ORIGINAL PAPER

Indian summer monsoon onset vortex formationduring recent decades

R. Deepa & Jai Ho Oh

Received: 18 December 2012 /Accepted: 27 November 2013# Springer-Verlag Wien 2013

Abstract This study investigates the decrease in the frequencyof onset vortex of summer monsoon during recent decadesusing the National Center for Environmental Prediction–National Center for Atmospheric Research reanalysis (1982–2011) data. Onset vortices are known to occur over the ArabianSea mini warm pool where the sea surface temperature peaksjust before the onset of monsoon. Even though the Arabian Seamini warm pool intensifies during the recent decades, they arenot seen as a regular feature. It is found from the analysis ofirrotational and non-divergent wind component at 850 and200 hPa that during the recent decades, convergent windsdominate at upper levels and divergent winds at lower levelswhich inhibits convection. Moreover, the cyclonic shear vortic-ity shows a decrease in the recent decades which tend to reducethe boundary layer moisture convergence and lower tropospher-ic humidity which is an important component for the initiationof a cyclonic system. The recent decades are characterized byweak convection due to the presence of strong northerlies anddescending motion at lower levels in the southeast Arabian Sea.The response of atmospheric circulation to the interdecadalvariations in the warm pool and the corresponding decrease inthe frequency of onset vortex formation is analyzed in detail.

1 Introduction

The sea surface temperature (SST) in the north Indian Oceanpeaks (>28.5 °C) just before the onset of summer monsoon,

making it the warmest region among the world oceans (Joseph1990; Vinayachandran and Shetye 1991; Rao and Sivakumar1999). In the Indian Ocean, three warmest regions are in thethree preferred locations: in thewestern equatorial IndianOcean,in the southeast Arabian Sea (SEAS) and in the eastern Bay ofBengal. The region in the SEAS with SST >30 °C is named asthe Arabian Sea mini warm pool (ASMWP), first reported bySeetaramayya and Master (1984). Later studies (Rao andSivakumar 1999; Shenoi et al. 1999; Vinayachandran et al.2007; Deepa et al. 2007) examined the evolution and dynamicsof the warm pool using both observational and reanalysis data.

The importance of the mini warm pool over the ArabianSea in the prediction of monsoon onset is well represented byKershaw (1985, 1988). The onset of the summer monsoonover India is often, but not always, accompanied by the for-mation of an onset vortex (Krishnamurti et al. 1981) in theSEAS, and this suggests a possible link between this cyclonicsystem and the warm pool. The vortex, a low-pressure systemwhich forms over the east-central Arabian Sea (ECAS) on theleading edge of the monsoon current, brings monsoon flowand sets the monsoon over the south peninsular India. It oftendeepens into a cyclonic storm, resulting in establishment of aregion of convergence over the SEAS, which is followed bythe strengthening of monsoon westerlies (Ananthakrishnan1964; De and Joshi 1995). These synoptic systems(lows/depressions) which form in the monsoon flow and thesemi-permanent systems which establish as convective fea-tures contribute significantly to the onset and advance of themonsoon over India and the associated rainfall. The intensityof the system (Rao 1976) may vary between that of a trough oflow pressure (45 % cases) and cyclonic storms (8 % cases).Ananthakrishnan et al. (1968) have documented this type ofphenomenon from the data of 68 years from 1901 to 1968 andfound that in 75 % of the occasions, the onset of monsoon wasassociated with some cyclonic systems in the Arabian Sea.Among these 75 occasions, about 50 % are feeble troughs onthe low levels and 25 % depressions and storms.

