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1. Late Holocene Shifts in Indian Summer Monsoon Rainfall from Central Nepal Stalagmites and Coupled Climate Model Simulations

   Wiggins, C.C. ; Ummenhofer, C.C. ; Denniston, R.F. ; Wanamaker, A.D. ; Asmerom, Y. ; Polyak, V.J. ; Whitney, N.M. ; Basnet, K. ; Poudel, K. ; Baral, S. ( April 2022 , American Geophysical Union Ocean Sciences Meeting)

   The Indian Summer Monsoon [ISM] provides approximately 80% of South Asia’s annual average precipitation. Nepal represents a particularly important sector of the ISM because of its location at the base of the Himalayas, Asia’s water tower, and in the zone of influence of the mid-latitude westerlies. Late Holocene ISM variability has previously been examined using high resolution resolved stable isotope records of stalagmites from northern, northeastern, and central India, but as of yet, no such records have been published from Nepal. We present high resolution stable isotopic time series from two precisely-dated and partially overlapping stalagmites spanning the last 2400 years from Siddha Baba Cave, central Nepal, as well as a year of isotopic data from rainwater collected near the cave. It has been suggested that the amount effect has only a minor effect on the oxygen isotope variability in precipitation in this area. As a result, we couple oxygen and carbon isotopes from these stalagmites to examine both regional and local-scale ISM dynamics. The Siddha Baba record reveals two periods suggestive of changes in the ISM: an apparent increase in rainfall during approximately CE 1350-1550 and a reduction in rainfall characterizing the last two centuries. We investigate these intervals using the Last Millennium Ensemble, a state-of-the-art suite of climate model simulations conducted by the National Center for Atmospheric Research with the Community Earth System Model. A primary focus is on links between Indo-Pacific ocean-atmosphere interactions and subsequent changes in the monsoon circulation over the Indian subcontinent, as well as regional moisture transport into Nepal between these periods. 

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2. A precisely-dated, composite stalagmite record of Indian summer monsoon variability from Siddha Baba cave, central Nepal, for the last 2700 yr

   Denniston, R.F. ; Sharafuddin, E.K. ; Bransel, L.R. ; Creech, P.D. ; Hughes, E.K. ; Wanamaker, A.D. ; Asmerom, Y. ; Polyak, V.J. ; Ummenhofer, C.C. ; Thatcher, D.L. ; et al ( January 2022 , Transactions American Geophysical Union)

   The Indian summer monsoon (ISM), which today supplies ~75% of annual precipitation to South Asia, has been reconstructed across previous centuries using a variety of hydroclimate-sensitive proxies. In some of these cases, ISM variability far exceeds that observed in the century-and-a-half-long instrumental record. Understanding the origins of these events is best addressed by developing a wide-ranging, multi-proxy network of high-resolution ISM reconstructions. In Nepal, ISM variability has been examined through tree rings, glacial ice, and lake sediments, but no stalagmite isotopic records of ISM rainfall have yet been published. Here we present a sub-decadally-resolved, precisely-dated, composite aragonite stalagmite record of ISM variability from Siddha Baba cave, central Nepal, for the last 2.7 kyr. A rainwater sampling program near the cave site, and a published study from Kathmandu (Adhikari et al., 2020), 150 km to the southeast, reveal that rainfall amount explains little of the observed variance in d18O values. Local hydroclimate is thus reconstructed from stalagmite 13C values, which we interpret as reflecting prior aragonite precipitation driven by changes in effective precipitation above the cave. ISM variability is apparent across a number of time scales, including centennial periods of reduced or enhanced rainfall coincident with societally-relevant precipitation regimes identified at other sites across South Asia. These include the Neo-Assyrian drought in the eastern Mediterranean and Middle East (2.7-2.5 kyr BP; Kathayat et al., 2019), the Mauria Empire (2.1-1.9 kyr BP), and the Guge Kingdom (0.9-0.3 kyr BP) pluvials in India and Tibet (Kathayat et al., 2017). A secular shift toward drier conditions since 0.5 kyr BP in the Siddha Baba record tracks the 18O records from Dasuopu glacier, Nepal Himalaya, and Sahiya cave, North India. Numerous multidecadal oscillations are also evident, including markedly wetter conditions during the 18th century, in the late Little Ice Age, apparent in the Dasuopu and Sahiya records. References Adhikari et al. (2020) Tellus B: Chem. Phys. Meteor., 72, 1-17. Kathayat et al. (2017) Sc. Adv., 7, e1701296. Kathayat et al. (2019) Sci. Adv., 5, eaax6656. 

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3. Drivers of Variability in the Position of the Subtropical Jet Over Nepal in the Last Millennium Ensemble

   Wiggins, C. ( January 2022 , Transactions American Geophysical Union)

