Estimation of Evaporation Rates in the Southern Red Sea Based on the AVHRR Sea Surface Temperature Data

Transcription

1 JKAU: Mar. Sci., Vol. 23, No. 1, pp: (2012 A.D. / 1433 A.H.) DOI : /Mar Estimation of Evaporation Rates in the Southern Red Sea Based on the AVHRR Sea Surface Temperature Data Abdullah M. Al-Subhi Faculty of Marine Science, King Abdulaziz University Jeddah Saudi Arabia amalsubnhi@kau.edu.sa Abstract. Dalton's equation is used to study the evaporation in the southern Red Sea. Evaporation is estimated for three regions; (A) Jeddah, (B) Jazan and (C) Hodeidah (Yemen). The annual averages of evaporation for Jeddah, Jazan and Hodeidah are 2.04, 1.29 and 1.25 m/yr respectively. In Jeddah region, the evaporation rate is higher than those of Jazan and Hodeidah. Evaporation is higher in Jeddah during November, December and January, while it is higher in Hodeidah region in the month of October. In Jazan evaporation rate is higher in July. The correlation between the humidity gradient (e w -e a ) and monthly average of evaporation is strong in all regions being approximately 0.8, whereas the correlation between monthly average of evaporation and wind speed is about 0.5. The main cause of higher evaporation is the humidity gradient (e w -e a ) i.e. higher humidity gradient. In Jeddah region higher annual average evaporation is observed during 2000 and 2006 with lower values in 2004 and In Jazan evaporation is higher in 1999 and lower in 2002 whereas in Hodidah the higher values are observed in 2006 and the lower values in These variations are related to the changes in humidity and the wind speed at these locations. Keywords: Southern Red Sea, Evaporation rate, AVHRR SST. Introduction The Red Sea, being geographically located between tropical and subtropical regions, provides a classic example of the interaction between Monsoon (seasonal) meteorological conditions at the sea surface and the thermohaline circulation of the basin. It is a long narrow basin lies 77

2 78 Abdullah M. Al-Subhi between latitudes 12 o N and 28 o N. Its north-south extent is about 2300km and the average width about 280km. The Red Sea mainly exchanges water with the Gulf of Aden through the strait of Bab el Mandeb with a small amount via the Suez Canal. The Red Sea is characterized by low rainfall and high evaporation which controls the properties of the Red Sea and the flow exchange through the Strait of Bab el Mandeb (Thunellet al.,1988; Rohlinget al.,1998; Siddallet al.,2004 and Arzet al.,2007). The average annual evaporation is about 2 m/yr (Morcos, 1970; Behairyet al., 1981; Ahmad and Sultan, 1987, 1989; Al-Barakati, 2005; and Matdoukaset al., 2007). Osman (1984) estimated the evaporation rate near the coast of Port Sudan as 2.04 m/yr. The computed annual average evaporation in the Red Sea presented in Sofianos et al.(2002) and Siddallet al.(2003) is about 2.06±0.22 m/yr. The air temperature over the Red Sea is usually relatively low in the northern part (Morcos, 1970 and Edwards, 1987) and increases southward. The warmest region over the Red Sea is between 20 o N and 16 o N (Edwards, 1987). The average air temperature during February is approximately 18 o C in the northern part. It increases gradually toward southern part and reaches about 26 o C. In August, the average air temperature increases from 29 o C in the north to 33 o C in southern part(ahmad and Sultan, 1989). Monthly average sea surface temperature in the southern Red sea is high compared to the northern part (Morcos, 1970 and Siddallet al., 2004). The amplitude of annual temperature variation in coastal water is greater than those of open water due to the impact of the land, and shallow coastal waters (Morcos, 1970). The wind in southern part of the Red Sea reverses direction with monsoon system over the Indian Ocean. This system controls the water circulation and the exchange of water with the Gulf of Aden (Neumann and McGill, 1962; Phillips, 1966; Siedler, 1969 and Patzert, 1974). The flow of water from the Gulf of Aden to the Red Sea during winter is greater than that during summer (Neumann and McGill, 1962 and Rohlinget al., 1998). In winter along the northeast coast strong wind leads to increase the evaporation and heat loss (Jiang et al., 2009). The possible maximum extent and differences associated with these strong flows along the axis of the Red Sea is associated with a strong decline in limits of relative humidity by both increase of temperature and reducing specific humidity (Eshel and Heavens, 2007).

