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On the heat balance terms in the central region of the Red Sea
Research, Vol.34. Printed inGreatBritain. Deep-Sea
019&0149/87 $3.00+ 0.00 0 1987Pergamon Journals Ltd.
No. 10. pp. 1757-1760, 19X7
NOTE On the heat balance terms in the central region of the Red Sea F. (Received 8 December
AHMAD*
and S. A. R.
SULTAN*
1986; in revised form 6 April 1987; accepted 27 April 1987)
Abstract-Estimates of the heat balance terms based on the monthly mean meteorological variables have been made in the central region of the Red Sea between 21”N and 22”N latitudes. The evaporative flux of 165 W m-’ (208 CL y-‘) is close to the accepted annual evaporation of 210 cm. The sensible heat flux and the net long-wave radiation terms are -3 and 57 W mm’, respectively. The observed solar radiation at land stations around the area of study Bverages about 220 W mm’, indicating that the heat lost from the sea surface is balanced by the heat gain.
INTRODUCTION THE
various terms which contribute to the heat balance at the air-sea interface are absorbed solar radiation, net long-wave radiation, and evaporative and sensible heat transfer. The annual contributions of the heat fluxes are normally in balance if the heat carried by ocean currents and other processes is neglected. The main factors governing the general circulation and the water budget in the Red Sea are the high rate of evaporation, the reversal of wind regime in its southern part, and the narrowness and shallowness of Bab-el-Mandab. Consequently most workers have emphasized quantifying evaporation in the area. Following the principle of conservation of volume and salt, GRASSHOFF(1969) estimated annual evaporation as 200 cm y-l. Using water balance, other estimates are 266 cm y-’ (BOGDANOVA, 1966), 200 cm y-’ (SIEDLER, 1968), and 205 cm y-’ (MORCOS, 1970). YEGOROV (1950) and PRIVET-T(1959), using the vertical water vapour flux in the lower atmosphere, obtained values of 230 and 183 cm y-‘, respectively. From the heat balance equation and Bowen’s formula, NEUMANN (1952) estimated the annual evaporation in the Daedalus Reef area as 215 cm y-‘. Based on these previous studies MORCOS (1970) concluded that the average annual evaporation in the Red Sea is 210 cm y-l. More recently BUNKER and GOLDSMITH(1979). using BUDYKO’S (1963) method, revised the evaporation values to 183 W m-*. This estimate was taken by BUNKER et al. (1982) for their annual mean heat balance of the Red Sea. BUNKER (1976) and BUNKER et al. (1982) estimated the sensible heat flux from the sea as 5 and 3 W m-*, respectively, in contrast to a negative value by NEUMANN(1952). The published work on the net long-wave radiation includes HASTENRATH and LAMB (1978), 75 W m-*, BUNKER(1976), 48 W rnm2, and BUNKERand GOLDSMITH (1979), 76 W m-‘. * Faculty of Marine Science, King Abdulaziz University, P.O. Box 1540. Jeddah 21441. Saudi Arabia. 1757
F. AHMAD and S. A. R. SLWA~
1758 Tuhle
1.
Monthly
uverrrges of yea surfuce
temperature, heat
Sea
Relative humidity (“A,)
surface
temp. (L)
Air temp. (L)
Jan. Feb. Mar. Apr. Ma) Jun. Jul. Aug.
24.X 23.5 25.2 36.5 2x.0 2X.X 30.s 30.x
25.0 25.1 25.6 26.5 2x.2 30.5 31.-1 31.2
Sep.
29.5
29.6
Oct. Nov. Dee.