   Nepal is positioned at the intersection of the Indian Summer Monsoon (ISM) and Subtropical Jet (SJ). Although the ISM is responsible for ~two thirds of annual precipitation, the SJ supplies precipitation in the winter and spring, with the jet migrating southwards to the subcontinent beginning in October and reaching its most southerly position in May before moving northward in June. Using the state-of-the-art Community Earth System Model Last Millennium Ensemble, we investigated potential drivers of the latitudinal position of the SJ over Nepal (referred to as the Himalayan Jet) between 850-2005 CE. The Himalayan Jet Latitude [HJL] is defined as the latitude with the highest wind speed at 200 mb for every longitude containing Nepal (Thapa et al., 2022). In order to identify dominant periodicities in HJL positioning, power-spectral-density analyses were used. For the purpose of evaluating drivers of HJL position, we identified years with a northward or southward displaced HJL, defined as being two standard deviations above or below the average annual HJL position, and used anomaly composites of precipitation, winds (upper- and lower-level), sea surface temperature, moisture transport (lower-level at 850mb), and geopotential height (upper-level at 200mb). Our analyses seem to point toward a link between HJL and the phases of the El Niño Southern Oscillation and Indian Ocean Dipole (IOD): Southerly HJL years often occur during years with an El Niño and a positive IOD event. Northerly HJL years often occur when a Rossby wave train appears to be present over Nepal, indicative of a remote teleconnection. We provide an initial quantification of the physical mechanics of how these climate modes in the Pacific, Indian, and Atlantic Oceans, including remote teleconnections transmitted via atmospheric Rossby Waves, affect HJL. These climate model simulation results are also compared with a sub-decadally-resolved, precisely-dated, composite stalagmite isotope record of ISM variability from Siddha Baba cave, central Nepal. 

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4. Drivers of Variability in the Position of the Subtropical Jet Over Nepal in the Last Millennium Ensemble

   Wiggins, C. ; Ummenhofer, C.C. ; Tiger, B.H. ; Denniston, R.F. ( January 2022 , Transactions American Geophysical Union)

   Nepal is positioned at the intersection of the Indian Summer Monsoon (ISM) and Subtropical Jet (SJ). Although the ISM is responsible for ~two thirds of annual precipitation, the SJ supplies precipitation in the winter and spring, with the jet migrating southwards to the subcontinent beginning in October and reaching its most southerly position in May before moving northward in June. Using the state-of-the-art Community Earth System Model Last Millennium Ensemble, we investigated potential drivers of the latitudinal position of the SJ over Nepal (referred to as the Himalayan Jet) between 850-2005 CE. The Himalayan Jet Latitude [HJL] is defined as the latitude with the highest wind speed at 200 mb for every longitude containing Nepal (Thapa et al., 2022). In order to identify dominant periodicities in HJL positioning, power-spectral-density analyses were used. For the purpose of evaluating drivers of HJL position, we identified years with a northward or southward displaced HJL, defined as being two standard deviations above or below the average annual HJL position, and used anomaly composites of precipitation, winds (upper- and lower-level), sea surface temperature, moisture transport (lower-level at 850mb), and geopotential height (upper-level at 200mb). Our analyses seem to point toward a link between HJL and the phases of the El Niño Southern Oscillation and Indian Ocean Dipole (IOD): Southerly HJL years often occur during years with an El Niño and a positive IOD event. Northerly HJL years often occur when a Rossby wave train appears to be present over Nepal, indicative of a remote teleconnection. We provide an initial quantification of the physical mechanics of how these climate modes in the Pacific, Indian, and Atlantic Oceans, including remote teleconnections transmitted via atmospheric Rossby Waves, affect HJL. These climate model simulation results are also compared with a sub-decadally-resolved, precisely-dated, composite stalagmite isotope record of ISM variability from Siddha Baba cave, central Nepal. 

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5. The Link Between Southern Portuguese Climate and the East Atlantic Mode from Highly Resolved Speleothem Records

   Chormann, A. ( January 2022 , PAGES Open Science Meeting)

   Climate in the Iberian Peninsula is impacted by both internal and external climate modes, which are expected to shift in position and intensity due to anthropogenic climate change. Examples of such modes include the North Atlantic Oscillation (NAO) and the East Atlantic mode (EA). Changes in the behavior in these regional climate modes could significantly alter water availability in the Iberian Peninsula, a region identified by model projections as particularly sensitive to future warming scenarios. There has been extensive research and paleoclimate reconstructions of the NAO and its impacts on Iberian climate. However, to date few paleoclimate records have been developed to evaluate the behavior of the EA over the late Holocene and into the present. The development of highly resolved regional paleoclimate records from Iberia is critical for improving the predictive capability of regional climate models under future warming scenarios and to determine the extent to which different teleconnection patterns are influencing climate. Here we present a near annually resolved stable carbon isotope (δ13C) and oxygen (δ18O) isotope time-series from three stalagmites from the Algarve region of southern Portugal from two caves within 2.3 km of each other. The southern coast of Portugal offers an ideal location to study the behavior of the EA due to the modulation of storm tracks coming across the North Atlantic Ocean into Iberia associated with the EA. U/Th dating indicates that our composite record spans the last millennia continuously through 2018 CE. Two stalagmites (GIA-19-1 and C-18-1) stopped growing around 1600 CE, during a dry interval, and sample GIA-19-2 grew continuously since the 15th century. GIA-19-2, with sub-annual resolution, is compared to modern instrumental records to evaluate the influence of specific environmental controls, including temperature and precipitation amounts. Isotope data from all three stalagmites exhibit substantial multidecadal variability indicating relatively wet and dry intervals. Based on our initial results, it is likely that both temperature and precipitation amount effects are the dominant controls on isotopic variability in these stalagmites. Comparison of the GIA-19-2 oxygen isotope time-series with the instrumental index (1950 to present) and reconstructed index (1650 CE to present) of the EA mode shows strong coherence with both index records. Hence, multidecadal variability observed in our stalagmite isotope time series may provide insight into the historical behavior of the EA mode and its resulting impacts on southern Portuguese climate. 

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