FIGURE 2.11 Time series for the period 1948 to 1998 of ocean heat content (1022J) in the upper 300 m for the Atlantic, Indian, Pacific, and world oceans. Note that 1.5 × 1022J equals 1 W•year•m–2 (averaged over the entire surface of earth). Vertical lines through each yearly estimate represent ±1 standard error (SE) of the estimate of heat content. (SOURCE: Levitus et al., 1999.)

cal warming since 1950 is a cause of enhanced AO/NAO activity in the North Atlantic. This constitutes one pathway from anthropogenic forcing to high latitudes, and another comes via the stratosphere. Intensification of the stratospheric wintertime polar vortex under greenhouse-gas forcing has been a major prediction of models (e.g., Shindell et al., 2001). The AO/NAO is a mode of variability that grows more symmetric but no less energetic as one moves upward from troposphere to stratosphere (Perlwitz and Graf, 1995). Baldwin and Dunkerton (1999) demonstrated downward phase propagation of intense stratospheric anomalies. The possible intensification of the AO/NAO beneath an enhanced polar vortex is being investigated with models and observations (Shindell et al., 2001). Yet, crucial

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