Climatic warming of Atlantic Intermediate Waters

In this section, the interdecadal temperature variability of the Atlantic Ocean is investigated by comparing hydrographic sections worked in the 1920s, 1950s, 1980s and 1990s. Previous studies (Roemmich and Wunsch, 1984; Parrilla et al., 1994; Bryden et al., 1996; Joyce and Robbins, 1996; Arhan et al., 1998; Levitus, 1989; Joyce et al., 1999) have concentrated on changes in the North Atlantic since the 1950s. Here, we reanalyse these changes and extend the analysis to the South Atlantic and back to the 1920s.

For this study we extracted data from the following sources: three South Atlantic sections from the Meteor expedition of 1925-27 (Defant, 1936), plus a total of seven North Atlantic and four South Atlantic sections from the International Geophysical Year (IGY, 1956-60), (Fuglister, 1960; Worthington and Wright, 1970), five North Atlantic sections from 1981-85 (Roemmich and Wunsch, 1984; McCartney, 1992; Knapp, 1988; Hendry, 1989), one from 1992 (Parrilla et al., 1994), and one from 1993 (Arhan et al, 1998). These were supplemented by two 1983 sections (McCartney and Woodgate-Jones, 1991; Warren and Speer, 1991) and one 1994 section (Holfort et al., 2000) from the South Atlantic. All section data were mapped onto standard grids, regularly spaced in the vertical and horizontal, before differencing. Pressure is the vertical coordinate used in the grid.

Figure 7.3.4 (see Plate 7.3.4, p. 588) summarizes the calculated changes, averaged over the 10002000 dbar depth interval, except at 48°N where the maximum cooling signal was between 500 and 1500 dbar. The changes are partitioned into three components: (a) the isobaric 6 change, (b) the isopy-cnal 6 change, and (c) changes in 6 due to isopycnal displacements. The components of the 1950s-minus-1920s changes are calculated on the assumption that the titrated Meteor salinities, although containing large random errors, do not contain significant systematic errors (Mantyla, 1994).

The measurement errors of Meteor and IGY temperatures (Fuglister, 1960), as well as those of the more recent Conductivity-Temperature-Depth (CTD) temperatures, are much smaller than the changes reported here. Errors caused by sparser sampling of the Meteor and IGY sections, and by the non-coincidence of cruise tracks from different decades, were investigated and found to be small. For all of our sections, the largest source of error in the changes between 1000 and 2000 dbar is that due to mesoscale eddies. Following previous work (Parrilla et al., 1994), we estimate eddy errors by taking the standard deviation of the section-averaged signals and dividing by the square root of the number of eddy diameters (taken to be 100 km) across the section. The eddy errors of the vertically averaged isobaric changes are shown in Figure 7.3.4a (see Plate 7.3.4a, p. 588).

Except at 48°N, the section changes show an isobaric warming, of 0.5°Ccentury—1, between 1000 and 2000 dbar. This warming rate is consistent with the trend seen at the Bermuda time series station (Roemmich and Wunsch, 1984; Joyce and Robbins, 1996). Our map, with its comprehensive geographical coverage, is a good complement to the Bermuda time series, which samples only one point in the ocean, but has excellent temporal coverage. Together, they argue convincingly that there have been widespread changes in temperature at intermediate depths over the entire Atlantic Ocean since at least the 1920s. In the North Atlantic between the 1950s and 1980s, the warmings are mostly due to downward displacements of isopcy-nal (neutral) surfaces, while isopycnal change dominates all but the most recent of the South Atlantic signals.

Though depressions of isopycnal surfaces in the North Atlantic may arise from a range of causes, such coherent and large-scale changes as we see here suggest that the cause may involve variations in the formation rate of intermediate water masses at source. Correlations between time series of the source-thickness of Labrador Sea Water (LSW) -the major water mass in the North Atlantic between 1000 and 2000 dbar - and the temperature anomalies at Bermuda have led to the proposal (Curry et al., 1998) that the thickness of the LSW layer is a major controlling factor in intermediate-depth temperature anomalies of the subtropical North Atlantic. We suggest that, additionally, there may have been changes in the thickness of the layers above or below the LSW intermediate-layer. If the intermediate-layer temperature changes were due only to thickening and thinning of the LSW, one might expect to see isopycnal displacements of opposite sign at the top and bottom of the LSW layer - roughly at 1000 and 2000dbar, respectively. However, the 1980s-minus-1950s differences on all of our North Atlantic sections from 8 to 36°N show a consistent downward displacement of isopycnals of —30-100 dbar (not shown) throughout the entire extent of the 1000-2000-dbar layer. This is consistent with either a deepening thermocline or a shallowing of the abyssal layer (Bacon, 1998). Our calculations indicate that the thermocline motions concurrent with the wind- driven sea-level changes calculated by Sturges et al. (1998) are not consistent with the isopycnal displacements we observe. Thus, our analysis does not support the idea that these changes are simply due the effect of wind forcing on thermo-cline depth. Another possibility is that the thermo-cline has deepened due to changes in the amount of fluid being pumped downwards from its surface outcrops (Huang and Pedlosky, 1999), but we do not yet have a quantitative measure of this effect.

In contrast to the North Atlantic warmings, those in the South Atlantic are generally dominated by isopycnal change, the 1990s-minus-1980s signal at 11°S being an exception. This isopycnal warming extends north into the southern North Atlantic. There are several sources for waters between 1000 and 2000 dbar: deep convection in the Labrador Sea, overflows from the Mediterranean Sea, and Antarctic Intermediate Water (AAIW). Although the core of the AAIW layer is shallower, salinity data (Bainbridge, 1981) suggest that AAIW

comprises a large fraction of the 1000-2000-dbar layer in the South Atlantic and a lesser fraction of this layer in the southern North Atlantic. For this reason, the large warmings (and compensating increases in salinity) along isopycnals in the South Atlantic, and the lesser warmings along isopycnals in the southern North Atlantic, are suggestive of changes in AAIW, especially since the warmings in the southern North Atlantic are more pronounced in the western part (Fig. 7.3.4b, see Plate 7.3.4b, p. 588), where the presence of AAIW is strongest. Likewise, the coolings along isopycnals in the northern North Atlantic are suggestive of changes in LSW (Fig. 7.3.4b, see Plate 7.3.4b, p. 588).

Our study has broadened the geographical extent of the warmings at intermediate depths reported by previous authors (Roemmich and Wunsch, 1984; Parrilla et al., 1994; Bryden et al., 1996; Joyce and Robbins, 1996; Arhan et al., 1998; Levitus, 1989; Joyce et al., 1999) by showing that these warmings extend into the South Atlantic. We find that the isopycnal-displacement mechanism, which is of widespread significance in the North Atlantic (Levitus, 1989), is not the only one of importance. Changes on isopycnals also form a large and widespread component of the changes observed on isobars. However, unlike the displacement mechanism, changes along isopycnals respond very slowly to changes in forcing, so that by serving as integrators over long time scales, they might be the more sensitive indicator of long-term climate change (suggestion of anonymous reviewer). In this light, the isopycnal warmings we observe, extending back into the 1920s, may well indicate that recent changes in the climate of the ocean are a response to climatic forcing over longer periods than we have previously realized.

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