Description: Variations in oxygen isotope ratios (δ18O) measured from modern precipitation and geologic archives provide a promising tool for understanding modern and past climate dynamics and tracking elevation changes over geologic time. In areas of extreme topography, such as the Tibetan Plateau, the interpretation of δ18O has proven challenging. This study investigates the climate controls on temporal (daily and 6 h intervals) and spatial variations in present-day precipitation δ18O (δ18Op) across the Tibetan Plateau using a 30 year record produced from the European Centre/Hamburg ECHAM5-wiso global atmospheric general circulation model (GCM). Results indicate spatial and temporal agreement between model-predicted δ18Op and observations. Large daily δ18Op variations of 25 to +5‰ occur over the Tibetan Plateau throughout the 30 simulation years, along with interannual δ18Op variations of ~2‰. Analysis of extreme daily δ18Op indicates that extreme low values coincide with extreme highs in precipitation amount. During the summer, monsoon vapor transport from the north and southwest of the plateau generally corresponds with high δ18Op, whereas vapor transport from the Indian Ocean corresponds with average to low δ18Op. Thus, vapor source variations are one important cause of the spatial-temporal differences in δ18Op. Comparison of GCM and Rayleigh Distillation Model (RDM)-predicted δ18Op indicates a modest agreement for the Himalaya region (averaged over 86°–94°E), confirming application of the simpler RDM approach for estimating δ18Op lapse rates across Himalaya.

Description: Variations in oxygen isotope compositions (δ18O) provide insight into modern climate and past changes in climate and topography. In addition, in regions such as Tibet, geologic archives of isotope ratios record climate change driven by plateau uplift and therefore also provide information about the surface uplift history. A good understanding of modern-day controls on δ18O is crucial for interpreting geologic δ18O in this context. We use the ECHAM5-wiso global atmospheric general circulation model to calculate δ18O in precipitation (δ18Op) for the present-day climate. In the region of the Tibetan Plateau, spatial variations of monthly means of δ18Op are statistically related to spatial variations of 2 m air temperature and precipitation rate, as well as to topography. The size and location of investigated regions are varied in our study to capture regional differences in these relationships and the processes governing the modern δ18Op. In addition to correlation analyses, a cross-validated stepwise multiple regression is carried out using δ18Op as the predictand, and topography and atmospheric variables (temperature and precipitation amount) as predictors. The 2 m air temperature and topography yield the highest spatial correlation coefficients of 〉0.9 and 〈 0.9, respectively, throughout most of the year. Particularly high correlation coefficients are calculated for the region along the Himalayan orogeny and parts of western China. The predictors explain 〉90% of the δ18Op spatial variance in the same regions. The 2 m air temperature is the dominant predictor and contributes 93.6% to the total explained spatial variance on average. The results demonstrate that most of the δ18Op pattern on and around the Tibetan Plateau can be explained by variation in 2 m air temperature and altitude. Correlation of the dependent predictors indicate that in low-altitude regions where topography does not determine temperature variability, the high correlation of temperature and δ18Op may partially be explained by variations in precipitation rates.

Description: The Himalaya–Tibet orogen contains one of the largest modern topographic and climate gradients on Earth. Proxy data from the region provide a basis for understanding Tibetan Plateau paleo climate and paleo elevation reconstructions. Paleo climate model comparisons to proxy data compliment sparsely located data and can improve climate reconstructions. This study investigates temporal changes in precipitation, temperature and precipitation δ18O(δ18Op) over the Himalaya–Tibet from the Last Glacial Maximum (LGM) to present. We conduct a series of atmospheric General Circulation Model (GCM, ECHAM5-wiso) experiments at discrete time slices including a Pre-industrial (PI, Pre-1850 AD), Mid Holocene (MH, 6 ka BP) and LGM (21 ka BP) simulations. Model predictions are compared with existing proxy records. Model results show muted climate changes across the plateau during the MH and larger changes occurring during the LGM. During the LGM surface temperatures are ∼2.0–4.0◦C lower across the Himalaya and Tibet, and 〉5.0◦C lower at the northwest and northeast edge of the Tibetan Plateau. LGM mean annual precipitation is 200–600 mm/yr lower over on the Tibetan Plateau. Model and proxy data comparison shows a good agreement for the LGM, but large differences for the MH. Large differences are also present between MH proxy studies near each other. The precipitation weighted annual mean δ18Op lapse rate at the Himalaya is about 0.4h/km larger during the MH and 0.2h/km smaller during the LGM than during the PI. Finally, rainfall associated with the continental Indian monsoon (between 70◦E–110◦E and 10◦N–30◦N) is about 44% less in the LGM than during PI times. The LGM monsoon period is about one month shorter than in PI times. Taken together, these results document significant spatial and temporal changes in temperature, precipitation, and δ18Op over the last ∼21 ka. These changes are large enough to impact interpretations of proxy data and the intensity of the Indian monsoon.

Description: The denudation history of active orogens is often interpreted in the context of modern climate gradients. Here we address the validity of this approach and ask what are the spatial and temporal variations in palaeoclimate for a latitudinally diverse range of active orogens? We do this using high-resolution (T159, ca. 80 × 80 km at the Equator) palaeoclimate simulations from the ECHAM5 global atmospheric general circulation model and a statistical cluster analysis of climate over different orogens (Andes, Himalayas, SE Alaska, Pacific NW USA). Time periods and boundary conditions considered include the Pliocene (PLIO, ∼3Ma), the Last Glacial Maximum (LGM, ∼21ka), mid-Holocene (MH, ∼6ka), and pre-industrial (PI, reference year 1850). The regional simulated climates of each orogen are described by means of cluster analyses based on the variability in precipitation, 2 m air temperature, the intra-annual amplitude of these values, and monsoonal wind speeds where appropriate. Results indicate the largest differences in the PI climate existed for the LGM and PLIO climates in the form of widespread cooling and reduced precipitation in the LGM and warming and enhanced precipitation during the PLIO. The LGM climate shows the largest deviation in annual precipitation from the PI climate and shows enhanced precipitation in the temperate Andes and coastal regions for both SE Alaska and the US Pacific Northwest. Furthermore, LGM precipitation is reduced in the western Himalayas and enhanced in the eastern Himalayas, resulting in a shift of the wettest regional climates eastward along the orogen. The cluster-analysis results also suggest more climatic variability across latitudes east of the Andes in the PLIO climate than in other time slice experiments conducted here. Taken together, these results highlight significant changes in late Cenozoic regional climatology over the last ∼3Myr. Comparison of simulated climate with proxy-based reconstructions for the MH and LGM reveal satisfactory to good performance of the model in reproducing precipitation changes, although in some cases discrepancies between neighbouring proxy observations highlight contradictions between proxy observations themselves. Finally, we document regions where the largest magnitudes of late Cenozoic changes in precipitation and temperature occur and offer the highest potential for future observational studies that quantify the impact of climate change on denudation and weathering rates.