Moist dynamics of active/break cycle of Indian summer monsoon rainfall from NCEPR2 and MERRA reanalysis

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1 INTERNATIONAL JOURNAL OF CLIMATOLOGY Int. J. Climatol. 34: (2014) Published online 27 June 2013 in Wiley Online Library (wileyonlinelibrary.com) DOI: /joc.3774 Moist dynamics of active/break cycle of Indian summer monsoon rainfall from NCEPR2 and MERRA reanalysis Prasanth A. Pillai* and A. K. Sahai Indian Institute of Tropical Meteorology, Pune, India ABSTRACT: The vertically integrated moisture and moist static energy (MSE) budgets during the active and break phases of Indian summer monsoon are studied using National Centers for Environmental Prediction Reanalysis2 (NCEPR2) and Modern Era Retrospective Analysis for Research and Applications (MERRA) data. The different parameterization procedures adopted by the reanalysis models will impose inevitable differences between the reanalysis products, which will make an impact on their intraseasonal oscillation (ISO) also. At the same time, the agreement of the moisture budget terms of the reanalysis products has enabled us to come up with a common moisture mechanism responsible for the ISO. Estimates of the moisture budget indicates that the rainfall anomalies over the Indian monsoon region in both active and break phases are mainly (70 80%) balanced by local moisture divergence. The remaining 20 25% is accounted by the horizontal moisture advection. The moisture advection is found to lead the rainfall by 8 9 d, while there is no leadlag relationship is observed between the moisture divergence and rainfall. The anomalous moisture advection from the northwest pacific initiates the active phase of Indian monsoon and the dry air advection from the northwest of India after the onset of active convection sets off the break conditions. MSE budget over India is balanced by MSE divergence and moisture advection, indicating that the variation in MSE is mainly governed by moisture in the intraseasonal timescales. A positive covariance between radiative and surface fluxes and precipitation anomalies indicates that these flux terms slowdown the variation of MSE during active/break phase and may help to extend the ISO. The anomalous moisture advection has a major role in northward propagation of ISO also. A large residual of MSE budget during onset of ISO is the main caveat of the reanalysis products and it is the major constrain in the present analysis also. KEY WORDS Indian summer monsoon; intraseasonal oscillation; moisture budget; moist static energy; moisture advection Received 11 February 2013; Revised 23 April 2013; Accepted 25 May Introduction Indian summer monsoon, which is the prominent component of Asian summer monsoon, regulates the life and culture of the vast population of the Indian subcontinent through its wide spectrum of variability. While the defining variability of Indian summer monsoon is seasonal (June to September), its variability about the season (intraseasonal) is also important. This is due to the fact that this variability is as large as or larger than the variability associated with other time scales and is most prominent over the Indian monsoon region (Goswami and Ajayamohan, 2001). The intraseasonal oscillation (ISO) of the Indian monsoon consists of active periods of high rainfall and break periods of deficient or no rainfall during the summer season (see review by Webster et al., 1998). The ISO of the Indian monsoon has been found to be mainly consisting of fluctuations on time scales of and d (Krishnamurti and * Correspondence to: P. A Pillai, Indian Institute of Tropical Meteorology, Dr Homi Bhaba Road, Pashan, Pune, India. prasanth@tropmet.res.in, prasanthap2@yahoo.co.in Bhalme, 1976; Yasunari, 1979; Annamalai and Slingo, 2001; Goswami, 2005). The intraseasonal variability of the Indian rainfall has been described in terms of northward propagation of convection from the central equatorial Indian Ocean to the Indian subcontinent (e.g. Yasunari, 1979; 1980; Sikka and Gadgil, 1980; Lawrence and Webster, 2002; Goswami, 2005). This northward propagation during boreal summer accompanies the eastward movement of the ISO that can be found in all seasons (Lau and Chan, 1986; Wang and Rui, 1990; Wheeler and Hendon, 2004). Lawrence and Webster (2001) argue that the eastward propagation of convection is always related to the northward movement and is fundamental to the variability over both the Indian Ocean and the Indian subcontinent. They propose that the summer intraseasonal variability should be thought of as wintertime variability (i.e. Madden Julian Oscillation (MJO)) modified by the basic state. Independent northward moving intraseasonal oscillations due to internal dynamical mechanisms are also shown to exist in atmospheric general circulation model (AGCM) simulations (Jiang et al., 2004). In addition to the eastward propagation, weak westward movement of convection towards 2013 Royal Meteorological Society

