Observations of Decadal Time Scale Salinity Changes in the Subtropical Thermocline of the North Pacific Ocean. Li Ren 1, 2 and Stephen C.

Transcription

1 Observations of Decadal Time Scale Salinity Changes in the Subtropical Thermocline of the North Pacific Ocean Li Ren 1, 2 and Stephen C. Riser 1 1 School of Oceanography, University of Washington, Seattle, WA USA riser@ocean.washington.edu 2 Now at: Dept. of Oceanography, Florida State University, Tallahassee, FL USA DSR2 paper DSR2-D R1 [Revised submission to Deep-Sea Research II, special Suginohara issue, 13 July 2009]

2 ABSTRACT Data from Argo floats indicate that significant salinity changes have occurred in the North Pacific thermocline relative to data collected in the previous two decades, including observations obtained as part of the WOCE hydrographic program. Such a salinity decrease on both isopyncals and isobars implies a freshening scenario in the nearsurface source region of this water mass. The frequently repeated meridional section P16 supports this inference. The subsurface salinity freshening likely began in the early 1990s, strengthened through 1997, and continued into the 2000s; the surface salinity freshening had commenced by 1984 and continued through the first decade of the 21st century. The spatial distribution of salinity change on the density surface σ θ = 25.5 is examined through comparisons of Argo and most of the North Pacific WOCE sections ( ) and between Argo and the Hydrobase climatology, largely composed of data from the late 1970s through the mid-1980s. Both comparisons show a large-scale, basinwide decrease in subsurface salinity through the Argo time period used in this analysis ( ). The salinity difference is maximum in the northeast area and spreads southward and westward, approximately following geostrophic streamlines. Keywords: Argo floats; Main thermocline; North Pacific; Subduction; Outcrop; Mixed layer; Hydrological cycle 2

3 1. INTRODUCTION Water properties in the ocean exhibit variability on many time scales. Below the winter mixed layer, inter-annual, decadal, or even longer time scale variability can predominate. Decadal-scale changes in subsurface Pacific water properties have been previously studied using hydrographic section data (Mecking et al. 2005, Emerson et al. 2004, Emerson et al. 2001, Wong et al. 2001; Mecking 2000, Johnson and Orsi 1997; Bindoff and McDougall 1994), mainly for the period between the 1960s and 1980s/1990s. In this paper we examine decadal-scale water property changes in the North Pacific main thermocline of the subtropical North Pacific Ocean that were likely remotely forced near the sea surface. Such changes have also been found in earlier studies: Deser et al. (1996) first observed the subduction of temperature anomalies in the North Pacific, and there are numerous subsequent studies (Zhang and Levitus 1997; Inui and Hanawa 1997; Yasuda and Hanawa 1997; Miller et al. 1998; Tourret et al. 1999: Schneider et al. 1999) supporting the downward and equatorward movement of such anomalies in the central North Pacific. However, similar studies of salinity, an equally-important component of the ocean circulation indicative of freshwater transport, are lacking. Relatively low surface salinities at higher latitudes in the N. Pacific prevent deepwater formation (Warren 1983), making the N. Pacific deep water much older than analogous water in the North Atlantic. In addition, because of the fresh surface layer, a strong stratification exists in the North Pacific that leads to a shallower wind-driven circulation than in the N. Atlantic (Talley 1985; Huang and Russell 1994). The distribution of salinity and its changes can provide valuable information about the hydrological cycle. Yet due to limited high-quality 3

4 observations, changes in salinity are generally poorly documented compared to temperature change. Lukas (2001) has reported that the main thermocline (25.0 σ θ 26.0) near the Hawaii Ocean Time Series (HOT) station (22 45 N, 158 W) freshened from 1988 to 2001 and suggested that this trend could be due to anomalous subduction. This result provides additional motivation for our larger scale study using Argo data. Since the Argo project began in 2000 more than 3000 profiling floats have been deployed in the world ocean, with a nominal spatial resolution of 300 km. Once a profiling float is deployed, it descends to its parking depth (1000 m) and drifts there for approximately 10 days, descends to 2000 m, and then collects profiles of temperature and salinity as functions of pressure as it ascends to the sea surface. The thousands of temperature and salinity profiles from Argo floats provide a valuable source of data for studying upper ocean water properties. Another data source used in this paper consists of shipboard CTD profiles collected during the World Ocean Circulation Experiment (WOCE). The aim of the WOCE program was to obtain a baseline dataset against which future changes in ocean properties could be assessed. Here, the WOCE hydrographic shipboard CTD dataset is compared with Argo float data to study decadal-scale water mass changes in the N. Pacific, focusing on salinity changes in the thermocline. Because of the high spatial resolution of the Argo float deployments, we are able to compare Argo data to most of the available WOCE sections in the North Pacific in order to examine any large-scale spatial pattern of change in the intervening decades. In what follows, the data and methods are described in Sections 2 and 3, with analysis of data along a zonal section and several repeats of a meridional section given in Section 4. The large-scale spatial pattern 4