J. H. OhDepartment of Environmental Atmospheric Sciences,Pukyong National University, 599-1, Daeyon-3 Dong, Nam-Gu,Busan 608-737, South Korea

Present Address:R. Deepa (*)Centre National De Recherches Meteorologiques, Meteo France,42, Avenue G. Coriolis, Toulouse Cedex 31057, Francee-mail: deepalinoj@gmail.com

Theor Appl ClimatolDOI 10.1007/s00704-013-1057-z

The India Meteorological Department (IMD) classifiesthese systems as lows, depressions, cyclonic storms, severecyclonic storms and hurricanes based on surface winds circu-lating around the systems up to 8.5, 8.5–16.5, 17–23.5, 24–31.5 and greater than 32m/s, respectively (Krishnamurthy andAjayamohan 2010). The structure, dynamics of formation,movement and direct influence of rainfall on these low-pressure systems have formed an active area of research(Krishnamurti et al. 1975, 1976; Sikka 1977; Mooley andShukla 1987; Sikka 2006). Jenamani and Dash (2001) haveexamined the characteristics of monsoon disturbances andtheir relationship with Indian summer monsoon rainfall.Recently, Dash et al. (2004) have examined the decreasingtrend in the frequency of monsoon depressions over the Indianregion. They have come to a conclusion that this trend maynot be due to the change in SST, but due to unfavourableatmospheric dynamical parameters which do not support theformation. The considerable reduction in the frequency ofhigher intensity systems since the 1970s to the recent decadesin spite of warm SSTs is noteworthy. This type of synoptic-scale cyclonic system which forms just prior to the northwardadvance of southwest monsoon over the ECAS and the westcoast during late May and early June is called the onset vortex(Krishnamurti et al. 1981). Mukherjee and Paul (1980) havealso analyzed the synoptic features over the Arabian Seaassociated with the onset of monsoons for 10 years from1969 to 1978. They have revealed that the monsoon set inover the Arabian Sea and south peninsular India in all these10 years under the influence of either a weak system (troughor low) or a strong system like a northward moving depressionor cyclonic storm.

The onset vortex can be identified in the wind field fromthe synoptic charts from the surface to 500 hPa. However,Krishnamurti et al. (1981) and Fein and Kuettner (1980) haveshown that the onset vortex can be visualized on the northernflank of the low-level jet at 850 hPa when the jet is broughtclose to the southern tip of India along 10°N. The onset vortexhas a three-dimensional structure. It has a zonal wavelength of3,000 km and a vertical extent up to the middle troposphere.Its time scale is 4–5 days. Generally, it forms in the latitudinalbelt 10°–15°N and east of 69°E and moves in a west–north-westerly direction with a speed of about 2° per day in thebeginning. Later, it moves in a northwesterly direction at aspeed of about 3°–5° per day. Based on 80 years of observa-tions, it is noticed that the onset vortex occurs in 67 % casesand has a wind speed of about less than 33 knots(Krishnamurti et al. 1981). It frequently moves meridionallytowards the northern Arabian Sea, and subsequently, its mo-tions are more westward usually towards the Arabian coast,where it dissipates. The monsoon onset vortex's associationwith the monsoon onset (Krishnamurti et al. 1981) was notwidely appreciated until after the Summer MonsoonExperiment 1979 (SMONEX).

Rao and Sivakumar (1999) found the geographical coinci-dence between the ASMWP and onset vortex for a 30-yearperiod (1961–1990). Recently, Deepa et al. (2007) made apreliminary survey of the relationship between ASMWP andonset vortex of summer monsoon over ECAS by consideringsix consecutive years of monsoon onset from 2000 to 2005.They have concluded that in the formation of OV, though theocean is fully cooperative with high thermal energy in theupper part, the onset vortex of summer monsoon does notform (over the ASMWP) unless the shear line forms along thenorthern flank of LLJ at 850 hPa near the west coast of Indiaalong 12°–14°N. They further found that there exists a stronginterannual variability in the formation and intensification ofthe mini warm pool. Later, Deepa et al. (2011), using the PSU/NCAR MM5 model, pointed out that ASMWP is playing akey role in the development of onset vortex by increasing thehorizontal shear and decreasing the vertical shear. Recently,Ramesh Kumar et al. (2012) have examined whether the onsetvortex is an important ingredient for the monsoon over Kerala.They considered the monsoon onset vortex as any synopticdisturbance which forms in the SEAS region. They alsosuggested that the monsoon onset vortex which forms overthe SEAS region upsets the delicate balance between convec-tion, buildup of moisture and strengthening and deepening ofthe westerlies over the SEAS that is needed for the setting upof the MOK. Sijikumar and Rajeev (2012) reiterated that thepeak location and amplitude of Arabian Sea SSTs do have adominant effect on the monsoon onset process and the char-acteristics of the ASMWP could be a potential predictor thathelps improve the seasonal forecast of the Indian summermonsoon onset.