2 1430 P. A. PILLAI AND A. K. SAHAI the Indian continent has also been noted (Krishnamurti and Ardanuy, 1980; Wang and Rui, 1990; Annamalai and Slingo, 2001). The most comprehensive knowledge of ISO can be obtained from Goswami (2005); Lau and Waliser (2005); Wang (2005); Waliser (2006a) and Zhang (2005). Due to its importance in tropical circulation, a lot of efforts have been taken for the prediction of ISO using both statistical and numerical models (Hendon et al., 2000; Goswami and Xavier, 2003; Webster and Hoyos, 2004; Waliser et al., 2006b, etc.). Recent model intercomparison project study by Lin (2006) showed that ISO simulation by the state-of-the-art climate models has improved significantly in the recent periods. However, understanding of the mechanisms of ISO, especially the energy source and the role of moisture and surface fluxes, remain limited. Although various studies noted the movements of convection zones over to the Indian continent with the active and break phases of monsoon (e.g. Krishnan et al., 2000; Annamalai and Slingo, 2001; Lawrence and Webster, 2002; Gadgil and Jospeh, 2003; Annamalai and Sperber, 2005), the direct relationship with the rainfall over India is not fully explained. Some observational and modelling studies identified the possible role of moisture and moist static energy (MSE) in the intraseasonal oscillations. Gross moist stability (GMS), which represent the net export of MSE, has been hypothesized to be important for the phase speed of the ISO (Neelin and Yu, 1994). The model results of Wang and Xie (1997) showed that boreal summer ISOs are strongly influenced by the background circulation and low-level moisture. They further identified local air sea interaction as one of the factors responsible for pre-conditioning the boundary layer moisture ahead of convection. Kemball-Cook and Weare (2001) suggested that the destabilization associated with the ISO is brought about by a combination of a low-level build up of MSE and drying of the middle atmosphere due to subsidence from the wake of a previous ISO event. Further, Roxy and Tanimoto (2007, 2012) indicated that ocean-atmospheric processes during an active (break) phase of the ISO induce positive (negative) MSE anomalies, which makes the lower atmospheric column unstable (stable) and enhance the intraseasonal convective activity. Krishnamurti et al. (2010) and Prasanna and Annamalai (2012) noted the advection of dry air from north during extended breaks. Thus all these studies point out the necessity of understanding the moist dynamics that can initiate and control the intraseasonal oscillations of Indian summer monsoon. The MSE budget has recently been cited as important tool for understanding Madden- Julian Oscillation (MJO) and tropical disturbances (Maloney, 2009; Boos and Kuang, 2010; Kiranmayi and Maloney, 2011). Prasanna and Annamalai (2012) used moisture and MSE budget for studying extended breaks over India. Earlier Ajaymohan et al. (2011) used the same budget for explaining the northward propagation of boreal summer ISOs. However, most of these studies used model outputs to perform these budget estimates. The role of moisture and the surface fluxes in the evolution of active/break cycle of ISO is yet to be documented. This study compares the moisture and MSE budgets of active/break phases of the Indian summer monsoon using two different reanalysis data sets. The budget analysis will enable us to understand the interaction between the moisture dynamics and circulation and to make out the various competing processes. Though the reanalysis data sets are not true observations and are strongly influenced by physical parameterizations employed in the model, we use these reanalysis products to represent state-of-the-art replication of global scale observations. 2. Data and methodology 2.1. Data This study identifies active and break days using National Oceanic and Atmospheric Administration (NOAA) daily Outgoing Long Waver Radiation (OLR) data for the 32- year period, from 1979 to As a first step, daily climatological OLR time series is constructed from this 32-year daily data. This daily climatological time series is used for calculating the daily anomalies for each year. The same procedure is repeated for calculating anomalies of all the parameters used. If the OLR anomalies over the central Indian region (18 28 N, E) exceeds a critical value of ±10 W m 2 for at least four consecutive days, those days are defined as break (active) days of the monsoon ISO (Krishnan et al., 2000). Apart from attaining this threshold value, we also cross check the pattern of OLR anomalies of these days with rainfall distribution presented by Ramamurthy (1969) and compare the break and active composites from earlier works like Krishnan et al. (2000) and Rajeevan et al. (2010). National Centers for Environmental Prediction Reanalysis2 (NCEPR2) data (Kanamitsu et al., 2002) and Modern Era Retrospective Analysis for Research and Applications (MERRA) project (Rienecker et al., 2008) data are used to calculate the vertically integrated moisture and MSE budgets. The budgets are calculated using the vertical level data of zonal wind, meridional wind, vertical velocity, specific humidity and air temperature along with surface temperature and surface pressure. The latent heat and sensible heat, along with longwave, shortwave radiation fluxes at the surface and top of the atmosphere are also used for budget calculations (equations for budget estimations are provided in Section 2.2.). While calculating moisture budget in these reanalysis data sets, respective precipitation data are also used for the accuracy of the budget. Here active (break) days are defined as the days in which the rainfall anomaly over the central Indian region has anomalies above ±1 standard deviation for at least four consecutive days. Those active and break days where the reanalysis precipitation anomalies and OLR anomalies coincide were used for constructing composites. NCEPR2 data set are on 2.5 latitude 2.5 longitude horizontal grid resolution, with 28 vertical levels. The analysis covers the satellite period of 1979 to the present

3 MONSOON ISO AND MOISTURE BUDGET 1431 and uses an updated forecast model, updated data assimilation system (DAS), improved diagnostic outputs and fixes for the known processing problems of the National Centers for Environmental Prediction (NCEP) and the National Center for Atmospheric Research (NCAR) reanalysis. The NCEPR2 uses improved radiation and planetary boundary layer schemes (see Kanamitsu, 2002 and Kanamitsu et al., 2000 for full details). MERRA was generated with the Goddard Earth Observing System-5 (GEOS-5) atmospheric model and DAS, version The state-of-the-art Gridpoint Statistical Interpolation (GSI) Package, originally developed at NCEP serves as the DAS. Rienecker et al. (2011) present an overview of the MERRA reanalysis. For more details on the GEOS-5 model, the assimilation system and its configuration, see Rienecker et al. (2008). Earlier studies by Robertson and Roberts (2012) indicate that state variables of atmospheric temperature and moisture are well represented in MERRA on intraseasonal time scales and thus at least give the model physics good forcing with which to operate. We note that the reanalysis data sets used here are not true observations, but rather, are data assimilation products that ingest observations. Hence, the reanalysis fields we use may be strongly constrained by parameterization assumptions made in the models (Kiranmayi and Maloney, 2011). For example, NCEPR2 has simple Arakawa Schubert (SAS, Pan and Wu, 1995) scheme for convection parameterization, while MERRA uses relaxed Arakawa Schubert (RAS, Moorthi and Suarez, 1992) scheme. Tian et al. (2010) showed that the ability of reanalysis data sets to capture the phase and amplitude of moisture and temperature anomalies varied depending on reanalysis procedure. This point should be noted when considering the results from reanalysis fields shown below. Nevertheless, we hope that by using two different reanalysis data sets we will be able to assess the robustness of basic dynamics of ISO. We focus on the active and break days during the period from 15 June to 15 September to avoid the vagueness due to delayed onset of monsoon in June and early withdrawal in September. In this study, day 0 of break and active is defined as the day in which the rainfall/olr anomaly attains the prescribed critical value. The days before day 0 are represented as lag and the days after onset as lead days. The lead/lag notations throughout the manuscript follow the same criteria. The moist and MSE budget methods (as Su and Neelin, 2002; Pillai and Annamalai, 2012; Kiranmayi and Maloney 2011) are explained briefly below Moist and MSE budget formulations The vertical integrated temperature (T ) and moisture (q) equations for the perturbations have the following form: t T + D T T + ω p s = Q c + g ( F rad P + H ) T (1) t q + D T q + ω p q = g ( Q q + E ) (2) P T where both T and q are in energy units (W m 2 ) after absorbing the heat capacity at constant pressure (C p )and latent heat of condensation (L) respectively. s = T + is the dry static energy, with the geopotential. Q c and Q q are anomalous convective heating and moisture sink, respectively. Here, g is the acceleration due to gravity, ω is vertical pressure velocity, and P T is the reference pressure depth of the troposphere. F rad is the net radiative flux convergence into the atmospheric column. The surface sensible and latent heat fluxes (SHF and LHF) are H and E, respectively. The symbol indicates vertical integration. Combining Equations (1) and (2), the vertically integrated anomalous MSE equation is dh = V h ω p h + gpt (F rad dt + H + E ) (3) where ω p h is the anomalous (MSE) convergence. Also, h = s + q, is the MSE. Here, the large terms in individual temperature and moisture equations, Q c and Q q, cancel each other and the resultant balance is between the net flux into the column and stability. Since Q c = ( ) Q q = g P T P, anomalous precipitation P can be calculated from Equation 2). 3. Results The first part of this section compares the composite pattern and evolution of active and break phase from OLR and the reanalysis rainfall. This will enable us to understand the similarities/differences of observed and reanalysis convection, which should be taken into consideration while analysing the budget Active/break composite from OLR and reanalysis precipitation Figure 1 shows the composite of active (left panel) and break (right panel) days based on NOAA OLR (top panel), NCEPR2 rainfall (middle panel) and MERRA rainfall (bottom panel). Active (break) composite in Figure 1 (d) shows quadrupole structure with increased (decreased) convection over India (10 N 30 N, 65 E 95 E) and equatorial west Pacific (EWP,10 S 10 N, 110 E 150 E) and decreased (increased) convection over equatorial east Indian Ocean (EEIO, 10 S 0, 70 E 100 E) and northwest Pacific (NWP, N,110 E 150 E). This convection pattern is similar to that observed by Krishnan et al. (2000) and Rajeevan et al. (2010) and confirms the selection of active and break days in this study. In NCEPR2 active composite, the convective maximum over central India is shifted southward along with an additional reduced rainfall over north central Indian Ocean (Figure 1). In MERRA composite, there is increased convection over central India and EWP and reduced rainfall over EEIO for active composites (Figure 1). In both the reanalysis products, there is an additional convection

