14 



BJ0RN HELLAND-HANSEN 



(REP. OF THE "MICHAEL SARS" NORTH 



To begin with we may lay down following propo- 

 sitions: 



The conditions in the atmosphere have a dynamical 

 and a thermal effect upon the ocean. Dynamically, the 

 essential direct effect is the creation of wind-currents, and 

 of convection currents due to local variations of precipi- 

 tation and evaporation. The wind pushes the surface 

 water forwards, generally in a direction cum sole from 

 the direction of the wind. The water is displaced from 

 its normal^ position by variations of the wind, and such 

 displacements may be recognized in annual or monthly 

 averages from fixed geographical areas. The geographical 

 variations of] precipitation and evaporation on the one 

 hand and of temperature on the other will, upon the 

 whole, give a certain positive correlation between tempe- 

 rature and salinity. In the North Atlantic these quantities 

 as a general rule increase or decrease simultaneously (cf. 

 section 18). We shall mostly find that the isohalines and 

 the isotherms, even at the surface, go parallel, and that 

 a close agreement between the variations in salinity and 

 temperature is established. 



The thermal effect of the atmosphere upon the sea 

 is chiefly of two kinds: indirectly by the variations in 

 transparency which will evoke variations in the quantity 

 of solar radiation absorbed by the sea and of radiation 

 from the sea, and directly by the heating or cooling effect 

 of the air by its contact with the water. Investigations 

 have shown that, on an average, 2/3 of the solar radiation 

 penetrating into the atmosphere is absorbed by the latter, 

 while the rest reaches the sea or the solid crust of the 

 earth. The solar radiation is subject to variations, as is 

 also the quantities of solar radiation absorbed by the 

 atmosphere in proportion to the total radiation. The 

 variations in atmospheric absorption create variations in 

 air pressure and winds, and thereby indirectly affect the 

 ocean currents. The variations in oceanic absorption 

 affect the temperature of the sea directly and give rise to 

 various hydrographic variations (thermally and dynamically). 

 The direct thermal effect of the atmosphere upon the sea, 

 due to contact between the two elements, is but small 

 when observed for a brief period, but the cumulative 

 effect may be rather substantial, especially as regards the 

 cooling in winter. 



The sea must, on the other hand, exercise a powerful 

 influence upon the atmosphere. The air at the sea surface 

 very quickly adopts a temperature which is near that of 

 the sea. The consequence is that the air temperature 

 above the ocean will show on an average much the same 

 distribution as the surface temperature. The comparatively 

 slight differences of temperature between water and air 

 are however of the utmost importance to the atmospheric 

 processes. A positive difference (sea temperature minus 



air temperature) means a state of instability whereby the 

 heat as well as the moisture will be propagated upwards 

 into the atmosphere. This process is highly intensified 

 by turbulence, and the amount of heat or moisture carried 

 to distant regions and released there depends to a great 

 extent upon the wind^velocity, as well as upon the value 

 of the temperature difference. In the state of instability 

 the total amount of moisture (or the evaporation from 

 the sea) tends to increase, although the humidity measured 

 on board a ship may show a decrease, at any rate to 

 begin with, under ordinary circumstances. A negative 

 difference of sea temperature minus air temperature means 

 stability. The cooling of the atmosphere will not be 

 propagated as quickly upwards as the heating in the former 

 case. It is limited to a thinner stratum, in the lower 

 part of which the relative humidity rises, ultimately with 

 the effect that the evaporation decreases, or even comes 

 to a stand-still. The cooling of the air from the under 

 surface will, therefore, quantitatively have little effect upon 

 the atmosphere in comparison with the heating. As the 

 sea-surface in most regions and during the greater part 

 of the year is warmer than the air, the levelling influence 

 of the sea upon the climate must be a levelling "upwards" 

 as far as temperature and precipitation are concerned: 

 the sea gives on an average a higher temperature (not 

 only a smaller amplitude) and a greater precipitation. 



One consequence of the above conclusions is that 

 there is an intimate connection between the surface 

 salinity, the surface temperature, the air temperature, the 

 humidity, and even to some extent the cloudiness. The 

 relations between these various elements are modified by 

 the variations in wind, which (at least to a great extent) 

 means the variations in solar activity and in the absorp- 

 tion-conditions of the troposphere. 



The atmospheric conditions tend to be relatively 

 uniform above an oceanic region with but slight differences 

 in temperature. Places where abrupt hydrographic changes 

 exist, however, are the scene of many and rapid mete- 

 orological variations. Such a region is found along the 

 northern border of the "Gulf Stream" across the Atlantic. 

 Even short displacements of the air may here create 

 pronounced stability or pronounced instability with corres- 

 ponding variations in heating effect, evaporation and 

 cloudiness. It is especially important that a comparatively 

 high degree of instability is often evoked by "Polar" air 

 coming into the "Gulf Stream" area. A great deal of 

 water vapour will thereby come into the atmosphere. 

 When more than ordinary quantities of such "Polar" 

 air move across the warm areas of the North Atlantic and 

 proceed to Northern Europe we may expect to find, on 

 the average, in the latter region a surplus of rainfall with a 

 reduced annual amplitude of air temperature; possibly this 



