RSSUMÉ : O n considbre le problkrne de la détermination des composantes du bilan des eaux des océans. On discute les méthodes de la détermination des précipitations et de l’évaporation i la surface des océans. O n citc les données des valeurs moyennes des composantes du bilan des eaux des divers océans et de l’ensemble des océans.
BALANCE HIDKOLÓGJCO D E LOS O C k A N O S
RESUMEN: Se estudia la cuestión relativa al cálculo de los componentes del balance hidrológico de los océanos. También se examinan los métodos para determinar la prccipitación y la evapora- ción sobre los océanos, y se prescntan datos sobre los vulorcs medios de los componciites del balance hidrológico de difercntcs océanos y del conjunto de los océanos.
The Wuter Bularice u/ the Oceuns
ATJIaHTU¶eCK€iÜ OKeaH 89 124
TlzuIIfi OKeaH 133 132
EFHAIIÜCKHÜ OKeaH 117 132
MHPOBOÜ oKeaH 114 126
23 7 8 12
The water exchange over the oceans has a determining influence on the water balance of our planet. Evaporation from the oceans amounts to 90% of the total evaporation from the earth’s surface. A little less is the ratio of precipitation at the surface of the oceans to the total amount of precipitation at the globe. Thus, the accuracy of estimating the components of the water balance of the Earth depends considerably on the accuracy of data on the water balance of the oceans.
Meanwhile the water balance of the oceans has so far been studied insufficiently, which is explained to a considerable extent by difficulty in estimating the components of the ocean water balance from the available observations.
The equation of water balance of the world ocean for the annual mean conditions can be given in the form
where:
r is precipitation;
f is runoff from the continents to the oceans;
E
is evaporation.r+f
= E
(11This evaporation characterizes the water balance o€ individual oceans or individual ocean areas, the componentfrepresenting, in the latter case, a total amount of runoff and horizontal redistribution of oceanic waters as a result of circulation in the hydrosphere.
Data on precipitation over the oceans, that are obtained from ship observations, characterize only the îrequeiicy of prccipitution fallout and do not contain information on amounts of precipitation.
In this conncction, most of charts of precipitation over the oceans were prepared from the data of observations at island and coastal stations. It is evident that using lhis method it is difficult to estimate the value of precipitation Tor many open ocean areas. Besides, an opinion was expressed more than once that the observations on land near coasts give appreciable systematic errors when estimating precipitation over the oceans since along the coastal line there are often developed air updrafts increasing the ainount of rainfall. Therefcre it was assumed that actual values of precipitation over the oceans are smaller than those obtained from observations at island and coastal stations.
The world charts of precipitation distribution over the oceans were prepared by Meinardus (1934), Schott (1926, 1935), O. A. Drozdov and I.A. Berlin (1953), Albrecht (1960), L.P. Kuznetsova and V.Ya. Sharova (1964) and olher authors. W h e n comp- uting the components of water balance of the oceans from the charts available at that time, Wüst (1936) paid attention to the fact that the gain of water from precipitation and runoff is considerably greater than evaporation from the oceans. In this connection h e concluded that for the reason given above the actual values of precipitation over the oceans are by 30% smaller than thosegiven in the charts. Such a view-point was then taken into account in the works by Jacobs (1951) and Möller (1951) who, when drawing charts of precipitation over the oceans, corrected lhe results obtained by the water balance, which decreased considerably the values of precipitation.
In the work by Albrecht (1951) for estimating precipitation over the oceans there were used data of ship observations on the frequency of precipitation fallout. For this purpose, Albrecht estimated, from the results of observations at land, values of precipitation intensity for various latitudes and then computed amounts of precipitation over the oceans as the product of the mean values for the corresponding area by the frequency of precipi- tation estimated from data of observations. Charts of precipitation over the Indian and Pacific Oceans were obtained by this method for several months.
Similar method was used by Tucker (1961) for drawing a chart of precipitation over the North Atlantic, and by V. S. Samoilenko (1966) w h o prepared the annual and monthly charts of precipitation over the Pacific Ocean.
It should be noted that along with the use of observational data on precipitation there is a possibility of computing precipitation amounts over the oceans using empirical or theoretical dependences connecting precipitation with other ineteorological elements.
