Deep-Sea Research, Vol. 35, No. 3, pp. 333-346, 1988. 0198-0149/88 $3.00 + 0.00 Printed in Great Britain. Pergamon Press plc.

Vertical turbulent mass flux below the sea surface and air-sea interact ion--monsoon region of the Indian Ocean

L. HARENDUPRAKASH* and ASHIS KUMAR MITRA*

(Received 28 January 1987; in revised form 25 June 1987; accepted 14 September 1987)

Abstract--Analysis of the quarterly distributions of vertical turbulent mass flux below the sea surface induced by atmospheric factors in the intertropical Indian Ocean shows large spatial and seasonal variations in mass flux, variations being strongest during the Southwest Monsoon. Estimates show that the freshwater flux dominates the mass flux over extensive areas. The seasonal cycle of sea surface temperature in the monsoon region of the Indian Ocean may be influenced by surface fluxes of freshwater (evaporation and rainfall).

INTRODUCTION

SEASONAL and spatial variations of fluxes of heat and freshwater across the sea surface are very large in the intertropical Indian Ocean, especially in its northern half. These variations are integral characteristics of the monsoon climate of the region. The beginning of the Southwest Monsoon brings about large-scale and rapid shifts in the rainfall pattern over southern Asia and the Indian Ocean (RAO et al., 1976; JAEGER, 1977). Monsoon rainfall in the northern Bay of Bengal is one of the highest in the World Ocean. As a consequence of the strong seasonal variation of oceanic rainfall and river runoff, the northern Bay of Bengal experiences one of the largest amplitudes of seasonal variation of surface water salinity in the open ocean (LEvITUS, 1986). On the other hand, the western half of the northwest Indian Ocean has a dry climate with little rainfall and strong evaporation. In addition to the rainfall, evaporation also shows marked seasonal variations (HASTENRATH and LAMB, 1979). The climatic conditions described above lead to the formation of warm saline waters in the northwest Indian Ocean and warm fresh waters in the Bay of Bengal.

As the monsoonal variation of surface fluxes of freshwater is regular and of large amplitude--perhaps the most pronounced seasonal variation of this nature in the Tropical Ocean--it is relevant to examine whether the surface fluxes of freshwater can affect atmospheric processes in the region by modifying the upper ocean structure. This aspect of air-sea interaction in the monsoon region of the Indian Ocean, i.e. ocean feedback associated with the water cycle of the Monsoon, has not been investigated explicitly until now. A particular problem in this regard is the nature of ocean feedback to the monsoon rains.

The importance of surface buoyancy flux in the evolution of the annual cycle of sea

* Centre for Atmospheric Sciences, Indian Institute of Technology, New Delhi 110016, India.

333

334 L. HARENDUPRAKASH and A. K. MITRA

surface temperature has been demonstrated by WOODS and BARKMAN (1986). The extent to which surface buoyancy fluxes influence the annual cycle of sea surface temperature in the North Indian Ocean is not clear. Its estimation is complicated because the dynamical response of the Indian Ocean to the seasonally varying winds is very strong and is known to alter the sea surface temperature in a decisive manner. As it is necessary to estimate the relative importance of the surface buoyancy fluxes and changes in surface circulation on the seasonal evolution of sea surface temperature, the problem is a complex one. Therefore the identification of ocean feedback phenomena associated with the fresh- water fluxes (if any) and the estimation of their relative significance for monsoon variability require much work. But, as a first step in this direction it is possible to examine in a simple manner whether the necessary conditions for the existence of such feedback exist in the monsoon region of the Indian Ocean.

