scholarly journals Tropical and mid-latitude teleconnections interacting with the Indian summer monsoon rainfall: A Theory-Guided Causal Effect Network approach

2019 ◽  
Author(s):  
Giorgia Di Capua ◽  
Marlene Kretschmer ◽  
Reik V. Donner ◽  
Bart van den Hurk ◽  
Ramesh Vellore ◽  
...  
Keyword(s):  
Standard Deviation ◽  
Summer Monsoon ◽  
Indian Summer Monsoon ◽  
Causal Effect ◽  
Convective Cell ◽  
Teleconnection Pattern ◽  
Boreal Summer ◽  
Seasonal Forecasts ◽  
Internal Dynamics ◽  
Circumglobal Teleconnection

Abstract. The alternation of active and break phases in the Indian summer monsoon (ISM) rainfall at sub-seasonal timescales characterizes each ISM season. Tropical and mid-latitude drivers influence this sub-seasonal ISM variability. The circumglobal teleconnection observed in boreal summer drives sub-seasonal variability across the mid-latitudes and a two-way interaction between ISM and the circumglobal teleconnection pattern has been hypothesized. We use causal discovery algorithms to test the ISM-circumglobal teleconnection hypothesis in a causal framework. Our analysis shows a robust causal link from the circumglobal teleconnection pattern and the North Atlantic region to ISM rainfall. We estimate the normalized causal effect (CE) of this link to be about 0.2 (a one standard deviation shift in the circumglobal teleconnection causes a 0.2 standard deviation shift in the ISM rainfall one week later). In turn, the ISM rainfall influences back the circumglobal teleconnection pattern, however weakly. Moreover, we identify causal links that represent the internal dynamics of the ISM convective cell at weekly timescales: Periods with strong updraft lead to strong rainfall one week later, but the resulting increase in static stability suppresses convection again. In our analyses, this internal ISM dynamics has the strongest CE of 0.5. Tropical Madden-Julian Oscillation variability has a CE of 0.2–0.3. Our results show that the most of the ISM variability on weekly timescales is due to internal dynamics of convective cell, but both tropical and mid-latitude teleconnections have a substantial influence. Identifying these local and remote drivers paves the way for improved sub-seasonal forecasts.

scholarly journals Tropical and mid-latitude teleconnections interacting with the Indian summer monsoon rainfall: a theory-guided causal effect network approach

2020 ◽  
Vol 11 (1) ◽  
pp. 17-34 ◽  
Author(s):  
Giorgia Di Capua ◽  
Marlene Kretschmer ◽  
Reik V. Donner ◽  
Bart van den Hurk ◽  
Ramesh Vellore ◽  
...  
Keyword(s):  
Standard Deviation ◽  
Summer Monsoon ◽  
Indian Summer Monsoon ◽  
Causal Effect ◽  
Intraseasonal Variability ◽  
Intraseasonal Oscillation ◽  
Causal Link ◽  
Teleconnection Pattern ◽  
Boreal Summer ◽  
Circumglobal Teleconnection

Abstract. The alternation of active and break phases in Indian summer monsoon (ISM) rainfall at intraseasonal timescales characterizes each ISM season. Both tropical and mid-latitude drivers influence this intraseasonal ISM variability. The circumglobal teleconnection observed in boreal summer drives intraseasonal variability across the mid-latitudes, and a two-way interaction between the ISM and the circumglobal teleconnection pattern has been hypothesized. We use causal discovery algorithms to test the ISM circumglobal teleconnection hypothesis in a causal framework. A robust causal link from the circumglobal teleconnection pattern and the North Atlantic region to ISM rainfall is identified, and we estimate the normalized causal effect (CE) of this link to be about 0.2 (a 1 standard deviation shift in the circumglobal teleconnection causes a 0.2 standard deviation shift in the ISM rainfall 1 week later). The ISM rainfall feeds back on the circumglobal teleconnection pattern, however weakly. Moreover, we identify a negative feedback between strong updraft located over India and the Bay of Bengal and the ISM rainfall acting at a biweekly timescale, with enhanced ISM rainfall following strong updraft by 1 week. This mechanism is possibly related to the boreal summer intraseasonal oscillation. The updraft has the strongest CE of 0.5, while the Madden–Julian oscillation variability has a CE of 0.2–0.3. Our results show that most of the ISM variability on weekly timescales comes from these tropical drivers, though the mid-latitude teleconnection also exerts a substantial influence. Identifying these local and remote drivers paves the way for improved subseasonal forecasts.

scholarly journals Tropical and mid-latitude teleconnections interacting with the Indian summer monsoon rainfall: A Theory-Guided Causal Effect Network approach

