ORIGINAL PAPER

Dominant impact of South Asian low heat on summermonsoon rainfall over Central India

Muhammad Jawed Iqbal & Mirza Jawwad Baig &

Saba Naz

Received: 26 July 2011 /Accepted: 19 October 2011 /Published online: 1 December 2011# Saudi Society for Geosciences 2011

Abstract Although previous literature have consideredSouthern Oscillation Index (SOI), Indian Dipole, and SSTas the major teleconnection patterns to explain thevariability of summer monsoon rainfall over India. SouthAsia low pressure and Indian Ocean high are the centers ofaction that dominates atmospheric circulations in Indiancontinent. This paper examines the possible impact ofSouth Asian low pressure distribution on the variability ofsummer monsoon rainfall of India using centers of actionapproach. Our analysis demonstrates that the explanation ofsummer monsoon rainfall variability over Central India isimproved significantly if the SOI is replaced by SouthAsian low heat. This contribution also explains the physicalmechanisms to establish the relationships between theSouth Asian low heat and regional climate by examiningcomposite maps of large-scale circulation fields usingNCEP/NCAR Reanalysis data.

Keywords Summer monsoon rainfall . South Asian lowheat . SOI

Introduction

The Asian monsoon circulation influences most of the tropicsand subtropics of the Eastern hemisphere and more than 60%of the earth’s population (Webster et al. 1998). Monsoon

variations, particularly if they are unanticipated, impartsignificant economic and social consequences. An accuratelong-lead prediction of monsoon rainfall can improveplanning to mitigate the adverse impacts of monsoonvariability and to take advantage of beneficial conditions(Webster et al. 1998). A better understanding of the monsooncycle is clearly of scientific and social value. Monsoonprediction studies have utilized the indicators of atmosphericcirculation, land surface conditions, and Indian and PacificOcean’s sea surface temperatures (SSTs) (summarized inWebster et al. 1998). As far as the monsoon study of Indiansubcontinent is concerned, an empirical forecasting of Indianmonsoon rainfall has been performed using a combination ofclimatic parameters (Parthasarathy et al. 1988; Shukla andMooley 1987) including atmospheric pressure, wind, snowcover, SST, and phases of El Niño-Southern Oscillation(ENSO) (Hastenrath 1986, 1987; Wu 1985; Iqbal andQuamar 2008). Moreover, regression models based on theseand other empirical correlations (Harzallah and Sadourny1997; Sadhuram 1997) have been able to yield a predictionof 60–80% of total seasonal Indian rainfall by May, i.e.,preceding the summer monsoon (Hastenrath 1994).

The studies of Hastenrath (1986, 1987) and Parthasarathy etal. (1988) established that for the prediction of Indianmonsoon rainfall, local parameters (such as seasonal pressuretendency at Bombay and premonsoon surface air temperatureover West Central Indian region) are correlated with Indianmonsoon rainfall along with the other parameters reflectingthe ENSO cycle and meridional circulation over the IndianOcean region. Parthasarathy et al. (1991) have shown theimpact of sea level pressure over Bombay on the IndianMonsoon. Recently, new statistical models developed andadopted by IMD for long-range forecasts of southwestmonsoon rainfall (Rajeevan et al. 2004). The periodicrevision of regression models has also to be viewed in the

M. J. Iqbal (*) : S. NazDepartment of Mathematics, University of Karachi,Karachi, Pakistane-mail: javiqbal@uok.edu.pk

M. J. BaigInstitute of Space and Planetary Astrophysics,University of Karachi,Karachi, Pakistan

Arab J Geosci (2013) 6:2001–2008DOI 10.1007/s12517-011-0458-5

light of the possible impact of global warming and climatechange on the inter-annual variability of the Indian summermonsoon rainfall. The likely influence of processes likeENSO and the North Atlantic Oscillation should beexamined in greater detail. It is an accepted fact that long-range forecasts with a higher resolution in time and spacescales can only be generated by dynamical models which canhandle far better, the complex regional-scale interactions andmanifestations of regional rainfall variability.