2X.5 2J.S 26.5
2x.2 27.2 26.0
mereorologicd
hulunce
wriuhles
trnrl the computed
vul~tes
of
ierrns
Wind speed (ms ‘)
72 71 73 76 7X 7X 71 77 Xl 7X 70 6X
Q,, (W m ‘)
4.x 5.x X. I 6.3 5.1 3.x 4.2 5.2 5.6 5.1 6.2 5.0
-2 -9 -X 0 _ 3 -16 -9 -5 - I 3 5 6
Q,. (W m ‘)
Q/> (W m ‘)
141 162 215 176 117 65 15x 164 I40 IS6 217 206
67 67 64 60 54 16 49 46 47 S4 63 6X
Estimates of solar radiation include HASTENRATII and LAMB (1979), 230 W m-‘. BUNKER and GOL~SMIII~ (1979), using BUDYKO’S (1963) method, gave an estimate of 263 W m-‘, and BUNKER et al. (1982) support this value for their annual heat balance of the Red Sea. The present study is based on data made available by the Saudi-Sudanese commission for the exploitation of the Red Sea resources, and was collected during the “Environmental Survey Programme” 1977-1978, Atlantis II Deep Project. Sea surface temperature and meteorological parameters were observed on board R.V. Soefa at 3-hourly intervals during the programme, 14 June, 1977 to 15 June, 197X. from 21 stations on four longitudinal sections between 21”N and 22”N latitudes. Monthly averages of sea surface temperature, air temperature, relative humidity, and wind speed are given in Table 1. EVAI’ORATIVE For the evaporative
and sensible
ANDSENSIBLEHEATFLUX
heat fluxes the well-known
bulk equations
are used:
Qc = P,, L C (40 - qo) W Qt, = ~a C,, C (to - t,) W. where q(, is saturated specific humidity at sea surface temerature, ql, is specific humidity in the air, pa is density of the air, C,, is specific heat of air at constant pressure, L is latent heat of evaporation, W is wind speed, t, and t,, are the sea surface and air temperatures, respectively, and C = 2.1 x lo-” (BUNKER et al., 1982). These equations reduce to Q, = 3.8 (e, - e,,)
W
Qh = 2.5 (to - t,) w
W mm2 W mm2
where W is measured in m s-’ and t, and t, in “C. e, is saturation vapour pressure at the sea surface temperature and e, is the vapour pressure at the air temperature, both expressed in millibars. The saturation vapour pressure for seawater, which is about 2% lower than freshwater, is taken for these calculations. The computed values of Q, and Q,, based on the average monthly data are given in Table 1.
Heat balance terms of the Red Sea
1759
NET LONG-WAVE RADIATION The effective back radiation depends on sea surface temperature, humidity, and the amount of cloud cover. BERLIANDand BERLIAND(1952) established an expression under cloudless skies: Qh = E(sT~ [0.39 - 0.0.58(e,)“], where E is the coefficient of emissivity with an average value of 0.985 (KLAUS, 1972), o is the Stefan-Boltzman constant, To the and e, is in millimetres of mercury. The above absolute sea surface temperature, expression was favoured by EFIMOVA(1961), for average and high humidities, in processing many of the observational data obtained during the period of the International Geophysical Year. For the decrease of net long-wave radiation with the increase of cloudiness, BUDYKO(1974) favoured an expression: I = I, (1 - C r?), where C is a factor which depends on latitude and n is the cloudiness in fraction of unity. Putting the numerical values and expressing e, in millibars, the expression becomes: Qh = 5.7 x lo-’ T; [0.39 - O.O504(e,)+j[l - 0.59 nL] W m-*. The computed values of the net long-wave radiation are given in Table 1 for an average cloud cover of 20%.
SOLAR RADIATION Recorded solar radiation from Port Sudan western coast of the Red Sea, between 1964 and 1968, corresponds to 225 W m-* (BUNKERet al., 1982). This value is supported by solar radiation from inland stations Taif, Biljurshi, Riyadh on the eastern side, recorded between 1975 and 1980,214,216 and 229 W m-*, respectively (MINISTRYOFAGRICULIURE AND WATER, 1975-1980); this suggests that BUNKERet al’s (1982) figure of 263 W m-* for the Red Sea was overestimated.
