Role of tropical Indian Ocean air sea interactions in modulating Indian summer monsoon in a coupled model

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1 ATMOSPHERIC SCIENCE LETTERS Atmos. Sci. Let. 16: (2015) Published online 24 January 2015 in Wiley Online Library (wileyonlinelibrary.com) DOI: /asl2.561 Role of tropical Indian Ocean air sea interactions in modulating Indian summer monsoon in a coupled model Jasti S. Chowdary, 1 * Arti B. Bandgar, 1,2,3 C. Gnanaseelan 1 and Jing-Jia Luo 4,5 1 T.S. Division, Indian Institute of Tropical Meteorology, Pune, India 2 Department of Atmospheric and Space Sciences, University of Pune, India 3 India Meteorological Department, Pune, India 4 Japan Agency for Marine-Earth Science and Technology, Yokosuka, Japan 5 Centre for Australian Weather and Climate Research, Melbourne, Australia *Correspondence to: J. S. Chowdary, Indian Institute of Tropical Meteorology, Dr. Homi Bhabha Road, Pashan, Pune , India. jasti@tropmet.res.in Received: 30 January 2014 Revised: 18 August 2014 Accepted: 2 January 2015 Abstract While the importance of El Niño to the Indian summer monsoon (ISM) rainfall variability has been widely recognized, the role of tropical Indian Ocean (TIO) is still unclear. Present results based on coupled model sensitivity experiments revealed that the TIO air sea interaction trims down suppressed rainfall locally over the ISM region by opposing the influence of tropical Pacific Ocean (TPO) during El Niño developing summers. Besides, El Niño-induced TIO basin-wide warming supports positive rainfall anomalies over the ISM region by maintaining local convection. TIO local air sea interaction opposes the Pacific impact even in the absence of Indian Ocean Dipole. Keywords: Indian Ocean; air sea interactions; Indian summer monsoon; El Niño and Southern Oscillation (ENSO) 1. Introduction Tropical Indian Ocean (TIO) and tropical Pacific Ocean (TPO) are known to play key roles in modulating Indian Summer Monsoon (ISM) rainfall (Shukla, 1975). The interdependence of the sea surface temperature (SST) variations in the Pacific and Indian Ocean can modulate SST anomalies in their counterparts remotely through surface heat flux and upper ocean changes via modifying the atmospheric Walker Cell and Indonesian Throughflow (Du and Qu, 2010; Luo et al., 2010; Yuan et al., 2011). This could change the monsoon circulation and rainfall over the Indian subcontinent. SST in the TIO varies significantly at interannual time scales (Xie et al., 2002; Du et al., 2009). The potential role of the Indian Ocean (IO) SST anomalies in predicting the south Asian summer monsoon has been well established (Sahai et al., 2003). The El Niño Southern Oscillation (ENSO) is the leading mode of climate variability and is predictable on interannual time scales (Luo et al., 2008a). Many seasonal extreme droughts (floods) of ISM are accompanied by the growing Phase of El Niño (La Niña) events in the Pacific Ocean (Krishnakumar et al., 1999). However, 1997 is an unusual case. South Asia experienced normal rainfall despite the strong El Niño event in 1997 (Slingo and Annamalai, 2000); this is attributed to a positive Indian Ocean dipole (IOD Saji et al., 1999) that co-occurred with the El Niño in the Pacific (Webster et al., 1999). IOD is known to play an important role in modulating the relationship between the ISM rainfall and ENSO (Ashok et al., 2001). Besides, it is important to note that El Niño-induced TIO basin-wide warming (from boreal winter to summer) would influence the ISM rainfall (Yang et al., 2007). However, the role of TIO air sea interaction in modulating monsoon during ISM rainfall deficit years (associated with El Niño) is not clear from the previous studies. In this study, coupled general circulation model (CGCM) hindcast experiments are used to examine the contribution of TIO SSTs to ISM during extreme years. The model used in this study is the Scale Interaction Experiment (SINTEX) global ocean atmosphere CGCM (Gualdi et al., 2003) modified and improved at the Frontier Research Centre for Global Change (SINTEX-F), Japan (Luo et al., 2005). 2. Model and data The global ocean atmosphere fully coupled model used in this study is SINTEX-F CGCM (Luo et al., 2005). We perform nine-member ensemble retrospective forecasts for 12 target months from the first day of each month during The nine members are generated on the basis of three different coupling physics (i.e. three different models) with three different initial conditions for each model (Luo et al., 2008a, 2008b). The results presented here are based on the nine-member ensemble mean anomalies. To study the 2015 Royal Meteorological Society