2019 ◽  
Author(s):  
Giorgia Di Capua ◽  
Marlene Kretschmer ◽  
Reik V. Donner ◽  
Bart van den Hurk ◽  
Ramesh Vellore ◽  
...  
Keyword(s):  
Time Scales ◽  
Summer Monsoon ◽  
Indian Summer Monsoon ◽  
Causal Effect ◽  
Causal Link ◽  
Teleconnection Pattern ◽  
Boreal Summer ◽  
Network Approach ◽  
Seasonal Forecasts ◽  
Circumglobal Teleconnection

Abstract. The Indian Summer Monsoon (ISM) is characterized by alternating active (wet) and break (dry) phases operating at sub-seasonal timescales, and various studies advocate tropical and mid-latitude teleconnection drivers influence the sub-seasonal ISM rainfall variability. One such driver is the circumglobal teleconnection pattern, which is commonly observed during boreal summer regulating the variability across the mid-latitudes at sub-seasonal time scales. In this study, a two-way interaction between ISM and circumglobal teleconnection is hypothesized and causal discovery algorithms are employed to examine and quantify the interaction linkage. Our analysis shows that there is a robust causal link from the circumglobal teleconnection pattern and the North Atlantic Oscillation (NAO) to ISM rainfall, and also a reverse causal link from the ISM rainfall to the circumglobal teleconnection pattern. Further, by including regional drivers in the framework, we identify the causal links that represent the internal dynamics associated with the ISM convective activity operating on weekly timescales, e.g., on weekly time scales, there is precedence of enhanced ascent to increased rainfall over the monsoon trough region which is followed by strong stabilization and convective inhibition. In our analyses, this internal ISM dynamics has the strongest effect, which is about twice as large as those of the mid-latitudes and of tropical MJO variability on the ISM dynamics. With our theory-guided causal effect network approach, we can (1) test physical hypotheses, (2) exploratively search for causal links and (3) quantify their relative causal contributions. This paves the way for improved (sub)seasonal forecasts.

Dependence of Indian summer monsoon rainfall forecast skill of CFSv2 on initial conditions and the role of bias in SST boundary forcing

2021 ◽  
Author(s):  
Stella Jes Varghese ◽  
Kavirajan Rajendran ◽  
Sajani Surendran ◽  
Arindam Chakraborty
Keyword(s):  
Summer Monsoon ◽  
Indian Summer Monsoon ◽  
Lead Time ◽  
Monsoon Rainfall ◽  
Indian Summer Monsoon Rainfall ◽  
Initial Conditions ◽  
Summer Monsoon Rainfall ◽  
Forecast Skill ◽  
Boreal Summer ◽  
Central Pacific

<p>Indian summer monsoon seasonal reforecasts by CFSv2, initiated from January (4-month lead time, L4) through May (0-month lead time, L0) initial conditions (ICs), are analysed to investigate causes for the highest Indian summer monsoon rainfall (ISMR) forecast skill of CFSv2 with February (3-month lead time, L3) ICs. Although theory suggests forecast skill should degrade with increase in lead-time, CFSv2 shows highest skill with L3, due to its forecasting of ISMR excess of 1983 which other ICs failed to forecast. In contrast to observation, in CFSv2, ISMR extremes are largely decided by sea surface temperature (SST) variation over central Pacific (NINO3.4) associated with El Niño-Southern Oscillation (ENSO), where ISMR excess (deficit) is associated with La Niña (El Niño) or cooling (warming) over NINO3.4. In 1983, CFSv2 with L3 ICs forecasted strong La Niña during summer, which resulted in 1983 ISMR excess. In contrast, in observation, near normal SSTs prevailed over NINO3.4 and ISMR excess was due to variation of convection over equatorial Indian Ocean, which CFSv2 fails to capture with all ICs. CFSv2 reforecasts with late-April/early-May ICs are found to have highest deterministic ISMR forecast skill, if 1983 is excluded and Indian monsoon seasonal biases are also reduced. During the transitional ENSO in Boreal summer of 1983, faster and intense cooling of NINO3.4 SSTs in L3, could be due to larger dynamical drift with longer lead time of forecasting, compared to L0. Boreal summer ENSO forecast skill is also found to be lowest for L3 which gradually decreases from June to September. Rainfall occurrence with strong cold bias over NINO3.4, is because of the existence of stronger ocean-atmosphere coupling in CFSv2, but with a shift of the SST-rainfall relationship pattern to slightly colder SSTs than the observed. Our analysis suggests the need for a systematic approach to minimize bias in SST boundary forcing in CFSv2, to achieve improved ISMR forecasts.</p>

scholarly journals The ENSO teleconnections to the Indian summer monsoon climate through the Last Millennium as simulated by the PMIP3

