5.3.2.3 Subtropical and Equatorial Atlantic

In the North Atlantic subtropical gyre, circulation, SST, the thickness of near-surface Subtropical Mode Water (STMW, Hanawa and Talley, 2001) and thermocline ventilation are all highly correlated with the NAO, with some time lags. A more positive NAO state, with westerlies shifted northwards, results in a decreased Florida Current transport (Baringer and Larsen, 2001), a likely delayed northward shift of the Gulf Stream position (Joyce et al., 2000; Seager et al., 2001; Molinari, 2004), and decreased subtropical eddy variability (Penduff et al., 2004). In the STMW, low thickness and production and higher temperature result from a high NAO index (e.g., Talley, 1996; e.g., Hazeleger and Drijfhout, 1998; Marsh, 2000). The volume of STMW is likely to lag changes in the NAO by two to three years, and low (high) volumes are associated with high (low) surface layer temperatures because of changes in both convective forcing and location of STMW formation. While quasi-cyclic variability in STMW renewal is apparent over the 1960 to 1980 period, the total volume of STMW has remained low through 2000 since a peak in 1983 to 1984, associated with a relatively persistent positive NAO phase during the late 1980s and early 1990s (Lazier et al., 2002; Kwon and Riser, 2004).

In the subtropics at depths of 1,000 to 2,000 m, the temperature has increased since the late 1950s at Bermuda, at 24°N, and at 52°W and 66°W in the Gulf Stream (Bryden et al., 1996; Joyce and Robbins, 1996; Joyce et al., 1999). These warming trends reflect reduced production of LSW (Lazier, 1995) and increased salinity and temperature of the waters from the Mediterranean (Roether et al., 1996; Potter and Lozier, 2004). After the mid-1990s at greater depths (1,500–2,500 m), temperature and salinity decreased, reversing the previous warming trend, most likely due to delayed appearance of the new colder and fresher Labrador Sea Water produced in the mid-1990s.

Intermediate water (800–1,200 m) in the mid-latitude eastern North Atlantic is strongly influenced by the saline Mediterranean Water (MW; Section 5.3.2.3). This saline layer joins the southward-flowing NADW and becomes part of it in the tropical Atlantic. This layer has warmed and become more saline since at least 1957 (Bryden et al., 1996), continuing during the last decade (1994–2003) at a rate of more than 0.2°C per decade with a rate of 0.4°C per decade at some levels (Vargas-Yáñez et al., 2004). In the Bay of Biscay (44°N; González-Pola et al., 2005) and at Gibraltar (Millot et al., 2006), similar warming was observed through the thermocline and into the core of the MW. From 1955 to 1993, the trend was about 0.1°C per decade in a zone west of Gibraltar (Potter and Lozier, 2004), and of almost the same magnitude even west of the mid-Atlantic Ridge (Curry et al., 2003).

Surface waters in the Southern Ocean, including the high-latitude South Atlantic, set the initial conditions for bottom water in the (SH). This extremely dense Antarctic Bottom Water (AABW), which is formed around the coast of Antarctica (see Section 5.3.5.2), spreads equatorward and enters the Brazil Basin through the narrow Vema Channel of the Rio Grande Rise at 31°S. Ongoing observations of the lowest bottom temperatures there have revealed a slow but consistent increase of the order 0.002°C yr^–1 in the abyssal layer over the last 30 years (Hogg and Zenk, 1997).

In the tropical Atlantic, the surface water changes are partly associated with the variability of the marine Inter-tropical Convergence Zone, which has strong seasonal variability (Mitchell and Wallace, 1992; Biasutti et al., 2003; Stramma et al., 2003). Tropical Atlantic variability on interannual to decadal time scales can be influenced by a South Atlantic dipole in SST (Venegas et al., 1998), associated with latent heat fluxes related to changes in the subtropical high (Sterl and Hazeleger, 2003). The South Equatorial Current provides a region for subduction of the water masses (Hazeleger et al., 2003) and may also maintain a propagation pathway for water mass anomalies towards the north (Lazar et al., 2002).

The North Atlantic Oscillation is an important driver of the oceanic water mass variations in the upper North Atlantic subtropical gyre. Its effects are also observed at depths greater than 1,500 m within the subtropical gyre consistent with the large-scale circulation and changes in source waters in the North Atlantic Ocean. While there are coherent changes in the long-term trends in temperature and salinity (Section 5.2), decadal variations are an important climate signal for this region.