2 Tropical Indian Ocean air sea interactions and Indian summer monsoon 171 role of TIO and TPO air sea interactions in ISM rainfall, two sensitivity experiments (NoTIO and NoTPO) are carried out using SINTEX-F in addition to the control run (CTL) in which atmosphere and ocean are fully interactive over the globe. In the NoTIO (NoTPO) experiments, ocean and atmosphere in the TIO (TPO) are decoupled where climatological monthly SST from Reynoldset al. (2002) is prescribed over the TIO (TPO) between 25 S and 25 N. More details of model, initialization and experiments are provided in Luo et al. (2005, 2008a, 2010). Model results are compared with the European Centre for Medium-Range Weather Forecasts (ECMWF) reanalysis Interim (ERA-Interim; Dee et al., 2011) sea level pressure (SLP), 850 and 200 hpa winds, Reynolds et al. (2002) SST and the Centre for Climate Prediction merged analysis of precipitation (CMAP; Xie and Arkin, 1997). 3. Model prediction skill of regional rainfall and circulation indices The SINTEX-F has shown remarkable skill in simulating and predicting the Indian and Pacific Ocean climate variability (Luo et al., 2005, 2008a, 2008b; Behera et al., 2005; Chowdary et al., 2011; Izumo et al., 2008, Figure S1, Supporting Information). Figure 1 illustrates the different regional indices such as rainfall for extended region (RER index; E and equator to 30 N; Goswami et al., 1999), Webster-Yang index (WYI; area average of the difference between zonal wind anomalies at 850 hpa and at 200 hpa over 0 20 N, E; Webster and Yang, 1992) during JJAS (mean of June to September) both for observations and model (CTL) 1-month lead prediction initiated from 1 May. The model shows some skill in predicting RER and WYI indices and the corresponding correlations are 0.38 and 0.5 respectively for 26 years (significant at 85% based on student s t-test). While the model shows significant skill in predicting the ISM, further improvement in skill is required. Large biases over the eastern TIO SST (Figures S2 and S3) may affect the ISM predictability. Similar problem is also noted in many of current Coupled Model Intercomparison Project Phase 5 (CMIP5) coupled models (Du et al., 2013; Liu et al., 2014). The model displays good skill in predicting the Niño 3.4 and dipole model indices with correlation above 0.6 (Figure 1(c) and (d)) at 1-month lead. The model also has good skills in predicting El Niño (IOD) at long lead times up to 12 (6) months (Luo et al., 2008a, 2008b). Both TIO and TPO SSTs (coupling to atmosphere) are necessary for better knowledge of the ISM rainfall prediction. Decoupling of either TIO/TPO air sea interactions in the model may alter the prediction skill. It is therefore essential to know the individual role of TIO and TPO air sea interaction in affecting the ISM rainfall prediction particularly during extreme years such as severe drought/flood, ENSO and IOD years. Figure 1. (a) Rainfall for extended region(rer; mm day 1 ) index, (b) Webster-Yang index (difference of 850 and 200 hpa zonal wind index; m s 1 ), (c) Niño 3.4 (sea surface temperature, SST anomalies; C) index and (d) Indian Ocean dipole mode (SST anomalies; C) index during JJAS ( ) both for observations (grey bars) and model (CTL run; line with solid circles) prediction at 1-month lead (i.e. initiated from 1 May). Correlation between model and observations is displayed at the top of the each panel. 4. Impact of TIO SST on the ISM during anomalously dry or wet years 4.1. ISM drought years that are associated with the developing phase of El Niño To diagnose the impact of TIO air sea interaction on the ISM, we have analysed the difference between the CTL and NoTIO/NoTPO model runs at 1-month lead (Figure 2). Composite anomalies of SST and atmospheric parameters over the TIO region are illustrated during the deficit ISM rainfall years (1987, 2002 and 2004). Note that drought (El Niño) years are selected for composite analysis based on RER (Niño 3.4) indices, when both the observations and model are in phase (Figure 1). The difference between the CTL and NoTIO

3 172 J. S. Chowdary et al. (CTL-NoTIO) run during the drought years shows strong (weak) 850 hpa westerlies over the equatorial (north) IO (Figure 2(a)). SST difference displays negative IOD-like conditions over the TIO. Slightly more rainfall is produced over the Indian subcontinent and southeast equatorial IO in the CTL compared with the NoTIO run. This is clearly evident in the spatial distribution of precipitation difference as well (Figure 2(b)). The model NoTIO experiment reveals that, without air sea interactions over the TIO, El Niño in the Pacific tends to cause strong negative precipitation anomalies in some parts of the ISM region (Figure 2(b)). In other words, our results show that TIO air sea coupling opposes the impact of Pacific during the drought years over the ISM region. This is further evident in SLP and 200 hpa winds over the ISM region (Figure 2(b)). SLP is slightly low in the CTL run compared with the NoTIO, supporting enhanced precipitation in the presence of the TIO SST variations. To verify our results further, the NoTPO experiment is carried out. The difference between the CTL and NoTIO (CTL NoTPO) runs displays weak easterlies (westerlies) over the Arabian Sea (Bay of Bengal) and strong negative rainfall over the Indian subcontinent during deficient ISM rainfall years that are associated with