2018 ◽  
Author(s):  
Charan Teja Tejavath ◽  
Karumuri Ashok ◽  
Supriyo Chakraborty ◽  
Rengaswamy Ramesh
Keyword(s):  
Summer Monsoon ◽  
Indian Summer Monsoon ◽  
Little Ice Age ◽  
Walker Circulation ◽  
Summer Monsoon Rainfall ◽  
Ice Age ◽  
Boreal Summer ◽  
Last Millennium ◽  
Enso Teleconnections ◽  
The Mean

Abstract. Using seven model simulations from the PMIP3, we study the mean summer (June–September) climate and its variability in India during the Last Millennium (LM; CE 850–1849) with emphasis on the Medieval Warm Period (MWP) and Little Ice Age (LIA), after validation of the simulated current day climate and trends. We find that the above (below) LM-mean summer global temperatures during the MWP (LIA) are associated with relatively higher (lower) number of concurrent El Niños as compared to La Niñas. The models simulate higher (lower) Indian summer monsoon rainfall (ISMR) during the MWP (LIA). This is notwithstanding a strong simulated negative correlation between the timeseries of NINO3.4 index and that of the area-averaged ISMR, Interestingly, the percentage of strong El Niños (La Niñas) causing negative (positive) ISMR anomalies is higher in the LIA (MWP), a non-linearity that apparently is important for causing higher ISMR in the MWP. Distribution of simulated boreal summer velocity potential at 850 hPa during MWP in models, in general, shows a zone of anomalous convergence in the central tropical Pacific flanked by two zones of divergence, suggesting a westward shift in the Walker circulation as compared to the simulations for LM as well as and a majority of historical simulations, and current day observed signal. The anomalous divergence centre in the west also extends into the equatorial eastern Indian Ocean, resulting in an anomalous convergence zone over India and therefore excess rainfall during the MWP as compared to the LM; the results are qualitative, given the inter-model spread.

scholarly journals The Indian summer monsoon climate during the Last Millennium, as simulated by the PMIP3

2017 ◽  
Author(s):  
Charan Teja Tejavath ◽  
Ashok Karumuri ◽  
Supriyo Chakraborty ◽  
Rengasamy Ramesh
Keyword(s):  
Summer Monsoon ◽  
Indian Summer Monsoon ◽  
Walker Circulation ◽  
Summer Monsoon Rainfall ◽  
Tropical Pacific ◽  
Ice Age ◽  
Boreal Summer ◽  
Last Millennium ◽  
Strong Negative Correlation ◽  
Summer Temperatures

Abstract. In this study, using the available model simulations from the PMIP3, we study the mean summer (June–September; JJAS) climate and its variability in India during the Last Millennium (CE 850–1849; LM) for which conventional observations are unavailable, with emphasis on the Medieval Warm Period (MWP; CE 1000–1199 as against the CE 950–AD1350 from the proxy-observations) and Little Ice Age (LIA; CE 1550–1749 as against the CE 1500–1850 proxy observations. Out of the eight available models, by validating the corresponding simulated global and Indian mean summer temperatures and mean Indian summer monsoon rainfall (ISMR), and their respective trends, from historical simulations (CMIP5) against the various observed/reanalysed datasets for the 1901–2005 period. From this exercise, we identify seven realistic models. The models simulate higher (lower) mean summer temperatures in India as well as globally during the MWP (LIA) as compared to the corresponding LM statistics, in conformation with several proxy studies. Our Analysis shows a strong negative correlation between the NINO3.4 index and the ISMR and a positive correlation between NINO3.4 and summer temperature over India during the LM, as is observed in the last one-and-half centuries. The magnitude of the simulated ISMR-NINO3.4 index correlations, as seen from the multi-model mean, is found to be higher for the MWP (−0.19; significant at 95 % confidence level) as compared to that for the LIA (−0.09; insignificant). Our analysis also shows that the above (below) LM-mean summer temperatures during the MWP (LIA) are associated with relatively more (less) number of concurrent El Niños as compared to the La Niñas. Distribution of boreal summer velocity potential at 850 hPa in the central tropical pacific and a zone of anomalous convergence in the central tropical pacific, flanked by two zones of divergence in the equatorial pacific, suggesting a westward shift in Walker circulation as compared to the current day signal. The anomalous divergence centre in the west also extends into the equatorial eastern Indian Ocean, which results in an anomalous convergence zone over India and therefore excess rainfall during the MWP as compared to the LM. The results are qualitative, given the inter-model spread.

