Deep-.Sin R,~em:A. VaL 37, No. 2. PP- 227-243, 19q0. 01~I-0149#~} 13.00 ÷ 0.0O pnnt~ ~- Gait agitm. O t~0 ~ P t m t ~

A case s tudy on the mixed layer variabi l i ty in the south central Arabian

Sea dur ing the onset phase o f M O N E X - 7 9

R. R. RAO* and BASIL MATHEW*

(Received 30 June 1988;/n revised /orm 18 laly 1989; accepted 8 September 1989)

Almract--The influence of the summer monsoonal onset event on properties such as tempera- ture, salinity and currents in the near-surface mixed layer and upper thermocline in the south central Arabian Sea is documented with the aid of time series measurements made from a stationary USSR four ship polygon during the onset phase of MONEX-79. An attempt is made to explain the observed mixed layer cooling of the order of I°C and deepening of about 20 m under the influence of the onset event with some of the known processes. The one-dimensional mixed layer model of KRAUS and TURNER (1967, Tellus, 19, 98--106), modified by DENMAN (1973, Journal of Physical Oceanography, 13, 173-184) and MILLER (1976, Journal of Physical Oceanography, 6, 29-35), is utilized to simulate the observed mixed layer variability. The cooling and deepening of the mixed layer showed a very good correspondence with the one-dimensional forcing process as forced and free mixing except at the northern location where thermal advection was also important. Temporal changes in the salinity profiles clearly revealed the relative importance of lateral advection process. The observed current in the mixed layer was nearly southerly at the northern, eastern and southern locations while the mean flow at the western location was northwesterly.

I N T R O D U C T I O N

DURING the last few years, monsoon meteorologists and oceanographers increasingly have appreciated the need for a more detailed knowledge on the interaction between the Arabian Sea and seasonally reversing monsoons on a variety of space-time scales. On a seasonal mode the wind forcing during both the summer and winter monsoons is identified as an important agent for a major part of the observed changes of the thermal structure of the upper layers of the Arabian Sea. Exchanges of heat, mass and momentum across the air-sea interface are known to influence not only the behaviour of both the monsoons but also the response characteristics of the upper layers of the sea to the variable monsoonal forcing. For example, the surface mixed layer exhibits significant variations in the Arabian Sea from May to August under the summer monsoonal forcing (WVRTrd, 1971; ROBINSON et al., 1979; MOUNARt et al., 1986a; RAG et aL, 1989). But the knowledge on the time-dependent development of the mixed layer in the Arabian Sea during the summer monsoon season is fragmentary due to non-availability of systematic time series measurements for complete seasonal cycles. Accordingly, more case studies on the development of the mixed layer in response to the monsoon winds are needed (CCCO, 1985; WOCE, 1987) to improve our understanding of the mixed layer variability.

* Naval Physical and Oceanographic Laboratory, Cochin - 682 004, India.

