Asian Monsoon Research

 The entire Asian Monsoon project is directed by one overriding research question:

How have the processes responsible for driving much of the observed Asian monsoon climate dynamics varied and interacted on inter-annual to centennial time scales?

This over-arching question will be investigated through within- and between-region analyses of centuries-long climate reconstructions developed from our ‘Monsoon Asia’ tree-ring network (an extension of the observed system of Meehl and Arblaster, 2002).  To do so actually requires answering a number of regionally specific research questions that are integral parts of our over-arching question.  For example, 

How have Asian surface air temperatures, Indian Ocean SSTs and TropicalPacific SSTs covaried and interacted over the past several centuries?

The TRL at LDEO has successfully engaged in tree-ring research in various parts of Asia and Australia for more than a decade.  Along the way we have developed important scientific collaborations with scientists, institutes and universities who will be directly involved in this effort.  We have also established laboratories and trained students and scientists in a variety of countries in monsoon Asia in the science of dendrochronology.  Existing and proposed tree-ring records will form a comprehensive network covering much of monsoon Asia for use in this extensive study of monsoon climate.  Considerable additional data is required, particularly in the tropics where such work is highly labor intensive, and where there is particular urgency in collecting this resource before valuable old-age forests are destroyed.  We will use these data to develop quantitative, high-resolution reconstructions of important monsoon indices over the past several centuries to millennia.  These reconstructions will be thoroughly investigated through various analyses in order to address the above and other questions.  The ultimate product will be a valuable resource of information on the processes that have impacted the past behavior and variability of Asian monsoon climate dynamics.

In terms of reconstructing climate variability associated with the three process regions shown in figure 1, we have enough past results from ‘Monsoon Asia’ to conclusively demonstrate the feasibility of our proposed research.  Figure 2 shows two well-calibrated and verified seasonal temperature reconstructions for Nepal (Cook et al., 2003).  Significantly, the seasonality of these temperature reconstructions precedes the summer monsoon over India, which suggests that they may be useful in understanding precursor thermal conditions that contribute to monsoon variability.  They also reveal the presence of multi-decadal timescale variability, thus demonstrating the importance of these dendroclimatic records in characterizing long-term climate behavior in ‘Monsoon Asia’.

Figure 2.  Two seasonal temperature reconstructions for Nepal centered at Kathmandu, based on a dense network of annual tree-ring chronologies. See figure 1 for the location of Nepal and its dense tree-ring network.

Figures 3 and 4 show correlations between Indian Ocean SSTs and the EOF PC#1 of tree-ring chronologies from Thailand (figure. 3) and Indonesia (figure. 4).  In both cases, there is a significant (p<0.05) association between tree growth and SSTs in areas of correlation >0.40 in figure. 3 and >0.20 in figure. 4.

Figure 3. (left) Indian Ocean SST correlations with Thailand pine tree-ring chronologies.

Figure 4. (right) Indian Ocean SST correlations with Indonesian teak tree-ring chronologies.

The seasonal associations are admittedly different (Jan-Aug for Thailand and Jul-Dec for Indonesia), reflecting in part the timing of the wet seasons in the two areas (e.g., a rainfall maximum in boreal summer for Bangkok vs. the boreal winter for Jakarta).  This difference in monsoon rainfall seasonality shows why it is necessary to develop a ‘Monsoon Asia’ network of tree-ring chronologies.  If we were to restrict our spatial coverage to, say, the Indian monsoon region, we would miss important spatial features of the Asian monsoon system.

Figure 5 shows the potential for developing a millennial-length teak tree-ring chronology in Asia (e.g., Thailand) from a combination of living trees, archeological timbers, wood from alluvial deposits, and log coffins.  There is similar potential in Mongolia, China, Bhutan, and other regions of ‘Monsoon Asia’.

Figure 5.  Thailand teak “floating” and fixed chronology timeline.  The cross-dated chronologies are based on living trees, wood from alluvial deposits, and log coffins in caves. The KNG-MAS series in the modern era is vertically offset for visual clarity only.  These results show the potential for developing millennial-length teak tree-ring chronologies in ‘Monsoon Asia’..