The accuracy of such computations, for a number of reasons, cannot be high, which does not exclude, however, the possibility of using them for comparison with the results of observations.
The existing data on the mean values of evaporation from the ocean surface are com- puted since direct observations of evaporation over the oceans are too scanty.
For this purpose the following formula is generally used
where:
q s is the specific humidity of the air saturated with water vapour at the tcmperature of oceanic surface;
y is the specific air humidity at the ship observations level;
A is coefficient depending mainly on wind speed.
Beginning with the investigalion of V. V. Shuleikin (1935) it is taken in many works that A
=
air where u is constant coefficient; u is wind speed. In some works a more complex dependence of parameter A on wind spccd is used, as well as thc efTecl of verticalThe Wr,fer Brrluncc of’the Oceuris
tcniperature stratification on coeficient A is considered (Deacon and Webb, 1962);
(Budyko and Gandin, 1966; and others).
Using formula (2) values of evaporation were estimated and charts of evaporation (or heat loss foi evaporation) prepared for some ocean regions. World charts of evaporation from the ocean surface for the year and for each month were prepared at the Main Geophysical Observatory and published in “Atlas of Meat Balance” (1955). Subsequently these charts were corrected by more complete data of observations and published in
“Atlas of the Heat Balance of the Globe” (1963).
It should be noted that the value of evaporation from the ocean surface is closely connected with the radiation balance of the ocean.
The value of turbulent heat flux between the ocean surface and atmosphere is obtained from formula
P =
c,A(Ow-O) (31where:
c, is the specific heat of the air at constant pressure;
O, is the temperature of water surface;
O
is air temperature.The equation of heat balance for the world ocean as a whole is determined by formula
R = L E + P
(4)where:
R is radiation balance;
L is the latent heat of vaporization.
Froin (2), (3) and (4) w e find that for the annual mean conditions the equation of the heat balance of the world ocean is
R =
LA(q,,-q)+ c,A(e,-o)
(51 It should be borne in mind that the terms of the right part of this equation are propor- tional to the sums of products of the coefficient A values by those of differences between humidity and temperature relating to every physical moment of time. These sums m a y differ from the products of the mean values of coeíficient A by the mean differences between humidity and temperature multiplied by the length of the averaging period.In the works by Malkus (1962), the author and L.S. Gandin (1966), and others, quantities A(q,-q) and A(O,-O) were compared with quantities A(q,y-q) and A(O,-f?) respectively. To obtain the first two products, the data of observations for individual titne intervals were used, to estimate the third and fourth ones, the monthly means for the corresponding regions. Such computations showed that the effect of separate averaging of the terms of the above formulae, when estimating turbulent heat fluxes and moisture over the oceans, changes usually their values by not more than a few per cent. Consider- ing this conclusion, from (2), (3) and (5) w e obtain the relation
-- _-
Using data of observations one can find that for the world ocean the value cp (O,
- - e)/
L(q,-q) is considerably smaller than unity and equals .approximately 0.1. Taking this into account, it follows from formula (6) that the total evaporation from the world
-
M. I. ßiidvko
ocean depends but little on temperature and air humidity, practically does not depend on wind speed, and is determined chieflly by the value of radiation balance of the ocean surface. The heat loss for evaporation is about 90% of the value of radiation balance.
It should be borne in mind in further analysis that for the world ocean the value of runoff is approximately an order of magnitude smaller than the other two components of water balance-precipitation and evaporation. Using the data of M.I. Lvovich (i 964) the water gain from runoff to the world ocean can be found to be equal to 10 c m per year.
From other data (Atlas of the Heat Balance of the Globe, 1963) this value equals 13 cm.
The difference between these values is negligible as compared to the accuracy of estimat- ing the main components of the water balance.
Thus, the basic task of studying the water balance of the world ocean is associated with the estimation of values of precipitation and evaporation that should be in agreement with each other.
As is seen from the above, several independent methods may be used for this purpose., including:
1) estimation of precipitation by data of observations;
2) computation of precipitation;
3) estimation of evaporation by means of formula (2) the coefficients of which have been 4) estimation of evaporation, taking into account the data on radiation balance the value 5) estimation of evaporation, taking into account the data on radiation balance found
found without the use of data on radiation balance;
of which has been computed;
by data of observations.