227

228 R.R. RAo and B. MATHEW

Intense solar heating during the pre-monsoon season (February-May) progressively warms up the upper layers of the Arabian Sea. By the end of May/early June, over much of the Arabian Sea the temperature of the topmost 30 m water column is around 29°C ( H A ~ T H and L~,MB, 1979; RoensSON et al., 1979; L~vrrus, 1982), with the near- surface mixed layer depth (MLD) of about 30-40 m (RosnssoN et al., 1979; M o L n s ~ et al., 1986a; RAO et al., 1989). The onset and sway of the suwaner monsoon over the Arabian Sea produce a dramatic drop in the temperature of the upper layers and deepening of the mixed layer (INDEX, 1977). However, these observed changes differ in space. The cooling is mostly manifested by the increased turbulent heat fluxes due to strengthened winds and decreased insolation under the monsoon cloud cover (COLON, 1964; K~SHNAMURTL 1981; MCPHADEN, 1982; RAO, 1984, 1987), coastal upwelling (WoosTER et al., 1967; DUING and LEETMA, 1980; MOLINARI, 1983), advection of these cold upwelled waters into the interior basin (TLrN~EL, 1963; SAHA, 1974) and entrainment of colder waters from thermocline into the mixed layer due to wind, wave and buoyancy mixing (MURa'HV et al., 1983; RAMESH BABU and SASa'RY, 1984). The layer deepening is mostly attributed to mixing in the upper layer caused by wind/wave action and convective overturn caused by buoyancy flux, Ekman type of convergence due to clockwise surface wind stress curl, and enhanced vertical shear in the horizontal flow in the upper layers, etc. (W,atTrd, 1971; COLBORN, 1975; RAO et al., 1976; SASTRY and RAMESH BABU, 1979; ROaL~sos et al., 1979; RAO, 1987). GILL (1974) and BERtqSTEIN (1974) showed that the mesoscale eddy field also can produce strong local advectioa of the surface patterns. A few studies indicate that the Arabian Sea is rich in eddy fields. Dutr~G (1972) showed the importance of mesoscale eddies with horizontal scales around 420 km in the Arabian Sea. Eddies are likely to be found anywhere in the Arabian Sea, with horizontal dimensions in the range of 200-500 km, vertical extent of some hundreds of meters and typical current of 20-30 cm s -t (SWALLOW, 1983b). However, detailed information is still meagre on the relative importance of these processes during the monsoon season (SWALLOW, 1983a) in producing these observed changes in the Arabian Sea, as no systematic collection of time series data sets covering at least one full seasonal cycle has been reported. With the available historic data sets SHETYE (1986) and MOLINARI et al. (1986b) have attempted to model the observed seasonal cycle of the SST at selected areas in the Arabian Sea by assessing the relative importance of various terms in the conservation of a heat equation.

During the summer monsoon season, on a synoptic scale, the characteristics of the atmosphere and ocean were monitored for short periods (1-2 weeks) before and after the onset of monsoon at selected areas over the Arabian Sea during ISMEX-73, MON- SOON-77 and MONEX-79 field experiments during the years 1973, 1977 and 1979, respectively. One of the basic aims of these monsoon experiments was to understand the time-dependent response of the Arabian Sea to the variable monsoon forcing and the nature of the feedback which the ocean produces on the monsoon regime itself. Improved understanding of the heat budget of the mixed layer in the Arabian Sea is considered to be important in the prediction of the monsoon rainfall (INDEX, 1977). Accordingly, with the aid of monsoon experimental data sets, an attempt is made to understand the development of the mixed layer in the Arabian Sea with particular attention towards the storage or removal of heat throughout the summer monsoon season. In the present study advantage is taken of the short time series data sets collected from a four-ship USSR stationary polygon to describe and explain the observed changes

Mixed layer variability. 229

25"N

2o"

ld'

%

170",,

16 JUN.~ 1979 '

ARABIAN SEA

N

INDIA

5"N,L_,_ SS'E 6# 6¢ ~ 7# 7£ BeE

II 13

12 14

Fig. 1. Caption on p.230.

230 R.R. RAo and B. MAT~EW

¢eD

~. .~.--~.~~ ~_._ .. ~ ~ . . . . . .

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SURFACE WIND VECTORS I i ,I., i I I I 2 4 6 8 10 12 1/.

JUNE 1979

Fig. 1. Localion of USSR four ship stationary polygon during MONEX-79. Satellite cloud imageries for 11-14 June 1979 and surface wind vectors for 2-14 June 1979 at all the l'~ur

locations.

Mixed layer variability 231

in the upper layers of the south central Arabian Sea under the forcing of the summer monsoonal onset event during MONEX-79.

Observations

Four USSR ships occupied a stationary polygon in the south central Arabian Sea (Fig. 1) from 2 to 14 June 1979. This observational period coincided with the onset of the summer monsoon and an associated embedded vortex system north of the observational array. Time series measurements of all standard marine meteorological elements (surface pressure, dry, wet bulb and sea surface temperatures, wind direction and speed, and visually observed cloud cover) were made at one hourly intervals. Direct solar radiation and surface albedo measurements at one hourly intervals, temperature (with BT) and salinity (with Nansen Casts) profiles at irregular depths at three to six hourly intervals made from these four ships are utilized in this study. Unfortunately the details of the surface meteorological instrumentation are not known. As the temperatures and salinity data are reported at irregular depths, a cubic spline technique was utilized to generate data at every 5 m interval in the vertical below the mixed layer (200 m depth). In the following discussions the observational sites are designated as N, E, S and W corresponding to northern, eastern, southern and western corners of the polygon (Fig. 1). Half hourly current measurements from I-moorings were made at 25, 50, 100, 150 and 200 m depths at all the four locations from 2 to 12 June 1979 using Alexyev Current Meters (accuracy of speed _+ 2 cm s -L and direction _+ 10°).

