The mean annual cycles of freshwater budget components from ERA-40 (P, E, P-E and atmospheric storage) averaged for the Arctic Ocean and its contributing terrestrial drainage (the same as used in Fig. 14.2, see caption) help to summarize some of the preceding discussion (Fig. 14.4).
For the Arctic Ocean as a whole, P-E peaks in July and is smallest in March. Because evaporation is always rather low, the shape of the annual cycle in P-E is broadly similar to that of precipitation. The July minimum in E is consistent with the melting sea ice surface. Its rise from July through October follows as specific humidity is falling (with cooling of the air) while open water is increasing to a maximum in September (and is still large in October), fostering stronger vapor gradients. Atmospheric storage of water vapor exhibits a fairly symmetric annual cycle, with a minimum in January and maximum in July, following the annual cycle of tropospheric temperatures and the water-holding ability of the atmosphere. The prominent feature of the terrestrial drainage (see also Walsh et al. 1994 and Serreze and Etringer 2003) is the opposing annual cycles of P and P-E.
It is useful to briefly examine the dominant pathways for the flow of water vapor into the Arctic. Figure 14.5 shows the vertically integrated meridional water vapor flow across the 70° N latitude circle by month (vertical axis) and longitude (horizontal axis). Poleward flows (inflows) are in red, while equatorward flows (outflows) are in blue.
For every month, inflows dominate, i.e., there is a vapor flux convergence into this "polar cap" domain. In the annual mean, this equates to a P-E of 193 mm (average water depth). Flows are larger in summer than in winter. Inflows during summer are prominent in four regions, near the prime meridian, about 90° E, about 165° W, and about 50° W. The peak at around 90° E is slightly east of the Urals trough, while the feature at about 165° W is located just east of the east Asian trough. Prominent inflows at about 50° W and near the prime meridian are separated by a region of equatorward flow in most months. This separation manifests blocking by the Greenland ice sheet. Most of the moisture flow occurs below 700 hPa (roughly 3,000 m). At 70° N, the highest ice sheet elevations of about 2,900 m are found at about 35° W longitude. The outflow centered at about 110° W corresponds to the descending leg of the western North American ridge.
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