NSFGEO-NERC: Understanding Trans-Hemispheric Modes of Climate Variability: A Novel Tree-Ring Data Transect spanning the Himalaya to the Southern Ocean

The mechanisms that govern trans-hemispheric climate linkages and those between the tropics and higher latitudes are not well understood, especially before the Anthropogenic period. Hence there are gaps in knowledge that limit our understanding of broader-scale climate teleconnections on interannual to decadal and longer time scales. Unlike the western Americas, where symmetries and common oscillatory modes are observed due to geographic similarities between the two hemispheres, the western Pacific lacks such symmetry, and data coverage of high resolution, well-dated paleorecords is limited over much of the region. We propose to repurpose an existing tree-ring data transect spanning from the Himalaya to the Southern Ocean, based on wood samples from our archives collected from broader monsoon Asia down to southern middle latitudes. By creating a new data network that focuses on earlywood (EW) and latewood (LW) width and maximum and minimum Blue Intensity (BI), these data will be used to enhance prior climate analyses based almost entirely on the single total ring-width (RW) parameter. BI is an indirect proxy of relative wood density and has been shown to be potentially more responsive to temperature and hydroclimate than TRW alone at low cost and effort. This expanded data transect will be used to model and reconstruct key modes of atmosphere-ocean variability between the northern and southern hemispheres (NH and SH).

The NSF-NERC project PIs have jointly pioneered BI and EW-LW research in the tropics and SH mid-latitudes. Proof of concept data show that BI, EW and LW parameters for several Himalayan, tropical and mid-latitude SH conifers are robustly reflective of climate variability, placing our research team in a unique position to expand our knowledge of temperature and hydroclimate variability across the region. This new suite of data will provide the opportunity to model and reconstruct key indicators of atmosphere-ocean circulation dynamics across this transect, the tropical Indo-Pacific is a key region of interest, where the El Niño-Southern Oscillation (ENSO), Intertropical Convergence Zone (ITCZ), and Indian Ocean Dipole are dominant sources of climate variability with global impact. Volcanism perturbs climate across both hemispheres, while the Southern Annular Mode or Antarctic Oscillation impacts climate towards the SH mid latitudes at the southern end of our transect. The new data coverage, parameters, analyses and reconstructions will allow us to address several outstanding research questions, including: 1. What are the climatic impacts of volcanic events in the tropics and Southern Hemisphere? 2. How have the ITCZ and ENSO varied of the past 500-1000 years as expressed across our study region? And 3. How has the Southern Annular Mode varied over the past millennium, and is it driven by tropical forcing? To understand the global response to climate requires robust, high-resolution paleoclimate data from the tropics and SH, which our proposed transect will provide.

This project will lead to an improved understanding of past variability in climate-sensitive, yet data-sparse regions of the Eastern Hemisphere transect proposed herein, some of the most climatically vulnerable regions of the globe. These data will be disseminated to the broader community via the International Tree-Ring Data Bank and other venues and will be included in a future iteration of the Monsoon Asia Drought Atlas. Education will be an important component of this project as well. Data from this project will be used for instructional purposes in a Masters level class taught at Columbia University (CU) by the lead PI and an undergraduate level course at Rider University (RU) by the RU PI. RU students will participate in this project and travel between RU and CU for educational activities. Regular virtual student exchanges will take place between students from RU, CU and foreign partners.