Meridionally Extending Anomalous Wave Train over Asia During Breaks in the Indian Summer Monsoon
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Anomalous interactions between the Indian summer monsoon (ISM) circulation and subtropical westerlies are known to trigger breaks in the ISM on subseasonal time-scales, characterised by a pattern of suppressed rainfall over central-north India, and enhanced rainfall over the foothills of the central–eastern Himalayas (CEH). An intriguing feature during ISM breaks is the formation of a mid-tropospheric cyclonic circulation anomaly extending over the subtropical and mid-latitude areas of the Asian continent. This study investigates the mechanism of the aforesaid Asian continental mid-tropospheric cyclonic circulation (ACMCC) anomaly using observations and simplified model experiments. The results of our study indicate that the ACMCC during ISM breaks is part of a larger meridional wave train comprising of alternating anticyclonic and cyclonic anomalies that extend poleward from the monsoon region to the Arctic. A lead–lag analysis of mid-tropospheric circulation anomalies suggests that the meridional wave-train generation is linked to latent heating (LH) anomalies over the CEH foothills, Indo-China, and the Indian landmass during ISM breaks. By conducting sensitivity experiments using a simplified global atmospheric general circulation model forced with satellite-derived three-dimensional LH, it is demonstrated that the combined effects of the enhanced LH over the CEH foothills and Indo-China and decreased LH over the Indian landmass during ISM breaks are pivotal for generating the poleward extending meridional wave train and the ACMCC anomaly. At the same time, the spatial extent of the mid-latitude cyclonic anomaly over Far-East Asia is also influenced by the anomalous LH over central–eastern China. While the present findings provide interesting insights into the role of LH anomalies during ISM breaks on the poleward extending meridional wave train, the ACMCC anomaly is found to have important ramifications on the daily rainfall extremes over the Indo-China region. It is revealed from the present analysis that the frequency of extreme rainfall occurrences over Indo-China shows a twofold increase during ISM break periods as compared to active ISM conditions.
KeywordsIndian summer monsoon Active and break periods Meridional Rossby wave Monsoon and mid-latitude flows Extreme precipitation
The active and break monsoon cycles of the Indian summer monsoon (ISM) rainfall variability are a well-known aspect of the monsoon subseasonal variability and have been extensively studied by various researchers (e.g., Ramamurthy 1969; Raghavan 1973; Krishnamurti et al. 1989; Krishnan et al. 2000, 2009; Annamalai and Slingo 2001; Gadgil and Joseph 2003; Krishnamurthy and Kinter 2003; Rajeevan et al. 2010; Umakanth et al. 2014). One of the salient features of break periods in the ISM is the anomalous enhancement of rainfall activity along the southern slopes of central and eastern parts of the Himalayas (CEH), and also notably over the Indo-Chinese peninsular countries (e.g., Dhar et al. 1984; Dhar and Nandargi 2000; Krishnan et al. 2009; Ramesh Kumar et al. 2009; Vellore et al. 2014). Furthermore, precipitation enhancements over the CEH foothills during the ISM break periods are well recognised to have associations with the northward migration of the monsoon trough (Ramamurthy 1969), enhanced moisture convergence by the southerly flow from the Bay of Bengal (BoB, Vellore et al. 2014), and equatorward advancing large-amplitude troughs in the upper level westerlies (e.g., Ramaswamy 1962; Krishnan et al. 2000). Earlier studies have reported the presence of anomalous cyclonic circulation at upper levels on a planetary scale spanning the Asian mid-latitudes during weak phases of the ISM (e.g., Keshavamurthi and Awade 1974; Raman and Rao 1981; Krishnan and Sugi 2001; Krishnan et al. 2009). Vellore et al. (2014) further noted that the vorticity enhancement over the CEH foothill region during ISM breaks was associated with an eastward progression of 500 hPa height perturbations towards the southern part of the Tibetan Plateau.
