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Recent Bottom Water Warming in the Pacific Ocean

Gregory C. Johnson1, Sabine Mecking2, Bernadette M. Sloyan3and Susan E. Wijffels3

1NOAA/Pacific Marine Environmental Laboratory, Seattle, WA 98115-6349, USA
2Applied Physics Laboratory, University of Washington, Seattle, WA, USA
3CSIRO Marine and Atmospheric Research, Hobart, Tasmania, AUSTRALIA

J. Climate., 20 (21), 5365–5375, 2007.
Copyright 2007 American Meteorological Society. Further electronic distribution is not allowed.

4. Summary and discussion

Temperature differences have been analyzed for five repeat hydrographic sections, mostly between the decades of the 1990s and the 2000s. Three of these sections are zonal, and two meridional (Fig. 1, Table 1). They span much of the Pacific Ocean, albeit sparsely. Warming in the weakly stratified abyss is consistently present in all of the section differences except one: the 1991–84 abyssal section-mean Δθ values along 150°W north of Hawaii are near zero, although the 2006–1991 abyssal differences there are positive. Most of the section differences in potential temperature are statistically different from zero at the 95% confidence limit over some abyssal pressure range. Section-mean Δθ values generally increase toward the bottom from values near zero around 3000–4000 dbar. The abyssal section-mean Δθ values range from 0.004° to 0.01°C. The lack of warming in the northern portion of the section along 150°W between 1984 and 1991 suggests that the abyssal warming found in all other sections may have commenced in the 1990s. However, that speculation is based on a very limited set of data.

Oceanographers often make inferences about water-mass changes from other properties such as salinity and oxygen. Examination of the potential temperature–salinity (θ–S) relationships in the abyss along these sections (not shown) reveals no clear large-scale changes in which we have confidence given the measurement error of salinity. That is to say, section average θ–S curves for the various occupations of P15 and P16 differ by less than about 0.004 in salinity, either before or after adjustment for differences among the standard seawater batches (Kawano et al. 2006a) used to standardize salinity measurements during these cruises. Nor do we find significant differences in the section-average abyssal potential temperature–oxygen (θ–O2) curves for the various P16 occupations. Abyssal θ–O2 curves for these sections differ by less than the accuracy of the oxygen measurements of 1–2 μmol kg–1. These findings are in accord with the results of Fukasawa et al. (2004) and Kawano et al. (2006b), who also report no significant θ–S or θ–O2 changes in their analyses of the abyssal Pacific Ocean.

It appears the rate of Pacific abyssal warming over the 1990s and 2000s decreases with distance from the source. Generally, warming rates appear to increase toward the south, being largest in the southwest Pacific Basin, and decrease toward the north, consistent with the fact that the source waters for the abyssal Pacific enter from the south. However, the statistical uncertainties make this conclusion a tentative one, as most of the section-mean differences overlap within their error bars. It has been noted previously that these increases do not need to be the direct result of changes in source water properties advected into the Pacific, but could be the result of changes in source water formation regions communicated more rapidly by planetary waves (Fukasawa et al. 2004; Kawano et al. 2006b), which would be consonant with the absence of measurable changes in the abyssal θ–S and θ–O2 relationships discussed above. Given horizontal abyssal temperature gradients, the abyssal temperature increases reported here are consistent with a lower bound of less than 1 × 10−3 m s−1 for decadal variations in interior abyssal ocean horizontal advection.

With significant warming present throughout the abyssal Pacific between the decades of the 1990s and the 2000s, it is interesting to assess the potential significance of this signal in terms of the World Ocean heat budget. The temperature differences are small, being only about an order of magnitude smaller than overall temperature increases between 2003 and 1993 in the upper 750 m of the World Ocean (Willis et al. 2004). However, they occur over a thickness of thousands of meters.

The horizontal average of heat content change ΔQ can be computed as a function of pressure p for each section:

ΔQ(p) = <ρcp(TfTi)/(tf – ti)>x                        (1)

where ρ is the time-mean in situ density, cp is the specific heat capacity, T is the temperature, t is the time, the subscripts f and i indicate final and initial section occupations, and the <>x operator indicates a lateral section average where values are present for both section occupations. If the ocean were of a uniform depth, then the vertical integral of this quantity would provide an assessment of the heat content change in terms of an ocean surface heat flux.

However, while the surface ocean occupies about 0.71 of the earth’s surface area, that fraction decreases with increasing ocean depth (Fig. 9a). This hypsometry must be factored into any estimate of the heat content change from oceanographic data in terms of surface heat flux. The section estimates of ΔQ from (1) are regional, and their spatial resolution of the Pacific (Fig. 1) is too sparse to map horizontally. For a crude estimate of their significance in terms of the earth’s heat budget, each regional estimate is multiplied by the fractional area of the earth covered by the World Ocean at each pressure (Fig. 9a) before vertical integration. This calculation (Table 1) gives a rough estimate of what impact the regional changes in ocean heat content would have on a uniform surface heat flux distributed over the entire earth (not just the ocean), were they representative of changes throughout the abyssal World Ocean. Between 3000 m (or 4000 m in the case of P06) and the bottom these estimates of heat flux range from 0.01 W m−2 along 47°N (P01) to 0.06 W m−2 along 170°W south of the equator (P15S). These values are between 5 and 30% of the heating trend of 0.2 W m−2 estimated for the 0–3000-m World Ocean heat content change between 1955 and 1998 (Levitus et al. 2005) and between 2% and 10% of the heating trend of 0.6 W m−2 (per unit area of the earth’s surface) estimated for the 0–750-m World Ocean heat content change between 1993 and 2003 (Willis et al. 2004). Thus, abyssal Pacific Ocean heat content variations may contribute a small but significant fraction to the earth’s heat budget.

Figure 9

Figure 9. (a) Percentage of the earth’s surface area (0.510 × 1015 m2) occupied by the World Ocean as a function of depth. (b) Percentage of the World Ocean volume (1.33 × 1018 m3) below any given depth. Estimates are based on the bathymetry fields of Smith and Sandwell (1997).

The Pacific abyssal heat content changes generally decrease from south to north. Changes may be larger in the South Pacific because that region is closer to the southern source of bottom water feeding the Pacific than in the most remote reaches of the North Pacific. Interestingly, all the abyssal Pacific warming signals are smaller than those recently reported in an analysis of repeat section data in the western basins of the South Atlantic (Johnson and Doney 2006). The same calculations made above applied to these repeat section data result in a value of 0.2 W m−2 below 3000 m between 2005 and 1989–95 and from 60°S to the equator in the southwest Atlantic. Again, this number represents the impact that these regional changes in ocean heat content would have on a uniform surface heat flux distributed over the entire earth (not just the ocean) were they representative of changes throughout the abyssal World Ocean. The southwest Atlantic is adjacent to a major source of Antarctic Bottom Water in the Weddell Sea, so the large value estimated might be expected. As repeat hydrographic sections continue to be occupied in service of CLIVAR and Carbon Programs (more information is available online at http://ushydro. ucsd.edu, and http://www.clivar.org/resources/data/clivar-carbon-and-hydrographic-sections), the global distribution of these potentially important changes in heat content can be better assessed.

A simple calculation using global bathymetric data (Smith and Sandwell 1997) reveals that the upper 750 m (predominant XBT profiling depth) comprises only 19% of the World Ocean volume, the upper 2000 m (target Argo profiling depth) 48%, and the upper 3000 m (maximum published World Ocean heat budget depth) 70% (Fig. 9b). Analyses of historical data suggest there may be significant interannual variations in upper ocean heat content (Willis et al. 2004; Levitus et al. 2005). Repeat hydrographic sections are occupied sparsely in space (a few per ocean) and time (once per decade). The data from these repeat sections suggest that abyssal variations may contribute significantly to global heat, and hence sea level, budgets.

To close ocean heat, sea level, and likely freshwater budgets on interannual time scales, the ocean below 2000 m must be much better sampled in space and time than it has been, or is likely to be, relying on repeat hydrography alone. Given the extensive measurements that would be required, autonomous instruments, perhaps some combination of floats engineered to sample the abyss, gliders (e.g., Eriksen et al. 2001) similarly modified, and deep moorings may be the only viable means to achieve sampling needs.

Acknowledgments. The hard work of all contributors to the collection and processing of hydrographic section data analyzed here is gratefully acknowledged. The 1990s data were collected as part of the World Ocean Circulation Experiment (WOCE) Hydrographic Program, and the 1980s Marathon II expedition data, measured to WOCE standards, were adopted by that program. The 2001 reoccupation of WOCE section P15S was a contribution of the Australian Climate Change Research Program. The 2003 reoccupation of WOCE section P06 was part of the Blue Earth Global Expedition (BEAGLE) organized by Japan Agency for Marine-Earth Science and Technology (JAMSTEC). The 2004 reoccupation of WOCE section P02, as well as the 2005 and 2006 reoccupations of WOCE section P16, were part of the NOAA/NSF-funded U.S. CLIVAR/CO2 Repeat Hydrography Program. The comments of three anonymous reviewers improved the manuscript. The NOAA Office of Oceanic and Atmospheric Research and the NOAA Climate Program Office further supported GCJ and SM. The findings and conclusions in this article are those of the authors and do not necessarily represent the views of the National Oceanic and Atmospheric Administration. BMS and SEW were partly funded by the Australian Climate Change Science Program. This paper is a contribution to the CSIRO Climate Change Research Program and the CSIRO Wealth from Oceans Flagship.

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