5 of salinity changes in the N. Pacific is presented in Section 5, with an analysis of the causes of source region changes and a summary presented in Section DATA a. WOCE data WOCE shipboard CTD data in the North Pacific spans the time period from 1984 to The WOCE lines used for comparison in this paper are P1 (carried out in 1985), P2 (1994), P3 (1985), P4 (1989), P8 (1996), P9 (1994), P10 (1993), P13 (1992), P14n (1993), P15 (1994), P16 (1991), P17n (1993), P17c (1991), P18 (1994) and P19 (1993), as shown in figure 1. This dataset is provided by the CLIVAR and Carbon and Hydrographic Data Office (CHHDO; The accuracies of temperature and salinity in WOCE are taken to be ±0.002 C and ± Practical Salinity Units (PSS78) (Saunders et al. 1991). As it is standard to express salinity without units, the PSS78 notation is omitted hereafter. While in most cases these sections were carried out on only one occasion, there are a few cases with multiple occupations. One of the most frequently repeated CTD sections in the North Pacific occurs along longitude 152 W (figure 1). This section was occupied in 1984 (the Marathon cruise), 1994 (WOCE), and 1997 (a University of Washington student cruise). WOCE line P3 along latitude 24 N, also used in this analysis, was first occupied in 1985 and was partly repeated in 2000 during another University of Washington student cruise. 5

6 b. Argo profiling float data By 2003, Argo float coverage of the N. Pacific was beginning to be adequate for scientific studies, with 300 km spatial resolution achieved over much of the basin. The most recent data used in the calculations in this paper are from December The entire dataset used herein was downloaded from the US Argo Global Data Center ( In this paper we have used raw Argo profile data, since in most cases the high quality delayed mode data were not yet consistently available. While the WOCE shipboard CTD measurements were carefully calibrated by comparisons with concomitant bottle data, there are no accompanying bottle data available for the calibration of most of the float data. Thus, the use of raw, rather than delayed-mode data in our analysis, requires some check on the quality of the raw salinity observations. The usual form of float conductivity (i.e., salinity) sensor drift for SeaBird CTD units (the type used on nearly all floats in Argo) is a uniform offset throughout the whole water column (if and when drift occurs), which in principle can be detected at any depth. In the N. Pacific, ventilation is shallow and the time scale of change in the deep ocean is expected to be much longer than decadal. Thus, comparison with WOCE CTD data at unventilated levels can be used to filter the Argo salinity data. We define such a level, where we compare the Argo and WOCE data in order to filter out any suspect Argo data, as the reference level. Note that the quality of raw Argo salinity data are generally quite good, with errors on the order of 0.01 or less, and typically no more than 10% of floatmeasured salinities require any adjustment, as shown by Riser et al. (2008). Here, in 6

7 order to use the raw Argo float salinity data, we will use the following process to quantify the accuracy of Argo float data. In the editing process, use of a potential temperature as an independent variable is often more appropriate than isobars and isopycnals, because calculations in isobaric coordinates can produce an anomalous anomaly (Lozier et al. 1994), and the uncertainty in salinity can compromise the density calculation. However, due to the large area of the North Pacific, it is impossible to use one reference level throughout the whole domain. Thus, based on an examination of temperature and salinity profiles in the North Pacific, south of 32 N the 3 C potential temperature surface is selected as the reference level. North of 32 N, 3 C is not appropriate because there is obvious decadal or even shorter time scale variability on this surface. In addition, a temperature inversion often exists in this area. Therefore, north of 32 N, σ θ =27.6 is selected as the reference level. In the area north of 32 N, at about 1700 dbar (σ θ =27.6), with potential temperature 2 C and salinity 34.54, a salinity error of 0.02 will only cause a kg/m 3 uncertainty in potential density. Thus, the use of the potential density σ θ =27.6 rather than a potential temperature surface as the reference level north of 32 N is appropriate. Both of these levels were chosen due to the fact that salinity was found to have a minimum in spatial and temporal decadal-scale variability on these surfaces. In the next step in our quality control process, the area from 0 to 60 N and from 120 E to 80 W was divided into 5 longitude by 3 latitude boxes. In each box, the median salinity of the WOCE CTD data at the reference level was calculated and compared with the salinity of each Argo float profile in the same box at the reference 7

8 level. Any Argo float profile with salinity differing from the median WOCE salinity by more than 0.02 was then discarded. Using this procedure, it was found that between almost 90% of the Argo profile salinities differed from the WOCE measurements by an amount no greater than 0.02 in deep water; this value was chosen as the criterion for eliminating suspect data. In the frontal region, where the temperature-salinity relation switches from a subtropical to subpolar character, the variation of WOCE salinities at the reference depth was no more than Thus, the use of the 0.02 criteria does not eliminate good Argo profile data in this region. c. Hydrobase The Hydrobase climatology in North Pacific (Curry et al. 1997, Macdonald et al. 2001) was also used in this work. The main source of data for this climatology in the North Pacific is WOD01 (World Ocean Database 2001; Conkright et al., 2001). WOD01 spans the time period from 1948 to 2001 and includes shipboard bottle casts and CTD profiles, two types of bathythermograph profiles, moored and drifting buoy data, and data from profiling floats. The mean year of collection for the shipboard bottle and CTD data (we include observations which have both temperature and salinity measurements) is 1978, with an 11.7 yr standard deviation. Therefore we take this climatology to be representative of the time from 1966 to 1990, centered on the late 1970s and early 1980s. 8