The focus of the present study is to examine the decrease inthe frequency of onset vortex of summer monsoon duringrecent decades. In this study we define monsoon onset vortexas any synoptic disturbance that forms just before or with theonset of monsoon which drags the monsoon current north-wards and helps in the northward advance of monsoon.

2 Data and methods

The main data source for the study is the National Center forEnvironmental Prediction–National Center for AtmosphericResearch (NCEP–NCAR) Reanalyses fields from May toJune. The NCEP–NCAR Reanalyses uses a state-of-the-artglobal data assimilation system on a 2.5°lon by 2.5°lat grid(Kalnay et al. 1996). Variables used in this study are pentadatmospheric horizontal and vertical winds and mean sea levelpressure. The interdecadal warm pool variability is studiedusing NCEP Optimum Interpolated version 2 (OIV2) SSTdata (Reynolds et al. 2002). In order to examine the air–seainteraction over the warm pool, WHOI OA flux data from1985 to 2011 is used (Yu and Weller 2007). Horizontal wind

R. Deepa, J.H. Oh

velocity can be divided into non-divergent (rotational) anddivergent (irrotational) components (Krishnamurti 1971):

V ¼ Vψ þ Vχ ¼ K � ∇ψþ ∇χ

where ψ is the stream function and χ is the velocity potential.In the natural coordinate system, the vorticity is given by

ξ ¼ V ∂β∂S−

∂V∂n , where ξ represents the relative vorticity, V

represents the local wind vector, the two unit vectors n ands are pointed perpendicular (normal) and parallel (streamwise)to the local wind vector V, respectively, and β is the winddirection relative to the coordinate system.

According to the above equation, vorticity is determined bycurvature and shear.

The quantity −∂V∂n

� �is called shear vorticity, which is a

measure of the change of the wind speed perpendicular to the

wind vector. The quantity V ∂β∂S

� �, the curvature vorticity, is

positive (negative) in a clockwise (counterclockwise) flow(Holton 1979).

Since Reynold's weekly SSTs are available from 1982onwards, we have taken the first decade (1982–1990) for theanalysis. The decades 1982–1990, 1991–2000 and 2001–2011are represented as DE1, DE2 and DE3, respectively. Also, thenotation ‘onset-2’ represents two pentads before onset, ‘onset-1’ represents one pentad before onset and so on. The 850 hPalevel is the main forcing region for the monsoon onset vortex(Kershaw 1985, 1988; Deepa et al. 2007). So here in thisstudy, the 850 and 200 hPa levels are taken as representativelevels for studying the atmospheric interdecadal circulations.The study area is represented by 10°S–25°N, 40°–100°E; forthe warm pool evolution and for understanding the convergentand divergent centres of atmospheric circulation, the region40°–100°E and 10°S–30°N is considered. Table 1 representsthe onset dates for the corresponding study period. The fre-quency of onset vortex during the decades DE1, DE2 and DE3is 90, 20 and 27 %, respectively.

