Indian Monsoon Variability, Himalayan Climate, and Terrestrial Ecology from Bhutanese Lake Sediments



The Indian monsoon system represents one of the largest hydrological features on Earth and is a critical water resource for over a billion people.  This proposal aims to investigate decadal to centennial variability in monsoon rainfall using unique sedimentary archives from Bhutanese lakes and novel organic geochemical approaches.  The records provided by these lakes will offer an unparalleled view of past changes in Indian monsoon circulation and its relationship to regional and global temperature changes.  Diagnostic biomarkers will be used to link these climatic changes to Himalayan forest ecology.  The proposed research will be undertaken in collaboration with the Watershed Management Division of the Ministry of Agriculture in Bhutan and will set the foundation for future joint research efforts.

 Introduction and Rationale

Climate changes associated with the boreal summer Indian Monsoon (IM) directly affect the livelihoods of the ca.  1.5 billion people that live on the Indian subcontinent.  The importance of the Indian Monsoon to agronomy and society in Southeast Asia has long made the IM a target for meteorologic and climatic study - Sir Gilbert Walker, who first identified the Southern Oscillation, was charged with this task at the turn of the 20th century.  However, many aspects of IM variability are not well understood; for example, there is considerable uncertainty surrounding the dynamical controls on decadal to centennial variability in the monsoon, such as decadal shifts in the coupling between the IM and ENSO [Kucharski et al., 2007] and the role of multidecadal fluctuations in SSTs of the Atlantic, Indian, and Pacific Oceans [Zhang and Delworth, 2006; Meehl and Hu, 2006].  These aspects of IM climatology cannot be studied from the instrumental record alone, as it is too short to encompass the full range of natural variability.

Paleoclimatic records from the heart of the Indian monsoon region provide a valuable perspective on variability in the Indian Monsoon, the relationship of IM rainfall to global and regional temperature changes, and the impact of IM variability on the terrestrial environment.  Presently, there are relatively few terrestrial paleoclimate records that describe decadal-millennial scale climate changes from the Indian subcontinent, and many are low in temporal resolution [c.f.  Fleitmann et al., 2007].  The tree-ring based Monsoon Asia Drought Atlas (MADA) [Cook et al.,2010] provides annually-resolved historical drought estimates over India, the Himalaya, China and Southeast Asia, but only covers the last 700 years, leaving centennial and longer-scale variability underconstrained.  Conversely, marine sediment records from the nearby Arabian Sea, Pakistan Margin, and Bay of Bengal provide orbital to millennial-scale perspective on ocean dynamics related to the IM [e.g.  Schulz et al., 1998; Ponton et al., 2012] but do not typically resolve Late Holocene climate - and in particular, terrestrial hydroclimate - at a decadal resolution.  Therefore, we have relatively little data that describe multidecadal to centennial-scale changes in the IM during the Late Holocene.  Understanding how the monsoon has varied in recent millennia is critical; the Late Holocene is our best analog to present-day conditions and offers perspective on both externally-forced climate events (e.g., the response to cooling during the Little Ice Age, or warmer conditions during the Holocene Climatic Optimum) and internally-forced natural variability.