According to the equation of conservation for turbulent kinetic energy derived for integral mixed-layer models (NIILER and KRAUS, 1977), the relative contribution of freshwater flux to surface buoyancy flux is an appropriate parameter for discussing the effect of rain or evaporation on the upper ocean structure. If the freshwater flux is to exert a significant influence on the annual cycle of sea surface temperature, then the relative contribution of freshwater flux to the turbulent mass flux below the sea surface must be of the same magnitude as that of the heat flux. It is necessary to point out here the limitations of this criterion when applied to oceanic areas where the upper ocean structure is strongly influenced by river runoff. The runoff of the major rivers in the monsoon region is strongly seasonal; in the case of Ganges and Brahmaputra up to 80% of the annual input occurs during the Southwest Monsoon. The physico-geographical conditions of South Asia are such that most of the land drainage during the Southwest Monsoon is diverted into the Bay of Bengal, especially into the northern Bay. According to GORSHKOV (1980) the mean annual inputs from Ganges and Brahmaputra are 395 and 644 km 3, respectively. If this water were spread over the Bay north of 15°N, the resulting layer of freshwater would have a thickness of about 1.2 m. This value is more than twice the mean annual net freshwater gain from the atmosphere (i.e. the excess of rainfall over evaporation which is about 0.5 m). Thus the land drainage must have an even greater influence on the upper ocean salinity in the northern Bay than oceanic rainfall. The massive runoff which is sediment-laden can also alter the optical characteristics of the upper waters. But, as our primary concern is with the influence of freshwater fluxes on air-sea interactions over the whole of the monsoon region of the Indian Ocean and as the river effect is confined to relatively small areas, we may exclude for the present a study of the transformation of upper ocean structure by river runoff. While discussing the role of freshwater fluxes across the sea surface we will assume that the after effect of river runoff and oceanic rainfall on upper ocean salinity is cumulative, even though the mechanisms of modification of upper ocean structure by them and the manner in which such mechanisms are coupled with atmospheric processes may be different.

In the present paper we investigate the relative contribution of surface fluxes of freshwater (evaporation and rainfall) on the total turbulent mass flux below the sea surface in the monsoon region of the Indian Ocean, based on climatic data on evaporation, rainfall and net oceanic heat gain at the sea surface. On the basis of the analysis we try to locate the areas where ocean feedback associated with the water cycle is more likely to occur.

Vertical turbulent mass flux below the sea surface 335

METHOD AND MATERIALS

The vertical turbulent mass flux below the sea surface as induced by exchanges of heat and freshwater with the atmosphere is given by the expression (DORRESTEIN, 1979; MONIN, 1985),

M = S]3(E - P) - ac -1 Q - aTP ,

where M (kg m -2 s -1) is the vertical turbulent mass flux (positive when directed into the ocean), S is the salinity of the surface water expressed as a mass ratio, [3 is the coefficient of salinity contraction (p-I(op/OS), p being the density of seawater), E and P are the rates of evaporation and precipitation, respectively, a is the coefficient of thermal expansion of water, c is specific heat of seawater, Q is the net oceanic heat gain at the ocean surface (positive when the ocean gains heat), and T is the excess temperature of the precipitation over that of the surface water.

In the present study the heat transfer by rainwater is neglected as its contribution is relatively small. The computations were carried out for a total of 118 5 ° latitude- longitude squares that cover the Intertropical Indian Ocean. Salinity was taken to be variable, and the quarterly mean salinity of the upper 10 m layer was used. The values of a, ]3 and c were constant (for seawater of salinity 35 and temperature 25°C), their respective values being 3 x 10-4(°C)-1, 0.74 and 4 × 10 3 J kg -1 K -1.

In the above expression for mass flux M, we can denote the contribution of freshwater flux by M w and that of heat flux by Mh. Then the mass flux

M = M w + Mh .

Both M w and M h can either be negative or positive, and it is possible that M vanishes even when M w and M h make definite contributions. For a proper understanding of the relative contribution of freshwater flux to M we require two more quantities,

M t = [Mw I + [Mhl and

IMwl P.C.W. = x 100,

IMwl + IMhl

where P.C.W. is the relative contribution of freshwater flux to Mt. It may be noted that the parameter M t is useful in estimating the surface water transformation brought about by the surface fluxes of heat and freshwater.

The following published data were utilized in the present study: (a) monthly charts of evaporation and net oceanic heat gain (HASTENRATH and LAMB, 1979); (b) quarterly charts of rainfall (MOLLER, 1951); and (c) mean monthly norms of salinity in the Indian Ocean (ANONYMOUS, 1981).

In the case of the first three data sets, the values were read from the charts. Mass flux was computed for the whole year and for the quarterly periods December-January- February, March-April-May, June-July-August and September-October-November. The computed values were assigned to the centres of 5 ° squares and isolines were drawn.