As described by Clemens et al. (1991), the summermonsoon is driven by differential sensible heating andtropospheric latent heat. Differential sensible heating resultsin seasonal formation of low atmospheric pressure overIndian Plateau and high pressure over the relatively coldsouthern subtropical Indian Ocean and thus the initiation ofmonsoon circulation. Much of the intensity of the summerrainfall drives from heating of the troposphere throughlatest heat collected over southern subtropical Indian

Fig. 1 Note that pressures lessthan 1,002 mb in the South Asialow extends from north India toSaudi Arabia (a). The cyclonicflow around the low brings themoisture laden winds from theArabian Sea into the CentralIndian region (b)

Fig. 2 Area of significant correlation between JJAS rainfall and JJASSALPS during 1952 to 2002. Positive and negative correlation areshown by red and blue colors with p=0.05

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Ocean, transported across the equator, and released byprecipitation over Asian continent. Let us see the distribu-tion of sea level pressure and wind pattern over Asia in themonths from June to September averaged for 1971–2000 asshown in Fig. 1. The presence of the very low pressure overPakistan, Northern India, and extending to eastern SaudiArabia is visible in the figure. The cyclonic flow around thelow brings the moisture laden winds from the Arabian Seainto the Central Indian region. This low pressure system iscaused by the heating of the surface is called the SouthAsian low heat. The pressure and the distribution of theSouth Asia low fluctuate from year to year with changes inthe global and regional circulations and rainfall. Thus, thisstudy is an attempt for investigating the possible impact ofSouth Asian low heat on the variability of summer monsoonrainfall over India using the centers of action (COA) approach(Bakalian et al. 2007). The large-scale semi-permanent highand low pressure centers which are prominent on a globalmap of monthly averaged sea level pressure were called the“centers of action” by Rossby (1939). A key point noted byRossby was that changes not only in the pressure but also theposition of a center of action influence regional circulation.In the scheme used in this paper, a COA is characterized by

three indices representing its area averaged longitude,latitude, and pressure. Several recent studies have illustratedthe advantages of the COA approach (Iqbal and Ilyas 2011;Hameed et al. 2011; Riemer et al. 2006; Bakalian et al. 2007;Piontkovski and Hameed 2002; Hameed and Piontkovski2004). This contribution also explains the physical mecha-nisms to establish the relationships between the South Asianlow heat and regional climate by examining the compositemaps of large-scale circulation fields using NCEP/NCARReanalysis data.

Data

We use monthly rainfall and temperature data obtainedfrom Climate Research Unit, University of East Anglia(http://www.cru.uea.ac.uk/cru/data/hrg/cru_ts_2.10) for theperiod from 1951 to 2002. Monthly averaged gridded sealevel pressure (SLP) data were also used for calculatingobjective COA indices for the monthly averaged pressure,latitude, and longitude of the Indian Ocean low pressure asdescribed by Bakalian et al. (2007). SOI monthly indicesare available at the Climate Data Center, National Centerfor Environmental Prediction.

Table 2 Partial correlation matrix for summer rainfall over CentralIndia with respect to SOI, South Asia low pressure (SALP)

No. Variables Partial correlation coefficients

1 SOI index and rain 0.16

2 SALP −0.45

SOI Southern Oscillation Index, SALPS South Asia low pressure

Fig. 4 A comparison of the summer rainfall in Central India and ourmodeled values based on linear regression model for the years 1952–2002. The independent variable in our regression model is June, July,August, and September (JJAS) averaged South Asia low pressure(SALP). The variance in the winter rainfall explained by ourregression model is R2=0.28

Table 1 Correlation matrix of summer monsoon rainfall for CentralIndia (67–82° E, 170–25° N)

Summer monsoon rainfall

SOI JJAS 0.335356

SALPS JJAS −0.526748SALT JJAS 0.011014

SALN JJAS −0.031927

SOI Southern Oscillation Index, JJAS June, July, August, andSeptember, SALPS South Asia low pressure

SOIjjas&PrecipJJAS

45° E 60 ° E 75 ° E 90 ° E 0°

15° N

30° N

Fig. 3 Area of significant correlation between JJAS rainfall and JJASSOI during 1952 to 2002. Positive and negative correlations areshown by red and blue colors with p=0.05

Arab J Geosci (2013) 6:2001–2008 2003

Methodology

We can determine the impact of atmospheric pressurefluctuations on rainfall variability over South Asia and itcan be attained through a quantitative assessment of thefluctuation in the pressure and locations of the South Asialow. The pressure index Ip of a low pressure system isdefined as an area-weighted pressure departure from athreshold value over the domain (I, J):