Subseasonal fluctuations of ISM precipitation and associated diabatic heating are known to modulate the zonal and meridional propagation characteristics of monsoon intra-seasonal oscillations (ISOs, Hazra and Krishnamurthy 2015). Yasunari (1986) suggested that diabatic heating fluctuations over the ISM environment can induce low-frequency oscillatory behaviour of the extra-tropical upper level westerlies on intra-seasonal time-scales. A major component of diabatic heating over the tropics and monsoon dominated areas involves LH release from organised convection (e.g., Houze 1997; Yanai and Tomita 1998; Schumacher et al. 2004; Houze et al. 2007; Krishnamurti et al. 2010; Choudhury et al. 2018). Studies have noted that interactions between the LH from the Asian monsoon precipitation and the mean subtropical westerly flow environment can generate a westward extending stationary Rossby waves, leading to pronounced summer-time descent over the Mediterranean and Sahara regions (Rodwell and Hoskins 1996; Liu et al. 2001). Conversely, eastward propagation of mid-latitude Rossby wave trains originating from the Atlantic can also modulate the rainfall fluctuations over the northwestern part of the Indian subcontinent on intra-seasonal time-scales (Ding and Wang, 2007, 2009; Saeed et al. 2011a).
Furthermore, teleconnections between the ISM and circulation over East Asia have also been noted in connection with setting up of a quasi-stationary Rossby wave pattern along the upper level Asian jet in response to ISM heating (e.g., Kripalani et al. 1997; Krishnan and Sugi 2001; Enomoto et al. 2003). Stan et al. (2017) have recently provided a comprehensive review of the current scientific understanding, as well as the gap areas, related to various aspects of tropical and extra-tropical interactions on intra-seasonal time-scales.
An important aspect of Rossby wave-train excitation in response to diabatic heating over the tropics and monsoon areas is the formation of meridionally extending patterns of large-scale circulation anomalies towards the extra-tropics and polar latitudes (e.g., Sardeshmukh and Hoskins 1988; Liu et al. 2001; Enomoto et al. 2003; Krishnan et al. 2009, Krishnamurti et al. 2015). While a decrease in upper tropospheric divergence during ISM breaks due to suppressed monsoon convection over the Indian subcontinent can induce a downstream Rossby wave response towards East Asia (Krishnan et al. 2009), it is not yet clear if the monsoonal heating anomalies can have any further consequences on large-scale circulation patterns. Murata et al. (2017) noted that extreme rain periods over Cherrapunji, the wettest place on Earth located over the CEH, had associations with westward propagating low-level circulation anomalies from the western North Pacific. Their results further indicated that a large number of extreme rain periods over Cherrapunji were coincident with ISM breaks. While taking note of the aforesaid studies, it must be mentioned that several aspects of the coupled interplay between the ISM heating and large-scale circulation variations on intra-seasonal time-scales over the broader Asian monsoon region and extra-tropics are not fully understood. This study intends to focus on the large-scale circulation response to LH variations during ISM breaks through the use of observations and model simulations. Details about the data sets and numerical experiments are described in the following section.
2 Data Sets and Model Details
List of break spells for the period 1979–2014. Only the spells which last for at least 5 consecutive days are considered for the analysis. Here, the symbol J represents July and A represents August
Spells with period ≥ 5 days
The atmospheric general circulation model (GCM) used in this study is only for the purpose of understanding the thermally driven large-scale circulation response to imposed heating in a forced-damped setup. Therefore, the complex representations of physical processes and boundary forcing are excluded for simplicity. The model is forced with observed three-dimensional heating and steady-state solutions are constructed by integrating the model. The model starts from rest state and the initial three-dimensional thermal state of the atmosphere is set to the global mean values of temperature. The dynamical core of this model is based on a spectral formulation (Bourke 1974) having a rhomboidal truncation at zonal wavenumber 40 (R40), which corresponds to a Gaussian grid with an approximately horizontal resolution of 2.835° (longitude) and 1.76° (latitude) and employs sigma coordinates in the vertical direction with 25 vertical levels. The simplicity of the model makes it particularly helpful to derive dynamical insights on different aspects of monsoonal phenomena (e.g., Kasture et al. 1991; Krishnan and Kasture 1995; Sundaram et al. 2010; Choudhury and Krishnan 2011; Choudhury et al. 2018).