Studies on the distribution of M in the World Ocean are few in number. KUKSA (1971) computed the turbulent mass flux using mean monthly data on fluxes of heat and

336 L. HARENDUPRAKASH and A. K. MITRA

freshwater in the Pacific Ocean between 10 ° and 30°N latitudes and 150 ° and 180°E longitudes. He compared the mass flux with the seasonal variation of surface water density. The correlation between the two was poor, probably as a result of the relative importance of internal processes in the ocean as well as due to the errors in the determination of the fluxes and surface water density. His work shows that it is not possible to expect a simple correlation between the surface fluxes of heat and freshwater and the changes in surface water density. The mean annual distribution of mass flux (without considering salinity contraction) in the World Ocean was presented by ACAFONOVA et al. (1972). According to these authors their results agreed with the prevailing ideas on the generation of thermohaline circulation in the ocean.

The major difficulty in the estimation of mass flux is the uncertainty in the presently available estimates of the surface fluxes of heat and freshwater (WOODS, 1984). At the same time any significant change in the quality of data on climatic fluxes of heat and freshwater is not expected in the near future. In the case of the Indian Ocean, recent works on the heat budget are comprehensive enough to undertake the study of seasonal variations of mass flux (HASTENRATH and LAMB, 1979; ESBENSEN and KUSHNIR, 1981). According to HASTENRATH and LAMB (1979, 1980) the net oceanic heat gain and evaporation in their atlas have percentage errors of about 15%.

BUNKER et al. (1982) note that evaporative losses and radiative surpluses in the atlas of Hastenrath and Lamb may be underestimated.

Data on rainfall intensity over the oceans poses a more difficult problem (DORMAN, 1982). The presently available climatic data on oceanic rainfall intensity are based on measurements of rainfall intensity at coastal and island stations and on rainfall frequency observations reported by ships. The rainfall intensity at coastal and island stations are interpolated over extensive oceanic areas after giving weightage to the rainfall fre- quency. In this situation it is not possible to determine objectively the errors in the rainfall values over the oceans. The quarterly charts of MOLLER (1951) were selected for the present study because the data is climatic and also to be in conformity with HASTENRATH and LAMB (1980).

Irrespective of the above-said problems, certain features of rainfall distribution in the Indian Ocean make it possible to use the estimates of freshwater fluxes with more confidence, namely, the strong east-west asymmetry in rainfall (between the Arabian Sea and the Bay of Bengal as well as within the northwest Indian Ocean itself) and the abruptness of seasonal changes in precipitation associated with the beginning of the Southwest Monsoon. Further, the large intensities of the fluxes lead to a reduction of the relative error. Therefore, while examining the implications of the observed distribution of mass flux we will confine our discussion to the Southwest Moonsoon season (June- July-August). Since it is not possible to make an objective estimate of the absolute error in the computation of mass flux, all isolines were drawn at large intervals paying attention only to the large-scale features.

RESULTS

The charts of M, M t and P.C.W. presented here may be interpreted in several contexts and are self explanatory. Therefore discussion of the charts is brief.

Vertical turbulent mass flux below the sea surface 337

20ON

20%

40 60 80 100 120°E

/ ,,-,, \

j / I

I _.--'3

~ ----------- 0 "'---- / i / ' 2 1

)-o, i l /

d

) \ U 40 60 80 10("

20 ° N

20os

120OE

Fig. 1. Distribution of annual mean turbulent mass flux (× 10 ~ kg m -2 s-l).

Annual distribution

The annual distribution of M (Fig. 1) generally agrees with that presented by AGAFONOVA et al. (1972). In particular, the position of the zero isoline and the distribution pattern south of it are in good agreement. The extreme values of M in the present work are higher (-5 x 10 -6 kg m -2 s -1 and +4 x 10 4 kg m -2 s -1) than in AGAFONOVA et al. (1972) (-1.5 X 10.6 kg m -2 s -1 and +3 x 10 .6 kg m -2 s-l). It may be noted that elsewhere in the World Ocean AGAFONOVA et al. (1972) show extreme values that are higher than the extreme values in the present work. These differences can be attributed to differences in the basic data on heat budget and water balance, the non- inclusion of salinity contraction in AGAFONOVA et al. (1972) and the better resolution in the present work. The major differences in the distribution pattern are observed north of the equator. Whereas AGAFONOVA et al. (1972) showed a region of weak positive flux north of a line joining Cape Guardafui and Bombay, such a feature is absent in our chart. Instead, areas of minimum negative flux (<1 x 10 -6 kg m -2 s -1) are observed in the Arabian Sea and the Bay of Bengal, but their locations are different from those in AGAFONOVA et al. (1972).