Ip;Δt ¼

PI

i¼1

PJ

j¼1Pij;Δt � Pt

� �cosϕijð�1ÞMdij;Δt

PI

i¼1

PJ

j¼1cosϕijdij;Δt

where Pij, Δt is the SLP value at grid point (i, j) averaged overa time interval Δt. In this case, monthly SLP values aretaken from NCEP reanalysis, Pt is the threshold SLP value,and ϕij is the latitude of the grid point (i, j).M=0 for the highand 1 for the low. δ=1 if (Pij, Δt−Pt)>0 and δ=0 if

(Pij, Δt−Pt)<0. This ensures that the pressure difference isdue to the low pressure system. The intensity is thus a measureof the anomaly of the atmospheric mass over the section (I, J)(Bakalian et al. 2007). The domain of the South Asia lowwas chosen as 10° N to 35° N and 35° E to 95° E. Thedomain of the low and their threshold values Pt (1,013 mb)were chosen by examining their geographical ranges inNCEP Reanalysis data over the period 1948–2006.

Similarly, the latitudinal index is defined as:

Iϕ;Δt ¼

PI

i¼1

PJ

j¼1Pij;Δt � Pt

� �ϕij cosϕijð�ÞMdij;Δt

PI

i¼1

PJ

j¼1Pij;Δt � Pt

� �cosϕijdij;Δt

and the longitudinal index Iλ, Δt is defined in an analogousmanner.

The COA approach examines not only the impact of theintensity of South Asia low pressure on climate variabilitybut also determines the influence of its zonal and meridinal

Fig. 5 When the SE low isdeeper than normal, there is astrengthening of monsoon windsentering Central India

Fig. 6 When the SE low isdeeper than normal, the surfacepressure over Central India isseen to be decreased by about3 mb as a seasonal average

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movement on climate variability. To show that Indiansummer rainfall is closely connected to large-scale atmo-sphere circulations such as semi-permanent atmospherecenters of action. Summer monsoon rainfall is calculatedfor the season from June to September (JJAS). Followingthe procedure as explained by Hameed and Piontkovski(2004), we apply two rounds of linear regression analysis.In the first round, a multiple linear regression will becalculated between the rainfall and all COA indices andSOI. We ignore any collinear ties among the independentvariables at this stage. The purpose is to recognize regionswhere a significant amount of variation can be possiblyexplained by further study. Rainfall averaged over theidentified regions is calculated. Next, we will identify theindices with large contributions that could be significant.Correlations are calculated among the indices. Thus,significant independent COA indices will be identified forthe second round of regression.

The second round of multiple linear regressions will becalculated between the regional average or rainfall and thepreviously identified independent and significant indicesonly. The variance associated with each index designatesthe relative importance of the index in modulating the

regional variability. Moreover, the total variance givesconfidence in relating and explaining these inter-annualvariations.

Results

With p=0.05, we have plotted correlation map in Fig. 2 forexamining the impact of South Asian low heat on summer-time (JJAS) monsoon rainfall. It shows that summer rainfallover a large region of Central India (67−82° E, 170–25° N)is strongly correlated with South Asia low pressure. It showsthat summer rainfall over Central India is significantlyinfluence by the intensity of low pressure in the ArabianSea (see Fig. 2). While, Fig. 3 shows that summer monsoonrainfall of Central India is also strongly correlated with SOI,we have selected the region of Central India (67–82° E, 170–25° N) for further investigation.

Then, we next compute correlations between threeindices of COA (South Asia low pressure, South Asialatitude, and South Asia low longitude) and SOI with themean of summer rainfall for the mentioned region of India.Results are summarized in Table 1. The average summer

Fig. 7 When South Asia low ishigher than normal pressure, thewind anomalies over theArabian Sea are southerly, thus areduction in rainfall over CentralIndia

Fig. 8 When South Asia low ishigher than normal pressure,surface pressure increment of0.8 mb is seen over CentralIndia for the seasonal average

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monsoon rainfall over Central India has significant corre-lation −0.53 and 0.33 respectively with South Asia lowpressure and SOI (see Table 1). It shows South Asia Lowpressure (SALP) explain 28% variability of summer rainfallover Central India while SOI explain only 11% of thevariability of the summer rainfall.

