Continental Shelf Research 32 (2012) 62–70

Contents lists available at SciVerse ScienceDirect

Continental Shelf Research

0278-43

doi:10.1

� Corr

E-m

journal homepage: www.elsevier.com/locate/csr

Research papers

Response of southwest monsoon winds on shelf circulation off KeralaCoast, India

Madhu Joshi, A.D. Rao �

Centre for Atmospheric Sciences, Indian Institute of Technology Delhi, Hauz Khas, New Delhi 110016, India

a r t i c l e i n f o

Article history:

Received 29 December 2010

Received in revised form

21 October 2011

Accepted 24 October 2011Available online 6 November 2011

Keywords:

Coastal upwelling

QSCAT winds

Currents

Buoy data

43/$ - see front matter & 2011 Elsevier Ltd. A

016/j.csr.2011.10.015

esponding author. Tel.: þ91 11 26522317; fa

ail address: adrao@cas.iitd.ernet.in (A.D. Rao)

a b s t r a c t

Three-dimensional Princeton Ocean Model is used to investigate the dynamics of the shelf flow

response to the spatial and temporal variability of the wind stress forcing along the Kerala Coast. The

model is configured for the west coast of India with higher horizontal resolution near the coast. In the

vertical, a terrain following sigma coordinate is used, having finer resolution near the surface and

bottom. In the present study, an attempt is made to understand the variability of the surface circulation

and associated SST during July–August for 2000 and 2002 as a response of daily wind stress forcing

derived from the QSCAT winds. The southerly current during July–August supports upwelling processes

along the Kerala Coast. From the available buoy observations and simulations, it is noted that the

coastal current reverses its direction to either off-shore or northward if the wind magnitude reduces

suddenly below 3 m s�1 towards the coast. In this event, the coastal upwelling is inhibited by the

reversal of currents and it results an increase in the local SST. The reversal of currents is explained in

terms of local vorticity generated as a result of reduction of wind magnitude near the coast.

& 2011 Elsevier Ltd. All rights reserved.

1. Introduction

During south west (SW) monsoon the wind-driven circulationdominates the entire west coast of India (Shetye and Shenoi,1988; Wyrtki, 1971). The observational study shows that there isa substantial surface cooling at the southern Indian coast (parti-cularly, Kerala Coast) during July–September that is attributed tocoastal upwelling, promoted by the alongshore wind stresscomponent (Johannessen et al., 1987; Shetye et al., 1990). Thecoastal upwelling is a local phenomena while, the SW monsoon isglobal. Hence, the local SST change due to the upwelling may notrelate directly to the monsoon activity and associated precipita-tion. However, the upwelling process is of concern as thereare important fisheries along this coast. The field experimentsconducted by Sanilkumar et al. (2004) during July 2003 offsouthwest coast of India also suggests the same. Wind stress isidentified as the most important driving force for coastal upwel-ling. Although the wind stress is mostly directed southerly alongthe coast, i.e. favorable to coastal upwelling, the observationssuggest considerable variability over the South-Eastern ArabianSea (SEAS) on spatial and temporal scales. The variability maymodify the coastal circulation and hence affect the upwellingprocesses along the coast. The intense upwelling along the coast

ll rights reserved.

x: þ91 11 26591386.

.

reduces the sea surface temperature (SST) of the region signifi-cantly by 2–3 1C. Haugen et al. (2002) studied the seasonalcirculation and coastal upwelling off the southwest Indian coastusing high resolution Miami Isopycnic Coordinate Ocean Model(MICOM). The model simulations by Rao et al. (2005) during SWmonsoon show that the coastal upwelling predominates in southcoast along the west coast of India as compared to that of northcoast. Further, it is found that the upwelling is more intense whenthe monsoon is at its peak in July compared to May andSeptember. Rao et al. (2006) also studied this region usingsatellite data and named it as Mini Cold Pool (MCP). This observedMCP primarily appears to be a consequence of coastal upwellingcaused by the divergence in the near-surface circulation.

Whenever coastal Kelvin waves propagate along the easternmargin of a basin (e.g. west coast of India), they radiate westwardpropagating Rossby waves. Johns et al. (1992) illustrated theeffect made by the alongshore propagation of coastally trappedright bounded Kelvin waves on the shelf circulation and coastalupwelling. It has two indefinable effects the first of which is toexercise a coastally confining influence on the upwelling-relatedsurface jet and the second is suppressive influence on theupwelling tendency. The westward propagating Rossby wavesinfluence the coastal circulation in a longer time of the order offew months, when its dispersion of the coastal jet becomesimportant (Philander and Yoon, 1982).