ANALYSIS AND D I S C U S S I O N

Surface meteorological conditions

The satellite cloud imageries in the visible band over the north Indian Ocean taken from Environmental Satellite Imagery (TIROS-N) for the period 11-14 June 1979 are shown in Fig. 1. On 11 June, the oceanic area was almost cloud-free, with transient appearance of cloud clusters between the equator and 10°N. A massive cloud cluster dramatically developed on 12 June in the same area attributed to the burst of the monsoonal onset. During the following 2 days this cloud system moved further north in an organized manner with the formation of the onset vortex system on 14 June 1979. The near-surface winds were unsteady and fluctuating until 12 June, after which they blew towards east in association with the developing meteorological disturbance (Fig. 1).

In association with the northward movement and subsequent disappearance of anti- cyclonic flow (SIKg.A and GaOSSMAN, 1980) the surface pressure (PR) registered a drop of about 4 mb with superposed propagating perturbations of 4-5 day periods (Fig. 2). The wind field (FF) near the surface also progressively strengthened, but with a rapid rate during the last 4 days especially at N and E (15 m s-l), which were closer to the onset vortex system formed on 14 June. Thus, the last 4 days (11-14 June 1979) may be considered representative of disturbed weather regime. Total cloudiness (TCL) appears as a mirror image of the surface pressure. The coincidence of the peaks in the cloudiness and the minima in pressure distribution is conspicuous and obvious. Sea minus air temperature (SMA) was positive throughout, suggesting an unstable regime over the observational array. The peaks in SMA generally coincided with those of TCL due to reduction in air temperature on overcast days. The overall pattern of vapour pressure gradient (VPG) mostly resembled that of SMA without showing any trend. This

232 R . R . RAO and B. MA'n-mw

- 1012r ('~ ( ~

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g .

m 1o

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31

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6 810121t, 2 4 £ e ~,~I71L ? /, 6 81012V. 2 /* 6 810121/. JUNE 1979

Dai ly progrcssion of surface marinc meteorological clcmcnts.

coincidence suggests that greater evaporation may have occurred on days of higher VPG probably resulting in an increase in local cloudiness. The sea surface temperature (SST) registered a gradual fall but with varying magnitudes over the array. During the observational period the E location showed a drop of 1.I°C, while the N, S and W experienced a cooling of 0.65, 0.75 and 0.8°C, respectively.

Surface heat budget estimates

The local surface heat exchange processes are known to be important in producing these observed changes in SST on short time scales of less than I week. Daily totals of the surface heat budget estimates (Fig. 3) are evaluated for all the four locations. Net longwave radiative flux was estimated following REED'S (1975) formulation using the cloud correction terms of LAEVASTU and AVERS (1966). The turbulent heat fluxes were estimated employing the bulk aerodynamic approach with KONDO'S (1975) formulation for heat exchange coefficients. The details on flux estimates are described in RAO et al. (1985). The observed solar radiation is corrected for albedo losses. The ocean surface heat budget equation can be written as

Q = Q I + Q L + Q s + QE, (1)

where Q is net oceanic heat gain, QI is insolation, QL is net Iongwave radiation, Qs is sensible heat flux and QE is latent heat flux.

Positive values ( W m -2) imply heat gain to the ocean. During the undisturbed days (winds < 5 m s -l) Qz varied between 200 and 300 W m -2, and on disturbed days it was less than 100 W m -2 due to overcast skies. In general, the observed fluctuations in QI

Mixed layer variability 233

O~

-60 ( / ', ~O 200 : .." ~ -,

- ooL 1 v), 2 6 10 IL, 2

® ® ®

Qs

I I i I i i P 6 tO I/, 2 6 10 lZ.

