Climate variability in the North Pacific thermocline

diagnosed from oxygen measurements: An update based

on the U.S. CLIVAR/CO2 Repeat Hydrography cruises

Sabine Mecking,1 Chris Langdon,2 Richard A. Feely,3 Christopher L. Sabine,3

Curtis A. Deutsch,4 and Dong-Ha Min5

Received 1 September 2007; revised 12 April 2008; accepted 21 May 2008; published 13 August 2008.

[1] New observations of oxygen variability in the North Pacific Ocean are reported onthe basis of comparison of the U.S. Climate Variability and Predictability and Carbon(CLIVAR/CO2) Repeat Hydrography sections conducted along 30�N (2004) and 152�W(2006) with the earlier World Ocean Circulation Experiment (WOCE) data and othercruises along these sections. The largest changes in apparent oxygen utilization (AOU)continue to occur, as found in earlier North Pacific repeat section analyses, within thethermocline on sq = 26.6 kg m�3, which is the densest isopycnal to outcrop in the openNorth Pacific in climatological data. In the northeastern North Pacific along 152�W,where a total of five cruises (1980, 1984, 1991, 1997, and 2006) spanning a period of26 years are available, the AOU changes correspond to an overall increase in AOU onsq = 26.6 kg m�3 from the 1980s/early 1990s to 2006. However, from 1997 to 2006 adecrease in AOU is observed within the boundary region between the subtropical andsubpolar gyres at 40�–45�N. Along the center axis of the subtropical gyre at 30�N,where two cruises are available (1994 and 2004), AOU has also substantially increased onsq = 26.6 kg m�3 from 1994 to 2004 in the eastern part of the section. The repeat sectiondata along 152�W and 30�N are consistent with a pattern of decadal-scale ventilationanomalies that originate in the northwestern Pacific, possibly through variability(including cessation) of the sq = 26.6 kg m�3 outcrop, travel eastward along thesubtropical-subpolar gyre boundary, and enter the northern portion of the subtropical gyrealong the way. For the 152�WAOU data within the gyre boundary region (40�–45�N),good agreement exists with the close-by time series data from Ocean Station P (50�N,145�W) where a bidecadal cycle in AOU has been observed. In contrast, a sensiblecorrelation with the Pacific Decadal Oscillation could not be found.

Citation: Mecking, S., C. Langdon, R. A. Feely, C. L. Sabine, C. A. Deutsch, and D.-H. Min (2008), Climate variability in the North

Pacific thermocline diagnosed from oxygen measurements: An update based on the U.S. CLIVAR/CO2 Repeat Hydrography cruises,

Global Biogeochem. Cycles, 22, GB3015, doi:10.1029/2007GB003101.

1. Introduction

[2] Among the objectives of the Climate Variability andPredictability and Carbon (CLIVAR/CO2) Repeat Hydrog-raphy Program (see http://ushydro.ucsd.edu) was to deter-mine the large-scale variability of biogeochemical tracers as

well as of pathways of ocean ventilation. Oxygen (O2)measurements are particularly useful in addressing theseobjectives because a large historical record of high-qualityO2 data exists that exceeds that of any other biogeochemicaltracer and because of the close relation of oxygen concen-trations to other biogeochemical properties through stochio-metric ratios [Redfield et al., 1963; Anderson and Sarmiento,1994]. The purpose of this paper is to examine O2 changesalong the two North Pacific U.S. CLIVAR/CO2 sections, P2and P16N (Figure 1), conducted in 2004 and 2006, respec-tively, and to compare them with earlier observations of O2

variability in the North Pacific Ocean.[3] To account for variations in the O2 equilibrium con-

centrations with the atmosphere (O2, equil), O2 concentrationsare often converted to apparent oxygen utilization (AOU),which is defined as AOU = O2, equil(q, S) � O2, meas, whereO2, meas are the measured O2 concentrations in the oceanand q and S are potential temperature and salinity. In the

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1Applied Physics Laboratory, University of Washington, Seattle,Washington, USA.

2Rosenstiel School of Marine and Atmospheric Sciences, University ofMiami, Miami, Florida, USA.

3PacificMarine Environmental Laboratory, NOAA, Seattle,Washington,USA.

4Department of Atmospheric and Oceanic Sciences, University ofCalifornia, Los Angeles, California, USA.

5Marine Science Institute, University of Texas at Austin, Port Aransas,Texas, USA.

Copyright 2008 by the American Geophysical Union.0886-6236/08/2007GB003101$12.00

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subsurface ocean, below the mixed layer and the euphoticzone, AOU gives a measure of how much O2 has beenconsumed because of respiration since a water parcel has leftthe surface layer. Temporal variations in AOU can occur inthe subsurface ocean because of changes in respiration rates(biological effects) or changes in the age of a water parcel(physical effects) which changes the time the water parcelhas been exposed to respiring organisms. Since temporalvariations in O2, equil are usually small when AOU varia-tions are examined on isopycnal surfaces (where variationsin q and S are comparatively small), temporal changes inAOU are about equivalent to O2 changes of equal size butopposite in sign.[4] Several papers have examined temporal AOU (or O2)

variability in the North Pacific over the past several years.Investigations of repeated hydrographic sections [Watanabeet al., 2001; Emerson et al., 2001, 2004; Kumamoto et al.2004] (see Figure 1) have shown that there has been anincrease in AOU in the subpolar gyre and the northern