Luis and Kawamura (2002a,b) studied the upwelling processesnear the southern tip of India as a result of gap wind event

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Fig. 1. Model analysis area and bathymetry (m).

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Fig. 2. QSCAT winds for

M. Joshi, A.D. Rao / Continental Shelf Research 32 (2012) 62–70 63

(blowing in the Palk Strait) during pre-monsoon season. The gapwinds are generally low-level strong winds of 20–40 knotsblowing through a gap between mountains. Gan and Allen(2002a,b) studied the upwelling processes and correspondingcirculation along the California coast. Their study demonstratesthe existence of northward currents during relaxation of south-ward upwelling favorable winds. The coastal upwelling along thewest coast of India is recently studied (Rao et al., 2008) duringpre-monsoon and monsoon seasons using Princeton Ocean Model(POM). The study includes temperature inversions, reversal ofsurface and sub-surface currents in the SEAS region. Luis andKawamura (2003) noticed that during SW monsoon the surfacecooling due to coastal upwelling promoted by the alongshorewind stress overwhelms the surface heat loss. In the presentstudy, an attempt is made to comprehend coastal variability ofshelf circulation and associated upwelling processes as a responseof wind alone on the temporal and spatial scales.

2. Numerical experiments

The POM model (described in detail by Blumberg and Mellor,1987) is implemented for the eastern Arabian Sea (AS). As shownin Fig. 1, the analysis area (bounded by dashed line) extendsapproximately from 41N to 24.51N along the eastern AS, which isalmost parallel to the coast with a maximum offshore extent of�800 km on the southern open boundary and reduces towardsnorth. The model is fitted with realistic bottom topographyderived from modified ETOPO2 (Sindhu et al., 2007). The modifieddataset is more accurate in depths below 200 m and therefore

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July—August 2000.

M. Joshi, A.D. Rao / Continental Shelf Research 32 (2012) 62–7064

improves the performance of the numerical model in the coastalzone. Resolution in zonal direction varies from 6 to 9 km withfiner resolution near the coast while, it is �10 km in meridionaldirection with 175�250�26 computational grid points. Themodel grid size near the coast resolves internal Rossby radius ofdeformation. The model uses mode splitting technique for solvingbarotropic and baroclinic modes separately in order to savethe computational time. The two dimensional external modeuses a short time-step of 11.25 s based on the Courant–Freidrch–Levy condition and the external wave speed, while,three-dimensional internal mode uses a long time-step of 480 sbased on the internal wave speed. Along the open-boundaries,radiation type of conditions are used to allow disturbancesgenerated from the interior to travel outwards as progressivewaves. For temperature and salinity, upstream advection schemeis applied. Initial data fields of temperature and salinity areobtained from World Ocean Atlas 2001 (Conkright and Boyer,2002). Detailed description about the model setup and boundaryconditions can be referred from Rao et al. (2008). The modelsurface forcing is derived from the daily QuickSCAT winds of 0.251horizontal resolution.

To study the variability in shelf circulation and associatedthermal response during July–August for 2000 and 2002, twoexperiments have been carried out keeping the same initialconditions for temperature and salinity. The model is forcedwith the corresponding wind stress after interpolating at themodel grid points. Initially, the model is integrated in thediagnostic mode for 25 days until the kinetic energy of the systemachieves nearly a steady state (Ezer and Mellor, 1994). The model

INDIA

INDIA

16 Jul 2002

02 Aug 2002

Fig. 3. QSCAT winds for

integration is then further carried out for 50 days starting from 21June and simulations are analyzed from 1 July focusing the resultsalong the Kerala Coast. Four coastal stations A–D are selected asshown in Fig. 1 to study the circulation at different latitudes. Thebuoy observations of SW4, which is located near to the station C isused for validation of the model simulations.

As the wind stress has a direct bearing on the upper layers of theocean and hence on the upwelling processes, it is pertinent here tohave an overview of the wind field in the analysis region. In order tostudy spatial and temporal variations of the winds over the region,selected dates are picked up during July and August when the winddirection is almost uniform but the magnitude vary significantlytowards the Kerala Coast in 2000 and 2002. Fig. 2 shows the QSCATdaily winds of 2000 for selected days, that is, July 7, 16 and August10, 16. The magnitude of the winds is shown in color code and thedirection by the vectors. During this period, the general winddirection is mainly westerly. Strong winds (410 m s�1) over thecentral AS indicate the low-level jet (Findlater, 1977), whichextends up to the northern part of the west coast of India. Thewinds over the SEAS are found to be ranging from 2 to 10 m s�1

with a maximum variability in wind magnitude off Kerala Coast.The high intensity winds (46 m s�1) are noticed from 1 July to 12July (figures for all these dates are not depicted here). It can be seenclearly from Fig. 2 that the winds along the Kerala Coast arestronger (�10 m s�1) on 7 July. It suddenly reduces and becomeso3 m s�1 on 16 July. The low-wind field over the SEAS remains fora period of 3–5 days. It started picking up the strength slowly andmaintains the wind strength more than 6 m s�1 till 12 August.Another representative field of high-wind intensity on 10 August is

INDIA

INDIA

23 Jul 2002

08 Aug 2002

July–August 2002.

