda [2005]. T-inversions with DT smaller than
0.01°C were omitted. A T-min in an Argo profile wasconsidered to outcrop when the difference between theT-min density and the sea surface (10 m depth) density
was smaller than 0.03 kg m 3, as per Ueno et al. [2005].
A T-min obtained from XBT data was considered to outcrop
when the difference between the T-min temperature and thesea surface (average 10 – 20 m depth) temperature wassmaller than 0.1°C. This value (0.1°C) is the mediantemperature difference between the sea surface and themixed layer bottom obtained from the Argo data for theregions between 42°N and 60°N and 130°W and 160°W
using the density criterion of 0.03 kg m 3. We also used
NOAA optimum interpolation monthly sea surface temperature anomaly (SSTA: deviation from the monthly clima-[9] We examined T-inversions in the eastern SNP, focusing on two regions, Region A (52° –60°N, 130° –160°W,
excluding the Bering Sea; Figure 1) and Region B (42°–48°N, 140°– 160°W). These regions were characterized byhigh occurrence frequencies of T-inversions, compared tothe area between 48° and 52°N and the area east of Region
B, which was demonstrated by climatological data [Uenoand Yasuda, 2005] and Argo and PX38 XBT data in ourstudy. The infrequent T-inversions between 48° and 52°N
were considered to be because (1) winter surface cooling in
the area was weak, (2) the T-max was relatively cold, and(3) advection of the T-min layer water from the west was
weak [Ueno and Yasuda, 2005].
[10] We focused on variation in DT rather than T-min/T-max temperature variations, because DT was less affectedby eddies and areal sampling bias, and likely gave a morerobust measure of the regional impact of wintertime surface
cooling in the eastern SNP [Ueno et al., 2005]. For
example, both T-min and T-max in Region B, whererelatively small number of Argo profiles likely leads toareal sampling bias, changed abruptly in April 2003 andAugust 2004 (Figure 2b), because two floats were deployedin April 2003 and three in August 2004 in the southern areaof Region B where T-min/T-max temperatures were relatively high [Ueno and Yasuda, 2005]. However, on thoseoccasions, DT did not change abruptly.
[11] In Region A, T-min outcropped at considerable rates(>40%) in winter and DT reached a maximum in late winterFigure 2. Time series of DT (black line) and temperaturesat T-min (blue line) and T-max (red line) from Argo float
observations, averaged monthly in (a) Region A and(b) Region B. Vertical bars for DT represent ±1 standard
deviation. The green bar charts indicate monthly frequency
of T-min outcrops. Months with fewer than three observations are not included. Orange squares indicate SSTA
averaged from January to March in Region A (Figure 2a)and Region B’ (Figure 2b).

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Figure 3. Exemplary time-pressure sections of potential
temperature (°C) in (a) Region A and (b) Region B,
obtained from an Argo profiling float with WMO ID
4900344 and 4900119, respectively (WMO ID is a World
Meteorological Organization identifier). The float 4900344(4900119) drifted in the area of 53° – 58°N, 148° –156°W(46°– 47°N, 147° – 152°W). White stars (dots) represent
outcropping (non-outcropping) T-min, while red dots denote
T-max.
in 2002, 2004, and 2005 (Figures 2a and 3a). In these years,
DT changed seasonally, decreasing from late winter to thenext fall. The maximum DT in winter varied annually: 1.1°,
0.4°, and 0.2°C in 2002, 2004, and 2005, respectively. The
maximum DT was negatively correlated with winter SSTAin this region, which was 0.6°, 0.1°, and 0.3°C in therespective years. That is, when the winter mixed layer was
cold, the formation of a low T-min resulted in a large DT.
The T-min scarcely outcropped in winter 2003, when thewinter mixed layer was significantly warmer (SSTA =0.9°C) and shallower [Freeland and Cummins, 2005] thanin the previous year. Consequently, DT did not exhibit asignificant maximum in late winter 2003 and decreased
nearly monotonically from March 2002 to December 2003.
The DT variation in Region A was similar to that in theeastern SNP (45° –60°N, 120°– 160°W, except the BeringSea) presented by Ueno et al. [2005], because of therelatively high float distribution density and large DT
variability in Region A.
[12] In Region B, T-min scarcely outcropped throughout
2002 – 2005, and DT did not exhibit a significant maximum
in late winter (Figures 2b and 3b). T-inversions in thisregion were probably formed by subsurface advection of theT-min water that outcropped previously in the western,
upstream region, as suggested by Ueno and Yasuda[2000]. The potential density at T-min in Region B was
26.0 – 26.3 kg m 3. This density range outcropped in
winter west of Region B (160°W – 180°) [Suga et al.,2004, Figure 3c]. DT in Region B decreased in mid-2004,
likely in association with an increase in winter SSTA from2003 to 2004 averaged over 42° – 48°N and 160°W – 180°
(named Region B’) with some time lag (Figure 2b). This
relationship is further discussed below using the long-term

L05603XBT data. The Argo observations also demonstrated that
the T-min outcropped at a high rate in Region B’, much
more frequently than in Region B (Figure 1), supporting thesuggestion of Ueno and Yasuda [2000]. However, the Argo
data in Region B’ were temporally limited. Therefore, we
compared the DT in Region B with winter SSTA in Region
B’ here.
[13] We also examined the long-term variability ofT-inversions in the eastern SNP using line PX38 XBT datafor 1993 – 2005. In Region A, T-min outcropped at considerable rates (30 –90%) and DT exhibited a seasonal cyclewith a maximum in winter, except in 2003 (Figure 4a).
Winter DT was negatively correlated with winter SSTA near
the XBT line (correlation coefficient = 0.74). These data
indicate that the seasonal variability of T-inversions and its
relationship with SSTA in this region (as revealed by the
Argo data; Figure 2a) were maintained in the long term, with
only one exception (2003).
[14] In Region B, the frequency of winter T-min outcropping was relatively low (0 – 50%) and DT did not exhibit asignificant maximum in winter, except in 1998 and 1999,
indicating that T-inversions in this region were mainly
formed by horizontal advection (Figure 4b). However,during the 1998 and 1999 winters, T-min outcropped athigh rates (>90%) and was relatively colder, increasing DT
significantly. Net downward surface heat flux averaged
within Region B between the previous October and Marchwas 66 Wm 2 in 1998 and 70 Wm 2 in 1999, whichwere much higher than the average for 1993 – 2005
( 51 Wm 2). This cooling likely decreased the winter
SSTA in Region B from 0.5°C in 1997 to 0.9°C in
1999. In 1994, 1995, and 2003, T-min partially outcropped,
but DT did not increase in winter, possibly because wintercooling was not as strong. The DT during the years with
infrequent outcropping was small from 1994 to 1997 andFigure 4. As in Figure 2, but based on the PX38 XBT line
observations. Before averaging, some observations wereomitted so that the along-track resolution was nearly
uniform. Sea surface temperature anomalies (SSTA) wereaveraged in the areas of (a) 52°– 60°N, 144°– 153°W and(b) Region B’.

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2004 to 2005 and relatively large from 2000 to 2003. ThisDT variation corresponded to the winter SSTA variation in
the upstream (Region B’), as suggested by the Argo data
analysis. Lag correlation coefficients between winter SSTA
in Region B’ and winter DT in Region B for the infrequentoutcropping years (1994– 1997, 2000– 2005) were 0.54,
0.76, and 0.25 with time lags of 0, 1, and 2 years,
respectively. The highest correlation with a 1-year lag,which was significant at the 95% confidence level, suggeststhat the advection time of the T-min water from Region B’
to line PX38 by the North Pacific Current was roughly 1
year. This is consistent with the result of the numericalparticle-tracking experiment performed by Ueno and
Yasuda [2000].
[15] Differences in winter SSTA in Region B’ and DT in
Region B between 1994 and 1997 and 2000 and 2003 may
be associated with the 1998 – 1999 climate regime shift over
the North Pacific [e.g., Minobe, 2002]. The DT difference
may also have been affected by the intensification of theeastward North Pacific Current during 1996 and 1997, as
determined by analysis of PX38 XBT data and output of an
ECCO model [Douglass et al., 2006]. The intensified North
Pacific Current may have transported the T-min water withless vertical diffusion from Region B’ to Region B, thusincreasing DT in Region B.