Let us consider what results are obtained from estimating precipitation at the ocean surface by data of observations.
One of the latest world charts of the annual amounts of precipitation over the oceans that was prepared by L.P. Kuznetsova and V.Ya. Sharova has been published in the physical and geographical atlas of the world (1964). This chart is based mainly on the data of observations of precipitation amounts at coastal and island stations. The amount of precipitation for the year at the world ocean surface, according to this chart, equals 114 cm.
It should be found out what the difference is between the data of the above chart and the amounts of precipitation over the oceans that have been estimated from theship observations of precipitation frequency. Comparison of the precipitation charts drawn by such a method and the charts based on the data of observations at land shows considerable difference between them for individual ocean areas.
Considering this, one can believe that the use of data of ship observations allows LIS
to estimate more reliably totals of precipitation in the open ocean-areas, though in this case it is rather difficult to evaluate the accuracy of the results obtained. The methods for estimating precipitation totals using ship observations are based on consideration of the values of precipitation intensity that changes over the oceans within the wide range.
Since data on precipitation intensity over the oceans are obtained froin observations ut land, the estimates of intensity used in computations of precipitation may be charac- terized by appreciable random and systematic errors.
The random errors in determining precipitation for various regions are eliminated to a consideratle extent when estimating values of precipitation for whole oceuils or for the world ocean. Such a conclusion is confirmed when comparing the totals of precipi- tation for the world ocean estimated by different authors. Thus, in particular, Albrecht (1960, 1961), using data of ship observations, obtaincd anniial values of precipitation in the Pacific and Indian Oceans. Then he changed a little these data foi closing the water balance of the oceans. Howcver, if we compare the values obtained by Albreclit dircclly
The Wnter Balance of the Oceuris
from data of observations with analogous values in the chart of Kuznetsova and Sharova, w e shall find that the totals of precipitation obtained by Albrecht for the Indian Ocean are by 3% smaller and for the Pacific Ocean by 8% larger than the data obtained from the chart of Kuznetsova and Sharova.
The mean value of precipitation for the North Atlantic, that was estimated from Tucker’s chart (1961), is by 20% smaller than the corresponding value from Kuznetsova and Sharova’s chart.
In contrast to this, the amount of precipitation over the Pacific Ocean, that was obtained from V. S. Samoilenko’s chart (1966), is by 10% greater than the value obtained from the data of Kuznetsova and Sharova. The results of these comparisons show that the use of data of ship observations does not lead to marked changes in the estimations of mean totals of precipitation over extensive areas of the oceans. Tucker’s chart, that differs most greatly From Kuznetsova and Sharova’s chart, characterizes a comparatively small part of the world ocean.
One can think that the use of data of ship observations, which allows us to estimate more accurately the spatial distributions of precipitation over the oceans, does not solve the problem of eliminating systematic errors in values of precipitation.
These errors may be associated with two reasons. The first one is the difference between the precipitation intensity over land and that over the oceans. It is possible that because of greater roughness of the land surface and higher intensity of the thermal convection over the continents the intensity of precipitation over the land is, on an average, higher than that over the oceans. In such a case the charts, available give overestimated values of precipitation over the oceans. The second source of errors can be underestimation of precipitation values measured by existing instruments. The value of such error was studied in detail for the territory of the Soviet Union (Struzer and others, 1965; et id.),
and it was established that it was rather considerable for areas with great amount of solid precipitation. For the world ocean as a whole this error cannot be great since the relative role of solid precipitation over the oceans is not significant.
A rough estimation shows that the above-indicated error does not seem to exceed 10%. It is more difficult to estimate the value of error connected with the difference between the precipitation intensity over land and that over the oceans, which in general can be higher than 10%.
The use of existing calculation methods for the estimation of precipitation over the oceans senims not to a Ilow us to estimate precipitation with higher accuracy as compared to observational results. It is still difficult to use the methods of the general theory of cli- mate for this task though in the recent research works of this direction there have been compiled world charts of the amount of precipitation. To compute total of precipitatian over the oceans, it is possible to use empirical relations between precipitation and other meteorological elements determined in the works by A.P. Galtsov and other authors.