Evaluation of the ENSEMBLES multi-model seasonal forecasts of Indian summer monsoon variability

2011 ◽  
Vol 38 (11-12) ◽  
pp. 2257-2274 ◽  
Author(s):  
M. Rajeevan ◽  
C. K. Unnikrishnan ◽  
B. Preethi
Keyword(s):  
Summer Monsoon ◽  
Indian Summer Monsoon ◽  
Seasonal Forecasts ◽  
Monsoon Variability

scholarly journals A zonally-oriented teleconnection pattern induced by heating of the western Tibetan Plateau in boreal summer

2021 ◽  
Author(s):  
Qingquan Li ◽  
Mengchu Zhao ◽  
Song Yang ◽  
Xinyong Shen ◽  
Lili Dong ◽  
...  
Keyword(s):  
Tibetan Plateau ◽  
Heat Source ◽  
General Circulation ◽  
Numerical Models ◽  
Wave Train ◽  
Teleconnection Pattern ◽  
Radiative Heating ◽  
Boreal Summer ◽  
The Tibetan Plateau ◽  
Circumglobal Teleconnection

AbstractThe thermal effect of the Tibetan Plateau (TP) on the northern hemisphere climate has long been a hot topic of scientific research. However, the global effects of the TP heat source are still unclear. We investigate the teleconnection patterns coincident with the TP heat source in boreal summer using both observational data and numerical models including a linearized baroclinic model and an atmospheric general circulation model. The western TP shows the most intense variability in atmospheric heating and the most active connection to atmospheric circulations. The surface sensible heating component of the western TP heat source is associated with a high-latitude wave train propagating from North Japan to central North America through the Bering Sea and Canada. The radiative heating component is accompanied by a wavenumber-4 wave train over Eurasia. We focus on the global zonally-oriented pattern that is connected with the latent heat release from the western TP, referred to here as the TP–circumglobal teleconnection (TP-CGT). The TP-CGT pattern is triggered by the western TP latent heating in two parts starting from the TP: an eastward-propagating wave train trapped in the westerly jet stream and a westward Rossby wave response. The TP-CGT accounts for above 18% of the total variance of the circumglobal teleconnection pattern and modulates mid-latitude precipitation by superimposition. The western TP is the key region in which diabatic heating can initiate the two atmospheric responses concurrently, and the heating over northeastern Asia or the Indian Peninsula is unable to induce the circumglobal pattern directly. The unique geographical location and strong tropospheric heating also make the western TP as a “transit area” of transferring the indirect impact of the Indian summer monsoon (ISM) to the TP-CGT. These results enhance our understanding of the relationship between the circumglobal teleconnection and the ISM and is helpful for improving the prediction of the circumglobal teleconnection variability.

scholarly journals Interannual Variability of Indian Summer Monsoon arising from Interactions between Seasonal Mean and Intraseasonal Oscillations

2012 ◽  
Vol 69 (6) ◽  
pp. 1761-1774 ◽  
Author(s):  
E. Suhas ◽  
J. M. Neena ◽  
B. N. Goswami
Keyword(s):  
Interannual Variability ◽  
Summer Monsoon ◽  
Indian Summer Monsoon ◽  
Exchange Process ◽  
Alternative View ◽  
Monsoon Circulation ◽  
Internal Dynamics ◽  
Intraseasonal Oscillations ◽  
Internal Component ◽  
The Many

Abstract A significant fraction of interannual variability (IAV) of the Indian summer monsoon (ISM) is known to be governed by “internal” dynamics arising from interactions between high-frequency fluctuations and the annual cycle. While several studies indicate that monsoon intraseasonal oscillations (MISOs) are at the heart of such internal IAV of the monsoon, the exact mechanism through which MISOs influence the seasonal mean monsoon IAV has remained elusive so far. Here it is proposed that exchange of kinetic energy (KE) between the seasonal mean and MISOs provides a conceptual framework for understanding the role of intraseasonal oscillations (ISOs) in causing IAV and interdecadal variability (IDV) of the ISM. The rate of KE exchange between seasonal mean and ISOs is calculated in frequency domain for each Northern Hemispheric summer season over the ISM domain, using 44 yr of the 40-yr ECMWF Re-Analysis (ERA-40) data. The seasonal mean KE and the rate of KE exchange between seasonal mean and ISO shows a significant relationship at both the 850- and 200-hPa pressure levels. Since the rate of KE exchange between seasonal mean and ISO is found to be independent of known external forcing, the variability in seasonal mean KE arising from this exchange process can be considered as an internal component explaining about 20% of IAV and about 50% of IDV. Contrary to the many modeling studies attributing the weakening of tropical circulation to the stabilization of the atmosphere by global warming, this paper provides an alternative view that internal dynamics arising from scale interactions might be playing a significant role in determining the decreasing strength of the monsoon circulation.

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