9 3. METHODS 3.1 Comparison of WOCE Sections and Argo Data In the vertical dimension, the WOCE shipboard CTD data have a 2 m vertical resolution, while data from Argo floats profiles have considerably larger vertical spacing. Thus, in order to avoid bias due to the different sampling resolution, the WOCE CTD data were sub-sampled at levels similar to the Argo float data. First, for each WOCE station, we select the Argo float profiles within an enclosed area of an ellipse whose radius is 2.5 in longitude and 1.5 in latitude centered on the WOCE station; in most cases this method yields more than 100 Argo profiles that correspond to each WOCE station. Next, the WOCE cast is sub-sampled according to each selected Argo profile. In the next step, the Argo profiles and the subsampled WOCE data are labeled by potential density and interpolated to a common potential density level. Thus, for each selected Argo profile, we have a corresponding subsampled WOCE profile (since WOCE shipboard CTD data have 2 decibar vertical resolution, the maximum pressure difference between Argo data and the WOCE subsample is ±1 dbar). The subsampled WOCE profile is then subtracted from the Argo profile on each density surface. In the horizontal direction, the differences between each Argo profile and the corresponding WOCE station are then mapped onto the WOCE line coordinates (latitude and longitude) at each common density surface using an objective mapping method. The objective mapping method is based on the Gauss-Markov theorem and gives an optimal pointwise estimate in the least squares sense (Bretherton et al. 1976; McIntosh 1990). The Gaussian covariance decorrelation length scales are specified as 5 in longitude and 3 in latitude. This anisotropic spatial scale reflects the predominantly zonal currents in 9

10 the ocean interior. We varied the length scales used in the mapping and found that the signal is not unreasonably sensitive to parameter values chosen. The length scales selected here are the smallest that show the signal and also result in minimum gaps in the map. The use of larger length scales makes the maps smoother but does not qualitatively influence the nature of the signal. To obtain temperature and salinity differences on pressure surfaces, we applied the same sub-sampling steps as above. Differences on pressure surfaces are directly calculated between selected Argo profiles in the ellipse and the sub-sampled WOCE profiles on the same pressure surfaces. Objective mapping is then applied in order to map the differences onto the WOCE station locations. Boundary regions in each section are excluded because the mapping procedure at the boundaries produces larger errors. The widths of the excluded boundaries are different in each section and depend on the availability of Argo float profiles. To avoid seasonal noise in the upper layer we focus only on the water column below the winter outcrop (about 150 m). These methods are used in all the comparisons discussed below. 3.2 Comparisons on selected surfaces Two portions of the water column were selected to study the spatial pattern of the salinity changes. They are the winter mixed layer and the σ θ = 25.5 density surface. The depth of the bottom of the mixed layer is defined as the depth where potential density differs from the near-surface density (about 5-10 m depth for Argo) by σ θ units. For the Argo float data , we select all winter season (defined as January- March) profiles in the North Pacific. The salinity at σ θ = 25.5 is estimated by linear interpolation for each selected profile. The salinities on this surface are then objectively mapped onto a 2 by 2 grid from 120 E to 120 W and from 7 to 50 N. The salinity in 10

11 the winter mixed layer (i.e., above the defined lower limit of the mixed layer) is estimated in a similar fashion. For the Hydrobase climatology, the mixed layer salinity and the salinity on σ θ = 25.5 are estimated simply by calculating the mean value in the winter season (January-March) for each grid point. 3.3 Geostrophic Velocity The winter season absolute geostrophic streamlines in the North Pacific have been estimated following the method of Kwon and Riser (2005), using winter Argo float data from In their method, the absolute gesotrophic streamlines are estimated on pressure surfaces. Here, the σ θ = 25.5 absolute geostrophic streamlines were calculated by linear interpolation from nearby pressure surfaces. 4. SECTION ANALYSIS 4.1. Zonal section P3, along 24 N In this section we examine subsurface water mass property changes using WOCE sections and Argo data, beginning with differences along WOCE section P3. The temporal difference between P3, occupied in 1985, and a synthetic Argo line along the same latitudes is years. Roemmich et al. (1991) described in detail the large-scale features of the potential temperature, potential density, and salinity along this latitude. The most notable feature in salinity along 24 is the subsurface salinity minimum (figure 2a), corresponding to North Pacific Intermediate Water (NPIW; Talley 1993), located at potential densities between 26.6 and 27.0; this minimum in salinity typically lies roughly between 600 dbar and 900 dbar at 24 N. The NPIW layer characteristically has a salinity less than Below σ θ 25.0 ( at a depth of about 150 m), but above the NPIW, 11

12 salinity monotonically decreases with depth; in the horizontal direction, salinity declines from west to east (figure 2a). A high horizontal salinity gradient exists in the eastern portion of the section between 140 W and 130 W. The most striking feature of the salinity changes on density surfaces between WOCE and Argo (figure 2a) is the large-scale, basin wide salinity decrease in the density range 25.0 σ θ 26.2, with the signal stronger in the east than in the west. Potential density surfaces shallower than 25.0 show weak positive change west of the dateline (figure 2a), while east of the dateline negative changes occur from the winter outcrop to σ θ = Potential density surfaces deeper than 26.2, including the salinity minimum layer, do not show large differences (figure 2a). The potential temperature difference on density surfaces (not shown) has, by definition, a pattern similar to the salinity difference, with significant decreases in temperature in the density range 25.0 σ θ The salinity difference on isobars (figure 2b) is not as coherent as on density surfaces, but it is still obvious that negative signals dominate between 100 dbar and 500 dbar, corresponding to potential densities between 25.0 and Positive changes occur only west of 160 W, in very shallow layers. Deeper than 600 dbar, salinity shows no significant changes. Mesoscale features dominate the potential temperature difference on pressure surfaces (figure 2c). East of 160 E, negative differences in temperature predominate between 100 dbar and 500 dbar (figure 2c). We also compare the WOCE line with a University of Washington student cruise in 2000 that resampled the P3 section east of 152 W. In this region, salinity shows a significant decrease in 2000 relative to 1985 (figure 3), consistent with the comparison between Argo and WOCE line in figure 2(a). 12