4 1432 P. A. PILLAI AND A. K. SAHAI (d) (e) (f) Figure 1. Composite anomalies of active days from NOAA OLR, NCEPR2 rainfall and MERRA rainfall; (d) (f) are break days composites of OLR, NCEPR2 rainfall and MERRA rainfall, respectively. To represent convection by positive values negative of OLR anomalies are plotted in and (d). Anomalies in and (d) has units of W m 2 and others are in mm d 1 (1 mm d 1 = 28W m 2 ). centre over equatorial west Indian Ocean between 55 E and 65 E, making a dipole like convection pattern in the Indian Ocean. Reanalysis have a large number of regional convection centres over the ocean regions, which is absent in observations (OLR). Too many convection centres in MERRA can be attributed to its higher resolution (1 ) compared with OLR (2.5 ). Break composites have reverse pattern of active composite with reduced convection over India and EWP and increased convection over EEIO and (Figure 1(d f)). But the reanalysis have strong and organized convection in NWP and EIO in break composite than its active counterpart. Thus both the active and break composites have some important regional scale difference between the three data sets Space time evolution of ISO Space time evolution of ISO is examined from the sequence of OLR and rainfall anomalies from 20 day lag (lag20) to 20 day lead (lead20) of the ISO onset. The present analysis considers both active and break phases of monsoon. However, for brevity, the discussion mainly focuses on the active phases only, as the convection centres have opposite anomalies in active and break phases. Nevertheless, any differences arising between the two are discussed Active phase Figure 2 shows the sequence of composite OLR anomalies from lag20 to lead20 with each panel 5 day apart. In order to represent convection by positive anomalies, negative of OLR anomalies are plotted in all panels of Figure 2. Each panel is 3-day average (i.e. day 0 is the average of lag1, day 0 and lead1). Hereafter we will follow the 3-d averaging for all the figures showing ISO evolution. The convective activity over the equatorial western Indian Ocean (around 60 E) in lag20 (Figure 2) strengthens by lag15. Convection moves north eastward to the east Indian Ocean by lag10 (Figure 2) and reaches the southern part of India by next 5 d (lag5, Figure 2(d)). By day 0, the convention centre is over core monsoon region, extending to the maritime continent along with reduced convection over the EEIO and NWP (Figure 2(e)) resulting in a quadrupole structure (Annamalai and Slingo, 2001). The convective centre over India starts moving northeast by lead5, and reduced convection appears over monsoon region by lead20 (Figure 2(i)).

5 MONSOON ISO AND MOISTURE BUDGET 1433 (d) (e) (f) (g) (h) (i) Figure 2. Composited evolution of NOAA OLR anomalies (i) from lag20 to lead20 of active phase (day 0) with each panels separated by 5-d interval. Each panel is the average of 3 d (i.e. in lag20 day is the average of lag21, 20 and 19). All the units are in W m 2. To represent convection by positive values negative of OLR anomalies are plotted. Figure 3 is the composite evolution of active phases of NCEPR2 rainfall anomalies. The increased convection is around 75 E in the Indian ocean in lag20 and it extends eastward to EEIO and EWP by lag10 (Figure 3). EEIO convection starts moving north by lag5 (Figure 3(d)). By day 0 the convection centre is over India and increased convection over the EWP persists. There is reduced convection over north Indian Ocean between 60 E 90 E and NWP (Figure 3(e)). But this quadraploe structure is different from the structures noted for NOAA OLR in Figure 2(e). The convection centre extends northwest in lead5 and reaches NWP by lead15 (Figure 3(f h)). MERRA reanalysis have increased convection over the equatorial Indian Ocean, EWP and NWP and reduced