3 Results

3.1 Evolution of sea surface temperature

Evolution of SST 2 weeks before onset to 1 week after onset isrepresented in Fig. 1. The Arabian Sea warming is more in therecent decade (DE3) compared to DE1. The SST distribution2 weeks before onset shows 30 °C isotherm fully occupied inthe SEAS in DE1 (Fig. 1a) and a small tongue of 30.3 °Cisotherm in DE2 (Fig. 1e). The recent decade DE3 (Fig. 1i) isrepresented by SST >30.3 °C in the SEAS. In the subsequentweek, the isotherm of maximum SST area shrinks and isconfined as an elongated or concentric region in the SEASwith 30, 30.3 and 30.6 °C during DE1, DE2 and DE3, respec-tively (Fig. 1b, f, j). The SST during the onset week shows

29.5 °C isotherm in the SEAS for both DE1 and DE2, while therecent decade DE3 is still warm with SSTs greater than 30.3 °C(Fig. 1k). The cooling near the southern tip of India even 1weekafter onset in recent decades (DE2 and DE3) is less relative toDE1. The areal shrinking of warm SST isotherms and theirnorthward movement in accordance with the progress of mon-soon can be noticed (Fig. 1d, h, l). The warm pool in the SEAShas a critical impact on the monsoon onset over peninsularIndia during late May or early June. Kothawale et al. (2008)have shown that there is significant warming over the ArabianSea, Bay of Bengal as well as equatorial south Indian Oceanover the last hundred years, and the trend has been acceleratedin the period 1971–2002. Joseph et al. (1994) suggested that theregion of high SST can create large-scale moisture conver-gence, deep convective clouds and lowering of the surfacepressure, favourable for monsoon onset over Kerala. If theSSTs are above a critical temperature, what are the other factorsthat decide convection to occur? Graham and Barnett (1987)

Table 1 Onset dates forthe corresponding studyperiod

a Years of onset vortexformation

Year Onset dates

1982a 01 June

1983a 12 June

1984a 01 June

1985a 24 May

1986a 12 June

1987a 01 June

1988a 26 May

1989 03 June

1990a 19 May

1991 02 June

1992 05 June

1993 28 May

1994a 28 May

1995 05 June

1996 03 June

1997 09 June

1998a 02 June

1999 25 May

2000 01 June

2001a 23 May

2002 29 May

2003 08 June

2004 18 May

2005 07 June

2006 26 May

2007a 28 May

2008 31 May

2009a 23 May

2010 31 May

2011 29 May

Indian summer monsoon onset vortex formation during recent decades

suggested that the important component which determines thepresence or absence of convection over an area with SSTgreater than a critical temperature is surface divergence. Theyadvocated that in some regions convection appears to be sup-pressed by persistent divergent surface flow that reflects large-scale subsidence. So in the next section, the analysis of lowerand upper tropospheric circulation is represented.

3.2 Evolution of tropospheric circulation

Figure 2 represents the anomalies of tropospheric circulationone pentad before onset and on onset pentad. Centres of low(high) velocity potential are associated with divergent outflow(convergent inflow) winds for DE1, DE2 and DE3 at 850 and200 hPa. High velocity potential with strong convergent

Fig. 1 Evolution of SST (in °C) 2 weeks before onset (first row), 1 week before onset (second row), during onset (third row) and 1 week after onset (fourthrow) for the decades a–d DE1 (1982–1990), e–h DE2 (1991–2000) and i–l DE3 (2001–2011). The square box inside the figure represents the SEAS region

R. Deepa, J.H. Oh

inflow during DE1 and divergent outflow during DE2 isnoticed at 850 hPa over the SEAS region (Fig. 2a, e). In therecent decade, strong convergent inflow is noticed over thenorthwestern Arabian Sea. The circulation at 200 hPa duringone pentad before onset represents strong divergent wind for

DE1 and convergent inflow for DE2 (Fig. 2b, f), while for therecent decade, strong outflow is noticed over the near-equatorial western Arabian Sea. This flow converges towardsthe west coast of India (a weak convergence zone). Thisdepicts that the circulation in the SEAS (Fig. 2j) supports the

Fig. 2 Velocity potential (shaded ; in 10−06 m2 s−1) and divergent wind vectors for a–d DE1, e–h DE2 and i–l DE3 for 850 and 200 hPa levels duringone pentad before onset (upper two rows) and on onset pentad (lower two rows)

Indian summer monsoon onset vortex formation during recent decades

formation of onset vortex during DE1. If we observe thecirculation features of onset pentad during DE2, strong diver-gent (convergent) flow exists at 850 (200) hPa (Fig. 2g, h),whereas in DE3 the SEAS is marked by weak negative veloc-ity potential at 200 hPa with maximum divergence over thewestern Arabian Sea. As a result, the frequency of onsetvortex formation has considerably decreased during thesedecades (Fig. 2k, l).

The climatological and anomalous Hadley and Walkercirculations over the ASMWP region are depicted in Figs. 3and 4. The first row represents the climatological circulationand the second row the anomalous circulation during onepentad before onset, and the third row represents the climato-logical circulation and the fourth row the anomalous circula-tion during the onset pentad. The Hadley circulation onepentad before onset represents upward motion from the sur-face to 200 hPa in the onset vortex region during DE1(Fig. 3b). The intense convection in the SEAS and to its eastgive rise to a local Hadley circulation with upward motionover the area of convection and downward motion in the southIndian Ocean (Joseph et al. 2003). This flow then returns aftermerging with the low-level jet. The corresponding pattern inDE2 shows descending motion from mid-troposphere to thesurface over the onset vortex region while upward motionbetween the equator and 5°N (Fig. 3f), and the descendingHadley circulation from 200 to 600 hPa is observed.Descending circulation is observed during DE3 from500 hPa to the surface with ascending motion above 500 to200 hPa (Fig. 3e, f). The onset pentad Hadley circulation inDE1 is characterized by a zone of confluence between 8° and15°N from the surface to 300 hPa (Fig. 3c, d). In DE2, theanomalous Hadley cell circulation shows descending motionover the onset vortex region. While DE3 is dominated by acyclonic circulation between the equator and 10°N, descend-ing motion persists over the region 8°–15°N. The recentdecade DE3 is characterized by strong upward (ascending)motion at 200 hPa (Fig. 3k, l). Also, a weak upward motion isnoticed from the surface to 700 hPa between 70° and 75°E.The comparison of pressure anomalies (figure not shown) withthe Hadley circulation shows low pressure anomaly in DE1,high pressure in DE2 and again low pressure in DE3 over theSEAS, but near the equatorial Indian Ocean in accordancewith the descending motion in DE3, positive pressure anom-alies are noticed. Ramesh Kumar and Schlüssel (1998) haveshown that an early peaking of SST in the Arabian Sea willhelp in the development of a low-pressure area and thus helpsa stronger-than-average interhemispheric pressure gradientand strong cross-equatorial flow.

The Walker circulation which is driven by the gradient ofsea surface temperature characterized by the coupling betweenthe tropical atmosphere and oceans is represented in Fig. 4. Itcan be seen that during one pentad before onset, the majorrising branch of the Walker circulation is found in between

68° and 75°E in the SEAS region. Maximum upward motionis noticed in the upper troposphere. A strong descendingbranch is seen at 300–400 hPa at 65°E (Fig. 4a, b), while inDE2, weak upward motion is noticed at lower levels between1,000 and 700 hPa (Fig. 4f). This is dominated by strongdescending motion from 200 to 650 hPa, thus inhibiting theformation of onset vortex (Fig. 4i). According to Lindzen andNigam (1987), the SST gradients induce surface moistureconvergence that promotes large-scale lifting in the tropics.

3.3 OLR anomalies

Figure 5 depicts the outgoing longwave radiation (OLR)anomalies two pentads before onset to one pentad after onset.An interesting feature is that during decade DE1, the SEASand equatorial Indian Ocean is marked by strong convection(Fig. 4a) whereas during the recent two decades DE2 andDE3, the convection is weak (Fig. 5e, i). The convection startsnear the equator only during the pentad after onset (Fig. 5h). InDE1, convection begins near the equator two pentads beforeonset (Fig. 5a). As time advances, the convection intensifiesand the area of maximum convection moves north over theSEAS (Fig. 5b–d). In DE3, the convection is concentratedover the western Arabian Sea where the SSTs are higher. InDE1, the intense convection zone is present in the SEAS fromone pentad before onset and on the onset pentad and extendsfrom SEAS to the South China Sea. In DE2, the convection isvery weak with positive OLR anomalies over the SEASregion. So there is no flux transfer at the air–sea interfaceinhibiting the formation of onset vortex as seen from fluxanomalies (figure not shown). According to Fu et al. (1994),the necessary condition for deep convection can be achievedeither by warm SSTor by surface convergence. Seetaramayyaand Master (1986) suggested that the combination of ex-change processes of momentum and sensible and latent heatat the air–sea interface may play an important role in the initialestablishment of large-scale low-level convergence and con-vective instability facilitating large-scale convection over thewarm pool region. A recent study by Meenu et al. (2012)stressed the absence of convection at higher temperature.Their study revealed that over the Arabian Sea, BOB andTIO, the total cloud amount increases significantly with SSTup to 28.5 °C and decreases especially above 29.5 °C. HighSSTs can act as boundary conditions which influence deepconvection through both local and global air–sea interactions.However, regions of SST >30 °C are generally associatedwithclear-sky conditions (Gadgil et al. 1984; Graham and Barnett1987; Zhang 1993; Lau et al. 1997).

3.4 Shear and curvature vorticities

A parcel in the atmosphere has three rotational motions at thesame time: (1) rotation of the parcel about its own axis, (2)

R. Deepa, J.H. Oh

rotation of the parcel about the axis of a pressure system(curvature) and (3) rotation of the parcel due to the atmospher-ic rotation. The sum of the first two components is known asrelative vorticity, and the sum total of all three is known asabsolute vorticity. The details of shear and curvature vorticitycomputation are given in Section 2. The analysis of shearvorticity one pentad before onset shows positive anomaliesin the SEAS region in DE1 while in DE2 and DE3, weak

negative anomalies are noticed. This represents anticlockwise(cyclonic) flow in DE1 and clockwise (anticyclonic) flowduring DE2 and DE3 (Fig. 6a, e, i). The curvature vorticitydistribution also shows positive anomalies in DE1 and nega-tive anomalies in DE2 and DE3 (Fig. 6b, f, j). Schenkel et al.(2008) suggested that increasing curvature vorticity associatedwith the development of a tropical cyclone is due to shearvorticity being converted to curvature vorticity via horizontal

Fig. 3 Streamlines of climatological (first row) and anomalous (second row) Hadley circulation over 68°–75°E (first panel) for one pentad before onset.The third and fourth rows represent the same as above, but for the onset pentad. a–d DE1, e–h DE2 and i–l DE3

Indian summer monsoon onset vortex formation during recent decades

shear instabilities. This causes the convection to organizearound the centre of circulation. The meridional componentof wind pattern in Fig. 7 shows the dominance of southerliesin DE1, northerlies in DE2 and weak southerlies in DE3 overthe SEAS. So there exists a sharp frontal boundary in DE1which increases the moisture convergence necessary for theinitiation of convection.

