242 Evaporation from the Surface of the Sea and the Water Budget of the Earth 



side of continents than on the western side. This is mainly due to differences in 

 cloudiness. 



The annual heat loss by evaporation, Q^, according to Jacobs (1951) is given in 

 Fig. 107. Evaporation is particularly large in the v^estern parts of the two oceans, 

 where the currents carry warm water northward (Gulf Stream and Kuroshio). It is, 

 however, less in the eastern parts where there are cold currents flowing southward. 

 The extreme seasons show considerable quantitative differences in evaporation. In 

 middle and higher latitudes the evaporation is large in winter and small in summer, 

 but conditions may be rather complicated on the western sides of the oceans where in 

 winter cold air is advected out from the continents over the warmer sea. 



The heat loss Q^ of sensible heat by convection is shown in Fig. 108 for the same 

 oceans. Also one notices here a distinct increase on the western sides of the oceans 

 which is of the same type as in the distribution of Q^. In the over-all distribution of 

 energy given off from the sea as heat, the values for the evaporation predominate and 

 set the basic pattern. With respect to seasonal changes also the behaviour of both loss 

 items is rather similar. The sum —Qa = {Qe+ Qn) gives a final value for the total heat 

 turnover as far as it applies to a current-free ocean. If currents are present then the 

 equation Qv= Qr— Qa must apply, where Q^ is the energy surplus which is obtained 

 by each cm^ of the surface under influence of a complete heat exchange with the 

 atmosphere. This energy surplus, when positive, is carried away from the water mass 

 unber consideration by currents and mixing processes and represents that part of the 

 radiational gain Qr which is stored in the water. A negative surplus implies that energy 

 is supplied to the water mass by currents and mixing processes which then is dissi- 

 pated by the excess in radiation into the atmosphere (Sverdrup, 1945). Figure 109 shows 

 the total energy surplus of the oceanic water (g cal cm^^ day"^), and shows that in 

 the water of larger ocean surfaces, especially in middle and lower latitudes along and 

 near to the western coasts of continents, some energy is stored in the water while 

 enormous amounts of energy are dissipated (lost by the ocean) in the Gulf Stream and 

 Kuroshio systems. Thus, to a very noticeable extent, the areas in which large amounts 

 of energy are available to the atmosphere are localized in definitive oceanic regions. 

 Comparison of Figs. 109 and 107 clearly shows that the pattern of total energy ex- 

 change corresponds to that of evaporation. In order to recognize the seasonal varia- 

 tions in the energy turnover, it is of advantage to compare these quantities along 

 definite latitudes or meridians along the eastern and the western sides of the oceans 

 respectively. This can be seen from Fig. 110. The first diagram shows a marked con- 

 trast between western and the eastern sides of the oceans. A narrow band representing 

 the energy loss appears along with the Gulf Stream in the North Atlantic, while a 

 corresponding and more broad band is connected with the Kurishio. This is under- 

 standable from the direction of the two currents in the zone between 35° N and 40° N. 

 The contrast of the two sides of the ocean in a meridional direction is shown in the 

 other two diagrams. Along the western sides at all times of the year, except in summer, 

 the largest amounts of energy are given off between 25° N. and 40° N to 50° N. Along 

 the eastern sides there is a winter minimum in these latitudes. These energy transports 

 arc undoubtedly of decisive importance for the climatic conditions in the effected 

 regions and form the basis of the study of the inter-relation between ocean and atmos- 

 phere. 