6 1434 P. A. PILLAI AND A. K. SAHAI (d) (e) (f) (g) (h) (i) Figure 3. Composited anomalies of NCEPR2 rainfall for active phase; (i) is from lag20 to lead20 with each panels separated by 5-d interval. Units are in mm d 1. convection over India up to lag10 of the active phase (Figure 4(a c)). By lag5, the convection extends to Indian region also (Figure 4(d)). This is accompanied by increased convection over the entire monsoon region, EEIO and NWP. During day 0, the convection strengthens over the monsoon region and EWP, and at the same time, convection weakens over EEIO and NWP (Figure 4(e)). Convection starts moving north by lead5 onwards. The results point out notable differences in the structure and evolution of convective anomalies in the three data sets. In OLR composite there is a smooth pattern of onset and movement of convection from equatorial Indian Ocean to monsoon region. However, in the reanalysis products there are multiple centres of convection in the Indian and Pacific Oceans making the pattern noisy Break phase In the OLR composites, convection in the break phase is almost mirror image of active. In lag5, however, the reduced convection centre is in the north Indian Ocean,

7 MONSOON ISO AND MOISTURE BUDGET 1435 (d) (e) (f) (g) (h) (i) Figure 4. Composited anomalies of MERRA rainfall for active phase; (i) is from lag20 to lead20 with each panels separated by 5-d interval. Units are in mm d 1. while it is at southern part of India in active counterpart in Figure 2. The convective anomaly in the EIO becomes organized by lead5 only in break composite, while it has already established in day 0 in active. Rest of the features are mirror images of active (hence figures are not shown). Reanalysis rainfalls also have opposite patterns for the active and break phases Moisture and MSE budget of active/break cycle in NCEP reanalysis The analysis in the previous sections makes out the regional differences in the ISO scale convection between the reanalysis products and observed OLR. At the same time, all the three products have general agreement over the onset and northward movement of convection. This encourages us to look into the moisture dynamics of active/break phase using reanalysis products. The relative role of different terms like moisture advection, MSE divergence, surface fluxes, radiative fluxes, etc., in the moisture and MSE budget can give an insight into the effect of various processes contributing to the destabilization and propagation of ISO. In the present section the different components of the budget and their relative importance are identified.

8 1436 P. A. PILLAI AND A. K. SAHAI Active phase Figure 5 shows the three day average time series of major terms of anomalous moisture and MSE budgets (Equations (2) and (3) above) for active period from lag18 to lead18 averaged over the monsoon region 65 E 90 E, N. Figure 5 is the time series of rainfall, horizontal moisture advection, moisture divergence, LHF and tendency of specific humidity, the parameters which explain the moisture budget (Equation (2)). Positive values of all these parameters indicate increased rainfall, horizontal advection of moist air, moisture convergence, LHF into the atmosphere and moisture tendency. From the figure it is evident that the anomalous moisture advection to monsoon region leads all other terms and becomes positive by lag13. Moisture tendency becomes positive by lag8. The rainfall and moisture convergence become positive by lag4. Thus moisture advection leads convection by around two pentads. LHF also favours increased convection by lag3. But moisture advection reverses its sign by 2 3 d after the onset of active phase, while the other terms remain positive for another 8 10 d. Thus in the moisture budget (Equation (2)), the main balance is between rainfall and the moisture convergence (around 70%). The secondary contribution (25%) is from moisture advection up to the onset of ISO and after that it is by the surface fluxes. The small value of the tendency term also confirms the major role of horizontal moisture advection in moistening associated with active phase. The result is similar to that for wintertime MJO study of Kiranmayi and Maloney (2011). Figure 5 is the major terms of MSE budget (Equation (3)), except moisture advection and LHF, which are included in Figure 5. As the anomalous horizontal moisture advection to monsoon region starts by lag13, MSE tendency also becomes positive by lag11. The lead of horizontal moisture advection over MSE tendency also confirms the predominant role of moisture in ISO. Vertical advection of MSE (MSE divergence) becomes positive by lag8, indicating the low-level convergence of moist air and upward movement. The rest of the terms like temperature advection and fluxes have no significant anomalies up to the onset (day 0). So the MSE tendency is mainly balanced by moisture advection and MSE divergence up to day 0. By lead3, moisture advection reverses sign and then the main balance is between MSE divergence, the fluxes and net radiation. Thus fluxes maintain the MSE after the onset and it will help the destabilization of atmosphere, which extends the active phase further. Both moisture and MSE budget have considerable amount of residuals, which can influence the result and is discussed later. Figure 5. Three day running mean of different terms of moisture budget [rainfall, moisture advection (madv), moisture divergence (mdiv), latent heat flux (lhf) and moisture tendency (d(lq)/dt], different terms of MSE budget(mse tendency(dm/dt), MSE divergence (mse), temperature advection (tadv), sensible heat flux (shf) and net radiation (netrad) anomalies averaged over monsoon region 10 N 25 N, E using NCEPR2 from lag 18 to lead 18. The negative (positive) days in the x axis represent the lag (lead) with respect to onset of active phase. The different terms and budget equations are explained in text. Here all the parameters are in energy units W m 2.