El Niño (Figure 2(c) and (d)). The role of the TPO (El Niño) in the large-scale subsidence over the Indian region is evident. Negative rainfall anomalies over the ISM region are supported by high SLP anomalies and weak easterly jet at 200 hpa (Figure 2(d)). This shows that when the Pacific air sea interactions are decoupled, precipitation over the Indian subcontinent is enhanced in the drought composite (Figure 2(c) and (d)), which suggests the potential role of the TIO SSTs in reducing the TPO s effect on ISM rainfall. El Niño-induced subsidence over the TIO suppresses the convective activities over the ISM region through Walker circulation (Krishnakumar et al., 1999). At the same time, weakening of southwesterlies in the North IO (NIO) region enhances SST (Figure S2), which tends to increase surface evaporation and hence oppose the large-scale subsidence and dryness induced by El Niño. Resultant moisture transport by mean southwesterly winds to the ISM region increases the rainfall locally. The NoTIO experiment shows suppressed precipitation over the south Asian region during the developing phase of El Niño (Figure 2(b)). As there is no extra moisture supply because of the absence of local TIO air sea interactions, subsidence induced by El Niño can suppress more rainfall over the ISM region. All the above discussions are based on the model s 1-month lead predictions. The NoTIO and NoTPO experiment at 3-month lead also supports the above results (Figure S3) ISM excess rainfall years that are associated with decay phase of El Niño Basin-wide warming over the TIO persists from boreal spring to summer after the mature phase of El Niño (i.e. the developing phase of La Niña), and subsequently impacts precipitation over Asian monsoon region (Yang et al., 2007; Xie et al., 2009). JJAS composite based on 1983, 1998 and 2003 during the decay phase of El Niño are used for detailed analysis. It is important to note that the TIO basin-wide warming and La Niña conditions in the Pacific are apparent during the decay phase of El Niño. Observed wind anomalies at 850 hpa are easterlies/northeasterlies over the NIO (figure not shown) that support the persistence of basin-wide warming by reducing the surface latent heat flux loss. In response to the basin-wide warming, precipitation anomalies are positive over most of the TIO and Indian subcontinent. The stronger low-level moisture transport and reduced moist stability associated with the warmer TIO increase monsoon rainfall despite a weakened monsoon circulation (Yang et al., 2007). The composite maps display a cyclonic circulation in the low-level wind anomalies (850 hpa) over the NIO, covering from western Arabian Sea to east coast of India in the CTL-NoTIO (Figure 3(a)). Associated positive precipitation and negative SLP anomalies are also apparent (Figure 3(b)). Divergent wind anomalies at 200 hpa with a weak anticyclonic circulation are also evident over the NIO, which is the representation of baroclinic monsoon circulation. The model NoTIO experiment clearly brought out the importance of local air sea interactions (precipitation is strong in the CTL than in the NoTIO run) in association with the TIO basin-wide warming in summer. Variability in the Pacific SST (during both the decay phase of El Niño and the developing phase of La Niña) is important in inducing the TIO basin-wide warming in winter and spring. Figure 3(c) shows the direct impact of the TPO on the SST anomalies in the TIO (which is quite weak although positive). The difference between Figure 3(a) and (c) suggests that the TIO local air sea interactions play a major role in the TIO SST warming. Weak negative precipitation in some parts of the TIO including Indian subcontinent is evident in the CTL NoTPO (Figure 3(d)). This emphasizes that the TIO warming (CTL run) is important in enhancing the precipitation over this region. SLP and 200 hpa wind anomalies are consistent with negative rainfall over the TIO region in the CTL NoTPO case. Impact of the basin-wide TIO warming on the ISM rainfall is also evident in 3-month lead prediction (Figure S4) El Niño and IOD co-occurrence years Negative SST anomalies over southeastern equatorial IO and positive anomalies over central and western TIO are apparent in observations during 1997 and 2006, when El Niño and IOD co-occurred. Corresponding rainfall anomalies are negative over the eastern and positive over the western IO. Strong easterly wind anomalies are observed over the equatorial IO (figure not shown). SLP is high over the eastern and low over the western equatorial IO (Saji et al., 1999). Rainfall over the Indian subcontinent is positive especially over