Seasonal distribution

December-January-February (Fig. 2a). M is positive over most of the Arabian Sea and the Bay of Bengal. The chart of P.C.W. (Fig. 3a) shows that in both areas freshwater flux dominates the mass flux. Generally, the distribution is zonal in the North Indian Ocean. On the other hand, zonal asymmetry is observed south of the equator. The highest values of negative mass flux are observed in the western half and weak positive flux in the eastern half. Comparison of the charts of M, Mt (Fig. 4a) and P.C.W. shows the regions where the signs of freshwater flux and heat flux agree and disagree. In general the pattern of Mt is similar to that of M. It should be noted that we have considered the regions north of 25°S, and therefore features in the South Indian Ocean are incomplete.

338 L. HARENDUPRAKASH and A. K. MITRA

(a)

20 ° N

20os

40 60

, / - . . _ _ ~ ~ ~ c -5

) - 2

80 100 120°E

"-4 /

-2 "-----" J ( ~ - - - 0.

\ 40 60 80

I Dec- Feb /

o ~ ' ' ~ 20ON

100 120°E

Fig. 2a.

20ON

20os

b) 40 60 80 100 120°E

/

-2 ~ , J f

_0 j j

f 6 " " - " - "~ f

40 60 80

~ CI ~ , ~ 20°N

,S

~ ~ - 20"S

100 120°E

Fig. 2b.

March-April-May (Fig. 2b). The distribution of M changes in both hemispheres, with high values of negative flux being observed in the North Indian Ocean. The distribution of P.C.W. (Fig. 3b) shows that the relative contribution of heat flux has increased and dominates the mass flux. The agreement between the distribution pattern of M and Mt is better than in the previous season.

June-July-August (Fig. 2c). The distribution pattern of M is very different from that of the previous season. Zonal asymmetry is observed within the northwest Indian Ocean itself. Strong negative flux is observed in the Gulf of Aden and off the Arabian coast. In the Bay of Bengal the flux is negative but magnitudes are much lower than in the Arabian Sea. It is interesting to note that an area of positive flux develops in the Arabian Sea even

Vertical turbulent mass flux below the sea surface 339

( c ) 40 60 80 100

o

40 60 B0 100

120°E

20ON

20°S

120°E

Fig. 2c.

(d)

20°N

20os

40 60 80 100 120°E

Sep-Nov /

~\\ -3 t /

.7-,.\( J / 4 60 80 100

20°N

20%

12 0 ° E

Fig. 2. Quarterly distribution of mean turbulent mass flux (x 10 ~ kg m -2 s-l).

though it is summer in the Northern Hemisphere. Positive flux is present also in the southern Bay of Bengal. The charts of freshwater flux (Fig. 5a) and heat flux (Fig. 5b) show how the areas of positive flux form. In the Arabian Sea the positive flux is dominated by freshwater flux and in the Bay of Bengal by heat flux. On the other hand, the negative flux in the northern and eastern Bay of Bengal is dominated by rainfall. In the South Indian Ocean the mass flux is positive and intensity has doubled in comparision with the previous quarter.

September-October-November (Fig. 2d). Mass flux is negative in the entire North Indian Ocean. The maximum flux is seen parallel to the east African coast. In the Arabian Sea and generally in the western Indian Ocean the contribution of freshwater

340 L. HARENDUPRAKASH and A. K. MITRA

20~

20°S

o ) ~0 6o 8o lOO

4 0

120°E

60

f

/ 8O

25

5O ,~

IO0

20°N

io

~ _..~ 20os

'T20OE

Fig. 3a.

( b ) 40 60 80 100 120°E

20°N ~ ~ ~ ~ / 1 0

J /

~10~ - 25 ( . <___ /

20°¢ S / 4O 6O B0

S I0O

o 20°N

/ 2 5 ~ 50 ~ 75 \ ~ 20°s

120°E

Fig. 3b.

Vertical turbulent mass flux below the sea surface 341

20°N

0

20oS

( c ) 40 60 80 100 120°E

\50/

) 40 60

b75) x,,:,

/ ~_____~0

80

Fig. 3c.

Jun-Aug

f ' l J 20ON

, 5 +

~ o 25 . ^ f

~ ,,--,.~ 20%

100 120OE

20*N

20%

( d ) 40 60 80 100 120°E

20

.lo /

/ 50

t - / 2SJ

",J l ~

20ON

Fig. 3.