9 MONSOON ISO AND MOISTURE BUDGET 1437 The time series plot of budget terms indicate that anomalous moisture advection leads both moisture and MSE tendency and confirms its role in the onset of active phase of convection. Thus, it would be worthwhile to investigate the source of this anomalous moisture advection. Figure 6 shows the spatial pattern of anomalous horizontal moisture advection along with 850 hpa wind anomalies from lag15 to lead5 (as Figure 5 shows that moisture advection reverses sign by lead3, spatial pattern is restricted to lead5). During lag15 (Figure 6) the anomalous easterly winds from India advect moisture to the Arabian Sea region. By lag10, there is moisture advection to India and north Indian Ocean from the NWP by the anomalous easterlies present in these regions (Figure 6). This anomalous flow and moisture advection exists up to the onset of active period. By day 0 westerly wind anomalies over the Arabian Sea further enhances the moisture advection to India and Bay of Bengal (Figure 6(d)). At the same time there are north-westerly wind anomalies to Arabian Sea, which can bring dry air from northwest. This dry air advection extends to India as convection moves northeast by lead5 (Figure 6(e)) Break phase Moisture and MSE budget terms of break period have anomalies opposite to that of active phase (not shown). Dry air advection to monsoon region starts by lag10 and the rainfall and moisture divergence becomes negative by lag1. Here also moisture budget is mainly balanced by rainfall and moisture divergence. MSE tendency and MSE divergence indicate drying of monsoon region from lag10 onwards. MSE tendency reverses by lead5 along with reversal of moisture advection, but MSE divergence continues to lead10 extending the break phase further. The spatial pattern of moisture advection and lowlevel circulation during the break period is illustrated in Figure 7. The cyclonic circulation over the monsoon region advects anomalous moisture to northern part of India in lag15 (Figure 7). At the same time its western flank can bring dry air to Arabian Sea. The dry air advection from west strengthens in lag10 as the cross equatorial flow advects the climatological moisture to the east of India (Figure 7). By lag5, the dry air advection from the northwest of Arabian Sea extends to the monsoon region (Figure 7). During the onset of break, the westerlies over south Indian region are replaced by easterlies, and westerlies in the northern part of India extend to NWP. Thus the climatological moisture is advected to NWP and dry air is advected to monsoon region from dry region northwest of India (Figure 7(d)). After the onset of break, dry air advection from northwest terminates and the easterly wind anomalies over India export moisture to Arabian Sea. Thus anomalous dry air advection from dry northwest region to monsoon region preconditions the break monsoon by lag10. This dry air advection from north of India has been noted by Krishnamurti et al. (2010) and Prasanna and Annamalai (2012) also in their respective studies. (d) (e) Figure 6. Composited evolution of anomalies of moisture advection (shaded) and 850 hpa wind (vector) from NCEPR2 reanalysis for active phase; (e) from lag 15 to lead 5 of active day (day0). Each panel is separated by 5 d interval. Moisture advection is in Wm 2 and wind is in ms Moisture budget and northward propagation of ISO Figure 8 shows the northward propagation of rainfall, moisture divergence, moisture advection, LHF, MSE and net radiation over the E region. Precipitation which was on equatorial region in lag20 ( 20 in Figure 8) reaches central Indian region by day 0 and

10 1438 P. A. PILLAI AND A. K. SAHAI active period. Comparing moisture advection and MSE, it can be seen that MSE moves together with moisture advection up to the onset of active phase and after that it is in the same phase with LHF, net radiation and also with temperature advection (though small compared with other terms). The model result of Ajaymohan et al. (2011) also noted the role of moisture advection in northward propagation of convection. Similar type of northward movement is noticed for the moisture and MSE budget terms for break phase also (not shown). Thus, the analysis in the above two sections identify anomalous moisture advection as the major source of the ISO onset and its northward propagation in NCEPR2. For the robustness of the result we are repeating the same analysis in MERRA reanalysis also. (d) (e) Figure 7. Composited evolution of anomalies of moisture advection (shaded) and 850 hpa wind (vector) from MERRA reanalysis for break phase; (e) from lag 15 to lead 5 of break day (day0). Each panel is separated by 5 d interval. Moisture advection is in Wm 2 and wind is in ms 1. extends further north (Figure 8). Moisture divergence also has same northward propagation pattern overlying rainfall over the monsoon region (Figure 8). At the same time, moisture advection leads the convection by around 6 8 d (Figure 8). MSE is positive over the Indian region by 6 7 d ahead of convection and it remains the same over the N area for the entire 3.5. Major terms of moisture and MSE budget from MERRA Here the results obtained from NCEPR2 are compared with MERRA by performing same budget calculations. However, the analysis is confined to major terms like rainfall, moisture advection, moisture divergence and MSE divergence. Time series of major budget terms indicate that during active period, moisture advection to monsoon region starts by lag8 and continues to lead1 (Figure 9). MSE divergence also becomes positive by lag8. Rainfall and moisture convergence are the two major terms of moisture budget and they become positive by lag5 and remains up to lead12. Thus moisture advection has 3 4 d lead over convection, while it was around 9 d in NCEP2. Same is true for break period also, but anomalous dry air advection starts 3 4 d earlier than the active counterpart (i.e. by lag12), but rainfall and MSE divergence reverses sign by lag5 only (not shown). It is interesting that in NCEP2, anomalous moisture advection to monsoon region during active phase started earlier than break phase, where as it is reverse in MERRA reanalysis. Figure 10 shows the anomalous horizontal moisture advection and 850 hpa wind anomalies from lag15 to lead5 of active period for MERRA with each panel separated by 5 d. The anti-cyclonic circulation in the Arabian Sea and the monsoon region advect moisture to northwest Indian Ocean in lag15 (Figure 10). There is anomalous cross equatorial south-westerly wind anomalies to Arabian Sea in lag10 which can advect additional moisture there (Figure 10). At the same time the easterly flow from Bay of Bengal export moisture to the eastern parts of India. Five days later, the easterly flow from NWP and south China Sea region strengthens with anomalous moisture advection to monsoon region (Figure 10). During the onset of the active period, the cross equatorial wind anomalies to monsoon region become the main source of moisture. At the same time there is a north-westerly flow to the Arabian Sea bringing dry air (Figure 10(d)). The north-westerly wind anomalies extend to the Indian region by lead5, when the active convection starts moving northeast (Figure 10(e)).

11 MONSOON ISO AND MOISTURE BUDGET 1439 (d) (e) (f) Figure 8. Lag/lead-latitude diagram of anomalies of rainfall, moisture divergence, moisture advection, (d) evaporation, (e) MSE divergence and (f) net radiation averaged over 65 E 90 E for active phase of ISO from NCEPR2. Figure has unit of mm d 1 and (f) has W m 2. Shading is different in different panels. Days with negative sign ( 20 to 1) indicates lag days and the positive sign (1 to 20) indicate lead days with respect to onset of active convection (0). Figure 9. Three-day averaged time series of rainfall, moisture advection (madv), MSE divergence (MSE div) and moisture (MDIV) divergence averaged over monsoon region 10 N 25 N, 65 E 90 E for active phase from MERRA reanalysis. The negative (positive) days in x axis represent lag (lead) days with respect to the onset day (0) of active phase. All the parameters are in energy units W m 2. Figure 11 is the anomalous 850 hpa circulation and moisture advection for break period. In Figure 11, during lag15 the anomalous easterly from north India export moisture to Arabian Sea. The anomalous moisture advection to Arabian Sea continues in lag10 also. At the same time westerly anomalies in Bay of Bengal export climatological moisture to NWP (Figure 11). By lag5 the anomalous westerly flow to NWP starts from the Indian land mass, making the entire monsoon region dry (Figure 11). During onset of the break, in addition to this westerly flow in north Indian region, there are easterly wind anomalies from Bay of Bengal to the equatorial western Indian Ocean. This flow will export the climatological moisture from the monsoon region to the ocean, aggravating the dryness over India. After the onset, north-westerly flow to monsoon region ceases and only the easterly flow to Arabian Sea persists (Figure 11(e)). Thus, MERRA also verifies the dominant role of moisture advection in the moisture and MSE budget. At the same time the source of the anomalous moisture have some notable difference between the two reanalysis products.