JUNE 1979

[ I I t t

2 6 10 I~.

Fig. 3. Daily progression of surface heat budget estimates.

were caused mostly by the variations in the cloud cover associated with the propagating atmospheric perturbations. The magnitude of the variability in QL was relatively low at all the locations, while Qs showed a sudden increase during the last 2-3 days on account of large SMA and FF values. The increase in Qs is conspicuous at N and E, which were closer to the onset vortex system. QE generally increased progressively, but the increase was very rapid during the disturbed days towards the end. Higher loss of QE occurred at N and E compared to that of S and W. At N the loss through QE increased by a factor of five from the undisturbed to the disturbed regime. However the basic pattern of Q is similar for all the four locations although the magnitudes of the peaks differed.

All four locations received surplus energy during 2-11 June with the exception during the initial disturbed regime (8 and 9 June) at E, S and W locations. The sign of Q changed at all the locations from 12 June. During the later disturbed regime the net heat loss was relatively higher at N and E compared to that at S and W. The cumulative net heat gain (YQ) from 2 June was mostly positive throughout with the only exception at E and W on 13 and 14 June. The progressive accumulation was phenomenal at N during the initial 10 days compared to any other location. The drops in 5~Q from 11 June were of similar magnitude at N and E and at S and W. The general distribution of YQ should produce an increase in SST at all the locations with the only exceptions of E and W from 12 June when the sign of YQ reversed, if one-dimensional surface heat exchanges alone are considered important. But all the stations showed a drop in SST with different magnitudes. This may imply that other processes, such as lateral advection within the

2.34 R .R . RAO and B. M A ' r ~ w

mixed layer and entrainment of colder waters across the base of the mixed layer, also might be important. Entrainment cooling may not be significant during fair weather conditions in the absence of strong current shear below the mixed layer. A very good qualitative correspondence between the distributions of ZQ and SST drop exists at E and W locations.

Temperature field in the upper layers

Daily averaged vertical temperature data contoured at I°C interval with linear interpolation to show the depth-time sections of temperature are shown in Fig. 4. The daily averaging of three hourly or six hourly BT profiles suppresses the tidal variability, which will be a separate topic of study. A careful examination of this figure reveals some similarities and differences between the four locations. A near-isothermal layer with thickness of ~30 m can be noticed at all the locations. This layer cooled with different rates within the array. During this 2 week period maximum and minimum cooling occurred at E and N, respectively. Below this near-isothermal layer the pattern of the vertical thermal stratification also differed. Close resemblances in the isotherm topo- graphy are evident between N and W and between E and S. This may imply that these pairs of stations encountered simlar oceanographic regimes. Strong vertical thermal stratification occurred just below the near-isothermal layer at E and S, while the same occurred at deeper depths (100-150 m) at N and W. The strength of this stratification was also higher at E and S compared to that of N and W. Such strong stratification can be manifested under the influence of strong vertical motions in the thermocline. In the thermocline the isotherm surfaces show an ascending slope from northwest to southeast within the polygon. Short period waves (3-4 days) superposed on the isotherms reflect the influence of inertial oscillations.

Temporal changes in the observed BT profiles of the upper water column during this 12-day observational period are portrayed in Fig. 5. The observed variations below I00 m depth are insignificant, but mixed layer cooling and deepening are evident with unequal

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Mixed layer variability 2 3 5

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Fig. 5. Six hourly and initial and final BT profiles during the observational period.

magnitudes. The layer deepening increased from S to N while the cooling increased from S to E. The vertical displacement at the base of the mixed layer due to internal tide was less than 10 m. Hence the differences of the order of 20-30 m in MLD noticed between the initial and final profiles cannot be attributed to tidal variability. In addition, the observed cooling during the 12 day period cannot be accounted from the internal tidal forcing. The time series of vertical temperature clearly demonstrates the above-men- tioned features. The onset vortex system formed towards the end of the observational period over the east central Arabian Sea was relatively closer to N and E locations. Stronger wind and buoyancy forcing might have produced relatively larger variations in the surface layers at these two stations. The warming below the mixed layer appears to be relatively larger and thicker in extent at N and W locations. The magnitude of this warming depends upon the penetrative mixed layer deepening, and downward displace-

236 R . R . RAO and B. MATtlEW

merit of isotherms in the upper thermocline in conjunction with Ekman type of convergence.