13 We apply the methodology of Bindoff and McDougall (1994; hereafter BM94) to attempt to understand these water mass property changes, with the most important results of their analysis restated here. In their paper they considered all the possible θ (potential temperature) /S (salinity) slopes in the ocean in order to separate surface source region changes from subsurface changes. The stability ratio, defined as R =! "# z $ S z, is central to their analysis. Here θ z is the vertical gradient of potential temperature, S z is the vertical gradient of salinity, α is the thermal expansion coefficient, and β is the haline contraction coefficient. In the N. Pacific subtropics, between the ocean surface and the salinity minimum (NPIW), both salinity and temperature monotonically decrease with increasing depth or density. For such θ-s curves (with stability ratio 1 < R ρ < ), in the case of pure freshening the newly ventilated water is colder and fresher along density surfaces (figure 4(b)). Under pure warming at the formation region, the newly ventilated water is also cooler and fresher along density surfaces (figure 4(a)), with the opposite situation operative under pure cooling. In accordance with BM94 s method, we have estimated the mean zonal salinity and temperature differences between the WOCE line P3 and Argo on both density and pressure surfaces. The mean salinity difference on density surfaces (figure 5a) is negative in the range 25.0 σ θ At σ θ = 25.0, the magnitude of the average salinity change is 0.05, increasing in magnitude to 0.09 at about σ θ = 25.6, and decreasing in magnitude to zero near σ θ = Mean salinity changes larger in magnitude than 0.05 occur between 25.0 σ θ The mean temperature difference shows a similar pattern as salinity on density surfaces. The maximum mean temperature difference is about

14 C near about σ θ = 25.6 (figure 5(a); temperature and salinity are shown by a single curve, with separate scales). The mean salinity difference on pressure surfaces (figure 5b) is negative at pressures shallower than 600 dbar (with a maximum in magnitude around 200 dbar with a value of 0.06). The mean potential temperature difference on pressure surfaces (figure 5c) is negative in the upper 800 dbar, with a maximum magnitude of 0.3 C. As discussed above, for a stability ratio, 1 < R ρ <, these negative interior salinity changes on both isopyncals and isobars suggest the possibility of either a pure freshening or a pure warming process in the surface source region of the water mass (25.0 σ θ 26.0). The zonal mean temperature change is also negative both on isopycnals (figure 5a) and on isobars (figure 5c). Taken together, at the values of temperature, salinity, and pressure on the σ θ = 25.6 surface, a temperature change of 0.3 C would result in a 0.05 increase of the potential density, while a salinity change of 0.09 would cause the potential density to decrease by Thus, the water in the surface source region became both fresher and cooler, with the freshening more significant. In conjunction with this result, the zonally averaged θ-s curve along 24 N (figure 6) is shifted to lower salinities in the density range 25.0 σ θ 26.0 by , with no significant change in the core of the NPIW and below. There are other factors that could cause changes in these ventilated waters over time, such as changes in mixing along the path between outcrop and the observations, a path change between the outcrop and the observations, and a change in the location of the outcrops themselves; these factors will be discussed later in the paper. 14

15 These findings appear to complement previous studies. Wong et al. (2001) found that salinity decreased between γ n = 26.4 (where γ n denotes a surface of neutral stability; this surface is nearly coincident with the σ θ = 26.3 surface) and γ n = 27.4 (σ θ 27.2) via a comparison of historical data of the 1960s with WOCE line P3 in Those layers lie within the salinity minimum, deeper than our focus. Fukasawa et al. (2004) compared two occupations along 47 N (1985 and 1999) and observed that deep water (below 5000 m) warmed C with no significant change in salinity, while at shallower depths ( m), the salinity change on density surfaces was less than 0.003, suggesting that there were no significant water property changes. The freshening observed here in the potential density range is likely governed by a different mechanism (ventilation at the sea surface in winter, followed by subduction) and is decoupled from these deeper changes. Lukas (2001) also observed freshening in the density range 25.0 σ θ 26 at the HOT site from 1988 through Our results suggest that the freshening continued after 2001 and is part of a much larger scale spatial pattern. 4.2 Repeated meridional section P16, along 152 W The analysis using the 24 N zonal section was limited by the fact that it was reoccupied (partially) only once after WOCE. We now analyze results from the more frequently repeated meridional section P16 (figure 1). P16 was occupied at least partially in 1984, 1991 (WOCE), and 1997, prior to our synthetic Argo line in This section lies approximately along 152 W; here, we analyze data from the band of latitudes from N, the common spatial coverage for all the occupations. The section crosses the subtropical high salinity cell, located south of about 35 N (surface yellow area in figure 7a), with lower surface salinities at higher latitudes (the purple, near-surface region 15

