Differences in baroclinicity

Figure 2: Global mean annual sea-level air pressure, based on the long-term mean from 1948-2013, as determined by the NCEP-NCAR Reanalysis (Kalnay et al., 1996). Note the large cross-latitudinal differences air pressure (baroclinicity) over the mid-latitudes of the Southern Oceans, with weaker but still significant baroclinicity over the North Atlantic and North Pacific Oceans.

Baroclinicity (the amount of air pressure variation with latitude) is greatest during the respective winter seasons in both hemispheres, as the temperature contrast between Tropics and the Poles is largest at this time of year. This is because the Arctic and Antarctic circles lie in darkness for much of their respective winter-times, maximising infra-red radiation loss. This pressure differential leads to the movement of air, and the development of weather systems.

Changes in the moisture content of air also have an important role to play. Overall, year-round, baroclinicity (and thus storminess) is greater in the southern hemisphere than in the northern hemisphere, because the Antarctic continent is very much colder than the Arctic (see Figure 2; Kalnay et al., 1996).