12 1440 P. A. PILLAI AND A. K. SAHAI (d) (d) (e) (e) Figure 10. Composited evolution of anomalies of moisture advection (shaded) and 850 hpa wind (vector) from MERRA reanalysis for active phase; (e) from lag 15 to lead 5 of active day (day0). Each panel is separated by 5 d interval. Moisture advection is in Wm 2 and wind is in ms Discussion 4.1. Possible effect of the reanalysis procedures on the ISO In the previous sections, we noted that the active/break composites and their evolution have some regional differences between the reanalysis and the observed OLR patterns (Figures 1 4). Here we discuss the difference Figure 11. Composited evolution of anomalies of moisture advection (shaded) and 850 hpa wind (vector) for break phase in MERRA reanalysis from lag15 to lead5. Each panel differs by 5 d. Moisture advectionisinwm 2 andwindisinms 1. between the ISO activities of the two reanalysis products, which can be possible due to the reanalysis procedures adopted. In MERRA, the convectively produced clouds deepen too fast as precipitation increases. At the same time, it should be taken into consideration that the precipitation in the model is the product of convective parameterization. Hence this early rise in precipitation suggests that the RAS convective parameterization used

13 MONSOON ISO AND MOISTURE BUDGET 1441 in MERRA assimilation responds too quickly to the moisture and it fails to let the moisture build upward in the lower troposphere. In the ISO cycle this will reduce the lead-lag relationship of moisture and precipitation (4 d in Figure 9). But in NCEPR2 using SAS scheme, moisture advection leads rainfall by 9 10 d. These two parameterization schemes mainly differ in the development of clouds and the downdrafts within the clouds (Kanimastu et al.2002a). Import of moisture to produce the energy source for precipitation is stronger in the MERRA reanalysis than observation (Robertson and Roberts, 2012). This strong association of rainfall with moisture is evident in ISO scale also (Figure 9). Dry static energy increment is very sensitive in MERRA, which will reduce net increment of MSE (as MSE is the combination of dry static energy and moisture) compared with fluxes. In the ISO cycle also, rate of increase of MSE is negligible (not shown). This mismatch is reflected in the error term in the budget calculations, mainly at the onset of ISO. This strong dependence of rainfall to moisture and error in MSE during onset are similar in both the reanalysis products. The nonlocal vertical mixing scheme employed in NCEPR2 hinders the undesirable vertical eddy flux convergence of heat, moisture and momentum within the planetary boundary layer, making the vertical structure of moisture more reliable. The improvement in the long wave and short wave radiation schemes made them closer to observation. Changing the shortwave radiation significantly improved the surface radiation fluxes. In the radiation budget, the improvement in shortwave radiation is somewhat offset by the increase of outgoing longwave radiation ( W m 2 in tropics). This difference can be seen in the ISO scale analysis also. In Figure 5, the net radiation effect is unrealistically large after the onset of ISO, contributing to the error in MSE after the onset (Figure 12). In addition to the parameterization schemes, input datasets also have differences in both the reanalysis. For example, in the MERRA analysis, monthly mean blended sea surface temperature (SST) analysis of Reynolds (1988) are used, while NCEPR2 uses optimum interpolated SST (Reynolds and Smith 1994). The major difference of these two is the regions of high SST gradients such as the eastern Pacific (Reynolds and Smith 1994). Despite all these differences due to the reanalysis procedure between the two data, we are able to come up with a common mechanism for ISO, which is discussed in the next section Moist mechanism of ISO The detailed moisture and MSE budget analysis enable us to pinpoint the moist dynamic processes that contribute to the ISO in reanalysis data. Both the reanalysis products advocate the role of moisture advection in initiating ISO and its northward movement. They also establish a lead relationship of moisture advection with respect to convection. This enables us to come up with a common mechanism controlling the ISO. During active cycle, the easterly wind anomalies export moist air from NWP to monsoon region 7 8 d before the onset of organized convection (Figures 6 and 10). After the onset, north-westerly wind anomalies from the dry land region advect dry air to the Arabian Sea, and subsequently to the monsoon region (Figures 6(e) and 10(e)) along with the northeast movement of convection centre from India. This dry air advection from northwest will compel ISO to its reverse (break) phase (Figures 7 and 11). During the onset of break, in addition to the north-westerly flow, the easterlies in the southern part of Indian and westerlies from central India to NWP export the climatological moisture from India to equatorial Indian Ocean and NWP (Figures 7(d) and 11(d)). After the onset of break (active) the dry air advection from north (moisture advection to NWP) ceases. Thus the anomalous moisture advection is the initiator of ISO of Indian summer monsoon. Over the monsoon region, resultant rainfall is mainly balanced by moisture divergence, while over the ocean evaporation opposes the moisture advection (not shown). The fluxes like latent heat and net radiation are in phase with convection. These fluxes attain dominant role in the budget after the reversal of moisture advection only. So Figure 12. Three-day averaged time series of rainfall, MSE tendency (dm/dt) and MSE residual for active composites averaged over monsoon region 10 N 25 N, 65 E 90 E for NCEPR2. The negative (positive) days in x axis represent the lag (lead) days with respect to the onset of active phase (day0).

