U.S. Dept. of Commerce / NOAA / OAR / PMEL / Publications

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.

3. Analysis

In all the Δθ sections (e.g., Fig. 2), there is a good deal of vertically banded structure, with temperature difference fields sometimes alternating sign from band to band over at least parts of the water column. Generally the amplitude of these variations increases from the bottom to the surface. This pattern might be anticipated in the ocean for vertical excursions associated with eddies, waves, tides, or meanders, where vertical stratification generally increases toward the surface.

Figure 2

Figure 2. Sections of potential temperature difference (AO, °C) for P16N (nominally along 152°W) between Hawaii and Kodiak Island, color shaded as a function of latitude and pressure. (a) 2006–1984, (b) 2006–1991, and (c) 1991–84. Red areas indicate warming and blue areas indicate cooling, with color saturation at ±0.05°C. Mean potential temperatures from all the data (black lines) are contoured. Portions of the sections with either large data gaps [from 48.3°–52.4°N in (b) and (c)] or where section longitudes diverged at a given latitude [south of 27°N in (a) and (c)] are blanked out.

Differences of potential temperature fields (Δθ) among the three P16N occupations between Hawaii and Kodiak Island show that for 2006–1984, much of the deeper water column has warmed measurably throughout all but the northernmost reaches (Fig. 2a), both in the very weakly stratified region of θ < 1.2°C (P > 3500 dbar) and in the increasingly (but still weakly) stratified regions as shallow as 2000 dbar. Strong bottom-reaching subpolar currents, including the Alaskan Stream (Warren and Owens 1988) over the Aleutian Trench near 56°N and an offshore counter-flow over the Aleutian Rise to the south, may be associated with significant variability on time scales much shorter than decadal, so temperature changes estimated there from differences of snapshots separated by many years may not be representative of long-term changes. For 2006–1991 (Fig. 2b) warming is also evident over much of the section for θ < 1.2°C, but both warming and cooling tend to alternate above that level. In contrast, for 1991–84 differences (Fig. 2c) warming again appears dominant for θ > 1.2° C, but below that level warming is not obvious since there are also bands of cooling. This result suggests that abyssal warming may have commenced here sometime after 1991, subject to the assumption that basin-scale abyssal variations are dominated by decadal and longer time scales. However, the fact that the time interval for the 1991–84 difference is less than half that for the 2006–1991 estimate makes detecting abyssal temperature changes over the earlier time period more difficult.

Ascertaining the statistical significance of Δθ changes requires estimates of the effective number of degrees of freedom in Δθ fields. Integral spatial scales for Δθ are estimated from autocovariances (e.g., von Storch and Zwiers 2001). Here the effective number of degrees of freedom at each level, estimated as the latitude or longitude range sampled at each level (which varies because of topography) divided by the integral spatial scale for that level, is used throughout the error analysis, including application of Student’s t test for 95% confidence limits.

When latitudinal averages are computed for the Δθ fields along P16N (omitting regions with large data gaps and where the sections diverge by more than 1° longitude) they show abyssal warming of about 0.004°C between 2006 and 1984 (generally approaching statistically significant differences from zero at 95% confidence limits only for P > 4500 dbar), as well as between 2006 and 1991, but not between 1991 and 1984 (Fig. 3). Again, these results suggest that most of the warming observed in the abyss along P16N occurred after 1991. However, the differing latitudinal ranges used for these averages because of gaps or excessive zonal separation of the sections (Fig. 2) may affect these difference estimates if the temperature changes are not uniformly distributed in latitude. As might be expected from the vertical banding noted above, uncertainties increase with decreasing pressure. The Δθ fields from 2005/06 to 1991/92 for the portion of P16 south of Hawaii (P16S and P16C) generally warm in the weakly stratified abyssal layers (Fig. 4). This pattern persists throughout the entire section, even with mean abyssal temperatures generally increasing from south to north. Once again there are stronger variations above the abyss. A mean of the Δθ field for this section between the Pacific–Antarctic Ridge and Hawaii (Fig. 5) shows abyssal warming of about 0.004°–0.01°C, increasing from near zero for P = 3000 dbar, but it is only statistically different from zero at 95% confidence for P > 5500 dbar. The large uncertainty around 5300 dbar is caused by an isolated region of cooling near 39°S (Fig. 4).

Figure 3

Figure. 3. Section-mean differences of potential temperature differences (AO, °C, thick lines) for P16N (nominally along 152°W) between 27°N/Hawaii and Kodiak Island with 95% confidence limits (thin lines). (a) 2006–1984, (b) 2006–1991, and (c) 1991–84.