subtropical gyre from the 1980s/early 1990s to the late1990s/early 2000s. Modeling studies [Deutsch et al., 2005,2006] as well as examination of chlorofluorocarbon (CFC)age data in conjunction with AOU data [Watanabe et al.,2001; Mecking et al., 2006] suggest that most of thesechanges are due to physical processes rather than changes inbiology. Since the observed AOU differences consistently arethe largest near the same density surface, sq = 26.6 kg m�3,it has also been suggested that variations in the outcropping ofthis specific isopycnal (which is the densest isopycnal tooutcrop in the open North Pacific in climatological data; seegreen line in Figure 1 for outcrop location)may be contributingto the observed AOU increase [Emerson et al., 2004;Meckinget al., 2006] in addition to a possible reduction in verticalmixing [Ono et al., 2001] and an increase in the inflow ofAlaskan gyre waters into the western subarctic [Andreev andKusakabe, 2001].[5] In the following, we find that sq = 26.6 kg m�3 stands

out again in the comparisons of the new CLIVAR/CO2

Figure 1. Location of CLIVAR/CO2 Repeat Hydrography cruises in the North Pacific (black circles)together with previous observations of AOU (or O2) changes based on reoccupations of hydrographiccruises (small gray symbols) and on compilations of longer-term time series records (large gray symbols).The late winter outcrop of sq = 26.6 kg m�3 (green line), annually averaged acceleration potential[Montgomery, 1937; Reid, 1965] contours on sq = 26.6 kg m�3 relative to 1500 m (streamlines; bluelines), and annually averaged apparent oxygen utilization (AOU) on sq = 26.6 kg m�3 (color shading) arealso shown. The grid spacing of the map projection is 15� and 30� of latitude and longitude, respectively.The outcrop, streamline, and AOU maps were calculated using the World Ocean Atlas 1998 climatology[Antonov et al., 1998; Boyer et al., 1998; O’Brien et al., 1998]. Acceleration potential and AOU data tothe west/northwest of the sq = 26.6 kg m�3 outcrop in late winter are omitted.

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sections to earlier data and that there is an indication thatthe AOU signal has started to reverse from an AOU increaseto a AOU decrease in recent years. This change in signresembles the bidecadal cycles in AOU observed in longer-term time series data in the subarctic North Pacific [Andreevand Kusakabe, 2001; Ono et al., 2001; Kumamoto et al.,2004; Whitney et al., 2007] (see Figure 1), although theforcing mechanisms for these cycles still need to be ex-plored further.

2. Data and Methods

[6] Two Repeat Hydrography cruises, P2 and P16N (dataavailable at http://cchdo.ucsd.edu/pacific.htm), were con-ducted in the North Pacific as part of the U.S. CLIVAR/CO2 program (Figure 1). CLIVAR/CO2 P2 (15 June to 27August 2004) repeated the World Ocean Circulation Exper-iment (WOCE) section P2 along a nominal latitude of 30�N,which is at about the center of the subtropical gyre asindicated by climatological streamlines (blue contour linesin Figure 1). WOCE P2 consisted of four different cruisesconducted by Japanese scientists of which WOCE P2T

(7 January to 10 February 1994) was designed to obtainnutrient and chemical data in conjunction with conductivity-temperature-depth data at 59 stations covering the entirewidth of the Pacific Ocean. The WOCE P2T data are theones used here for comparison with CLIVAR/CO2 P2. Sincefor WOCE P2T only the measured sum of nitrate and nitriteconcentrations is reported to the CLIVAR and CarbonHydrographic Data Office (CCHDO), nitrate concentrationsin the subsurface ocean are approximated by the sum ofnitrate and nitrite for this cruise. This is justified by the factthat nitrite concentrations, away from denitrificationregions, are usually negligible below a nitrite spike at75–125 m [Vaccaro, 1965].[7] The northern leg of CLIVAR/CO2 P16N (10–

30 March 2006), which we focus on in this paper, followeda nominal longitude of 152�W in the eastern North Pacificbetween Hawaii (17�–23�N) and Kodiak (56�N) andcrossed through the North Pacific subtropical and subpolargyres (Figure 1). The streamlines on sq = 26.6 kg m�3 inFigure 1 indicate that at 152�W the boundary regionbetween the subtropical and subpolar gyres extends from40� to 50�N with its center at about 45�N. This boundary

Figure 2. AOU differences along 152�W. AOU from CLIVAR/CO2 P16N (10–30 March 2006) minusAOU from (a) Fiona (11–29 August 1980), (b) Marathon II (5 May to 7 June 1984), (c) WOCE P16N(7 March to 8 April 1991), and (d) STUD97 (1–21 November 1997). The differences are calculated ondensity surfaces and then projected onto the average depth of the density surfaces (see text). The averageisopycnal depths are shown as red contours except for sq = 26.6 kg m�3, which is shown as purple. Thegreen dashed lines mark late winter mixed depths estimated from Antonov et al. [1998] and Boyer et al.[1998], and interpolated onto the cruise tracks using the average longitude of the two cruises involved.Gray shading marks areas where there are insufficient data for calculating AOU differences (see text).

