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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.

1. Introduction

Since the bulk of the heating of the earth by greenhouse gas warming appears to have been taken up by the World Ocean (Levitus et al. 2005), estimates of interannual World Ocean heat storage changes (Willis et al. 2004; Levitus et al. 2005) are very important in evaluating climate model performance (Barnett et al. 2005), understanding the energy imbalance of the earth associated with global warming, and estimating the earth’s climate sensitivity to changes in greenhouse gas concentrations and other climate forcing (Hansen et al. 2005).

Most estimates of changes in World Ocean heat storage have been limited to the upper 750 m (Willis et al. 2004), or at most the upper 3000 m (Levitus et al. 2005), because historical temperature data become very sparse below the 750-m depth limit of most expendable bathythermographs (XBTs). The growing array of Argo floats (more information is available online at http://www.argo.net) promises to enhance routine ocean measurements of the ice-free World Ocean compared to the past XBT-based system by achieving even global coverage, adding measurements of ocean salinity to those of temperature, sampling to 2000 m, and sampling throughout the annual cycle. All of these things provide a great improvement for climate science, but data for estimates of deep (>2000 m) ocean heat storage changes will still be very sparse.

As might initially be expected for the case where heat is simply mixed down from the surface of a stratified fluid like the ocean, heat content changes do appear to be surface-intensified (Willis et al. 2004; Levitus et al. 2005). For example, simple linear fits to World Ocean heat content variations for 0–300-m and 0–700-m analyses of Levitus et al. (2005) between 1955 and 1998 have, respectively, slopes that are 35% and 59% of the slope for the 0–3000-m analysis (not shown), even though those layers span only 10% and 23% of the depth of the 0–3000-m layer, respectively.

However, the ocean is not ventilated solely by mixing from a shallow surface mixed layer into the thermocline. At high latitudes in locations such as the Labrador Sea (Lazier et al. 2002) and the Greenland Sea (Karstensen et al. 2005), very dense waters occasionally form where cooling in the open ocean is sufficiently strong to overcome the weak local stratification and create a surface mixed layer that extends deep into the water column, thus locally exposing the abyss to surface forcing. In addition, very dense waters are formed on ocean shelves around Antarctica, which then cascade down into the abyss (Orsi et al. 1999). Combinations of these North Atlantic Deep Waters and Antarctic Bottom Waters ventilate the cold deep abyss, mixing with waters above them (Mantyla and Reid 1983). As a result, while middepth waters in the Pacific and Indian Oceans are some of the “oldest” waters in the world in terms of the time since they have last been exposed to the surface (or ventilated), the bottom waters are significantly newer (England 1995).

Abyssal cooling of about 0.02°C has been reported in the southwest Pacific Ocean in 1990/91 relative to 1968/69 (Johnson and Orsi 1997). It should be borne in mind that the deep stations they analyzed were widely spaced in the horizontal, not all these deep stations were occupied all the way to the bottom, the 1968/69 stations had about 500-m vertical spacing between samples in the abyss, and 0.01°C is about the best instrumental accuracy expected for the reversing thermometers (Emery and Thomson 1998) that were used in 1968/69. In contrast, more recent analyses of modern closely sampled high-quality repeat hydrographic section data taken over the last decade or so reveal an abyssal warming of 0.005°–0.01°C at decadal intervals in the very coldest, nearly vertically homogenous abyssal waters of the main deep basins of the Pacific Ocean that are ventilated from the south (Fukasawa et al. 2004; Kawano et al. 2006b).

Here deep ocean temperature differences are presented from analyses of modern high-accuracy closely spaced hydrographic section data taken in the Pacific Ocean from the Antarctic Circumpolar Current to the Alaskan Stream and occupied at least twice during the past few decades (Fig. 1). Some of these differences are new and some are previously reported but are reanalyzed here in a slightly different way. The results presented here add to and confirm pioneering findings of abyssal Pacific Ocean warming in recent decades (Fukasawa et al. 2004; Kawano et al. 2006b). The warming is estimated to be statistically significant in many locations. Furthermore, the potential contribution of this abyssal warming to the global heat budget is discussed.

Figure 1

Figure 1. Locations of hydrographic sections (thick black lines) with WOCE designators P01 (47°N), P02 (30°N), P06 (32°S), P16 (150°W), and P15 (170°W). Designator labels are located above zonal sections and to the right of meridional sections. Bathymetry is color shaded from 6 km (blue) to 2 km (red) with shallower depths in gray (see color bar) and coastlines drawn as thin black lines

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