Monsoon Variability and Evolution

Monsoon circulation is a primary component of the tropical and global climate system and our ability to understand and model its short and long-term variability and evolution is of interest to both climate modelers and paleoclimatologists. The modern summer monsoon is driven by two primary heat sources: sensible heating of the Asian land mass and condensational (latent) heating within the troposphere over the Asian Plateau. Latent heat from moisture collected over the southern subtropical Indian Ocean istransported across the equator and released during precipitation over Asia and Africa. Both sensible and latent heat mechanisms contribute to the land-sea temperature and pressure differences that ultimately drive summer monsoon circulation.

Brown faculty investigate marine and lacustrine sediments that record monsoon variability on annual to million-year time-scales. Since modern interannual insolation is relatively constant, interannual variability in monsoon strength derives mainly from changes in the latent heat source or other internal feedbacks. However, over orbital time scales, insolation gradients change on the order of ±12%. Monsoon variability at the orbital time scale is thus linked to changes in both the sensible and latent heat sources as well as internal feedback processes which impact them [Clemens and Prell, 1991b; Clemens et al., 1991; Clemens et al., 1996].

At longer timescales, monsoon strength is sensitive to the Plio-Pleistocene evolution of the northern hemisphere ice sheets as well as tectonic development of the Asian Plateau, two internal climate factors which may be causally linked as suggested by the climate-uplift hypothesis. For the Plio-Pleistocene, monsoon sensitivity to orbital insolation and the long-term evolution of glacial boundary conditions yields a nonstationary phase response over the interval 0 to 3.5 Ma [Clemens et al., 1996]. We anticipate that similar non-stationary responses will occur in the Neogene due to sensitivity of the monsoon to internal feedback processes involving large-scale uplift, CO2, and vegetation change in Africa and Southern Asia [Kutzbach et al., 1997; Prell and Kutzbach, 1997].

Our paleoceanographic research has involved a series of integrated field and lab projects designed to identify and understand the evolution of the Asian Monsoon over seasonal to tectonic time scales. We have initiated or served as co-chief scientist on a number of monsoon-related expeditions, including: 1) Cruise RC27-04 to the Oman Margin to identify ODP drilling targets and collect cores for study of late Quaternary monsoon dynamics; 2) ODP Leg 117 coring on the Oman Margin for study of Neogene-scale monsoon dynamics and evolution; 3) JGOFS Arabian Sea Process Study (Cruises TN041 and TN047) to acquire sediment cores and sediment trap samples across the monsoon upwelling productivity zone, and 4) ODP Leg 184 to the South China Sea to core sediments to compare the variability and evolution of the East Asian Monsoon with the Arabian Sea Monsoon. These studies will range from sub-Milankovitch to Neogene-scale in scope. The combined results of these studies represent a progression from documentation of the stratigraphic integrity of recovered sections and the reliability of proxy climate indicators, to detailed interpretation of climate change and data/model comparisons based on a multiple high-resolution climate records over a variety of time scales.

One current project is focused on the South China Sea and how its monsoonal response compares on orbital and tectonic timescales to the Arabian Sea Indian Monsoon history. This work will emphasize development and analysis of multiple independent tracers of monsoon strength and regional runoff and aridity. Because all monsoon-strength proxies have some drawbacks, we emphasize the analysis and comparison of multiple indicators from continuous, high-resolution records to reliably identify monsoon responses. Our ongoing and anticipated projects will cover the Seasonal and Interannual timescales, utilizing our JGOFS sediment trap and surface-sediment data [Murray and Prell, 1996; Prell et al., 1996; Clemens, 1998], through Orbital timescales, which uses a variety of core types to identify how the timing of strong monsoons is related to sensible heating over the Asian plateau, latent heat availability from the southern subtropical Indian Ocean, and evolving glacial boundary conditions. We plan to test our observation that the timing of the monsoon response is nonstationary (i.e., that the monsoon phase systematically changed relative to the phase of global ice-volume and regional aridity) by developing new records in the South China Sea and other monsoon regions. At Tectonic timescales we continue to explore the intensification of modern monsoonal circulation at ~ 8 Ma, which is indicated by the increased abundance of endemic upwelling species, the distribution of C3-C4 vegetation, the onset of eolian sedimentation in China, and the uplift of the Himalayan-Tibetan complex. Our goal is to Climate model simulations indicate that such intensification may be a threshold response to the development of Tibetan orography at least 1/2 that of the present elevation [Prell and Kutzbach, 1992; Kutzbach et al., 1997; Prell and Kutzbach, 1997]. We continue to test the hypothesis that coeval long-term changes in ice volume, the terrestrial paleosol record, and monsoon nonstationarity are all linked through the effects of plateau uplift on global climate (e.g. [Raymo and Ruddiman, 1992; Kutzbach et al., 1997; Ruddiman et al., 1997a; Ruddiman and Prell, 1997]).