M. Joshi, A.D. Rao / Continental Shelf Research 32 (2012) 62–70 65

presented here in Fig. 2. The strength of the winds reduces again(o3 m s�1) on 16 August.

Similar trend of sudden reduction in winds is noticed in 2002during July to August. The winds on the selected days, that is, 16,23 July and 2, 8 August is presented in Fig. 3. The winds over theSEAS region remain high (46 m s�1) with a maximum intensityon 16 July. The intensity reduces towards the coast suddenly after20 July and becomes below 3 m s�1 on 23 July. The similar low-wind field over the area remains for less than a week and picks upthe strength again. Another representative wind of high-intensitywas seen on 2 August followed by sudden reduction on 8 August.As the coastal upwelling process is transient phenomenon, it istherefore analyzed the coastal circulation and associated SST as aresponse of real-time winds prevailing over the region.

3. Results and discussion

Fig. 4 gives the model simulation of surface currents for 7 and16 July and 10 and 16 August of 2000. The simulated coastalcurrent is found to be southward along the Kerala Coast when thewinds are uniform over the SEAS region on 7 July and 10 August.

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Fig. 4. Model surface current (ve

However, it is interesting to note that the simulated currents varyits direction on 16 July and 16 August when the wind strength isreduced towards the Kerala Coast. The near-shore current direc-tion shifted to either offshore or northward when the wind speedfell below 3 m s�1. In the case of 16 August, as the windmagnitude reduces significantly to 2 m s�1 over the larger area(about 400 km) near to the coast, the simulated currents becomenorthward along the Kerala Coast. It is well known that thesouthward current along this coast produces coastal upwelling(Rao et al., 2008). Hence, it is expected that the coastal surfacewaters would be cooled. Since the coastal currents along the coastare either off-shore or northward on 16 July and August, it is notconducive to coastal upwelling. Hence, the associated SST issupposedly warm compared to that of 7 July and 10 August. Thisis supported qualitatively by the TRMM Microwave Imager (TMI)SST as shown in Fig. 5. In the absence of AVHRR SST data due tocloud coverage most of the time during this period, the TMI datais used here, which is a available at a spatial resolution ofabout 25 km (3-day mean) for corresponding dates. Particularly,significant warming of about 30 1C is seen along the coast on 16August compared to that of 16 July. This can be explained throughthe model simulated northward currents along the coast on 16

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ctors) 2000 with wind only.

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Fig. 6. Model surface current (vectors) 2002.

Fig. 5. TMI SST for 2000. The area of interest is circled.

M. Joshi, A.D. Rao / Continental Shelf Research 32 (2012) 62–7066

2 Aug 2002 8 Aug 2002

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Fig. 7. TMI SST for 2002. The area of interest is circled.

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Fig. 8. Wind speed (m s�1) and direction of surface current (deg.) for 2000.

M. Joshi, A.D. Rao / Continental Shelf Research 32 (2012) 62–70 67

M. Joshi, A.D. Rao / Continental Shelf Research 32 (2012) 62–7068

August, which would unable to favor the local upwelling. In thiscase, it is expected that the temperature of the surface waterswould not decrease locally as the Ekman transport is not in thefavorable direction.

Similar experiment is carried out for 2002 for which SW4 buoyobservations (located near the station C) on currents, winds andSST are available. In Fig. 6, simulated surface currents (vectors)are shown on the selected dates chosen (refer Fig. 3). Accordingly,the currents are southerly on 16 July and 2 August when thewinds are mostly uniform over the SEAS region. These currentsalong the Kerala Coast enhance the prospects of coastal upwellingand hence a reduction in SST is expected. As shown in Fig. 7,

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a qualitative trend of low SST is seen in the TMI data on 16 July,while, it is not clearly seen on 2 August as most of the data in theregion is missing. The simulated currents along the region on 23July and 8 August are northward and off-shore, respectively. Asmentioned earlier, this is due to decrease of winds towards thecoast. The change of current direction to either off-shore ornorthward along the coast inhibits the upwelling processes ofthe region, thereby; the SST is seen locally high. A warming ofcoastal waters up to 30 1C along the region is seen in the TMI SSTon the respective dates depicted in Fig. 7.