RESULTS
AND
DISCUSSIONS
The sensible heat flux in the Red Sea is negative, with an annual average of -3 W m-* in agreement with NEUMANN(1952). The calculated evaporation of 165 W m-’ is close to the average evaporation of 167 W m-* (MORCOS, 1970). The back radiation term is 57 W m-* which is about 35% of evaporation. The observed solar radiations from land stations at the western and eastern sides of the central Red Sea amount to an average heat flux of about 220 W rnp2 at the sea surface, indicating that the heat lost is balanced by the heat gain at the sea surface. BUNKERet al. (1982) studied the annual mean heat balance of the Red Sea using both the BUNKER(1976) and BUDYKO(1963) formulae as well as a modification (BUNKERand GOLDSMIT~I, 1979) that corrected the outgoing longwave radiation fluxes for the effects of upper-air humidity. Available radiation fluxes for the Red Sea seem to suggest that BUNKERet al. (1982) overestimated this term and consequently the net long-wave radiation and evaporative fluxes. Our findings indicate that their radiative heat flux was overestimated by as much as 16-20%) the net long-wave radiation and evaporative fluxes by about 30 and 10% respectively.
Acknowledgemenf-Co-operation highly acknowledged.
of the Saudi-Sudanese Red Sea Commission for making available the data is
1760
F. AHMADand S. A. R. SULTAN REFERENCES
BERLIANDM. E. and T. G. BERLIAND(1952) Determining the net long wave radiations of the earth with consideration of the effect of cloudiness. Izvestiya Akademii Nauk SSSR. Seriya Geofiziches Kaya, No. 1. BOGDANOVAA. K. (1966) Oceanological research. Vol. 15, Soviet Geophysical Committee. Academy of Sciences, U.S.S.R., pp. 45-68. BUDYKOM. I. (1963) Atlas of the heat balance offhe earth. Glabnaia Geofiz. Obsev. BUDYKOM. 1. (1974) Climate and rife. International Geophysical Series 18. BUNKERA. F. (1976) Computation of surface energy flux and annual air-sea interaction cycles of the North Atlantic Ocean. Monthly Weather Review, 104. 1122-I 140. BUNKERA. F. and R. A. GOLDSMITH(1979) Archived time series of Atlantic Ocean meteorological variables and surface fluxes. Woods Hole Oceanographic Institution Technical Reports No. WH$l, 79-3. BUNKERA. F., H. CHARN~CKand R. A. GOLDSMITH (1982) A note on the heat balance of Mediterranean and Red Seas. Journal of Marine Research, 40. 73-84. EFIMOVAN. A. (1961) On methods of calculating monthly values of net long wave radiations. Mefeorologiya i Gidrologia, No. 10, pp. 28-33. GRASSHOFFK. (1969) Zur Chemie Roten Meers und des inneren Golfs von Aden nach Beobachtungen von F.S. “Meteor” wahrend der Indischen Ozean Expedition 1964/65. Meteor Forschungsergebnisse Gebrudrr Borntraeger,
Berlin Reihe A, 6, 1-76.
HASI-ENRATHS. and P. J. LAMB (1978) Heal budget adas of rhe tropical Atlanlic and eastern Pacific Ocean. University of Wisconsin Press, Madison. KRAUSE. B. (1972) Atmosphere-ocean interaction. Clarendon Press. Oxford. MINISTRYOF AGRICULTUREand WATER(1975-1980) Water atlas of Saudi Arabia. MOR~OSS. A. (1970) Oceanography and Marine Biology, Annual Review, 8. 73-202. NEUMANNJ. (1952) Evaporation from the Red Sea. Israel Exploration Journal, 2, 15_%162. PRIVEI-~D. W. (1959) Monthly chart of evaporation from the North Indian Ocean including the Red Sea and the Persian Gulf. Quarterly Journal of the Royal Meteorological Society. 85, 424-428. SIEDLER G. (1968) Schichtungs-Und Bewegungsverhlltisse am Siidausgang des Roten Meeres. Meteor Forschungsergebnisse
Reihe A4,
I.
YEGOROVN. I. (1950) Calculation of the heat balance of the Red Sea. Meteorologiya i Gidrologia. Moscow. No. 3, pp. 49-56.
