Oxygen isotopic variations in rainfall proxies such as tree rings and cave calcites from South and East Asia have been used to reconstruct past monsoon variability, mainly through the amount effect: the observed 18O depletion of rain with increasing amount, manifested as a negative correlation of the monthly amount of tropical rain with its δ18O, both measured at the same station. This relation exhibits a significant spatial variability, and at some sites (especially North-East and peninsular India), the rainfall proxies are not interpretable by this effect. We show here that relatively higher 18O-depletion in monsoon rain is not related necessarily to its amount, but rather, to large scale organized convection. Presenting δ18O analyses of ~654 samples of daily rain collected during summer 2012 across 9 stations in Kerala, southern India, we demonstrate that although the cross correlations between the amounts of rainfall in different stations is insignificant, the δ18O values of rain exhibit highly coherent variations (significant at P = 0.05). Significantly more 18O-depletion in the rain is caused by clouds only during events with a large spatial extent of clouds observable over in the south eastern Arabian Sea.

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[1] δD v and δ 18 O v of~70 water vapor samples collected at 6 and 25 m above sea level over the Bay of Bengal (BoB) during July-August 2012 are reported. This helps characterize the isotopic signature of monsoon vapor. No significant vertical variation is observed in δD v , δ 18 O v , or deuterium excess (defined as δD-8δ 18 O); δD v and δ 18 O v are significantly correlated (r = 0.92) at each height; the deuterium excess values do not, because the variation of δD v and δ 18 O v relative to their uncertainties is much larger than that of the latter. The temporal variations of δD v and δ 18 O v correlate well with air temperature rather than sea surface temperature. The control of normalized humidity on deuterium excess is less prominent. While the distribution of water vapor isotopologues over the BoB is primarily determined by the ocean surface conditions, they are significantly altered by laterally advected vapor from rain en route during the monsoon. Citation: Midhun, M., P. R. Lekshmy, and R. Ramesh (2013), Hydrogen and oxygen isotopic compositions of water vapor over the Bay of Bengal during monsoon, Geophys. Res. Lett., 40,[6324][6325][6326][6327][6328]

Understanding the factors that control the variability of oxygen isotopic ratios (δ18O) of Indian Summer Monsoon (ISM) rainfall (δ18Op) is of vital importance for the interpretation of δ18Op derived from climate proxies (e.g., speleothem and tree ring cellulose) of this region. Here we demonstrate the importance of moisture transport pathways on spatiotemporal variations of ISM δ18Op using a new set of daily observations from central and northern India and previously reported data aided by simulations from an isotope‐enabled General Circulation Model. 18O‐depleted rain events are characterized by a higher number of air parcel back trajectories through the Bay of Bengal branch of moisture transport, while those through the Arabian Sea branch are associated with 18O enriched rain events. This effect is observed on intraseasonal to interannual timescales in the long‐term observations at New Delhi as well. Thus, the shift in moisture transport regimes must be considered when interpreting δ 18Op from climate proxies of the ISM region.

The relationship between rain amount and rain δ18O of monsoon rain (amount effect) helps to reconstruct past monsoon variability from proxies (e.g., tree rings and speleothems). Analysis of new (and published) data of the δ18O of monsoon rains and vapor at nine stations shows that in regions of distinct seasonality in precipitation (e.g., peninsular India), the noise in such reconstructions can be minimized by a careful selection of sites. Peninsular India receives rain from both the Indian summer monsoon (ISM) and the northeast monsoon (NEM). Significant amount effect is observed only where the NEM rainfall is larger than or comparable to ISM rainfall. This is due to the higher quantity of NEM rain with more depleted 18O relative to ISM rain. NEM rain is more depleted in 18O because of cyclonic activity over Bay of Bengal, and the 18O depletion of Bay of Bengal surface waters due to post‐ISM river runoff.

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Abstract. The incorporation of water isotopologues into the hydrology of general circulation models (GCMs) facilitates the comparison between modeled and measured proxy data in paleoclimate archives. However, the variability and drivers of measured and modeled water isotopologues, as well as the diversity of their representation in different models, are not well constrained. Improving our understanding of this variability in past and present climates will help to better constrain future climate change projections and decrease their range of uncertainty. Speleothems are a precisely datable terrestrial paleoclimate archives and provide well-preserved (semi-)continuous multivariate isotope time series in the lower latitudes and mid-latitudes and are therefore well suited to assess climate and isotope variability on decadal and longer timescales. However, the relationships of speleothem oxygen and carbon isotopes to climate variables are influenced by site-specific parameters, and their comparison to GCMs is not always straightforward. Here we compare speleothem oxygen and carbon isotopic signatures from the Speleothem Isotopes Synthesis and Analysis database version 2 (SISALv2) to the output of five different water-isotope-enabled GCMs (ECHAM5-wiso, GISS-E2-R, iCESM, iHadCM3, and isoGSM) over the last millennium (850–1850 CE). We systematically evaluate differences and commonalities between the standardized model simulation outputs. The goal is to distinguish climatic drivers of variability for modeled isotopes and compare them to those of measured isotopes. We find strong regional differences in the oxygen isotope signatures between models that can partly be attributed to differences in modeled surface temperature. At low latitudes, precipitation amount is the dominant driver for stable water isotope variability; however, at cave locations the agreement between modeled temperature variability is higher than for precipitation variability. While modeled isotopic signatures at cave locations exhibited extreme events coinciding with changes in volcanic and solar forcing, such fingerprints are not apparent in the speleothem isotopes. This may be attributed to the lower temporal resolution of speleothem records compared to the events that are to be detected. Using spectral analysis, we can show that all models underestimate decadal and longer variability compared to speleothems (albeit to varying extents). We found that no model excels in all analyzed comparisons, although some perform better than the others in either mean or variability. Therefore, we advise a multi-model approach whenever comparing proxy data to modeled data. Considering karst and cave internal processes, e.g., through isotope-enabled karst models, may alter the variability in speleothem isotopes and play an important role in determining the most appropriate model. By exploring new ways of analyzing the relationship between the oxygen and carbon isotopes, their variability, and co-variability across timescales, we provide methods that may serve as a baseline for future studies with different models using, e.g., different isotopes, different climate archives, or different time periods.

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