4 Discussion and concluding remarks

The present study focuses on the decadal frequency of mon-soon onset vortex formation. We have taken DE1 (1982–1990), DE2 (1991–2000) and DE3 (2001–2011) as the de-cades for the analysis. The frequency of onset vortex duringthe decades DE1, DE2 and DE3 are 90, 20 and 27 %,

Fig. 4 Streamlines of climatological (first row) and anomalous (second row) Walker circulation over 8°–15°N (first panel) for one pentad before onset.The third and fourth rows represent the same as above, but for the onset pentad. a–d DE1, e–h DE2 and i–l DE3

R. Deepa, J.H. Oh

Fig. 5 OLR anomalies (W m−2) from two pentads before onset to one pentad after onset for the three decades. a–d DE1, e–h DE2 and i–l DE3

Indian summer monsoon onset vortex formation during recent decades

respectively. The examination of SSTcomposites two pentadsbefore onset in the southeast Arabian Sea (68°–85°, 8°–15°N)reveals SST >30 °C during DE1 and SST >30.3 °C duringDE2 and DE3. The link between these warm waters (Arabian

Sea mini warm pool) and onset vortex is well defined byrecent studies (Seetaramayya and Master 1986; Rao andSivakumar 1999; Deepa et al. 2007, 2010, 2011; RameshKumar et al. 2012). These studies have suggested warm pool

Fig. 6 Shear (top panel) and curvature (second panel) vorticities at 850 hPa for the pentad. The units are in 10−05 s−1. a–d DE1, e–h DE2 and i–l DE3

R. Deepa, J.H. Oh

as a necessary condition for the formation of onset vortex.Even though the necessary condition is satisfied, the frequen-cy of onset vortex is decreased as observed from Fig. 1 andTable 1. A critical question now arises, whether convection isinhibited at higher SSTs. When SSTs are above a criticaltemperature, remotely forced changes in vertical motion orstability or both play an important role in regulating convec-tion. Dash et al. (2004) have also suggested that warm SSTanomalies alone are not sufficient for initiation of intenseconvection over the Bay of Bengal or Arabian Sea area,leading to the genesis of mesoscale systems and their inten-sification. This reinforces the findings of our study in the caseof monsoon onset vortex. The differential heating betweenwarm pool and adjacent regions creates convergent and di-vergent centres in the atmosphere as depicted by Fig. 2. Theonset vortex region in DE1 is dominated by convergence at850 hPa and divergence at 200 hPa, thereby favouring theair–sea interface processes necessary for the formation ofonset vortex. The decade DE2 is characterized by conver-gence at 200 hPa and divergence at 850 hPa in the southeastArabian Sea, while in DE3, weak convergence and weakdivergence exist at 850 and 200 hPa with strong convergenceover the western Arabian Sea. This gives rise to anomalousHadley and Walker circulations with ascending motion overthe southeast Arabian Sea region in DE1, extending up to

upper levels and strong descending motion in DE2, whereasin DE3, strong descending motion is noticed over the equa-torial region and western Arabian Sea. This condition is notfavourable for the onset vortex to form. Therefore, convectionis suppressed during recent decades by large-scale subsidencewhich leads to surface divergence. Moreover, the southerliesdominate in DE1 over the SEAS, northerlies in DE2 andweak southerlies with maximum over the western ArabianSea. The competition between the southerlies and the north-erlies generates a sharp front in the boundary layer in DE1,while in DE2 and DE3, the northerlies dominate and mixwith the weak southerlies (Fig. 7). This results in the reduc-tion of lower and mid-tropospheric humidity which sup-presses the development of convection. Also, the presenceof weak shear vorticity (Fig. 6) in DE2 and DE3 causes areduction in boundary layer convergence and lower tropo-spheric humidity. This in turn causes the convection to dis-organize around the centre of circulation, thereby resulting ina reduction of surface fluxes.

Acknowledgments This work is funded by the Korea MeteorologicalAdministration Research and Development Program under Grant CA-TER 2012-7150. The authors are grateful to the various agencies forproviding the different data sets used in the present study. FreewareGrADS is used for preparing the figures.

Fig. 7 Meridional wind anomaly at 850 hPa for a , b DE1, c , d DE2 and e , f DE3 from one pentad before onset to onset pentad

Indian summer monsoon onset vortex formation during recent decades

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Indian summer monsoon onset vortex formation during recent decades