60 80 100

Quarterly distribution of P.C.W. (%).

20°S

120°E

342 L. HARENDUPRAKASH a n d A . K. MITRA

(a)

2.0

20

40 60 80

) ( / / , , ' "'.,,~' &O 60 80

100 120"E

~ d 20"N

j ; r' ~ 2o's

100 120 "E

Fig. 4a.

( b ) 40 60 8O

\

0

20 ,'~

....__. 2 ._.___..~ .------

_ ~ . ~ 4 .-----J

f 6 40 60 80

100 120 E

rvlar ~ Z0"N

, 5 °

20S

100 120"E

Fig. 4b.

Vertical turbulent mass flux below the sea surface 343

c ) 40 6O 80 100

o 60 80

120° E:

20"N

20"S

100 120"E

Fig. 4c.

( d ) 40 60 80 100 120"E

1 6

40 60 80 100 I~

20"N

2o%

Fig. 4. Quarterly distribution of Mt (× 10 -6 kg m -2 s l).

344 L. HARENDUPRAKASH and A. K. MITRA

( o ) 40 60 80 100 120"E

~° J ( 1 I (, r., f . ~o-~ 40 60 80 100 120"E

Fig. 5a.

(b ) 40 60 ao ~oo ~20"E

Jun-Aug /

/~0 60 80 100

Fig. 5. (a) Quarterly distribution of ( E - P ) (mm day-l). (b) Quarterly distribution of Q (x l0 w m 2).

flux is very low, whereas in the rest of the region its contribution is significant. The intensity of the positive flux in the South Indian Ocean has dropped sharply.

Charts of seasonal distribution of mass flux show that the seasonal variation in the North Indian Ocean and South Indian Ocean are different in nature. In the South Indian Ocean the variation has one minimum and one maximum. In the North Indian Ocean the cycle is clearly monsoonal.

DISCUSSION

The distributions of M and P.C.W. in the monsoon region of the Indian Ocean clearly demonstrate that the surface fluxes of freshwater have a definite and decisive influence over the surface buoyancy flux. Freshwater flux is more dominant during winter and the

Vertical turbulent mass flux below the sea surface 345

Southwest Monsoon season. The non-inclusion of freshwater flux in buoyancy flux also may change the sign of the buoyancy flux in certain cases. Therefore it is necessary to take into account the seasonal variation of surface fluxes of freshwater while modelling the seasonal cycle of sea surface temperature in the monsoon region of the Indian Ocean.

The above observation raises the question of possible relationships between monsoon variability and the effects of monsoon rainfall on upper ocean structure. In this regard the conditions in the northern Bay of Bengal demand special attention. Our analysis shows that surface fluxes of freshwater (rainfall) make the major contribution to the generation of mass flux in the northern Bay during the Southwest Monsoon. As mentioned in the Introduction, the river runoff (which is an integral part of the water cycle of the monsoon) is massive during the same period. Upper ocean structure in the northern Bay of Bengal is progressively transformed by oceanic rainfall and river runoff as the monsoon progresses. Presently available information on the evolution of upper ocean structure in the northern Bay is fragmentary, but it suggests that salinity has a dominant influence on the stability of the upper waters and the thickness of the mixed layer. Thus, according to GOLOVASTOV et al. (1982), the freshened layer and the mixed layer usually coincide in the northern Bay. RAO et al. (1981) and SASTRV et al. (1985) support the above observation and also show that the influence of salinity on vertical stability is much higher than that due to temperature stratification. How this salinity stratification affects the evolution of sea surface temperature and the atmospheric processes---especially the development of monsoon depressions--is not clear. But, in the light of the broader problem which is raised here, i.e. the possibility of ocean feedback to monsoon rains, the nature of air-sea interaction in the Bay of Bengal acquires special significance. It is curious that very little attention is being paid to the factors affecting the evolution of sea surface temperature in the Bay of Bengal in comparison to the attention being given to the evolution of sea surface temperature in the Arabian Sea. We hope that the present work will stimulate more attention to the problems of air-sea interaction in the Bay of Bengal, especially on the problem of the influence of surface fluxes of freshwater--rainfall and land drainage---on air-sea interactions in the region.

Acknowledgements--The authors thank Prof. M. P. Singh, Head of the Department, C.A.S., for the facilities provided. L. H. thanks C.S.I.R. for financial support during the course of this study. We are also grateful to the referees for their valuable suggestions.

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