Vertical thermal stratification, salinity and currents in the upper layers

The large vertical gradients (°C/10 m) in the thermocfine showed significant differ- ences in depth and strength within the array (Fig. 6). These strongly stratified layers appeared at relatively shallow depths at E and S compared to those of N and W. The gradients were strongest at E, and of similar magnitude at the other three locations. No significant variability in the magnitude of this gradient was seen with time. But the entire layer of strong thermal gradient showed an ascent throughout the observational period.

The daily averaged salinity generally decreased from N to S (Fig. 7). These meridional differences reduced with depth. The vertical profiles exhibit a strong subsurface maxima in the depth range of 50-75 m, which is typical of this region (W,~tty,1, 1971). By 14 June the salinity dropped at all the locations except at S where an increase was registered in the mixed layer. The temporal changes in the salinity field seem to be influenced by lateral advection. The halocline considerably strengthened at W where the mixed layer deepening rate was also low. MILLER (1976) showed the importance of salinity gradient below the mixed layer on the layer deepening rates.

The observed current records have been low-pass filtered to remove the semidiurnal tidal and higher frequency components above 0.08 cph. The means have not been removed from each time series. The current sticks at one hourly intervals (alternate points) were only presented to avoid congestion in the diagram for two typical depths, 25 m (mixed layer) and 100 m (thermocline) (Fig. 8). These vector diagrams illustrate the following features of the synoptic scale current fluctuations. Within the mixed layer the flow was mainly southwesterly at N and E locations. At N the currents were generally weak compared to those at E but registered an increase in strength with time. At S the flow was mostly towards southeast and was strongest (50 cm s-t). At W the currents were highly variable, with synoptic scale fluctuations with an approximate period of 3 days.

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Fig. 6. Depth-time variation of the vertical thermal gradient.

layer variability 237

10C

150 "i

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SALINITY (%01 J,

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Fig. 7. Daily averaged vertical salinity profiles for the initial and final days.

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This oscillation does not exactly match with the local inertial period of 4 days. The flow at W was more oscillatory, but the mean flow was weak northwesterly. The flow was much weaker at 100 m depth and the coherence in the vertical was very weak with the only exception at N. At all the other three locations the flow was oscillatory with some weak lateral coherence.

Mixed layer depth and heat content

The mixed layer deepened from about 20 m to over 40 m at all the locations during this 2 week period (Fig. 9). But the layer deepening rate during the last 4 days was significant at N and E; the stations being closer to the zone of developing weather. The factors

238 R.R. Rxo and B. MArt-~w

0

2

o @ @ @ @

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JUNE 1979

Fig. 9. Daily progression of mixed layer depth (denoted as the deepest depth where SST minus 0.2°C occurred in the daily averaged BT profile), below layer vertical thermal gradient and heat

content.

responsible for the observed changes in MLD will be discussed later with the aid of a one- dimensional numerical model. The thermal stratification below the mixed layer is one of the most important factors in determining the mixed layer deepening rate. The day to day variability of the bulk vertical thermal gradient (BLG) in the 30 m water column below the mixed layer base is also shown in Fig. 9. The temporal variation in BLG was relatively large at E and S, when it even exceeded I°C/10 m during the latter disturbed regime. This increase in BLG at E and S mostly resulted from the ascending tendency of strong stratified water column beneath the mixed layer.

The heat content in the upper layers with respect to three isotherms is estimated utilizing the following equation.

f° HC -- P% ( T - rR)dz, 0

where all the terms have standard notation. The reference temperature TR is chosen as 28, 25 and 20~C. The heat content with respect to 28°C isotherm (HC28) represents the heat energy available in the water column for the atmospheric processes, while the

,Mixed layer variability 239

variations in the heat content with respect to 25"C (HC~) and 20"C (HC2o) isotherms suggest the importance of the Ekman and geostrophic heat transports in the upper layers. Within the polygon HC2s was in general lowest at S location. During the observational period HCzs decreased rapidly at E and S compared to W, while the variation was minimal at N. This reduction might have resulted due to the strong upwelling noted in the thermocline at E and S compared to the other locations. The same pattern is also evident in HC25 and HC:0 but with variable magnitudes. The warming beneath the mixed layer is masked against the cooling caused by the uplift of the isotherms in the thermocline. The heat content values between 40 and 60 m depths clearly show an increase in association with the below layer warming (Fig. 9).