16 in figure 7a) due to high net precipitation. Potential density surfaces in the range 25.0 σ θ 26.0 outcrop north of 32 N along 152 W, corresponding to this low salinity region. As the P16 line was occupied prior to the beginning of WOCE, we have used the Hydrobase salinity climatology along 152 W here (figure 7a) as the baseline; the values along the section as given in the Hydrobase climatology were subtracted from each later occupation of P16 (shown in figures 7b, c, d and e). Along the P16 line the mean year of data in the Hydrobase climatology is 1978, with a standard deviation of 11.7 years (note that Figure 7 only shows the differences below the winter surface outcrop, in order to minimize seasonal effects). In 1984, the main thermocline (25.5 σ θ 26.0) in the subtropics (south of 35 N) does not show large differences from Hydrobase, but north of this latitude salinity significantly decreased in this density range (figure 7b). By 1991 (figure 7c), a weak salinity decrease appeared between potential densities 25.5 and 26.0 south of 35 N. Salinity north of 35 N remained low, similar to the 1984 results. In 1997 (figure 7d), a strong salinity decrease appeared in the range 25.0 σ θ 26.0 in the subtropics, with salinities at higher latitudes consistent with the earlier occupations. In the synthetic section created from Argo profiles (figure 7e), freshening is seen throughout most of the 25.0 to 26.0 potential density range, except in the vicinity of 35 N. Our interpretation of these comparisons is that, south of 35 N, a weak salinity decrease in the main thermocline at potential densities between 25.0 and 26.0 occurred in 1991, intensified in 1997 and continued through This subsurface decrease in salinity likely resulted from a wintertime freshening near the sea surface in the outcrop region of 16

17 these density surfaces (north of 35 N) in the years prior to and including 1984, with these relatively low surface salinities continuing through The repeat occupations of the P16 section have been studied previously (Mecking et al. 2006; Emerson et al. 2004; Emerson et al. 2001; and Mecking 2001), although the focus in those studies was on density surfaces deeper than σ θ Those studies noted a dramatic change in Apparent Oxygen Utilization (AOU) and salinity below the 26.6 potential density surface, the deepest ventilated density surface in the ocean of N. Pacific; AOU became depleted and salinity increased in the 1990s relative to the 1980s, which suggested a possible slowdown of ventilation in this area. In the subtropics, Emerson et al. (2001) observed freshening in 1997 in the main thermocline (25.0 σ θ 26.0) south of 32 N compared with the previous occupations, consistent with our results. Between 32 N and 36 N, in the layers below σ θ 26.0, they found a salinity increase; our analysis shows a similar but weaker signal, possibly due to the use of different baselines. Mecking (2001), comparing WOCE P16n (1991) and a University of Washington student cruise (1997), focused on the deeper density surfaces and found that between potential densities of 26.4 and 26.7 the salinity increased over this time period, a feature that can be seen in figure 7d. 5. SPATIAL ANALYSIS The observed subsurface salinity differences in time along 24 N and 152 W suggest that we are observing basin-scale salinity changes. In order to capture the spatial pattern of the changes found in the subsurface main thermocline, we first compare most of the WOCE sections in the North Pacific with Argo float data. The WOCE lines were occupied from and thus predate the subsurface salinity changes that were 17

18 observed to occur in the late 1990s and early 2000s. The σ θ = 25.5 surface is selected as the focus since the changes reach a maximum near this isopycnal. Most of the lines show a salinity decrease at this density surface between the WOCE and Argo time periods (figure 8). The salinity decrease has a coherent basin-wide structure with maximum amplitude in the northeast and becomes weaker to the south and west, suggesting that the source of the signal is found in the northeast part of the basin where this density surface outcrops in winter. Salinity differences on other density surfaces in the range 25.0 σ θ 26.0 in the North Pacific show similar patterns. We have used the Hydrobase climatology to make a comparison to Argo at σ θ = 25.5 (figure 9) and find a similar spatial pattern consisting of a basin-wide decrease with maximum intensity in the northeast. As discussed earlier, most of the data in the Hydrobase climatology were collected from the early 1970s to the late 1980s. Although the WOCE data are included in the Hydrobase climatology, they only account for about 3% of the total number of profiles in the database. It is thus reasonable to view the comparison of Argo with WOCE and Hydrobase as two nearly independent analyses that lead to similar conclusions. The absolute geostrophic streamlines on σ θ = 25.5 (figure 9), computed as discussed by Kwon and Riser (2005), suggest that along this density surface water moves from the northeast toward south and west. The ventilation region for most of this isopycnal is in the northeast, consistent with the location of the maximum salinity decrease (figure 8 and figure 9). Approximately 10 years are required for a water parcel to flow from its outcrop area in the northeast to the western boundary region on σ θ = 25.5 along these streamlines, consistent with Huang and Qiu (1994) s renewal rates of those density surfaces. They 18