Using the empirical dependenee, that was determined by A.P. Galtsov (1962), of precipitation totals on the baric field and air humidity, the author coniputed values of precipitation for the North Atlantic from the data of observation on board weather sh.ips. The precipitation chart drawn by these data is in satisfactory agreement with the chart of Kuznetsova and Sharova.
Such a comparison, however, does not solve the problem on the correctness of soine or other charts of precipitation over the oceans since the dependence obtained by Galtsov is based on observations of precipitation over the land and the use of it for oceans m a y cause certain systematic errors.
Thus, for estiinnting the components of the water balance of the world ocean, the esti- mation of value of evaporation from the oceans is of substantial importance.
To comparc the results of different methods for the estimation of evaporation froin the acean surface, we use the recent world chart of evaporation published in ‘‘Atlas of the Heat Balance of the Glove” (1963). According to the data of this chart, evaporation from
M. I. Buúyko
the world ocean surfax foi. the year is eqnal to 126 cm. Il should be notcd that when comparing the abovementioned chart there was used formula
E =
au íq,, -;4)where coefficienl LI was considered conslant and its value (2.5. IO-‘ g/cm3 was found from closing the equation of heat balance, i.e. from the radiation balance of the occan surface.
There is a number of charts of evaporation from the ocean surface which were prepared through the use of formula (7) or formulae close to it in which coefficient a was considered as a value that changes within the limited range.
Let us examine what results are given by the use of the formulae for estimating evapo- ration in which coefficient n is given without the use of data on radiation balance.
This group of formulae includes those the values of coefficients in which have been found from closing the equation of water balance for inland reservoirs, from. the measure- ments of turbulent fluxes of heat and moisture by meteorological elements pulsations, and by other methods.
In the survey by Robinson (1966) the results are presented of seven works of this direc- tion that were carried out by different authors. By averaging the values of coefficient a obtained in these works, w e find its mean value to be equal to (t.Sh0.3). g/cni3. It is evident that the mean value of evaporation from the world ocean calculated with such. u value of coefficient n will be by 78e/, smaller than the value obtained by the chart from “Atlas of the Heat Balance of the Globe”.
Proceeding to the values of coefficient a estimated by the data on the heat balance of the oceans it should be noted that they give, as a rule, large values of evaporation. In the survey by Robinson there are presented values of this coefficient that were obtained in six investigations in which the values of radiation balance of the oceans were. computed.
The mean value of this coefficient found from these data equals (2.4+0.4).10-6 g/cm3.
As is seen, this value differs but little from the coefficient used when compiling the chart of evaporation from the oceans in “Atlas of the Heat Balance of the Globe”.
For the estimation of the coefficient a by the heat balance, of considerable importance is the fact that the calculations of the radiation balance of the oceans made in the recent years by different authors give rather close results.
Thus, the values of radiation balance found by Albrecht (1960, 1961) are smaller than the data in “Atlas of the Heat Balance of the Globe” (1963), by 9% for the Atlantic Ocean, by 3% for the Jndian and Pacific Oceans, by 5% for the world ocean. The radia- tion balance of the oceans of the Southern Hemisphere, according to Privett (1960), is by 8% greater than the data of the above-indicated atlas. The radiation balance of the North Pacific obtained by Wyrtki (1965) is by 4% smaller than the data of Atlas. The value of radiation balance of the Pacific Ocean, according to the data of V. S. Sanioilenko (1966), is by 13% smaller than the values in “Atlas of the Heat Balance of the Globe”.
Thus, the values of radiation balance found by Albrecht (1960, 1961) are smaller than the data in “Atlas of the Heat Balance of the Globe” (1963), by 9% for the Atlantic Ocean, by 3% for the Jndian and Pacific Oceans, by 5% for the world ocean. The radia- tion balance of the oceans of the Southern Hemisphere, according to Privett (1960), is by 8% greater than the data of the above-indicated atlas. The radiation balance of the North Pacific obtained by Wyrtki (1965) is by 4% smaller than the data of Atlas. The value of radiation balance of the Pacific Ocean, according to the data of V. S. Sanioilenko (1966), is by 13% smaller than the values in “Atlas of the Heat Balance of the Globe”.