3 Circulation and Rainfall Diagnosis During ISM Breaks
A region of intense rainfall influenced by the Himalayan orography is generally prevalent over the Brahmaputra River Basin (encompassing the regions in the north-eastern parts of India, Tibet, Bhutan, Nepal, and Bangladesh) and coastal mountain ranges in Vietnam and Myanmar, and significant rainfall amounts are also noticeable over the oceanic regions of the eastern equatorial Indian Ocean and western North Pacific Ocean during the ISM break periods (Fig. 1b, d; see also Xie et al. 2006; Krishnamurthy and Shukla 2007; Vellore et al. 2014). The observed distribution of rainfall events during ISM break periods over the CEH foothills and the Indo-China region is positively skewed towards larger values as compared to active ISM periods (not shown). The 850 hPa wind field shows a low-level jet bypassing the peninsular part of India and also directed eastward towards Southeast Asia (Fig. 1a; see also Ramamurthy 1969; Krishnan et al. 2000), while anticyclonic circulation anomalies are noted in association with negative rainfall anomalies over the ISM-rainfed regions of central India (Fig. 1c). Positive rainfall anomalies during the break phase of ISM are mostly confined to the CEH foothill region (Fig. 1c), and various studies showed that these positive anomalies have associations with the migration of the monsoon trough to CEH foothill region (e.g., Raghavan 1973), moisture extraction by low-level southerlies from the Bay of Bengal (Medina et al. 2010), maximisation of the synoptic-scale low-pressure anomaly over the Meghalaya Plateau (Romatschke and Houze 2011), and interaction of the equatorward advancing mid-latitude upper level westerly trough with ISM circulation (e.g., Ramamurthy 1969; Raghavan 1973; Vellore et al. 2014). Negative rainfall anomalies over the central part of India (Fig. 1c, d) are shown to have apparent associations with Rossby wave response induced by convectively stable anomalies seen over the Bay of Bengal (Krishnan et al. 2000), and also in part with westward propagating low-level circulation anomalies from the western North Pacific (Murata et al. 2017). The circulation anomalies seen over East Asia (Fig. 1c) show slight similarities to the Pacific-Japan pattern (Nitta 1987), and the corresponding phase of rainfall anomaly pattern bears resemblance with the ‘‘southern flood-northern drought’’ pattern over China (Wang 2002; Yu et al. 2004).
3.1 Latent Heating and Mid-tropospheric Circulation
We performed a similar analysis to examine the large-scale circulation response during active ISM (Figure S2 in supporting information). The appearance of a meridionally extending pattern of circulation anomalies is also noted for the active ISM. While the polarity of circulation anomalies over the Indian subcontinent is opposite to that of the ISM break, it is noticed that poleward extending wave train for the active ISM is mostly located near the longitudinal band over East Asia and West Pacific. Also the spatial scale of the meridional wave-train anomalies appears to be relatively smaller during the active ISM conditions. This suggests that the generation of poleward extending meridional wave train is generic to the anomalous heating over the subtropical latitude belt, although the longitudinal band of the wave train may depend on the location of the forcing. In a more recent study, Krishnamurti et al. (2015) have proposed a monsoonal connection to the rapid Arctic ice melt since the mid-2000s through transport of atmospheric heat fluxes from the monsoon belt to the Arctic. They hypothesised that increased atmospheric heat content associated with heavy rainfall over northwest India and Pakistan is transported by upper level anticyclonic outflows from the South Asian monsoon region towards the mid-latitudes and the Canadian Arctic through a meridional wave train.
3.2 Anomalous Evolution of Heating and Circulation
The emerging point for our discussion here is the southwest–northeast-oriented anomalous meridional wave train at 500 hPa level consisting of alternating mass excess and deficits from the Asian monsoon region extending poleward into the high latitudes during ISM break periods (Fig. 4). A question arises on the associated direction/causality: Does the anomalous build-up of heat sources and sinks over the Indian subcontinent and Indo-China region during ISM break periods play a role in promoting the ACMCC?. We address this by conducting a suite of numerical sensitivity experiments using a simplified global atmospheric general circulation model (GCM) forced with observed heating.