19 found that the renewal rates of the water masses in the potential density ranges 25.2 σ θ 25.4 and 25.4 σ θ 25.6 are about 9.4 years and 17.3 years respectively. Thus, the potential density surfaces between 25.0 and 26.0 containing the signal of interest likely respond to surface ventilation changes over decadal time scales. An interesting feature of all these comparisons is a region of positive salinity change in the far northeast corner, suggesting a possible shift in the future to a subsurface positive salinity anomaly in the eastern subtropical N. Pacific. Many more years of Argo float data will be required in order to adequately assess this possibility. 6. DISCUSSION AND SUMMARY All of the comparisons carried out here support a subsurface decrease in salinity in the subtropical N. Pacific thermocline between the early 1990s and the first decade of the 21st century. In this section we turn our attention to the surface source region for these thermocline waters. The potential density surfaces 25.0 σ θ 26.0 lie in the ventilated thermocline of the North Pacific (Huang and Qiu 1994). According to the Stommel Demon idea (Stommel, 1979), ventilated water properties are strongly biased toward the late winter value in the surface outcrop area. The subsurface salinity changes found in previous sections suggest that late-winter salinity in the surface source region has decreased since the 1980s. Along the density surfaces studied here, the ventilated water age varies and depends on the distance from the observations to the outcrop region. In order to compare ventilated water parcels in different locations, it would be ideal to trace the signal with time following an individual parcel of fluid, but at this point this is not possible. Nevertheless, the available database does allow us to examine the changes in the outcropping region over time. 19

20 We have compared Hydrobase climatology to the modern Argo float data; in this comparison, the winter season (January-March) salinity change in the surface mixed layer has been estimated by subtracting Hydrobase data from the Argo float observations ( ), using the mapping procedure outlined in Section 2. A northeast-tosouthwest freshening pattern dominates the winter mixed layer (figure 10), consistent with the spatial pattern of decadal salinity variability found by Overland et al. (1999). South of about 35 N, there is little salinity change at the mixed layer depth. North of this latitude (corresponding to the surface outcrop region of potential densities between 25.0 and 26.0), substantial freshening appears in the Argo data compared to Hydrobase climatology (figure 10). Based on a few stations, Freeland et al. (1997) showed evidence of warming and freshening in the Northeast Pacific as far back as the 1940s. Our analysis suggests that this surface freshening occurs over a very broad spatial area and continues to the present day. In the absence of any isopycnal and diapycnal mixing along the paths taken by fluid parcels on the σ θ 25.5 surface between the outcrop and subsurface observation point, two distinct mechanisms could cause the observed subsurface salinity changes: (1) changes in the winter mixed layer salinity, and (2) the movement of surface isopyncal outcrops. Our analysis confirms a large-scale salinity change in the surface source region for the 25.0 σ θ 26.0 density surfaces in the early 21st century compared with previous decades. Yeager and Large (2004) argue (based on model studies) that the subsurface salinity decrease in the subtropical N. Pacific is primarily caused by horizontal advection, a result seemingly consistent with (1), and our results support a scenario where 20

21 freshening occurs in the winter mixed layer and then is subducted and mixed into the corresponding subsurface layers, approximately along geostrophic streamlines. However, it is also important to consider the outcrop displacement over time, as shown in figure 10. Density outcrop lines generally follow isotherms in this area, indicating that the surface density is mainly determined by the temperature distribution (for σ θ 25 the equation of state is only a weak function of salinity). A comparison of Hydrobase and Argo data reveals an outcrop displacement in the northeast area that occurs primarily in the zonal sense, with the westward movement of the density lines resulting in a salinity change for density surfaces in the range 25.0 σ θ 26.0 (figure 10). We estimate the spatially averaged salinity difference due to the shift in outcrop areas using either the Hydrobase climatology or the Argo data result in a salinity change due to outcrop movement over time of about 0.1. Given the observed subsurface salinity change of about 0.2, we conclude that shifts in the outcrop position likely make an important contribution to the observed subsurface salinity decrease, perhaps comparable to upstream changes induced by variability in the winter mixed layer salinity. The robust freshening of both the subsurface ocean and winter mixed layer over a large portion of the North Pacific implies the possibility of a substantial change in the hydrological cycle. Since the freshening area is mostly at the higher latitudes, where high net precipitation dominates the hydrological cycle, this could suggest an increase in net precipitation over time. Lukas s (2001) analysis of the winter evaporation minus precipitation (E-P) change showed that E-P has decadal-scale variability and that the region of strong mixed layer freshening shown in figure 10 corresponds to a higher value of precipitation after 1996 that likely continues into the 2000s (Lukas, 2001). If we 21

22 assume that all the surface salinity decrease is due to a precipitation change, an increase in precipitation of about 4 cm/year is required to produce a salinity decrease of 0.07 over the upper 500 m, similar in magnitude to the observed salinity decrease in the northeast outcrop area. Given the inadequacy of the precipitation datasets available at the present time, a change of this magnitude is likely to be difficult to discern directly and is perhaps just at the limits of our detection ability when examined over the course of years. Additionally, there are other factors that may contribute to the observed surface salinity change, including changes in wind forcing, geostrophic and Ekman advection, deep entrainment, and eddy diffusion. A detailed study of these terms in a salt budget for the outcrop region of shallow potential density surfaces in the northeast Pacific will be the topic of a subsequent paper. In this paper, we have carried out a comparison of a great deal of historical salinity data to salinity measured from Argo floats, in order to study decadal-scale salinity changes in the northeast Pacific. Comparison along both zonal (24 N) and meridional (152 W) sections reveal a significant and coherent basin-wide salinity decrease that apparently began in the 1990s on both isopyncals (25.0 σ θ 26.0) and isobars, implying a freshening of the surface source region for this water mass. The observed changes are on the order of 0.1 in salinity, far in excess of the measurement limitations of either modern shipboard hydrography or CTD measurements from Argo floats. The analysis of repeated occupations of the 152 W line confirm that this salinity change is generated in the surface ventilation area and supports the notion of a shift in salinity in the 1990s. Analysis of the horizontal pattern on the surface 25.5 potential density surface reveals that the salinity decrease occurred over a broad area of the North Pacific. The freshening 22