14 1442 P. A. PILLAI AND A. K. SAHAI the effects of these fluxes are to uphold the MSE for maintaining the ISO. Although the horizontal moisture advection has major role on the ISOs, the source of anomalous moisture is different in the two reanalysis. For example, the moisture advection to Arabian Sea in lag10 is from the east in NCEPR2, while it is by the anomalous cross equatorial flow in MERRA (Figures 6 and Figure 10). In MERRA, active composite shows anomalous cross equatorial flow to north Indian Ocean from lag10 onwards, but in NCEPR2, these anomalies are from the onset of active phase only. In the case of break also, the anomalous advection of moist air from monsoon region to NWP is strong by lag5 itself in MERRA, while it is by day 0 only in NCEPR2 (Figures 6 and Figure 10). Similarly, the easterly flow from monsoon region to the Arabian Sea is stronger in the MERRA reanalysis in day 0. These changes in the moisture advection and wind are also reflected in the regional precipitation patterns of these two reanalysis (Figures 3 and 4). In NCEPR2 all the budget terms show northward propagation, with moisture advection leading all other terms (discussed in previous section Figure 9). However, in MERRA, only the moisture advection and moisture convergence shows northward migration along with the rainfall. Moisture advection leads rainfall and moisture divergence and rainfall are in phase to phase (not shown). MSE is reproduced as a standing mode, attaining strength just before onset of active/break and continues as such to the entire period Residuals and error in the reanalysis budget Figure 12 shows the time series of 3-d averaged plot of rainfall and MSE tendency (already explained in Figure 5) along with MSE residual over the monsoon region for active period in NCEPR2. The figure shows significant amount of MSE residual (with in the range of ±20 W m 2 ) with maximum value from onset to next 6 7 d. Residual tends to oppose MSE tendency from lag20 to lead10. It is also opposite to the rainfall anomalies. In the break phase also, the residual is maximum after the onset (not shown). This is the period at which the moisture advection reverses and fluxes dominate the budget in both active and break phases. This indicates the possibility of an unidentified moisture source in the reanalysis. In MERRA also MSE budget residual is high and is a leading term for 5 6 d after onset of ISO. At the same time both the reanalysis have marginal amounts of residual in the moisture budget. Hence it is necessary to assess whether this is due to the imperfection of the moisture or the radiation parameterization schemes employed in the reanalysis models. But the present analysis is not sufficient to discriminate the errors due to moisture and radiation schemes. There are also some inevitable errors in the budget calculations due to the methods employed for budget analysis. The budget analyses are performed in the interpolated pressure levels and the advection schemes employed are different from those used in the native model, making some mismatch. This will become large for MSE, as MSE divergence is the residual of two large terms, the dry static energy divergence and moisture convergence (Neelin and Held, 1987). The best possible way to reduce the latter error is to perform the budget analysis in the native coordinates of the models used and with the same advection schemes. Kiranmayi and Maloney (2011) also agreed with the similar errors in reanalysis products. This unanswered large residual term in the budget after the onset of ISO is the main drawback of the study using reanalysis product to address the ISO. 5. Conclusion This study advances to forward our understanding of monsoon ISO by performing moisture and MSE budget analysis. The budget analysis brings out the importance of different moisture processes competing with each other for producing rainfall. The ISO scale convection and circulation has notable differences between the reanalysis products. However, both the reanalysis have a common agreement in the major budget terms controlling ISO convection. In spite of the differences in the parameterization schemes in reanalysis, the moisture budget is the balance of rainfall and moisture convergence, whereas moisture advection gives the precondition signal of ISO by 6 7 d ahead in both the reanalysis. For break phase, the necessary dry air is advected from the northwest of India and for active phase, the moisture advection is from the NWP Ocean. After the onset of convection, LHF and net radiation have dominant role. The moist mechanism put forward in this study stresses the importance of faithful representation of moisture and radiation fields in the dynamic models, as both the fields depend on the parameterization schemes of the model for the accurate simulation of monsoon ISO. The major limitation of this study is the residual terms of the budget which are inevitable due to the procedures adopted for the analysis and also as the budget is the cancellation of large terms. Even though the residual terms in the budget put a constraint in the budget analysis; the study underlines the role of moisture advection in ISO. Thus, a better observation of the moisture field in the tropics is vital for the models for the accurate prediction of ISOs well in advance. Acknowledgements The authors acknowledge Drs. H Annamalai and Jan Hafner, IPRC, University of Hawaii for their help the energy budget calculations. Prof B. N. Goswami, director of IITM is acknowledged for the support. The editor and the two anonymous reviewers are acknowledged for their comments and criticism, which made the manuscript better. The NCEPR2 and MERRA data sets are downloaded from respective websites.