4 Thermally Forced Atmospheric Circulation Response During ISM Breaks
The simplified atmospheric GCM is integrated into a forced-damped mode for steady-state solutions. Five experiments (CTRL, EXP1, EXP2, EXP3, and EXP4) are conducted which essentially differ in terms of LH prescription. Initial conditions correspond to a resting atmosphere and temperatures at different vertical levels are set to their global mean values in all experiments (see Choudhury and Krishnan 2011). The model also employs linear damping in the form of Rayleigh friction (in vorticity and divergence equations) and Newtonian cooling (in thermodynamic equation), and both are set to have an e-folding decay time-scale of 5 days. The model is integrated for 100 days with fixed heating and damping terms to obtain steady-state solutions.
In short, the results from the four sensitivity experiments indicate that the large-scale forced response to positive LH anomalies over the CEH foothills, Indo-China, and central–eastern China and negative anomalies over the Indian subcontinent has potential consequences to the observed poleward extending meridional wave train in the mid-tropospheric levels during ISM breaks. In particular, the meridional wave-train pattern including the anomalous high over the Indian landmass appears to be due to the combined effects of the anomalous heat source over the CEH foothills and the Indo-China region, and the anomalous heat sink over the Indian landmass during ISM breaks. On the other hand, the anomalous LH over central–eastern China associated with ACMCC during ISM breaks tends to enhance the magnitude and spatial extent of the mid-latitude cyclonic anomaly over central–eastern China.
5 Implications of Mid-tropospheric Circulation Anomaly for Heavy Precipitation
Regions in the vicinity of Indo-China and near the Sichuan basin of China often experience extreme precipitation and floods during the summer monsoon season (Fan et al. 2015; Kim et al. 2019). Vellore et al. (2014) reported that the intensification of mid-tropospheric cyclonic vorticity and upward motions over the CEH foothills, during ISM breaks, tends to promote deep convection and localised heavy rainfall. They further indicated, based on high-resolution regional climate model simulations, that heavy precipitation over the CEH foothills can increase twofold during ISM breaks as compared to active ISM periods.
Composites of total rainfall averaged over the Indo-China domain from lag − 6 to lag + 6 are shown in Fig. 8b for the ISM active and break cases. A distinctive increase in the total precipitation during ISM breaks is evident, as compared to the ISM active cases, for lag − 2 to lag + 3 over the Indo-China region. Our understanding suggests that the mid-tropospheric circulation anomaly over China and East Asia, associated with the ACMCC pattern during ISM breaks, can act as an envelope of large-scale cyclonic vorticity that can in turn facilitate the growth of deep convective systems and heavy precipitation over the southern flanks of the ACMCC. Basically, the large-scale circulation anomaly to the east of the Tibetan Plateau during ISM breaks corresponds to dispersing downstream Rossby waves which are triggered by anomalous heating over the Indian subcontinent and the Himalayan foothills (Krishnan et al. 2009). Furthermore, precipitation enhancement over the eastern side of the Tibetan Plateau is usually associated with eastward propagating synoptic-scale troughs embedded in the subtropical westerly flow (Vellore et al. 2014). As the subtropical westerlies move towards the lower latitudes during ISM breaks, they tend to produce heavy rains over the CEH foothills and the adjoining areas of Indo-China in association with strong 500 hPa convergence of moisture transported from the Bay of Bengal towards the CEH foothills, together with additional contributions from mesoscale orographic effects which tend to amplify the vertical motions in the region (Vellore et al. 2014). In summary, the increase in the frequency of heavy precipitation occurrences over the Indo-China region during ISM breaks is a multi-scale interactive process and supported by the background large-scale cyclonic vorticity associated with the ACMCC anomaly pattern.