Modelling the mixed layer temperature and depth

A one-dimensional numerical model developed by Kotaus and TUIL'~ER (1967), modi- fied by DEmaA~ (1973) and extended by MILLER (1976) for salinity effects was utilized to simulate the observed cooling and deepening of the mixed layer. RAO (1986) carried out similar integrations of these model equations to simulate the mixed layer cooling and deepening in the central Arabian Sea during the onset phase of MONSOON-77 with reasonable success. Wind stress, surface heat and salt budgets as destabilizing factors with below layer thermohaline gradients (stabilizing factors) are considered in the simulation of MLD. Surface heat budget processes and entrainment at the base of the mixed layer are considered for the simulation of mixed layer temperature (MLT). The governing equations following MILLER (1976) are

dh 2[G - O + Qt(l - e-Vh)] - h[aL + QE + Os + Qt(l + e - v h ) - 2g/gt S(P - E)] - - = ( 2 ) dt h i ( T - Th) + X / a ( S - &)l

aT l [ - d h ] d--7=h ~ ( T - T h ) + O L + Q E + Q s + O t ( l - e -rh) + V.VT (3)

dTh dh dt - 7Q#-vh - -'~LT (4)

[ dh ] d S _ 1 _ ( S - Sh) - S ( P - e ) dt h

(5)

dSh dh

dt dt Ls, (6)

where h is mixed layer depth, t is time, G--D is turbulent kinetic energy derived from the wind (G-D = -m'cU/pag), Ql is insolation, y is the extinction coefficient (0.002 cm-t), Qt. is net longwave radiation, QE is latent heat flux, Qs is sensible heat flux, S is mean salinity of the mixed layer, T is mean temperature of the mixed layer, P is precipitation, E is evaporation, ct is the coefficient of thermal expansion, k is the coefficient of haline contraction, Th is temperature just below the mixed layer, Sh is salinity just below the mixed layer, LT and Ls are temperature and salinity gradients just below the mixed layer, x is wind stress, U is wind speed at 10 m and p is density of the mixed layer.

2 4 0 R . R . RAO and B. MATt-nEw . , . . , ,

These equations (2-6) are solved using the Runge--Kutta numerical integration scheme with a time step of 1 h when the surface winds were weaker than 10 m s -l. A time step of 5 rain is used when the winds strengthened over 10 m s -l. When the mixed layer shoals the term involving below layer density jump loses its significance and "h" is solved using Newton's iterative technique. To estimate the G-D term, several choices of "m" were tried. The value of 0.0009 for "m" gave the best comparable result. Similarly the choice of 0.002 for y produced the best agreement.

In the present study two sets of integrations without (run 1) and with (run 2) the advection term were performed for the total observational period (2-14 June). The distributions of the hourly simulated and three hourly observed (only six hourly at E) MLT and MLD are shown in Fig. 10. In general the agreement between the observed and simulated (run 1) MLT is quite encouraging at all the locations with the only exception of N. At N the observed MLT registered a mild steady drop during the initial undisturbed regime (2-12 June), while MLT from run 1 showed a progressive rise until 12 June. During the undisturbed regime the estimated accumulated heat across the air-sea interface is expected to produce an increase in MLT. The disagreement between observed and simulated MLT from run 1 probably suggests the importance of lateral

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Mixed layer variability 241

Table 1. r.m.s. Errors

MLD (m) MLT ( 'C)