019&0149/87 $3.00+ 0.00 0 1987Pergamon Journals Ltd.
No. 10. pp. 1757-1760, 19X7
NOTE On the heat balance terms in the central region of the Red Sea F. (Received 8 December
AHMAD*
and S. A. R.
SULTAN*
1986; in revised form 6 April 1987; accepted 27 April 1987)
Abstract-Estimates of the heat balance terms based on the monthly mean meteorological variables have been made in the central region of the Red Sea between 21”N and 22”N latitudes. The evaporative flux of 165 W m-’ (208 CL y-‘) is close to the accepted annual evaporation of 210 cm. The sensible heat flux and the net long-wave radiation terms are -3 and 57 W mm’, respectively. The observed solar radiation at land stations around the area of study Bverages about 220 W mm’, indicating that the heat lost from the sea surface is balanced by the heat gain.
INTRODUCTION THE
various terms which contribute to the heat balance at the air-sea interface are absorbed solar radiation, net long-wave radiation, and evaporative and sensible heat transfer. The annual contributions of the heat fluxes are normally in balance if the heat carried by ocean currents and other processes is neglected. The main factors governing the general circulation and the water budget in the Red Sea are the high rate of evaporation, the reversal of wind regime in its southern part, and the narrowness and shallowness of Bab-el-Mandab. Consequently most workers have emphasized quantifying evaporation in the area. Following the principle of conservation of volume and salt, GRASSHOFF(1969) estimated annual evaporation as 200 cm y-l. Using water balance, other estimates are 266 cm y-’ (BOGDANOVA, 1966), 200 cm y-’ (SIEDLER, 1968), and 205 cm y-’ (MORCOS, 1970). YEGOROV (1950) and PRIVET-T(1959), using the vertical water vapour flux in the lower atmosphere, obtained values of 230 and 183 cm y-‘, respectively. From the heat balance equation and Bowen’s formula, NEUMANN (1952) estimated the annual evaporation in the Daedalus Reef area as 215 cm y-‘. Based on these previous studies MORCOS (1970) concluded that the average annual evaporation in the Red Sea is 210 cm y-l. More recently BUNKER and GOLDSMITH(1979). using BUDYKO’S (1963) method, revised the evaporation values to 183 W m-*. This estimate was taken by BUNKER et al. (1982) for their annual mean heat balance of the Red Sea. BUNKER (1976) and BUNKER et al. (1982) estimated the sensible heat flux from the sea as 5 and 3 W m-*, respectively, in contrast to a negative value by NEUMANN(1952). The published work on the net long-wave radiation includes HASTENRATH and LAMB (1978), 75 W m-*, BUNKER(1976), 48 W rnm2, and BUNKERand GOLDSMITH (1979), 76 W m-‘. * Faculty of Marine Science, King Abdulaziz University, P.O. Box 1540. Jeddah 21441. Saudi Arabia. 1757
F. AHMAD and S. A. R. SLWA~
1758 Tuhle
1.
Monthly
uverrrges of yea surfuce
temperature, heat
Sea
Relative humidity (“A,)
surface
temp. (L)
Air temp. (L)
Jan. Feb. Mar. Apr. Ma) Jun. Jul. Aug.
24.X 23.5 25.2 36.5 2x.0 2X.X 30.s 30.x
25.0 25.1 25.6 26.5 2x.2 30.5 31.-1 31.2
Sep.
29.5
29.6
Oct. Nov. Dee.