4 Tropical Indian Ocean air sea interactions and Indian summer monsoon 173 (a) (b) (c) (d) 40 E 60 E 80 E 100 E 40 E 60 E 80 E 100 E Figure 2. Composite (JJAS) anomalies of model CTL minus NoTIO run for (a) sea surface temperature (shaded; C) and 850 hpa winds (vectors; m s 1 ), (b) precipitation (shaded; mm day 1 ), sea level pressure (contours; hpa) and 200 hpa wind (vectors; m s 1 ) during developing phase of El Niño (bad monsoon). (c) and (d) as in (a) and (b) but for model CTL minus NoTPO run. The model results are based on 1-month lead prediction. the Western Ghats, head Bay of Bengal and monsoon trough region. The above features are well represented by the model CTL run at 1-month lead but with weaker magnitude (figure not shown). The strong positive rainfall in some parts of the Indian subcontinent in the CTL-NoTIO indicates that the TIO air sea interactions (i.e. IOD in this case) exert strong impacts on the ISM in the presence of El Niño (Figure 4). SLP and wind patterns at 850 hpa further support the fact that the TIO SSTs reduce the impact of Pacific during the co-occurrence years. It is important to note that in CTL NoTPO similar surface easterly winds prevail in the eastern equatorial IO (Figure 4(a) and (c)), with major differences over the NIO. The CTL NoTPO shows negative rainfall anomalies over the Indian subcontinent, indicating strengthening of rainfall activities over the ISM region when Pacific ocean atmosphere is decoupled. This further supports that the TIO local air sea interaction contributes positively to the ISM rainfall by suppressing the Pacific impact. Similar results are obtained in 3-month lead prediction as well (figure not shown). The major differences are evident in the 200 hpa circulations. Our results highlight the importance of air sea interaction over the TIO in affecting local and ISM rainfall even when Pacific air sea interactions are active during boreal summer. 5. Summary and discussion In general it is known that most of El Niño events in the Pacific induce droughts over the ISM region (e.g. Krishnakumar et al., 1999; Ashok et al., 2001). However, the role of the TIO air sea interaction during deficit ISM rainfall years has been paid less attention. Using coupled model experiments, the present analysis demonstrates that the TIO air sea interactions undermine the impact of Pacific on the ISM rainfall especially during deficit rainfall years that are associated with El Niño. These are explored by coupled model sensitivity experiments with air sea interactions being decoupled respectively over the TIO and TPO. Warm SST anomalies over the TIO during El Niño developing summers enhance evaporation and trim down suppressed rainfall locally over the ISM region. These results are not only supported by the NoTIO experiment but also by the NoTPO experiment in the coupled model.

5 174 J. S. Chowdary et al. (a) (b) (c) (d) 40 E 60 E 80 E 100 E 40 E 60 E 80 E 100 E Figure 3. Same as Figure 2 but for composites of (JJAS) tropical Indian Ocean basin-wide warming years that are associated with decay phase of El Niño. In the El Niño decaying/la Niña developing summer, the TIO SST is anomalously high and exerts strong impact on the ISM. The dominant role of the TIO basin-wide warming in enhancing the precipitation over most of Indian subcontinent is clearly evident in the CTL-NoTIO run. The highlight of our experiments is that the TIO local air sea interactions oppose the Pacific impact on the ISM rainfall even when IOD is absent. Our result shows that although climate signals are overly strong in the equatorial Pacific, it is essential to consider the influence of the TIO local air sea interactions in predicting the ISM rainfall up to 6 months lead. Both 3 and 6 months lead predictions reveal similar conclusions as in 1-month lead but with weaker magnitude. Acknowledgements We thank Director IITM for support. We thank the anonymous reviewers for their valuable comments that helped us to improve the manuscript. Figures are prepared in GrADS. Supporting information The following supporting information is available: Appendix S1. Model Climatology and long lead prediction skills. Figure S1. Summer (JJAS) observed climatology for SST (shaded; C) and 850 hpa winds (vectors; m s 1 ), (b) precipitation (shaded and contours; mm day 1 ) and 200 hpa wind (vectors; m s 1 ). (c) and (d) is same as in (a) and (b) but for model CTL run 1-month lead prediction (SINTEX-F hindcast). Observed and model climatology is based on the period of Figure S2. Observed composite anomalies of (a) SST (shaded and contours; C) and 850 hpa wind (vectors; m s 1 ), (b) precipitation (shaded; mm day 1 ), 200 hpa wind (vectors; m s 1 )and SLP (shaded; hpa) during bad monsoon (JJAS) years that are associated with developing phase of El Niño. (c) and (d) is same as in (a) and (b) but for model CTL run. Figure S3. Composite (JJAS) anomalies of model CTL-NoTIO run for (a) SST (shaded and contours; C) and 850 hpa winds (vectors; m s 1 ), (b) precipitation (shaded; mm day 1 ), SLP (contours; hpa) and 200 hpa wind (vectors; m s 1 ) during developing phase of El Niño (bad monsoon). (c) and (d) same as in (a) and (b) but for model CTL NoTPO run. The model results are based on 3-month lead prediction. Figure S4. Same as Figure S3 but for composites of (JJAS) TIO basin-wide warming years that are associated with decay

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