Figure 4

Figure 4. Section of potential temperature difference (AO, °C) for P16S/C (nominally along 150°W south of Hawaii) color shaded as a function of latitude and pressure using 2005/06–1991/92 data. Details are the same as in Figure 2.

Figure 5

Figure 5. Section-mean potential temperature differences (AO, °C, thick lines) for P16S/C (nominally along 150°W) between the Pacific–Antarctic Ridge (near 59.5°S) and Hawaii (near 19°N) with 95% confidence limits (thin lines) using 2005/06–1991/92 data.

WOCE section P15S lies nominally along 170°W (Fig. 1). Again, a very consistent pattern of warming in the weakly stratified abyssal layer is evident in the Δθ field for P15S (Fig. 6). While warming appears stronger in the southern portion of the section, south of the Chatham Rise near 43°S, abyssal warming persists throughout the entire section. The mean Δθ for this section is about 0.003°–0.01°C, but is only statistically significantly different from zero at the 95% confidence level around P = 5500 dbar (Fig. 7). The time interval for this difference is much shorter than for the other sections, and since the mean warming signal is similar a faster rate of change is implied. The faster warming along P15S is perhaps the result of the section being closest to the entry of abyssal waters into the Pacific Ocean (Mantyla and Reid 1983), and is also consistent with the largest warming observed in this section being at its southern end.

Figure 6

Figure 6. Section of potential temperature difference (AO, °C) for P15S (nominally along 170°W, south of the equator) color shaded as a function of latitude and pressure using 2001–1996 data. Details are the same as in Figure 2.

Figure 7

Figure 7. Section-mean potential temperature differences (AO, °C, thick lines) for P15S (nominally along 170°W, south of the equator) with 95% confidence limits (thin lines) using 2001–1996 data.

These results are in close accord with those previously reported for differences of zonal sections at 47°N, 30°N, and 32°S (Fukasawa et al. 2004; Kawano et al. 2006b), all of which show slight but consistent warming over the past few decades in the weakly stratified abyss. Excellent sections of Δθ fields for those sections have already been presented in these previous studies, and so are not reproduced here. Suffice it to say that in the difference sections for P01 and P02 at 47° and 30°N, slight warming is present throughout the abyss, and in that for P06 at 32°S the abyss primarily warms, although weak cooling is observed in the South Fiji Basin, a small, relatively isolated deep basin just west of the date line, and relative strong warming is observed in the northern reaches of the Tasman Basin sampled by this section. The section-mean results reported below are all in agreement with the changes in areas between deep isotherms previously reported (Fukasawa et al. 2004; Kawano et al. 2006b). Here the analysis is slightly different, being presented as temperature differences on pressure surfaces with 95% confidence limits estimated.

Simply using the 32°S section as a whole to estimate mean Δθ values for this latitude suggests warming for P > 4000 dbar, with significant warming for P > 5200 dbar between 2003 and 1992, again in the weakly stratified abyss (Fig. 8a). The mean differences of about 0.01°C for P > 5000 dbar are larger than any other section-mean differences reported here, again consistent with the relative proximity of the section to the entry point for abyssal waters into the Pacific Ocean from the south. The abyssal warming seen here may be highly significant statistically because it is localized in the deepest portion of the southwest Pacific Basin, where the abyssal waters are relatively uniform in temperature both zonally and vertically.

Figure 8

Figure 8. Section-mean potential temperature differences (AO, °C, thick lines) for (a) section P06 (nominally along 32°S) with 95% confidence limits (thin lines) using 2003–1992 data, (b) section P02 (nominally along 30°N) with 95% confidence limits (thin lines) using 2004–1993/94 data, and (c) section P01 (nominally along 47°N) with 95% confidence limits (thin lines) using 1999–85 data.

Much further to the north, the P02 section along 30°N has positive mean Δθ values between 2004 and 1993/94 (Fig. 8b) for P > 2000 dbar. There is a relative minimum in section-averaged Δθ near P = 4000 dbar, and an increase toward the bottom, reaching around 0.008°C by 6000 dbar. Only the very deepest differences are statistically different from zero at the 95% confidence level.

Finally, the P01 section along 47°N has positive abyssal section-averaged Δθ values between 1999 and 1985 (Fig. 8c) for P > 3500 dbar. These differences are nearly statistically different from zero at 95% confidence for P > 4800 dbar, with a magnitude approaching 0.005°C near 6000 dbar. These relatively small temperature changes over a relatively long time interval are consistent with the northern location of this section being farthest from the abyssal source waters, and therefore the most buffered of the sections analyzed here in terms of abyssal variability. In addition, this buffering appears to result in very small error bars, again because of zonal and vertical gradients in deep potential temperatures that are very small, even compared with the rest of the abyssal Pacific.

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