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area, as shown in Figure 1, also corresponds to a meridionalminimum in AOU because water with low AOU concen-trations is carried from the isopycnal outcrops into the oceaninterior along the gyre boundary and AOU increases as thewater moves farther along streamlines into the ocean gyres.Data from four other cruises along the 152�W meridian(Fiona, 11–29 August 1980 (nominally 155�W); MarathonII, 5 May to 7 June 1984; WOCE P16N, 7 March to 8 April1991; and STUD97, 1–21 November 1997) will be used forcomparison with CLIVAR/CO2 P16N to the north ofHawaii, whereas Fiona and STUD97 only covered thesubtropical portion of the section to the south of 44� and45�N, respectively. AOU differences between these fourearlier sections have been analyzed in the past [Emerson etal., 2001, 2004; Mecking et al., 2006]. Comparison onisopycnal surfaces at depths of �2000 m (the deepest depthoccupied on STUD97) showed that systematic offsets inoxygen concentrations between these cruises are at most 1–2 mmol kg�1 [Emerson et al., 2001]. A similar comparisonof the CLIVAR/CO2 P2 and P16N sections with thecorresponding WOCE sections at depths �2000 m indicatesthat the deep oxygen data between the two programs agreewithin 1 mmol kg�1 or less.[8] In order to compare AOU data between cruises, the

AOU data are first objectively mapped on vertical sectionsfor each cruise following procedures by Roemmich [1983].To eliminate biases from the up and down movement ofisopycnals, the mapping is done in density space usingvertical correlation scales of 0.4 kg m�3 for sq � 26.0 kg m�3

and 0.3 kg m�3 for sq > 26.0 kg m�3. Horizontal correlationscales are set to 2� of latitude for the meridional sectionsalong 152�W. Larger horizontal correlation scales (4� oflongitude) are used for the mapping of the zonal sectionsalong 30�N because zonal gradients in AOU in the NorthPacific Ocean (on isopycnals) are much smaller than me-ridional AOU gradients which implies a larger horizontalcorrelation. AOU differences are calculated by subtractingthe maps for the earlier cruises from the maps for theCLIVAR/CO2 sections. The resulting differences are thenprojected onto the depth of the isopycnals (also objectivelymapped and averaged between the two cruises involved) so

that the AOU difference maps can be viewed in depth space(Figures 2 and 3) as illustrated, for instance, by Mecking etal. [2006, Figures 5 and 9]. Areas where there are insuffi-cient data for either cruise used for the differentiation aremarked by gray shading. Most prominently, there is a datagap for WOCE P16N between 48�200N and 52�300N thatwas caused by bad weather during the cruise. This producesthe gray vertical bar around 50�N in the CLIVAR/CO2

P16N minus WOCE P16N difference map (Figure 2c). Inaddition, because difference maps are calculated in densityspace, large gray areas occur at depths shallower than theclimatological winter mixed layer (dashed green lines inFigures 2 and 3) if an isopycnal exists during one cruise butnot the other because of the occupation of the cruises indifferent seasons of the year.[9] Differences are also not calculated where cruise tracks

deviate more than 3� from the CLIVAR/CO2 sections. Sucha deviation mainly occurs because the 152�W sections varyin their approach to Hawaii so that the AOU comparisonswith CLIVAR/CO2 P16N are terminated at 24�N for Fionaand Marathon II, at 21�N for WOCE P16N, and at 23.5�Nfor STUD97 (Figure 2). Also, while the nominal longitudeof the Fiona cruise is technically to the west of the other152�W cruises (by 3�), zonal AOU gradients within thebroad region of the subtropical gyre are small and much lessthan temporal variations observed in AOU [Emerson et al.,2001]. Hence, we treat the Fiona cruise for the purpose ofthis paper as being one of the ‘‘152�W sections.’’ ForWOCE P2T, the bottle oxygen data reported to the CCHDOare flagged as questionable or bad to the west of 147.5�Esuch that the AOU comparisons along 30�N are only doneto the east of this meridian (Figure 3).

3. Observed AOU Changes

[10] The AOU difference maps (calculated relative toCLIVAR/CO2) show that significant changes in AOU haveoccurred for both the 152�W and the 30�N sections. As inthe earlier North Pacific repeat section analyses [Watanabeet al., 2001; Emerson et al., 2001, 2004], the differences arethe most prominent at or near sq = 26.6 kg m�3 (purple

Figure 3. AOU differences along 30�N. AOU from CLIVAR/CO2 P2 (15 June to 27 August 2004)minus AOU from WOCE P2T (7 January to 10 February 1994). Details follow Figure 2.

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contours in Figures 2 and 3). From 1980 (Fiona) to 2006(CLIVAR/CO2 P16N), the AOU change along 152�Wcorresponds mostly to an increase with maximum values>35 mmol kg�1 just above sq = 26.6 kg m�3 at 39�N(Figure 2a). Differences within and at the base of theclimatological winter mixed layer (green dashed lines) canbe even larger, but since they are likely due to seasonaleffects, they are not considered here.[11] From 1984 (Marathon II) to 2006 (CLIVAR/CO2

P16N), AOU along 152�W has also mostly increased(Figure 2b) except for the vertical band of AOU decreaseat 25�N that probably results from shifts within the zonalcurrent system to the north of Hawaii [Emerson et al., 2001]and some reductions of AOU above the base of the wintermixed layer. Similar to the 1984 to 1997 comparisons byEmerson et al. [2001], the AOU increase relative toMarathon II (from 1984 to 2006) extends over a widerdensity range than for the Fiona (1980) section (Figure 2a)and also the WOCE P16N (1991) section (Figure 2c). Thisindicates that shallower parts of the permanent thermoclinemay also experience significant interannual AOU variations.Nevertheless, there is also a clear AOU difference between1984 and 2006 that is associated with sq = 26.6 kg m�3

(Figure 2b). At 43�–47�N, which is at the boundary between

the subtropical and subpolar gyres, an increase in AOU thatexceeds 20 mmol kg�1 occurs at sq = 26.6 kg m�3 andslightly below. This signal extends (with interruptions) to thesouth and to the north into the subtropical and subpolar gyres,respectively.[12] From 1991 (WOCE P16N) to 2006 (CLIVAR/CO2

P16N), changes in AOU along 152�W are still mostlypositive (i.e., an AOU increase with time) and still exceedvalues of 20 mmol kg�1 (Figure 2c), meaning that they areof similar magnitude as in the Fiona and Marathon IIcomparisons with CLIVAR/CO2 P16N. The AOU increaseis clearly centered around sq = 26.6 kg m�3, is the largestbetween 37� and 47�N, and extends from there southwardinto the subtropical regions where AOU differences aresmaller than for Fiona and Marathon II. A hint of AOUincrease also continues to be found in the subpolar gyre tothe north of the WOCE P16N data gap.[13] A reversal of the AOU signal is finally apparent from

1997 (STUD97) to 2006 (CLIVAR/CO2 P16N). Between40� and 45�N, the northern end of the STUD97 section,AOU on sq = 26.6 kg m�3 has decreased by as much as>10 mmol kg�1. Farther to the south, there is still anincrease in AOU on sq = 26.6 kg m�3 that is of the sameorder of magnitude, whereas on shallower isopycnals southof 35�N, AOU has also decreased by a small amount. Asdiscussed in section 2, the boundary between the subpolarand subtropical gyres at 152�W is centered at about 45�N.The onset of the AOU decrease on sq = 26.6 kg m�3 withina few degrees of the latitude of this location is consistentwith circulation patterns since the gyre boundary region isdirectly connected to the isopycnal outcrop (green line inFigure 1) through strong eastward flow within the NorthPacific Current (see blue streamline contours in Figure 1).Any ventilation anomaly carried in from the isopycnaloutcrop region would hence first arrive at 152�W near thegyre boundary. A time lag is then expected for the anomalyto reach the interior of the subpolar and the subtropicalgyres.[14] At 30�N, AOU differences between 1994 (WOCE

P2) and 2004 (CLIVAR/CO2 P2) show a maximum increaseon sq = 26.6 kg m�3 to the east of �160�W and are morevariable in the western part of the section (Figure 3).Decreases in AOU occur at shallower densities and depths(100–400 m), similar to the ones observed south of 35�N at152�W between 1997 and 2006 (Figure 2d). In contrast tothe increase in AOU in the eastern part of the 30�N section(Figure 3), no significant changes inAOUwere found on sq =26.6 kg m�3 in a zonal repeat section comparison thatwas done farther south at 24�N [Mecking et al., 2006] (seeFigure 1 for location of this section). Together, the 30� and24�N sections suggest that the AOU signal on sq = 26.6 kgm�3 that is carried in along the subtropical-subpolar gyreboundary, as seen in the 152�W sections (see above), isconfined to the eastern part of the subtropical gyre andreaches 30�N but not 24�N. The meridional AOU differencesections along 152�W support this conclusion since theAOU anomalies on sq = 26.6 kg m�3 coming from the northdo not extend southward of 25�–30�N (Figure 2). Thisdistribution pattern of the AOU anomalies is also found inmodeling studies [Deutsch et al., 2005, 2006] which show

Figure 4. AOU versus (a) salinity and (b) nitrate for the152�W sections (south of 45�N). Data are interpolated tosq = 26.6 kg m�3.

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two pathways of oxygen variability in the North Pacific: onethat originates from decadal variability in ventilation rates inthe northwestern Pacific and extends eastward into thenorthern portion of the subtropical gyre and another thatresults from decadal variability in the position of the south-eastern boundary of the subtropical gyre with the separationbetween the pathways occurring at 25�–30�N [see alsoSabine and Gruber, 2006]. The former, northern one ofthese pathways is the one that we are mostly concerned within this paper.

4. Comparison With Salinity, Nitrate, and OtherBiogeochemical Properties

[15] Comparison of AOUwith salinity on sq = 26.6 kgm�3

shows that the large AOU increases observed along 152�Wand 30�N between the 1980s/early 1990s and the late 1990s/2000s are not compensated by a change in salinity (Figures 4aand 5a). Along 152�W, this is indicated by AOU values beinglarger, at a fixed salinity, during STUD97 (1997) andCLIVAR/CO2 P16N (2006) compared to the 1980s/early1990s cruises for S < 34.04 (Figure 4a), which correspondsto the region to the north of �25�N. Along 30�N, AOUvalues during CLIVAR/CO2 P2 (2004) are larger than thosein the early 1990s (WOCE P2) at almost all salinities(Figure 5a). Along this section there have also been sub-stantial changes in salinity [Kouketsu et al., 2007], as shownby plots of salinity versus longitude on sq = 26.6 kg m�3

(Figure 6a). A freshening occurred from 1994 to 2004 to thewest of about 170�W, whereas an increase in salinity from1994 to 2004 is evident within the lateral salinity minimumat 130�–140�W (Figure 6a). The corresponding AOUchanges, on the other hand, are most pronounced betweenthese two regions at 140�–170�W (Figure 6b) wheresalinity has remained approximately constant (Figure 6a).This decoupling of the AOU variations from variability insalinity in the 30�N sections as well as in the 152�Wsections (>25�N) reconfirms that the AOU changes in thenorthern portion of the subtropical gyre are larger thancould be explained by a simple shift in gyre position[Emerson et al., 2001; Mecking et al., 2006].[16] Plots of AOU versus nitrate on sq = 26.6 kg m�3 at

152�W (Figure 4b) and 30�N (Figure 5b) show, in contrastto the AOU versus salinity plots (Figures 4a and 5a), thatthe approximately linear relationship between AOU andnitrate has remained constant over time. This suggests, asobserved by Emerson et al. [2001], that nitrate changes arelinked to AOU changes in stochiometric proportions[Redfield et al., 1963; Anderson and Sarmiento, 1994].On the basis of comparison of deep values, measurementsof other nutrients, namely, phosphate and silicate, appear tobe less consistent (within their signal-to-noise ratios) amongcruises than the oxygen and also the nitrate measurements.But temporal variations in phosphate and silicate concen-trations (not shown), as for nitrate, also occur concurrentlywith the AOU changes on sq = 26.6 kg m�3 (i.e., high/lowAOU concentrations are correlated with high/low nutrients).In the case of phosphate and nitrate, a correlation with AOUis expected because these nutrients are produced andoxygen is consumed during the remineralization of organic

matter and waters with high AOU (low oxygen) concen-trations usually have high nitrate and phosphate concen-trations. In the case of silicate, a connection to AOU is lessobvious because the dissolution of silicon-containing shellsexported from the surface ocean does not involve theconsumption of dissolved oxygen. However, since silicateis regenerated in the subsurface ocean just as the othernutrients (nitrate, phosphate) and the North Pacific Oceancontains very active silicate cycling [Sarmiento et al.,2004], it is plausible that changes in ventilation processesthat are the likely cause of the temporal variability in AOU[Deutsch et al., 2005, 2006] (see also section 5), nitrate, andphosphate observed in the North Pacific Ocean may causesimilar variations in silicate.[17] Dissolved inorganic carbon (DIC) and pH are also

linked to AOU during organic matter remineralization, butthese properties are also affected by the input of anthropo-genic carbon into the ocean. Observed changes in DIC andpH (not shown) between the WOCE and CLIVAR/CO2 P2and P16N sections (the other 152�W data sets do notcontain these carbon parameters) exceed those that can beexplained by the AOU changes alone. In this case, the AOUchanges can be used to divide temporal variations in DIC

Figure 5. AOU versus (a) salinity and (b) nitrate for the30�N sections covering the limited range where bottleoxygen data are available on WOCEP2 (147.5�E–121�W).Data are interpolated to sq = 26.6 kg m�3.

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and pH into an AOU-related ventilation component and intoan anthropogenic component [Sabine et al., 2008; R. Byrneet al., manuscript in preparation, 2008].

5. Discussion

[18] While interannual AOU variability also occurs onshallower density surfaces, sq = 26.6 kg m�3 stands out asthe isopycnal with the most persistent AOU changes whichoccur on decadal time scales and which have been measuredalong 152�W over a period of 26 years (1980–2006). Thisisopycnal lies between the potential vorticity minimumassociated with North Pacific Central Mode Water (sq =26.0–26.5 kg m�3) and the North Pacific IntermediateWater salinity minimum (sq = 26.8 kg m�3) but does notcorrespond to a distinct thermocline water mass in itself. Atime series of AOU anomaly on sq = 26.6 kg m�3,constructed by averaging the gridded AOU data for each152�W cruise between 40� and 45�N (44�N for the Fionacruise since is does not extend farther north) and thensubtracting the mean of the cruises, summarizes the evolu-tion of AOU in the subtropical-subpolar gyre boundaryregion, as discussed in section 3: lowest AOU values inthe 1980s and early 1990s, highest AOU values in 1997followed by above average AOU values in 2006 (Figure 7a,red symbols).

5.1. Comparison With Ocean Station P

[19] Multidecadal time series of AOU in the subpolarregions suggest that the AOU variations may occur on a�20-year cycle that is superimposed on a small increasingtrend in AOU [Andreev and Kusakabe, 2001; Ono et al.,2001; Kumamoto et al., 2004; Whitney et al., 2007]. Ofthese time series, Ocean Station P at 50�N, 145�W, whereoxygen data have been collected since 1956 [Whitney et al.,2007], lies in close proximity to our 152�W study region

Figure 6. (a) Salinity versus longitude and (b) AOUversus longitude for the 30�N sections. Data are interpolatedto sq = 26.6 kg m�3.