23 is more intense in the area closer to the isopycnal outcrop and weakens to the south and west, a pathway supported by geostrophic streamlines. Our absolute geostrophic velocity estimates and the previous literature both suggest that the water on these upper ocean potential densities is renewed on a decadal time scale. These large-scale freshening observations are supported by a similar analysis using Hydrobase data, and substantial freshening of the winter mixed layer in the outcrop region suggests the possibility of both a net increase in precipitation in this area in recent decades as well as a contribution from the change in winter outcrop position due to wintertime warming. As the Argo dataset continues to grow in the coming years, the nature of such variability in the hydrological cycle should become clearer. 23

24 Acknowledgments. This paper is dedicated to the memory of Prof. Nobuo Suginohara, who had an important influence on the scientific programs that led to the data used in this work. In the 1980s and 1990s while at Tokyo University, Prof. Suginohara played an important role internationally in the leadership of WOCE. Later in his career, at JAMSTEC, he helped to guide the Japanese contribution to Argo. He hosted one of us (S.R.) in an early-career visit to Tokyo University in 1989 that will never be forgotten, and it was a pleasure over many years to renew our acquaintance and engage in spirited scientific discussions. He was an important friend and colleague and will be greatly missed. The Argo work at the University of Washington has been generously supported by the National Oceanographic and Atmospheric Administration through grant NA17RJ1232 Task 2. We thank Dr. Susan Hautala for her help in improving this paper. 24

25 REFRERENCES Bindoff, N. L. and T. J. McDougall, 1994: Diagnosing climate change and ocean ventilation using hydrographic data. J. Phys. Oceanogr., 24, Bretherton F., R. Davis and C. Fandry, 1976: A technique for objective analysis and design of oceanography experiments applied to MODE-73. Deep-Sea Res., 23, Conkright, M.E., T.D. O Brien, T.P. Boyer, C. Stephens, R.A. Locarnini, H.E. Garcia, P.P. Murphy, D. Johnson, O. Baranova, J.I. Antonov, R. Tatusko, R.Gelfeld and I. Smolyar, 2001: World Ocean Database Curry, R. G., Hydrobase - A database of hydrographic stations and tools for climatological analysis, Woods Hole Oceanog. Inst. Tech. Rep., WHOI-96-01, 44 pp., Woods Hole Oceanog. Inst., Woods Hole, MA, Deser, C., M. A. Alexander and M. S. Timlin, 1996: Upper-ocean thermal variations in the North Pacific during J. Climate, 9, Emerson, S., S. Mecking, J. Abell, 2001: The biological pump in the subtropical North Pacific Ocean: nutrient sources, redfield ratios, and recent changes. Global Biogeochem. Cycles, 15, Emerson, S., Y.W.Watanabe, T. Ono and S. Mecking, 2004: Temporal Trends in Apparent Oxygen Utilization in the upper pycnocline of the North Pacific: J. Oceanogr., 60, Freeland, H., K. Denman, C.S. Wong, F. Whitney and R. Jacques, 1997: Evidence of change in the winter mixed layer in the Northeast Pacific Ocean. Deep-Sea Res., 44, Fukasawa M., H. Freeland, R. Perkin, T. Watanabe, H. Uchida and A. Nishina, 2004: Bottom water warming in the North Pacific Ocean. Nature, 427, Huang, R. and B. Qiu, 1994: Three-dimensional structure of the wind-driven circulation in the subtropical North Pacific. J. Phys. Oceanogr., 24, Huang, R. and S. Russell, 1994: Ventilation of the subtropical North Pacific. J. Phys. Oceanogr., 24, Inui, T. and K. Hanawa, 1997: A numerical investigation of effects of a tilt of the zero wind stress curl line on the subduction process. J. Phys. Oceanogr., 27,