To examine the variable which has dominant influenceon summer rainfall over Central India, we compute thepartial correlation coefficients for SOI index, South Asialow pressure, and summer rainfall. The partial correlationcoefficients report the contribution from a given index onthe winter rainfall while holding all other independentvariable fixed. As shown in Table 2, South Asia lowpressure and summer rainfall are correlated at 95% percentconfidence level, when SOI indices are held fixed. Thus,the partial correlation, r=−0.45, which shows that theintensity of South Asia low has a direct and significanteffect on the summer rainfall over Central India. Similarly,the partial correlation between SOI index and the rainfall is0.16, keeping SALP variables as fixed. However, the partial

correlation between SOI index and rainfall is not signifi-cant. Thus, it shows that the intensity of South Asia low hasdominant influence on summer rainfall over Central India.

Therefore, we construct a linear model of summerrainfall over Central India (CISR) using South Asia lowwhich yields:

CISR ¼ 80124:54� 78:90� SALPSð ÞR2 for the region is 0.28, a significant enhancement over

the SOI value of R2=0.11. Moreover, the regression withSouth Asia low pressure captures the major patterns ofwintertime observed rainfall variations from 1952 to 2002over Central India (Fig. 4).

Mechanisms for the relationships between COAand the rainfall variability in central

Now, we present evidence that regional circulations of theatmosphere and the ocean are consistent with the empiri-

Fig. 9 When the SOI indexfor the months from June toAugust is higher than normal,there is a strengthening ofmonsoon winds enteringCentral India from theBay of Bengal

Fig. 10 When the SOI index forthe months from June to Augustis higher than normal, the sur-face pressure over Central Indiais seen to be decreased by about1.1 mb as a seasonal average

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cally determined relationships. For this purpose, we use theNCEP/NCAR Reanalysis monthly averaged fields ofwinds, temperature, pressure, and humidity.

We first construct different composites with the phase ofSALP for the relationships between COA and the rainfallvariability. Figure 5 shows the composite differences ofvector wind at 850 mb over central India for 10 years whensouth low pressure was minimum (more rain in CentralIndia). Figure 6 also shows the composite differences ofsurface pressure over Central India for 10 years when southlow pressure was minimum. These two figures manifestthat when the low pressure is intense there is a strengthen-ing of monsoon winds entering Central India. The surfacepressure over Central India is seen to be decreased by about3 mb as a seasonal average. Figures 7 and 8 respectivelyshow the composite differences of vector wind at 850 mband surface pressure over Central India for 10 years whenSouth Asia low is higher than normal pressure. They showthat surface pressure increment of 0.8 mb is seen overCentral India for the seasonal average. The wind anomaliesover the Arabian Sea are southerly, thus a reduction inrainfall over Central India.

Figures 9 and 10 respectively show the compositedifferences of vector wind at 850 mb and surface pressureover Central India when the SOI index for the months fromJune to August is higher than normal there is a strength-ening of monsoon winds entering Central India from theBay of Bengal. The surface pressure over Central India isseen to be decreased by about 1.1 mb as a seasonal average.

Conclusions

The variability of summertime rainfall over CentralIndia is connected with the atmospheric centers ofaction. Conventionally, SOI and sea surface temperaturehave been considered the dominant modulators ofregional variability in this region. The intensity ofSouth Asia low pressure explains 28% of variability ofsummer rainfall over Central India while SOI explainsonly 11%. A comparison of Figs. 5 and 9 shows thatalthough SOI and SALP are statistically correlated, theyrepresent two different phenomena because when SOI ishigher than normal, there is a strengthening of monsoonwinds (westerly) entering Central India from the Bay ofBengal while South Asia low is deeper than normal thaneasterly winds extending towards Central India. Chang etal. (2001) have shown that the empirical relationshipbetween summer rainfall over Indian and ENSO isweakening due to Atlantic circulation (Chang et al.2001). In fact, westerly winds extending towards Indiaare weakening due to western disturbance. However,South Asia low which explains the variability of summer

rainfall over Central India more accurately than SOIcorresponds to easterly flow. Thus, summer rainfall overCentral India is significantly influenced by South Asia lowpressure.

Thus, the results presented in this work have demon-strated that explanation of rainfall variability over CentralIndia is improved significantly if the SOI is replaced bySouth Asia low. This paper manifests that when the lowpressure is intense, there is a strengthening of monsooneastern winds entering Central India. It is important to notethat South Asia low pressure also has influence on thesummer rainfall over some parts of Iran and Middle East(see Fig. 2).

Acknowledgments I would like to thank Prof. Sultan Hameed(School of Marine and Atmospheric Sciences, Stony Brook Univer-sity, NY, USA) for his suggestions to develop this work.

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