Ship Run 1 Run 2 Run 1 Run 2

N 10.57 6.74 0.52 0.33 E 4.52 - 0.13 - S 9.68 - 0.17 - W 7.42 - 0.25 -

advection of colder waters or the underestimation of entrainment cooling from below. The entrainment cooling may not be significant during the undisturbed conditions. The mean monthly analysis of SST fields for June 1979 (MOLn~ARI et al. , 1986c) show the presence of colder waters (<29°C) off the southwest coast of India. These cold waters might have advected into the study area as evidenced by observed southwesterly currents at 25 m depth at N (Fig. 8). In general the correspondence between the observed and run 1 MLT is quite good during the disturbed regime (12-14 June) when the local one- dimensional forcing was strong. The available observed current field at 25 m depth at N was chosen to represent the mixed layer mean flow field. As no spatial surveys within the polygon were made MOtJ~^R! et al. 's (personal communication) SST fields on pentad scale were utilized to estimate the horizontal temperature gradient in the determination of advective term V.VT. Incorporating the term V. VT in the heat budget equation (3) the integration for run 2 was repeated. The inclusion of the advective term has significantly reduced the disagreement between the observed and simulated MLT from run 1, thus demonstrating the importance of lateral advection of colder waters at N. The r.m.s. errors of MLT for run 1 and run 2 are shown in Table 1.

In general the agreement between the observed and simulated MLD is quite encourag- ing in pattern despite some minor differences due to perturbations in MLD caused by internal oscillations. In the model output the large amplitude fluctuations are mostly caused by the shoaling of mixed layer during the heat-dominated regime of the diurnal cycle (dh/dt < 0). At N the simulated MLD for run 1 underestimated the observed MLD due to weak heat losses. On the other hand, incorporation of the heat advection term (run 2) improved the model performance due to reduced thermal stratification in the denominator of equation (2). On a synoptic scale these results are consistent with those of DAvis et al. (1981). The r.m.s, errors of observed and simulated MLD for runs 1 and 2 are shown in Table 1.

C O N C L U S I O N S

The surface meteorological elements showed sudden changes with the monsoon onset event. The wind field strengthened to over 15 m s -t and enhanced the turbulent heat losses across the air-sea interface. The SST dropped by about I°C and the mixed layer deepened by over 20 m. These variations in the mixed layer properties were relatively larger during the disturbed weather conditions at the sites located closer to the onset vortex system. Isotherms in the topmost 200 m water column showed an ascending tendency throughout the observational period. The location of the layer of stronger thermal stratification also varied within the observational array. The temporal changes in the heat content with respect to 28, 25 and 20~C appeared to have been controlled by

242 R.R.R.Ao and B. MA'rl-mw

ascending motion in the upper layers. The observed currents in the mixed layer (25 m depth) were relatively stronger compared to those in the thermocline (100 m depth). But the spatial coherence in the mixed layer currents was relatively weak within the array. In the salinity profiles, the temporal changes were mostly confined to the topmost 100 m water column. On the whole the salinity dropped at all the stations with the only exception of S where salinity showed an increase.

The observed day to day variability in the MLD was examined in relation to local surface meteorological forcing with the only exception at N. Accordingly, a one- dimensional numerical model was utilized to simulate the development of the MLT and MLD. The observed and simulated MLD and MLT showed good agreement, particularly during the last three disturbed days, implying the importance of local one-dimensional forcing. The correspondence between the observed and simulated MLT at N during the undisturbed days improved with the inclusion of the heat advection term. More case studies are however needed to establish the relative importance of various competing physical processes which regulate the observed mixed layer variability in the Arabian Sea with the onset and progress of the summer monsoon.

Acknowledgements--The authors express their greatest appreciation to all the planners of the MONEX and to the USSR scientists and technicians who collected very valuable data sets utilized in this study. Thanks are due to the Director General of Meteorology, India Meteorological Department, New Delhi. for making available these data sets. Thanks are due to Dr R. L, Molinari, NOAA/AOML, Miami, U.S.A., for sparing SST analysis for the Arabian Sea for June 1979. Mr M. X. Joseph's programming support is appreciated, Helpful comments from anonymous reviewers and the editor during the preparation of the manu~ript are gratefully appreciated. The authors wish to record their gratitude to the Director, Naval Physical and Oceanographic Laboratory for his constant encouragement and hcilitics during the course of this study.

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