3 Estimation of Evaporation Rates in the Southern Red Sea Based on the 79 Studies of evaporation process in the southern part of the Red Sea are scarce in comparison with the northern part of the sea. So, the present work is one of the few investigations carried out for estimating the evaporation rate from the southern part of the Red Sea based on meteorological data taken during the period In addition, it compares SST data collected from satellites for Jeddah region with results obtained in previous studies carried out in the region. Data and Method of calculation The standard meteorological measurements such as; air temperature, relative humidity and wind speed for the study regions A (Jeddah; N, E) and B (Jazan; N, E) were taken from the Presidency of Meteorology and Environment (PME) of Saudi Arabia. For region C (Hodeidah; N, E) the data were taken from the Civil Aviation and Meteorological Authority in Yemen. The SST data are from Physical Oceanography Distributed Active Archive Center (PO.DAAC) on (URL: Figure 1 illustrates the locations of the study areas. The water vapor pressure is estimated from daily observations of SST and air temperature for the period The mean Evaporation is estimated using the bulk aerodynamic method given by (Dalton, 1802) as follows: E = K(e w e a )w (1) Where; E = evaporation (mm/day), K = (Osman, 1984), w = wind speed (m/sec), e w = saturated vapor pressure at water temperature (mb), and e a = water vapor pressure of air (mb) The saturated vapor pressure at water temperature (e w ) and vapor pressure at air temperature (e a ) are calculated from the following equation (Csanady, 2001): e w = exp (17.67T/ T) (2)

4 80 Abdullah M. Al-Subhi wheree w is the saturation vapor pressure in mb, T is the temperature in C. The vapor pressure at air temperature is estimated using the following equation: e a =e w relative humidity (3) Fig. 1. The southern Red Sea and locations of the study regions A, B, and C. The plots of monthly SST and meteorological data from 1999 to 2007 for the three regions are shown in Fig. 2 (a to e). The composite gird of the monthly SST is about ( ) with spatial resolution of 4 km.

5 Estimation of Evaporation Rates in the Southern Red Sea Based on the 81 ( a ) ( b ) ( c ) ( d ) ( e ) Fig. 2. Monthly sea surface temperature and meteorological data along with computed monthly values of (e w -e a ) in Jeddah, Jazan, and Hodeidah for the study period ( ). Based on monthly values of SST, air temperature and relative humidity, the humidity gradient (e w -e a ) was calculated and the monthly values of humidity gradient from 1999 to 2007 for the three regions are given in Table 1 and plotted in Fig. 2(e). The monthly climatology of SST, air temperature, wind speed, relative humidity and humidity gradient (e w -e a ) for the three regions constructed from monthly values for eight years (1999 to 2007) are given in Table 2. The computed monthly

6 82 Abdullah M. Al-Subhi values of evaporation from are given in Table 3 also and plotted in Fig. 3. In addition, the monthly climatology values of the computed evaporation are given in Table 4 and plotted in Fig. 4. Table 1. Monthly values of (e w -e a in mb) from 1999 to 2007 for Jeddah, Jazan, and Hodeidah. Monthly values of (e w-e ain mb) Years Region Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Jeddah Jazan Hodeidah Jeddah Jazan Hodeidah Jeddah Jazan Hodeidah Jeddah Jazan Hodeidah Jeddah Jazan Hodeidah Jeddah Jazan Hodeidah Jeddah Jazan Hodeidah Jeddah Jazan Hodeidah Jeddah Jazan Hodeidah Table 2. Monthly average of Sea Surface temperature (SST),air temperature, wind speed, humidity gradient (e w -e a in mb) and relative humidity for Jeddah, Jazan and Hodeidah from 1999 to Months SST ( o C) Air Temperature ( o C) Wind Speed (m/sec) humidity gradient e w-e a (mb) Relative Humidity (%) Jed Jaz Hod Jed Jaz Hod Jed Jaz Hod Jed Jaz Hod Jed Jaz Hod Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec

7 Estimation of Evaporation Rates in the Southern Red Sea Based on the 83

8 84 Abdullah M. Al-Subhi Fig. 3. Monthly values of evaporation(mm/month) in Jeddah, Jazan, and Hodeidah for the study period ( ). Fig. 4. Monthly climatology of evaporation(m/year) for Jeddah, Jazan and Hodeidah regions based on data from 1999 to 2007.