Figure 7. (a) AOU anomalies in the northeastern Pacificto sq = 26.6 kg m�3. The anomalies at Ocean Station P(blue crosses) present annual AOU averages that areinterpolated to sq = 26.6 kg m�3 [Kumamoto et al.,2004] and demeaned. The anomalies at 152�W (red pluses)were determined by averaging the gridded AOU data usedin Figure 2 to sq = 26.6 kg m�3 between 40� and 45�N(44�N for the Fiona cruise) and by subtracting the temporalmean. The green dashed line represents a linear fit to theAOU anomaly data at Ocean Station P. (b) Annuallyaveraged PDO index (gray line; data available at http://jisao.washington.edu/pdo). A Hanning filter of 10-yearwidth is applied to smooth the index data (black line). Redpluses mark the AOU anomaly at 40�–45�N, 152�Wshown in Figure 7a divided by 30 mmol kg�1. For bettercomparison, the data in Figure 7a are shown on the sametime scale as in Figure 7b, even though the Ocean Station Precord does not begin until the late 1950s.

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(Figure 1). Hence, annual mean AOU values calculated fromthe Ocean Station P oxygen data and interpolated to sq =26.6 kg m�3 for the time period 1956 to 2001 [Kumamotoet al., 2004] are shown in Figure 7a (blue crosses; alsoexpressed as anomalies, i.e., as difference from the meanfor the entire time period) for comparison with the 40�–45�N, 152�W time series (red pluses). There is a goodagreement between the two time series, indicating that the152�W data may also be part of the 15- to 20-year AOUcycles observed at Ocean Station P. While AOU at 152�Wis generally low in the 1980s/early 1990s, the slight increasein the AOU anomaly in 1984 hints at the high AOU phaseseen in the mid-1980s at Ocean Station P. The trend towardhigh AOU at 152�Win the late 1990s clearly corresponds to apeak in the AOU anomaly at Ocean Station P data in 2000–2001. The subsequent decrease in AOU at 152�W toward2006 is also evident in the newer Ocean Station P data(not shown) which indicates an increase in oxygen (areduction of AOU) from 2001 to 2006 within the sq =26.6 kg m�3 range [Whitney et al., 2007].[20] In addition to the decadal-scale variations, there is a

small linear increase in AOU (a decrease in O2) at OceanStation P over the whole record that has been estimated tobe on the order of 0.5 mmol kg�1 a�1 [Kumamoto et al.,2004; Whitney et al., 2007] (see also green dashed line inFigure 7a). Such a trend cannot be adequately inferred fromthe 152�W repeat data for comparison because of the shorterduration and sparser temporal spacing that the 152�WRepeat Hydrography data provide. The trend toward along-term increase in AOU at Ocean Station P is consistent,though, with a long-term increase in ventilation ages inthe subtropical and subpolar North Pacific that has beensuggested to occur under global warming conditions[Gnanadesikan et al., 2007].

5.2. Pacific Decadal Oscillation as Potential ForcingMechanism

[21] A potential forcing mechanism for the observeddecadal-scale AOU variations is the Pacific Decadal Oscil-lation (PDO) which is defined as the leading component ofNorth Pacific sea surface temperature (SST) variability[Mantua et al., 1997] and is also related to changes in thewind forcing of the oceanic gyre circulation [Deser et al.,1999]. However, the peak-to-peak period of the (filtered)PDO index (Figure 7b) which is on the order of 40–60 yearsis much greater than that of the bidecadal AOU cycles atOcean Station P (Figure 7a). A connection between ourAOU anomaly time series at 40�–45�N, 152�W (alsomarked as red symbols in Figure 7b) and the PDO is alsoquestionable. Statistically, a significant correlation (r > 0.9)exists at a lag of �15 years. But this would imply that thelow AOU values at 40�–45�N, 152�W in the 1980s/early1990s are the ocean’s lagged response to the negative PDOvalues that occur before the well-documented 1976–1977climate shift [e.g., Mantua et al., 1997] and that the highAOU values in the late 1990s/2000s correspond to thepositive values of the PDO after the 1976–1977 climateshift. Since the time before/after the climate shift is associ-ated with a slowdown/spin-up of the circulation due tochanges in the wind forcing [Deser et al., 1999], this lagged

correlation does not agree with a slow circulation, causingan increase in AOU and vice versa. Without time lag, aninverse correlation seems to exist between the negativeAOU anomaly in the 1980s and the positive PDO valuesat that time. However, during the ‘‘dip’’ of the PDO around1990, AOU values (1991 data point) remain low, and thenwhen the AOU anomaly becomes positive (1997 and 2006data points), the PDO is positive as well. Hence, an inversecorrelation between the PDO and the AOU anomalies is notconclusive either. It is also unclear why changes in windforcing would affect one isopycnal, sq = 26.6 kg m�3, morethan any other.