To investigate further, the cause of reversal of coastal currentsin the month of July and August, time variation of QSCAT wind

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Fig. 11. Schematic of local vorticity due to zonal variation in wind magnitude (a) decreases towards the coast (b) increases towards the coast.

M. Joshi, A.D. Rao / Continental Shelf Research 32 (2012) 62–70 69

speed (used in the study) and associated simulated surfacecurrent is plotted for 2000 and 2002 in Figs. 8 and 9, respectively,at the locations of A–D. As shown in Fig. 1, stations A and B arelocated north of Kerala; stations C and D are located along theKerala Coast. If the current direction is between 1401 and 2201,the flow direction is then considered as southward and if thedirection is between 2701 and 3601 or 01 and 451, it is northward.The variation in wind magnitude is noticed along all the pointswith a maximum range at C and D locations. It is evident from thefigures that flow pattern at A and B is southward throughout theperiod but its direction varies at C and D. It is noticed wheneverthe wind speed near the coast (within 50 km from the coast)decreases (o3 m s�1) there is reversal of currents in the region.Strength of the reversed current is also inversely proportional tostrength of the wind near the coast. Particularly, the windmagnitude reduces to 2 m s�1 on 16 July 2000 at the location Cand hence the corresponding off-shore current (Fig. 4) is verystrong at this location. It is also observed that there is a lag ofabout a day between the reduction in winds and the reversal ofcurrents.

To verify further existence of reversal of currents near thecoast, time variation of SW4 buoy wind speed and currentdirection for July–August 2002 are depicted in Fig. 10. To avoidthe land and sea breeze effect, the early evening to late morningdata is removed. That means the observations between 9 a.m. and4 p.m. are considered here to get the daily variations from thebuoy data. It is noticed from the observations that the currentstending to change its direction either off-shore or northwardwhenever the winds fell below 3 m s�1 near the coast. This isparticularly observed during 7–9 and 17–27 July. It is found thatthe correlation coefficient is 0.8 between model simulated andobserved currents for 2002. The time lag is also seen about a daybetween the reduction in winds and reversal of currents in theobservations. It is important to mention here that the decrease/increase in temperature due to variation in wind speed andcurrent direction is also noticed in the buoy SST data (figure notincluded here). Hence, the model simulations described above areconsistent with the feature noticed in the observations.

It is also important to understand the possible mechanism forreversal of coastal currents in the event of any decrease of windstrength towards the coast. To make this point more apparent, aschematic diagram is shown in Fig. 11(a, b) depicting anti-clock-wise and clockwise eddy formation as result of zonal windvariation. As it understands from Fig. 11(a), the distribution ofwind field when its magnitude decreases towards the coast, it

would help to form a local anti-clockwise vorticity leading toreversal of coastal currents. On the contrary to the above situationwhen the winds increases towards the coast, the circulationfavors a clockwise vorticity (Fig. 11b) causing a southern flownear the coast. As the winds modulate on synoptic scale, thevorticity would play an important role on the local coastaldynamics. Hence, it may be concluded that the weak windsprevailing near the coast compared to that of far away from thecoast is the main cause for the reversal of the coastal currents.

4. Conclusions

Variability of winds over the SEAS is observed on both spatialand temporal scales. The variability affects the shelf circulation ofthe region significantly. During July–August, the winds over theSEAS generate, in general, southerly current, which is conduciveto coastal upwelling of the region. However, the reversal of thecurrents is observed from the buoy data whenever the windmagnitude decreases towards the coast. This has been demon-strated by a numerical model using QSCAT daily wind data for2000 and 2002 and explained a mechanism for the reversal ofcurrents in terms of a local vorticity in the region. Moreover,strength of the reversed current depends on the gradient of thewinds in the zonal direction. The reversal of the currents inhibitsthe local coastal upwelling and hence increases the SST of theregion. Both the simulations and observations suggest that thereis time delay of about a day between the reduction in winds andreversal of currents near the coast.

Acknowledgments

The research is supported under INDOMOD program by theINCOIS, Ministry of Earth Sciences, Government of India. Theauthors are thankful to Dr. Shailesh Nayak, Secretary, Ministry ofEarth Sciences for supporting the study. Thanks are also due toDr. M. Ravichandran, INCOIS for his valuable suggestions.

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