4. Conclusions[16] We studied the formation and variation of T-inversions in the eastern SNP through analyses of Argo profiling
float data for 2001– 2005 and PX38 XBT data for 1993 –2005. Both analyses indicated that formation and variationdiffered between the northern and southern regions of theeastern SNP (Regions A and B, respectively). In Region A,
T-min outcropped and was cooled nearly every winter. As aresult, DT had a significant maximum in winter, and thewinter DT tended to be large when winter SSTA in theregion was low. In Region B, the winter T-min outcropped
relatively infrequently, and DT did not have a significant
maximum in winter, except in 1998 and 1999. We suggestthat during 1994 – 1997 and 2000 – 2005, T-min in Region B
was formed in the upstream region near the date line andthen advected to Region B in roughly 1 year. During 1998
and 1999, T-min outcropped and was cooled in winterbecause of exceptionally strong surface cooling, resulting
in a significant increase in DT.
[17] 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 Observ-

L05603ing System. The authors thank S. Masuda for helpful comments and fruitful
discussions, and also thank two anonymous reviewers for helpful andconstructive comments.

References
Argo Science Team (2001), The global array of profiling floats, in Observing the Oceans in the 21st Century, edited by C. J. Koblinsky and N. R.
Smith, pp. 248 – 258, Bur. of Meteorol. Res. Cent., Melbourne, Victoria,
Australia.
Dodimead, A. J., F. Favorite, and T. Hirano (1963), Salmon of the North
Pacific Ocean. part II: Review of oceanography of the subarctic Pacificregion, Bull. 13, 195 pp., Int. N. Fish. Pac. Comm., Vancouver, B.C.,
Canada.
Douglass, E., D. Roemmich, and D. Stammer (2006), Interannual variability in northeast Pacific circulation, J. Geophys. Res., 111, C04001,
doi:10.1029/2005JC003015.
Favorite, F., A. J. Dodimead, and K. Nasu (1976), Oceanography of thesubarctic Pacific region, 1960 – 71, Bull. 33, 187 pp., Int. N. Fish. Pac.
Comm., Vancouver, B.C., Canada.
Freeland, H. J., and P. F. Cummins (2005), Argo: A new tool for environmental monitoring and assessment of the world’s oceans, an example
from the N. E. Pacific, Prog. Oceanogr., 64, 31 – 44.
Kistler, R., et al. (2001), The NCEP-NCAR 50-year reanalysis: Monthly
means CD-ROM and documentation, Bull. Am. Meteorol. Soc., 82, 247 –268.
Minobe, S. (2002), Interannual to interdecadal changes in the Bering Sea
and concurrent 1998/99 changes over the North Pacific, Prog. Oceanogr.,
55, 45 – 64.
Oka, E., L. D. Talley, and T. Suga (2007), Temporal variability of wintermixed layer in the mid- to high-latitude North Pacific, J. Oceanogr., 63,
293 – 307.
Reynolds, R. W., N. A. Rayner, T. M. Smith, D. C. Stokes, and W. Wang
(2002), An improved in situ and satellite SST analysis for climate,
J. Clim., 15, 1609 – 1625.
Roden, G. I. (1964), Shallow temperature inversion in the Pacific Ocean,
J. Geophys. Res., 69, 2899 – 2914.
Suga, T., K. Motoki, Y. Aoki, and A. M. Macdonald (2004), The North
Pacific climatology of winter mixed layer and mode waters, J. Phys.
Oceanogr., 34, 3 – 22.
Uda, M. (1963), Oceanography of the subarctic Pacific Ocean, J. Fish. Res.
Board Can., 20, 119 – 179.
Ueno, H., and I. Yasuda (2000), Distribution and formation of the mesothermal structure (temperature inversions) in the North Pacific subarctic region, J. Geophys. Res., 105, 16,885 – 16,898.
Ueno, H., and I. Yasuda (2005), Temperature inversions in the subarctic
North Pacific, J. Phys. Oceanogr., 35, 2444 – 2456.
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.
E. Oka, Ocean Research Institute, University of Tokyo, Tokyo 164-8639,
Japan.
H. Onishi, Division of Marine Bioresource and Environmental Science,
Graduate School of Fisheries Science, Hokkaido University, 3-1-1 Minatocho, Hakodate 041-8611, Japan.
D. Roemmich, Scripps Institution of Oceanography, 9500 Gilman Drive,
Department 0230, La Jolla, CA 92093, USA.
T. Suga, Department of Geophysics, Graduate School of Science, Tohoku
University, Aoba-ku, Sendai 980-8578, Japan.
H. 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. (uenohiro@jamstec.go.jp)

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