15 MONSOON ISO AND MOISTURE BUDGET 1443 References Ajaymohan RS, Annamalai H, Luo JJ, Hafner J, Yamagata T Poleward propagation of boreal summer intraseasonal oscillation in coupled models: role of internal processes. Climate Dynamics 37: Annamalai H, Slingo JM Active break cycles: diagnosis of the intraseasonal variability of the Asian summer monsoon. Climate Dynamics 18: Annamalai H, Sperber KR Regional heat sources and the active and break phases of boreal summer intraseasonal (30 50 day) variability. Journal of the Atmospheric Sciences 62: Boos WR, Kuang Z Mechanism of poleward propagating, intraseasonal convective anomalies in cloud system resolving models. Journal of the Atmospheric Sciences 67: Gadgil S, Joseph PV On breaks of the Indian monsoon. Proceedings of the Indian National Science Academy (Earth and Planetary Sciences) 112: Goswami BN South Asian monsoon. In Intraseasonal Variability in the Atmosphere ocean Climate System, Lau K, Waliser D (eds). Springer-Verlag, Berlin Heidelberg; Goswami BN, Ajayamohan RS Intraseasonal oscillations and interannual variability of the Indian summer monsoon. Journal of Climate 14: Goswami BN, Xavier P Potential predictability and extended range prediction of Indian summer monsoon breaks. Geophysical Research Letters 30: DOI: /2003 GL Hendon B, Liebmann B, Glick JD, Newman M, Schemm J Medium range forecast errors associated with active episodes of the Madden Julian oscillation. Monthly Weather Review 128: Jiang X, Li T, Wang B Structures and mechanisms of the northward propagating boreal summer intraseasonal oscillation. Journal of Climate 17: Kanamitsu M NCEP Dynamical Seasonal Forecast System Bulletin of the American Meteorological Society 83: Kanamitsu M, Ebisuzaki W, Wollen J, Yang S, Hnilo JJ, Fiorino M, Plotter GL NCEP DOE AMIP-II reanalysis (R-2). Bulletin of the American Meteorological Society 83: Kemball-Cook SR, Weare BC The onset of convection in the Madden-Julian oscillation. Journal of Climate 14: Kiranmayi L, Maloney E The intraseasonal moist static energy budget in reanalysis data. Journal of Geophysical Research 116: D2117. DOI: /2011JD Krishnamurti TN, Ardanuy P The day westward propagating mode and breaks in the monsoon. Tellus 32: Krishnamurti TN, Bhalme HN Oscillations of a Monsoon system. Part I. Observational aspects. Journal of the Atmospheric Sciences 33: Krishnamurti TN, Thomas A, Simon A, Kumar V Dessert air incursions, an overlooked aspect, for the dry spells of the Indian summer monsoon. Journal of the Atmospheric Sciences 67: Krishnan R, Zhang C, Sugi M Dynamics of breaks in the Indian summer monsoon. Journal of the Atmospheric Sciences 57: Lau KM, Chan PH Aspects of day oscillation during summer as inferred from outgoing longwave radiation. Monthly Weather Review 114: Lau KM, Waliser DE Intraseasonal Variability in the Atmosphere ocean Climate System. Praxis: Chichester; 436. Lawrence DM, Webster PJ Interannual variations of the intraseasonal oscillation in the south Asian summer monsoon region. Journal of Climate 14: Lawrence DM, Webster PJ The boreal summer intraseasonal oscillation: relationship between northward and eastward movement of convection. Journal of the Atmospheric Sciences 59: Lin J Tropical intraseasonal variability in 14 IPCC AR4 climate models. Part 1: convective signals. Journal of Climate 19: Maloney ED The moist static energy budget of a composite tropical intraseasonal oscillation in a climate model. Journal of Climate 22: Moorthi S, Suarez M Relaxed Arakawa Schubert: a parameterization of moist convection for general circulation models. Monthly Weather Review 120: Neelin JD, Held IM Modeling tropical convergence based on the moist 18 static energy budget. Monthly Weather Review 115: Neelin JD, Yu J Modes of tropical variability under convective adjustment and Madden-Julian oscillation. Part1: analytical results. Journal of the Atmospheric Sciences 51: Pan HL, Wu WS Implementing a mass flux convection parameterization package for the NMC Medium-Range Forecast Model. NMC Office Note 409: 40. Pillai PA, Annamalai H Moist dynamics of severe monsoons over south 22 Asia: role of the tropical SST. Journal of the Atmospheric Sciences 69: Prasanna V, Annamalai H Moist dynamics of extended monsoon breaks over South Asia. Journal of Climate 25: DOI: /JCLI-D in press. Rajeevan M, Gadgil S, Bhate J Active and Break spells of the Indian summer monsoon. Journal of Earth System Science 119: Ramamurthy K Some aspects of the Break in the Indian southwest monsoon during July and August. Forecasting manual FMU report No. IV Reynolds RW A real-time global sea surface temperature analysis. Journal of Climate 1: Reynolds RW, Smith TM Improved global sea surface temperature analyses using optimum interpolation. Journal of Climate 7: Rienecker MM, Suarez MJ, Todling R, Bacmeister J, Takacs L, Liu HC, Gu W, Sienkiewicz M, Koster RD, Gelaro R, Stajner I, Nielsen JE The GEOS-5 Data Assimilation System - Documentation of Versions 5.0.1, 5.1.0, and Technical Report Series on Global Modeling and Data Assimilation , v Rienecker MM, Suarez MJ, Gelaro R, Todling R, Bacmeister J, Liu E, Bosilovich MG, Schubert SD, Takacs L, Kim GK, Bloom S, Chen J, Collins D, Conaty A, Silva AD, Gu W, Joiner J, Koster RD, Lucchesi R, Molod A, Owens T, Pawson S, Pegion P, Redder CR, Reichle R, Robertson FR, Ruddick AG, Sienkiewicz M, Woollen J MERRA- NASA s modern-era retrospective analysis for research and applications. Journal of Climate 24(14): Robertson F, Roberts J Intraseasonal variability in MERRA energy fluxes over the tropical oceans. Journal of Climate 25: DOI: /JCLI-D (in press). Roxy M, Tanimoto Y Role of SST over the Indian Ocean in influencing the intraseasonal variability of the Indian summer monsoon. Journal of the Meteorological Society of Japan 85(3): DOI: /jmsj Roxy M, Tanimoto Y Influence of sea surface temperature on the intraseasonal variability of the South China Sea summer monsoon. Climate Dynamics 39(5): DOI: /s x. Sikka DR, Gadgil S On the maximum cloud zone and the ITCZ over Indian longitudes during the southwest monsoon. Monthly Weather Review 108: Su H, Neelin JD Teleconnection mechanism for tropical Pacific descent anomalies during El Nino. Journal of the Atmospheric Sciences 59: Tian B, Waliser DE, Fetzer EJ, Yung YL Vertical moist thermodynamic structure of the Madden-Julian oscillation in Atmospheric Infrared Sounder retrievals: an update and a comparison to ECMWF interim reanalysis. Monthly Weather Review 138: Waliser DE. 2006a. Intraseasonal variability. In The Asian Monsoon, Wang B (ed) chap 5. Praxis Springer: Chichester; Waliser D, Weickmann K, Dole R, Schubert S, Alves O, Jones C, Newman M, Pan HL, Roubicek A, Saha S, Smith C, Dool HV, Vitart F, Wheeler Whitaker J. 2006b. The experimental MJO prediction project. Bulletin of the American Meteorological Society 87: Wang B Theory. In Intraseasonal Variability in the Atmosphere Ocean Climate System, Lau WKM, Waliser DE (eds) chap 10. Praxis Springer: Berlin; Wang B, Rui H Synoptic climatology of transient tropical intraseasonal convection anomalies: Meteorolgy Atmospheric Physics 44: Wang B, Xie X A model for the boreal summer intraseasonal oscillations. Journal of the Atmospheric Sciences 54: Webster PJ, Hoyos C Prediction of monsoon rainfall and river discharge on day time scales. Bulletin of the American Meteorological Society 85: