Annual and monthly fields of PET

The mean annual field of aerological P — ET based on NCEP/NCAR over the period 1970-99 is provided in Figure 6.8. Fields for the mid-season months appear in Figure 6.9. The data have been smoothed to try and eliminate most of the problems mentioned earlier. The fields shown here are very similar to those from ERA-15 and ERA-40.

Figure 6.8 Aerological estimates of mean annual precipitation minus évapotranspiration (P - ET) based on NCEP/NCAR data for the period 1970-99 (mm). Contours are at every 100 mm up to 500 m (negative values dashed) and at every 200 mm for amounts of 600 mm and higher (dotted) (by the authors).

Figure 6.9 Aerological estimates of mean precipitation minus evapo-transpiration (P — ET) for the four mid-season months based on NCEP/NCAR data for the period 1970-99 (mm). Contours are at every 10 mm for amounts up to 50 mm (negative values dashed) and at every 20 mm for amounts of 60 mm and higher (dotted) (by the authors).

Figure 6.9 Aerological estimates of mean precipitation minus evapo-transpiration (P — ET) for the four mid-season months based on NCEP/NCAR data for the period 1970-99 (mm). Contours are at every 10 mm for amounts up to 50 mm (negative values dashed) and at every 20 mm for amounts of 60 mm and higher (dotted) (by the authors).

Annual P — ET over the central Arctic Ocean is 200-300 mm, broadly similar to annual precipitation. The implication is that annual ET is small in this region, which is borne out clearly from the SHEBA results discussed earlier. A further implication is that estimates of precipitation over the central Arctic Ocean can be obtained from the P — ET field in the absence of surface observations. The annual excess of P over ET is greatest off the Greenland coast and around Iceland as well southern Alaska where precipitation is high. Note the areas of negative P — ET south of Svalbard. Like the annual cycle in precipitation, P — ET has a summer maximum and cold season minimum over the Arctic Ocean while the areas off eastern Greenland and along southern Alaska have a cold season maximum and summer minimum. By comparison, P — ET over much of the land area is at a minimum in summer, in rough antiphase with precipitation. Over Eurasia, P — ET in June and July is negative over large areas. This points to fairly high summer ET rates. We will return to this issue shortly.

The regions of negative P — ET south of Svalbard are not present in the summer months. These features appear to be associated with strong evaporation in the cold months associated with the contrast between open water and the cold, dry overlying atmosphere. The impact of strong convective heating in this area on the heat budget of the polar cap was examined in Chapter 3.

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