2X.5 2J.S 26.5
2x.2 27.2 26.0
mereorologicd
hulunce
wriuhles
trnrl the computed
vul~tes
of
ierrns
Wind speed (ms ‘)
72 71 73 76 7X 7X 71 77 Xl 7X 70 6X
Q,, (W m ‘)
4.x 5.x X. I 6.3 5.1 3.x 4.2 5.2 5.6 5.1 6.2 5.0
-2 -9 -X 0 _ 3 -16 -9 -5 - I 3 5 6
Q,. (W m ‘)
Q/> (W m ‘)
141 162 215 176 117 65 15x 164 I40 IS6 217 206
67 67 64 60 54 16 49 46 47 S4 63 6X
Estimates of solar radiation include HASTENRATII and LAMB (1979), 230 W m-‘. BUNKER and GOL~SMIII~ (1979), using BUDYKO’S (1963) method, gave an estimate of 263 W m-‘, and BUNKER et al. (1982) support this value for their annual heat balance of the Red Sea. The present study is based on data made available by the Saudi-Sudanese commission for the exploitation of the Red Sea resources, and was collected during the “Environmental Survey Programme” 1977-1978, Atlantis II Deep Project. Sea surface temperature and meteorological parameters were observed on board R.V. Soefa at 3-hourly intervals during the programme, 14 June, 1977 to 15 June, 197X. from 21 stations on four longitudinal sections between 21”N and 22”N latitudes. Monthly averages of sea surface temperature, air temperature, relative humidity, and wind speed are given in Table 1. EVAI’ORATIVE For the evaporative
and sensible
ANDSENSIBLEHEATFLUX
heat fluxes the well-known
bulk equations
are used:
Qc = P,, L C (40 - qo) W Qt, = ~a C,, C (to - t,) W. where q(, is saturated specific humidity at sea surface temerature, ql, is specific humidity in the air, pa is density of the air, C,, is specific heat of air at constant pressure, L is latent heat of evaporation, W is wind speed, t, and t,, are the sea surface and air temperatures, respectively, and C = 2.1 x lo-” (BUNKER et al., 1982). These equations reduce to Q, = 3.8 (e, - e,,)
W
Qh = 2.5 (to - t,) w
W mm2 W mm2
where W is measured in m s-’ and t, and t, in “C. e, is saturation vapour pressure at the sea surface temperature and e, is the vapour pressure at the air temperature, both expressed in millibars. The saturation vapour pressure for seawater, which is about 2% lower than freshwater, is taken for these calculations. The computed values of Q, and Q,, based on the average monthly data are given in Table 1.
Heat balance terms of the Red Sea
1759
NET LONG-WAVE RADIATION The effective back radiation depends on sea surface temperature, humidity, and the amount of cloud cover. BERLIANDand BERLIAND(1952) established an expression under cloudless skies: Qh = E(sT~ [0.39 - 0.0.58(e,)“], where E is the coefficient of emissivity with an average value of 0.985 (KLAUS, 1972), o is the Stefan-Boltzman constant, To the and e, is in millimetres of mercury. The above absolute sea surface temperature, expression was favoured by EFIMOVA(1961), for average and high humidities, in processing many of the observational data obtained during the period of the International Geophysical Year. For the decrease of net long-wave radiation with the increase of cloudiness, BUDYKO(1974) favoured an expression: I = I, (1 - C r?), where C is a factor which depends on latitude and n is the cloudiness in fraction of unity. Putting the numerical values and expressing e, in millibars, the expression becomes: Qh = 5.7 x lo-’ T; [0.39 - O.O504(e,)+j[l - 0.59 nL] W m-*. The computed values of the net long-wave radiation are given in Table 1 for an average cloud cover of 20%.
SOLAR RADIATION Recorded solar radiation from Port Sudan western coast of the Red Sea, between 1964 and 1968, corresponds to 225 W m-* (BUNKERet al., 1982). This value is supported by solar radiation from inland stations Taif, Biljurshi, Riyadh on the eastern side, recorded between 1975 and 1980,214,216 and 229 W m-*, respectively (MINISTRYOFAGRICULIURE AND WATER, 1975-1980); this suggests that BUNKERet al’s (1982) figure of 263 W m-* for the Red Sea was overestimated.