5.3. Changes in Biological Pump and Water MassDistributions as Potential Forcing Mechanisms

[22] Other possible mechanisms for causing AOUchanges that have been discussed in the literature [e.g.,Emerson et al., 2001, 2004] include changes in the biolog-ical pump and changes in isopycnal depths as well as watermass variability caused, e.g., by changes in the positions ofthe subtropical and subpolar gyres. The latter mechanismwas ruled out as a dominant mechanism because there havebeen substantial changes in the AOU versus salinity rela-tionships (Figures 4a and 5a) that would not be expected inthe case of a simple shift in gyre positions (see section 4).[23] In the case of the biological pump, one would expect

an increase in AOU if carbon export from the surface oceanand, subsequently, remineralization of organic matter in thesubsurface ocean were to be enhanced or if the stochio-metric ratios of oxygen consumption to nutrient and inor-ganic carbon production during remineralization [Andersonand Sarmiento, 1994] were to increase. However, sinceremineralization occurs throughout the water column (withan approximate exponential decrease in remineralizationrates with depth) [Martin et al., 1987], it seems unlikelythat changes in the biological pump, like changes in windforcing, would cause AOU changes that clearly follow oneisopycnal surface (sq = 26.6 kg m�3) as observed in theNorth Pacific. Also, the constancy of the AOU to nitrateratios (Figures 4b and 5b) suggests that the stochiometricratios have not substantially changed. Additional supportfor the biological pump not being the main source of theobserved AOU variability stems from changes in CFC agesthat have been found to occur concurrently with AOUchanges during earlier repeat section analyses [Watanabeet al., 2001; Mecking et al., 2006] and from modelingstudies [Deutsch et al., 2005, 2006], all of which pointtoward the dominance of physical processes. For the newCLIVAR/CO2 data, the calculation of ventilation ages fromCFCs has become more difficult because of the slowing/halting of atmospheric CFC increases since the 1990s.Hence, the comparison of ventilation ages from CLIVAR/CO2 P16N and P2 with earlier data will be subject of moreextensive future analyses of CFC ages.[24] Finally, an increase in the depth of sq = 26.6 kg m�3

due to an increase in the subducted volume of lighter watersand/or in stratification could increase AOU given that thebackground structure of AOU (as well as of ventilationages) increases with depth. On the other hand, a temporaryuplifting of an isopycnal could also increase AOU values

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because the isopycnal surface is moved into an area oflarger organic matter degradation [Emerson et al., 2004].Examination of the depth of sq = 26.6 kg m�3 in the 152�Wand 30�N sections (Figure 8) provides an inconclusivepicture: At 152�W between 40� and 45�N, where the largestAOU variations on sq = 26.6 kg m�3 are observed (Figures2 and 7), the ispoycnal is the deepest in 2006 (CLIVAR/CO2

P16N) and the shallowest in 1984 (Marathon II), as shownin Figure 8a. AOU values, on the other hand, are the largestin 1997 and at low levels for all the 1980s/early 1990scruises. Farther to the south, between 24� and 27�N, adistinct difference in isopycnal depth is also present with thedepth of sq = 26.6 kg m�3 being about 50–75 m deeper inboth 1984 and 2006 than during the other cruises. Incontrast, the largest difference in AOU at this location isbetween the 1984 (Marathon II) and 2006 (CLIVAR/CO2

P16N) cruises (Figure 2b). At 30�N, sq = 26.6 kg m�3 isdeeper in 2004 (CLIVAR/CO2 P2) than in 1994 (WOCEP2)between 150� and 170�W by as much as 50 m (Figure 8b).This region overlaps with the area of largest AOU increase(Figures 3 and 6b) but does not cover the entire area.

5.4. Changes in Ventilation as Potential ForcingMechanism

[25] Given the shortcomings of the mechanisms discussedin sections 5.2 and 5.3, we return to the suggestion thatchanges in ventilation are responsible for the observeddecadal-scale AOU variations (as implicitly assumed insections 3 and 4). More specifically, it has been proposed thatperiodic changes in the outcrop position of sq = 26.6 kg m�3

including a complete cessation of the outcropping of thisisopycnal (since it is the densest one to outcrop in the openNorth Pacific in climatological data) may play a moreimportant role than circulation changes for producingAOU variations in the North Pacific Ocean [Emerson etal., 2004; Mecking et al., 2006]. The consistency withwhich AOU anomalies in the North Pacific have been foundon sq = 26.6 kg m�3, as pointed out by Emerson et al.[2004] and reconfirmed by the CLIVAR/CO2 data presentedin this paper, indicate that there must be something distinctlydifferent about this isopycnal compared to others. The factthat sq = 26.6 kg m�3 is the densest isopycnal to outcrop inthe open North Pacific in late winter (see Figure 1) provides

an explanation why this isopycnal would be affected themost by the periodic ventilation mechanism.[26] The idea is that oxygen concentrations are reset to

their equilibrium concentrations when the 26.6 kg m�3

isopycnal outcrops in the northwestern Pacific. In contrast,if sq = 26.6 kg m�3 stops outcropping, perhaps for severalyears in a row, old waters with high AOU values (and highCFC ages) [Mecking et al., 2006] are recirculated within thesubtropical and subpolar gyres without this resetting of theboundary condition. As a result, large temporal AOU differ-ences occur in the northwestern Pacific that are then carriedeastward with the North Pacific Current and southward withthe subtropical gyre circulation toward the 152�Wand 30�Nsections discussed in this paper. During this part of theprocess, the influence of recirculated waters is obviouslyincreased, consistent with the notion that a greater influenceof older Alaskan gyre waters may be the reason for theperiodic increases in AOU concentrations observed in thewestern subarctic gyre [Andreev and Kusakabe, 2001] (seeFigure 1 for Andreev and Kusakabe’s study location). Asindicated by the constant AOU to nitrate relationships(Figures 4b and 5b), nutrients and other biogeochemicalproperties (see section 4) appear to be affected similarly toAOU by the periodic ventilation of the 26.6 kg m�3