The Asian summer monsoon heating is known to force summer-time descent over the Mediterranean and Sahara regions through westward extending stationary Rossby waves (Rodwell and Hoskins 1996; Liu et al. 2001). Several researchers have investigated the large-scale extra-tropical circulation response to monsoon heating on seasonal and inter-annual time-scales (Lau et al. 2000; Wang et al. 2001; Wu 2002; Wu and Wang 2002; Ding and Wang 2005; Saeed et al. 2011b). On the other hand, the role of ISM subseasonal variations on the large-scale circulation response over the extra-tropics is a topic that needs adequate investigations. An intriguing feature during the ISM break periods is the formation of a mid-tropospheric continental-scale cyclonic circulation anomaly extending over the Asian mid-latitudes (as referred ACMCC in the text). While studies have reported that equatorward advancing large-amplitude troughs in the mid-latitude westerlies can induce ISM breaks (e.g., Ramaswamy 1962; Krishnan et al. 2000), the influence of latent heating anomalies over the Indian landmass and the CEH foothills on the large-scale extra-tropical circulation response is not well understood.
By performing detailed diagnostic analyses using observed data sets and reanalysis products, the present study provides a mechanistic understanding of the ACMCC pattern. Our results indicate that the ACMCC is part of a larger pattern of meridionally dispersing Rossby waves that extends from the tropics into higher latitudes during ISM breaks. It is further noted that the aforesaid large-scale meridional circulation pattern during ISM breaks is largely forced by LH anomalies over central-north India and the CEH foothills. Model experiments reproduce the meridional circulation pattern when forced by the ISM break pattern of increased LH over the CEH foothills and decreased LH over the plains of central-north India. While the enhancement of LH over the CEH foothills is found to be critical for the formation of the mid-tropospheric cyclonic anomaly around the Tibetan Plateau, the large-scale meridional pattern across the Asian continent is fostered by the north–south dipolar pattern of the LH anomaly over the Indian subcontinent. Our analysis further suggests that the mid-tropospheric circulation anomaly over China and East Asia associated with the ACMCC pattern facilitates the growth of deep convective systems and heavy precipitation over the southern flanks of the ACMCC. It is noted that the frequency of extreme rainfall occurrences over Indo-China shows a twofold increase during ISM break periods as compared to active ISM conditions. The enhanced frequency of heavy precipitation occurrences over the Indo-China region during ISM breaks involves the combined effects of eastward propagating synoptic-scale troughs in the subtropical westerlies, strong mid-level convergence of moisture transport from the Bay of Bengal, and mesoscale orographic effects (Vellore et al. 2014), supported by the background large-scale cyclonic vorticity associated with the ACMCC anomaly pattern.
In a recent study, Krishnamurti et al. (2015) have proposed that poleward transport of LH release from intense precipitation events over the northwestern part of India and Pakistan through meridional wave trains can reach as far as Canadian Arctic, resulting in accelerating the melting of Arctic sea ice. From a climate change point of view, this implies that changes in the frequency and intensity of ISM breaks and associated heating anomalies over the ISM and adjoining areas can potentially exert far reaching impacts over the Arctic region. There are significant uncertainties in the CMIP5 model projections of the northward propagating low-frequency intra-seasonal modes associated with the ISM active and break phases (Sharmila et al. 2015), thus warranting further research in this area.
The authors acknowledge the Director, Indian Institute of Tropical Meteorology (IITM), Pune, India, for the encouragement and support for this work. This work is carried out under the MoES-Belmont Project Globally Observed Teleconnections and their role and representation in Hierarchies of Atmospheric Models (GOTHAM). GdC, DC, and RVD acknowledge financial support by the German Federal Ministry for Education and Research via the GOTHAM, Sacre-X and CoSy-CC2. The data providers of all used observational precipitation data sets (IMD, APHRODITE, and TRMM) and the ECMWF reanalysis (ERA-Interim) circulation products are thankfully acknowledged. The model computations were performed on the IITM high-performance computing (HPC) facility at IITM.
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Conflict of interest
On behalf of all authors, the corresponding author states that there is no conflict of interest.
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