9 Estimation of Evaporation Rates in the Southern Red Sea Based on the 85 Table 4. Monthly averages of evaporation (m) based on data from1999 to Months Jeddah Jazan Hodeidah Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Annual (m/yr) The annual average evaporation for the three regions from 1999 to 2007 is given in Fig. 5. Table 5 gives the correlation coefficients between evaporation and the controlling parameters; wind speed and humidity gradient. Fig. 5. Annual averages of evaporation (m/year)for Jeddah, Jazan, and Hodeidah regions based on data from 1999 to Table 5. Correlation coefficients between evaporation, wind speed and humidity gradient for Jeddah, Jazan and Al-Hodeidah regions. Jeddah Region Jazan Region Hodeidah Region Correlation P-value Correlation P-value Correlation P-value Wind speed (e w -e a )

10 86 Abdullah M. Al-Subhi Results and Discussion The meteorological measurements over the study area indicated that, air temperature during winter is less than sea surface temperature in all regions, while in summer it is higher, the opposite (Fig. 2). The wind speed varies from one season to another; the highest value of wind speed is recorded in Hodeidah region. Based on monthly values of SST, air temperature and relative humidity, the monthly humidity gradient (e w -e a ) was calculated from 1999 to 2007 for the three regions and given in Table (1) and Fig. 2(e). From the table it was clear that, a gradual increase in the differences (e w -e a ) occurred from Hodeidah to Jeddah region, as a result of high relative humidity from Jeddah to Hodeidah. The monthly climatology of SST, air temperature, wind speed, relative humidity and humidity gradient (e w -e a ) for the three regions constructed from monthly values for eight years ( ) are given in Table 2. The computed monthly values of evaporation from are given in Table (3) and Fig. 3. In addition, the monthly climatology values of the computed evaporation are given in Table (4) and Fig. 4. The Monthly average of evaporation for the period ( ) in the southern part of the Red Sea (Fig. 4, Table 4) indicated that evaporation increases from the Strait of Bab al-mandab to the north. The highest evaporation was 2.04 m/yr is near Jeddah region due to comparatively lower relative humidity, while in Jazan and Hodeidah it was 1.29and 1.25 m/yr respectively as shown in Table 4. The results of present study are consistent with the result of other researches carried out in Jeddah region (Morcos, 1970; Ahmad and Sultan, 1987 and 1989 and Al-Barakati, 2005). The lower evaporation towards in the southern Red sea may be due to the higher relative humidity and lower humidity gradient (e w -e a ). In the northern Red Sea the evaporation is high because of the low relative humidity as shown in Fig. 2d. This shows consistency with previous studies carried out by Ahmad and Sultan (1989); Eshel and Naik (1997); and Eshel and Heavens (2007). During winter, the monthly averages of evaporation in Jeddah and Jazan regions are greater than those of Hodeidah, as a result of strong wind along northeast coast. In Hodeidah the highest value of monthly average of evaporation is in the transition months (May and October), whereas in Jazan region the highest value is in the late summer to December.

11 Estimation of Evaporation Rates in the Southern Red Sea Based on the 87 Correlation of monthly averages of evaporation with wind speed and the difference (e w -e a ) in all regions is shown in Table 5. The correlation between the humidity gradient (e w -e a ) and monthly climatology of evaporation is strong in all regions and is approximately 0.8, whereas the correlation between monthly average of evaporation and wind speed is weak in all regions (~ 0.5). The annual average evaporation for the three regions from 1999 to 2007 is given in Fig. (5). It shows higher annual averages of evaporation in 2000 and 2006 with lower values in 2004 and 2005 in the Jeddah region and is believed to be related to relative humidity. In Jazan region, the higher value is in 1999 while the lower one is in 2002 and seems to be due to the variation of wind speed. In Hodeidah, the higher value is in 2006 while the lower is in 1999 and is mainly due to the variations of relative humidity and wind speed. Conclusion Dalton's Equations are used to study the evaporation in the southern Red Sea. Evaporation is estimated in three regions Jeddah, Jazan and Hodeidah. In Jeddah region evaporation is greater than Jazan and Hodeidah. Evaporation is higher in Jeddah region in winter, while in Hodeidah region it is higher in the transition months. In Jazan it is higher in the summer months. The correlation between the humidity gradient (e w -e a ) and monthly climatology of evaporation is strong in all regions and is approximately with a correlation coefficient of 0.8, while the correlation with the wind speed is about 0.5. Therefore the evaporation mostly depends on the humidity gradient over the southern part of the Red Sea and the wind speed. Acknowledgment This work has been achieved with the help of Prof. Fazal A. Chaudhry who gave lots of advices during the preparation of the manuscript. References Ahmad, F. and Sultan, S.A.R. (1987) On the Heat Terms in the Central Region of the Red. Sea. Deep-Sea Research,Part A, 34: Ahmad, F. and Sultan, S.A.R. (1989) Surface Heat Fluxes and their Comparison with the Oceanic Heat Flow in the Red Sea. Oceanology Acta,12:

12 88 Abdullah M. Al-Subhi Al-Barakati, A.M.A. (2005)Advective Heat Transport to the Red Sea at the Strait of Bab-el- Mandab.JKAU: Mar. Sci., 16: Arz, H. W., Lamy F., Ganopolski, A., Nowaczyk, N. and Patzold, J. (2007) Dominant Northern Hemisphere Climate Control over Millennial-scale Glacial Sea-Level Variability. Quaternary Science Reviews, 26: Behairy, A.K.A, Meshal, A.H. and Osman, M.M. (1981) Evaporation from the Central Zone of the Red Sea.JKAU: Mar. Sci.,1: 3-9. Csanady, G.T. (2001) Air-Sea Interaction Laws and Mechanisms. University of Cambridge, United Kingdom, 239. Dalton, J. (1802) Experimental Essays on the Constitution of Mixed Gases: on the Force of Steam or Vapour from Water or other Liquids in Different Temperatures, both in a Torricelli Vacuum and in air; on Evaporation ; and on Expansion of Gases by Heat. Manchester Lit.Phil. Soc. Mem. Proc., 5: Edwards, A.J. (1987) Climate and Oceanography. In Edwards, A. J. and Head, S. M. (eds) Red Sea Pergamon Press. Oxford, Eshel, G. and Heavens, N.(2007) Climatological Evaporation Seasonality in the Northern Red Seas. Paleoceangraphy.22: Eshel, G. and Naik, N.H. (1997) Climatological Coastal Jet Collision, Intermediate Water Formation, and the General Circulation of the Red Se., Journal of Physical oceanography, 27: Jiang, H., Thomas, F.J., Beardsley, R.C., Chen, R. and Chen, C. (2009) Zonal Surface Wind jets across the Red Sea Due to Mountain Gap Forcing Along both Sides of the Red Sea. Geophysical Research Letters, 36: 1-6. Matsoukas, C., Banks, A.C., Pavlakis, K.G., Hatzianastassiou, N., Stackhouse, Jr. P.W. and Vardavas, I. (2007) Seasonal Heat Budgets of the Red and Black Seas, Journal of Geophsical Research, 112: Morcos, S.A. (1970) Physical and Chemical Oceanography of the Red Sea.Oceanography and Marine Biology Annual Review, 8: Neumann, A.C. and McGill, D.A. (1962) Circulation of the Red Sea in early summer. Deep Sea Research, 8: Osman, M.M. (1984) Seasonal and secular variations of sea-level at Port Sudan, JKAU: Mar. Sci., 4: Patzert, W.C. (1974) Wind-induced Reversal in Red Sea Circulation.Deep Sea Research, 21: Phillips, O.M. (1966) On Turbulent Convection Currents and the Circulation of the Red Sea. Deep Sea Research,13: Rohling, E.J., Fenton, M., Jorissen, F.J., Bertrand, P., Ganssen, G. and Caulet, J.P. (1998) Magnitudes of Sea-level lowstands of the Past 500,000 years, Nature, 394: Siddall, M., Rohling, E.J., Almogi-Labin, A., Hemleben, C., Melschner, D., Schmelzer, I. and Smeed, D.A. (2003) Sea-level Fluctuations During the Last Glacial Cycle, Nature, 423: Siddall, M., Smeed, D.A., Hemleben, C., Rohling, E.J., Schmelzer, I. and Peltier, W.R. (2004) Understanding the Red Sea Response to Sea Level,Earth and Planetary Science Letters, 225: Siedler, G. (1969) General Circulation of the Water Masses in the Red Sea, in Hot Brines and Recent Heavy Metal Deposits in the Red Sea. edited by E. T. Degens and D. A. Ross, Springer-Verlag, New York, pp: Sofianos, S.S., Johns, W.E. and Murray, S.P. (2002) Heat and Freshwater Budgets in the Red Sea from Direct Observations at Bab el Mandeb, Deep Sea Research, part II. 49: Thunell, R.C., Locke, S.M. and Williams, D.E. (1988) Galcio-eustatic sea-level control on Red Sea Salinity. Nature, 334:

13 Estimation of Evaporation Rates in the Southern Red Sea Based on the 89 AVHRR ( ) ( ) ( ) ( ) :, :. /,,. (e w -e a ),,.(e w -e a )...