RESULTS
AND
DISCUSSIONS
The sensible heat flux in the Red Sea is negative, with an annual average of -3 W m-* in agreement with NEUMANN(1952). The calculated evaporation of 165 W m-’ is close to the average evaporation of 167 W m-* (MORCOS, 1970). The back radiation term is 57 W m-* which is about 35% of evaporation. The observed solar radiations from land stations at the western and eastern sides of the central Red Sea amount to an average heat flux of about 220 W rnp2 at the sea surface, indicating that the heat lost is balanced by the heat gain at the sea surface. BUNKERet al. (1982) studied the annual mean heat balance of the Red Sea using both the BUNKER(1976) and BUDYKO(1963) formulae as well as a modification (BUNKERand GOLDSMIT~I, 1979) that corrected the outgoing longwave radiation fluxes for the effects of upper-air humidity. Available radiation fluxes for the Red Sea seem to suggest that BUNKERet al. (1982) overestimated this term and consequently the net long-wave radiation and evaporative fluxes. Our findings indicate that their radiative heat flux was overestimated by as much as 16-20%) the net long-wave radiation and evaporative fluxes by about 30 and 10% respectively.
Acknowledgemenf-Co-operation highly acknowledged.
of the Saudi-Sudanese Red Sea Commission for making available the data is
1760
F. AHMADand S. A. R. SULTAN REFERENCES
BERLIANDM. E. and T. G. BERLIAND(1952) Determining the net long wave radiations of the earth with consideration of the effect of cloudiness. Izvestiya Akademii Nauk SSSR. Seriya Geofiziches Kaya, No. 1. BOGDANOVAA. K. (1966) Oceanological research. Vol. 15, Soviet Geophysical Committee. Academy of Sciences, U.S.S.R., pp. 45-68. BUDYKOM. I. (1963) Atlas of the heat balance offhe earth. Glabnaia Geofiz. Obsev. BUDYKOM. 1. (1974) Climate and rife. International Geophysical Series 18. BUNKERA. F. (1976) Computation of surface energy flux and annual air-sea interaction cycles of the North Atlantic Ocean. Monthly Weather Review, 104. 1122-I 140. BUNKERA. F. and R. A. GOLDSMITH(1979) Archived time series of Atlantic Ocean meteorological variables and surface fluxes. Woods Hole Oceanographic Institution Technical Reports No. WH$l, 79-3. BUNKERA. F., H. CHARN~CKand R. A. GOLDSMITH (1982) A note on the heat balance of Mediterranean and Red Seas. Journal of Marine Research, 40. 73-84. EFIMOVAN. A. (1961) On methods of calculating monthly values of net long wave radiations. Mefeorologiya i Gidrologia, No. 10, pp. 28-33. GRASSHOFFK. (1969) Zur Chemie Roten Meers und des inneren Golfs von Aden nach Beobachtungen von F.S. “Meteor” wahrend der Indischen Ozean Expedition 1964/65. Meteor Forschungsergebnisse Gebrudrr Borntraeger,
Berlin Reihe A, 6, 1-76.
HASI-ENRATHS. and P. J. LAMB (1978) Heal budget adas of rhe tropical Atlanlic and eastern Pacific Ocean. University of Wisconsin Press, Madison. KRAUSE. B. (1972) Atmosphere-ocean interaction. Clarendon Press. Oxford. MINISTRYOF AGRICULTUREand WATER(1975-1980) Water atlas of Saudi Arabia. MOR~OSS. A. (1970) Oceanography and Marine Biology, Annual Review, 8. 73-202. NEUMANNJ. (1952) Evaporation from the Red Sea. Israel Exploration Journal, 2, 15_%162. PRIVEI-~D. W. (1959) Monthly chart of evaporation from the North Indian Ocean including the Red Sea and the Persian Gulf. Quarterly Journal of the Royal Meteorological Society. 85, 424-428. SIEDLER G. (1968) Schichtungs-Und Bewegungsverhlltisse am Siidausgang des Roten Meeres. Meteor Forschungsergebnisse
Reihe A4,
I.
YEGOROVN. I. (1950) Calculation of the heat balance of the Red Sea. Meteorologiya i Gidrologia. Moscow. No. 3, pp. 49-56.