Variability in the marine carbon cycle and ocean carbon sources and sinks has been studied most thoroughly in the tropical Pacific in connection with the El Niño Southern Oscillation (ENSO). El Niño has profound impacts on weather and climate globally. By contrast, little is known about the contribution of the subtropical and subpolar gyres to atmospheric CO₂ variations, despite the fact that these gyres cover more than half of the world's ocean. Natural climate phenomena, such as the North Atlantic Oscillation (NAO) and the Arctic Oscillation (AO) also have strong regional (particularly in Europe) and global impacts on weather and climate. The dominant mode of atmospheric and climate variability in the North Atlantic region is the NAO, which is a dipole meridional oscillation in atmospheric pressure between the Iceland Low and Açores High (Hurrell, 1995; Hurrell and Van Loon, 1997; Hurrell et al., 2002; Figure 1 and 2). The NAO is linked to the Arctic Oscillation (AO), which is a tripolar oscillation between the North Pacific and North Atlantic, centered over the Arctic region (Visbeck et al., 2001).
 

The NAO has significant effects on climate and atmospheric variability. Strong eastward airflow between the Iceland Low and Açores High carries storms towards western Europe from North America. If the NAO index is negative (Figure 3), storm tracks are thought to shift southward, cooling surface waters, enhancing 18°C mode water formation and deepening winter mixed layers (Rodwell et al., 1999). In western Europe, for example, there is an increase in winter storms and precipitation during these periods. During positive NAO winters, westerlies that usually prevail in the region between Florida and Cape Hatteras (west of the Açores High) weaken. Reduced wind stress and heat exchange leads to the development of warm temperature anomalies in the subtropical gyre (Bjerknes, 1964; Cayan, 1992a,b) with a magnitude of ~0.2 to 0.4°C (Davies et al., 1997; Kapala et al., 1998). Although the NAO is the dominant mode of mid-latitude atmospheric variation, dynamical relations between El Ninõ and Atlantic climate have long been documented (e.g., Enfield and Mayer, 1997; see references in Penland and Matrosova, 1998). For example, the Gulf Stream position shifts northwards after El Niño events and during NAO positive phases with a lag of ~2 years (Taylor et al, 1998; Taylor and Stephens, 1998). Within 4-12 months of El Niño warming in the Pacific Ocean, warming is observed in the tropical North Atlantic, Caribbean Sea and the SE subtropical gyre (e.g., Zhang et al., 1996; Borariu, 1997; Penland and Matrosova, 1998). The mechanism for this apparently relates to a reduction in cloud cover in the drier, more stable Atlantic atmosphere (e.g., Zhang et al., 1996; Davies et al., 1997; Jones and Thorncroft, 1998).

The NAO, like El Niño, also has profound impacts on ocean biogeochemical dynamics and the fate of CO2. In the North Atlantic, we have recently discovered that NAO plays a large role in determining the exchange of CO2 between the ocean and atmosphere (Bates, 2001; Bates et al., 2002; Gruber et al., 2002). We have hypothesized that there is a coordinated basinwide response in the North Atlantic Ocean to climate phenomena such as NAO (Figure 4). There is also increasing evidence that shows a strong linkage between NAO and atmospheric dust transport variability which consequently control the variability of ecosystem structure and functioning in the North Atlantic subtropical gyre (Bates and Hansell, 2004). In the tropical and subtropical ocean, it has also been hypothesized that tropical cyclones (i.e., hurricanes and typhoons) play a significant role in determining the exchange of CO₂ between ocean and atmosphere (Bates et al., 1998a,b; Bates and Merlivat, 2001; Bates, 2002). Better scientific knowledge about the interannual variability in this feedback (and interactions with NAO and ENSO also), and the impact of temperate storms (e.g., North Atlantic wind storm events) on ocean-atmosphere exchange of CO₂ is critically needed.