isopycnal.[27] Most recently, Andreev and Baturina [2006] and

Whitney et al. [2007] suggested that the 18.6-year nodaltidal cycle due to lunar orbital fluctuations that maysubstantially alter mixing in the Kuril Straits and modulatethe PDO [Yasuda et al., 2006] could be the source of theoxygen (and AOU) cycles observed at Ocean Station Pbecause it has a similar period. Future oxygen observationsin the North Pacific will be needed to confirm the connec-tion between ventilation events and the tidal forcing cycle[Whitney et al., 2007]. Since variations in the tidal mixing inthe Kuril Straits affects sea surface density in the north-western Pacific [Yasuda et al., 2006], this mechanism wouldbe consistent with the idea that variations in the outcrop ofsq = 26.6 kg m�3 (see above) are causing the AOUvariability observed in the ocean interior.[28] Unfortunately, measurements of sea surface density

in the northwestern North Pacific remain sparse, particularlyin winter. Ono et al. [2001] constructed a time series of

Figure 8. Depth of sq = 26.6 kg m�3 for (a) the 152�W sections and (b) the 30�N sections.

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wintertime sea surface density in the Oyashio region offJapan which shows decadal variations in density, but datafarther to the north and northeast would be needed to fullyexplore interannual variability in the sq = 26.6 kg m�3

outcrop (see Figure 1 for location of the outcrop and of thestudy region by Ono et al.). Argo floats do not cover thenorthwestern North Pacific very well either, as indicated bythe lack of data in the region to the west of 165�W and tothe north of �42�N in analyses of mixed layer depth anddensity from Argo data [Ohno et al., 2004; Whitney et al.,2007]. However, measurement of ocean salinity through thelaunch of the Aquarius satellite within the next few years[Lagerloef, 2004] in combination with data from alreadyorbiting SST satellites will provide the opportunity to bettermonitor sea surface density. This will be particularly usefulin remote locations such as the northwestern North Pacificand will help to better identify the sources of AOUvariability in the ocean interior.

6. Conclusions

[29] The CLIVAR/CO2 measurements in the North Pacif-ic reveal AOU changes that remain predominant on sq =26.6 kg m�3 and include the onset of a reversal in the AOUsignal between 1997 and 2006 from AOU increase todecrease. Different from other oceans where decadal varia-tions in AOU have been observed within different kinds ofmode waters [McDonagh et al., 2005; Johnson and Gruber,2007], the changes in the North Pacific are not confined to aparticular water mass. Instead, they occur on or close to theisopycnal that is the densest to outcrop in the open NorthPacific according to climatological data (sq = 26.6 kg m�3).It is suggested that periodic changes in the outcrop locationof this isopycnal (including complete cessation of outcrop-ping) may be the reason why the decadal-scale AOUchanges are so closely confined to this isopycnal. Long-term monitoring of wintertime sea surface density in thenorthwestern North Pacific will be required to fully exploreinterannual and decadal variability in the ventilation of thisisopycnal as well as its forcing mechanisms.

[30] Acknowledgments. We would like to thank the captains, crew,and science parties of the North Pacific CLIVAR/CO2 cruises and of allother cruises shown for their hard work in collecting the data. YuichiroKumamoto kindly provided us with the annual averaged and interpolatedAOU data at Station P (bottle data available at http://www.pac.dfo-mpo.gc.ca/sci/osap/projects/linepdata/default_e.htm). Constructive com-ments by two anonymous reviewers and the Associate Editor are greatlyappreciated. We would also like to thank the funding agencies, NOAA andNSF, for their continuous support of the CLIVAR/CO2 Repeat HydrographyProgram. This publication is partially funded by the Joint Institute of theAtmosphere and Ocean (JISAO) under NOAA Cooperative agreementNA17RJ1232, contribution 1428. This is NOAA/PMEL contribution3093 and the University of Texas Marine Science Institute contribution1469.

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�������������������������C. A. Deutsch, Department of Atmospheric and Oceanic Sciences,

University of California, 405 Hilgard Avenue, Los Angeles, CA 90095,USA. (cdeutsch@atmos.ucla.edu)R. A. Feely and C. L. Sabine, Pacific Marine Environmental Laboratory,

NOAA, 7600 Sand Point Way NE, Seattle, WA 98115, USA. (richard.a.feely@noaa.gov; Chris.Sabine@noaa.gov)C. Langdon, Rosenstiel School of Marine and Atmospheric Sciences,

University of Miami, 4600 Rickenbacker Causeway, Miami, FL 33149,USA. (clangdon@rsmas.miami.edu)S. Mecking, Applied Physics Laboratory, University ofWashington, 1013

NE 40th Street, Seattle, WA 98105, USA. (smecking@apl.washington.edu)D.-H. Min, Marine Science Institute, University of Texas at Austin, 750

Channel View Drive, Port Aransas, TX 78373, USA. (dongha@mail.utexas.edu)

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