26 Johnson, G. C. and A. H. Orsi, 1997: Southwest Pacific Ocean water-mass changes between 1968/69 and 1990/91. J. Climate, 10, Kwon, Y-O and S. C. Riser, 2005: General circulation of the western subtropical North Atlantic observed using profiling floats. J. Geophys. Res,, 110 (c10), c11012, doi: /2005jc Lozier, M. S., M.S. McCartney and W.B. Owens, 1994: Anomalous anomalies in averaged hydrographic data. J. Phys. Oceanogr., 24, Lukas, R., 2001: Freshening of the upper thermocline in the North Pacific subtropical gyre associated with decadal changes of rainfall. J. Geophys. Res. Lett., 28, Macdonald A. M., T. Suga, and R. G. Curry, 2001: An isopycnally averaged North Pacific climatology, Journal of Oceanic and Atmospheric Technology, 18, McIntosh, P., 1990: Oceanographic data interpolation: objective analysis and splines, J. Geophys Res,, 95 (C8), Mecking, S. 2000: Spatial and temporal patterns of chlorofluorocarbons in the North Pacific thermocline: A data and modeling study. Ph.D dissertation, University of Washington, pp Mecking, S., M.J. Warner, J.L. Bullister, 2005: Temporal changes in pcfc-12 ages and AOU along two hydrographic sections in the eastern subtropical North Pacific, Deep-Sea Res., 53, Miller, A.J., D.R. Cayan and W.B. White, 1998: A westward-intensified decadal change in the North Pacific thermocline and gyre-scale circulation. J. Climate, 11, Overland J. E., S. Salo and J.M. Adams, 1999: Salinity signature of the Pacific Decadal Oscillation. J. Geophys. Res. Lett., 26, Riser, S.C., Li Ren, and A. Wong, Salinity in Argo: a modern view of a changing ocean. Oceanography, 20, Roemmich D., T. McCallister and J. Swift, 1991: A transpacific hydrographic section along latitude 24 N: the distribution of properties in the subtropical gyre. Deep-Sea Res., 38, S1-S20. Saunders, P.M., K-H Mahrt and R. T. Williams, 1991: Standards and laboratory calibration. WHP Operations and Methods. Schneider, N., A. J. Miller, C. A. Alexander and C. Deser, 1999: Subdution of decadal North Pacific temperature anomalies: observations and dynamics. J. Phys. Oceanogr., 29,

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28 FIGURE CAPTIONS Figure 1: WOCE sections in North Pacific Ocean. The red and blue lines denote sections P3 and P16, used extensively in the analysis in this paper. Figure 2: Temporal differences of salinity and potential temperature along P3 at 24 N. (a) Temporal differences of salinity on density surfaces between 1985 (WOCE) and (Argo) with mean WOCE/Argo salinity in white lines (i.e., the subtractions denote Argo minus WOCE). (b) Temporal differences of salinity on pressure surfaces with mean WOCE/Argo potential temperature in white lines. (c) Temporal differences of potential temperature on pressure surfaces with mean WOCE/Argo potential density surfaces in white lines. The mean properties contours are the average of the WOCE P3 data and the synthetic Argo section. Figure 3: Temporal differences of salinity on density surfaces between WOCE P3 (1985) and a University of Washington student cruise (2000). Figure 4: Schematic drawing showing (a) how surface warming can cause cooling and freshening along isopycnals and (b) how surface freshening can cause cooling and freshening along isopycnals of water with stability ratio 1 < R ρ <. The light dashed line is the original thermocline, the light solid line is new thermocline; the dark dashed line is the original isopycnal, and the dark solid line is new isopycnal. Surface warming in the formation region decreases the density of the subducted water (from point 1 on the original isopyncal to point 2 the new isopycnal). The mixing of newly ventilated water at point 2 with the original thermocline water at point 3 shifts the θ-s curve to colder and fresher values on the new isopycnal. Surface freshening also decreases the density of the subducted water (from point 1 on original isopyncal to point 2 the new isopycnal). The mixing of newly ventilated water at point 2 with the original thermocline water at point 3 shifts the θ-s curve to colder and fresher values on the new isopyncal. Figure 5: Zonally averaged salinity and potential temperature differences between Argo ( ) and WOCE P3 from 130 E to 130 W at 24 N (a) Zonally averaged salinity/ potential temperature differences as a function of potential density (one curve serves for both temperature and salinity, with different scales for the two variables).(b) Zonally averaged salinity differences as a function of pressure. (c) Zonally averaged potential temperature differences as a function of pressure. The black lines show the mean differences, and the shaded grey area shows the standard deviation. The error is estimated based on the Student-t test with 95% significance. Figure 6: Zonally-averaged T-S curves along 24 N. The solid line is the mean calculated from Argo and the dashed line is the mean calculated from WOCE P3. The gray contours denote constant values of potential density. Figure 7: Salinity change in 1984, 1991, 1997 and compared with the baseline Hyrobase climatology on potential density surfaces along P16 (~152 W). (a) Baseline spatial distribution in salinity. (b) Salinity change in 1984 (Marathon). (c) Salinity change in 1991 (WOCE). (d) Salinity change in 1997 ( a University of Washingtson student cruise). (e) Salinity change estimated using Argo data. The Hydrobase climatology and all the other section data have been objectively mapped 28

29 onto WOCE station locations, as described in the text. Blue areas in (b), (c), (d) and (e) shows where water has freshened. The salinity changes in 1984, 1991, 1997 and are estimated by subtracting the Hydrobase baseline. Figure 8: Temporal salinity differences between most of the WOCE ( ) lines in North Pacific and Argo ( ) at σ θ =25.5. The black line shows the winter outcrop of σ θ =25.5 estimated from winter (January-March) Argo float data in Figure 9: Temporal differences in salinity between Hydrobase climatology and Argo ( ) at σ θ =25.5. The white contours shows the winter absolute geostrophic streamlines estimated from winter (January-March) Argo float data from ; units shown on the streamlines are pressure in meters of water. The calculation of the absolute geostrophic streamlines follows the method of Kwon and Riser (2005). Figure 10: Salinity difference in the winter surface mixed layer between Argo and Hydrobase. The solid white line shows the outcrop of σ θ =25 and σ θ =26 estimated from Argo ( ) and the dotted white line shows the outcrop of σ θ =25 and σ θ =26 estimated from Hydrobase (1978). 29

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