Cooperative Institute for Research in Environmental Sciences

Tropical/subtropical Monsoon Studies

We are exploring the connection between the land and ocean surfaces and tropical monsoon systems. Chase et al. (2003) showed that monsoon circulations were diminishing over time in all major monsoon regions. This was a result consistent with model simulations of historical changes in landcover globally (Chase et al., 1996). More recently we have examined the East Asian summer monsoon (EASM) system and found that it is physically two distinct systems with a northern and southern component driven by distinct forcing mechanisms. For example, the northern East Asian summer monsoon (NEASM) is, for the years of highest SST anomalies in the tropical eastern Pacific (TEP), affected by the Pacific-East Asian (PEA) teleconnection pattern, which consists of a wave creating an anomalously strong western North Pacific (WNP) anticyclonic circulation and anomalously strong cyclonic circulation in the NEASM region, creating more monsoon rainfall. Increased southern East Asian summer monsoon (SEASM) precipitation is related to warm WNP SST anomalies which create a weaker WNP anticyclone. The proximity of the SEASM to the WNP anticyclone causes increased monsoon rainfall when the anticyclone is weaker or covers a smaller area. Physical linkages proposed between SST anomalies and the sub-EASMs are shown in Figure below.

We have also shown a relationship between Indian summer monsoon precipitation and spring season vegetation cover which is increasing due to massive irrigation over the Indian subcontinent and adjacent areas (Lee et al., In review). Future work will be to examine the effect of spring land cover on the NEASM and SEASM.

References

Lee, E., T. N. Chase, B. Rajagopalan (2007), Seasonal forecasting of East Asian summer monsoon based on oceanic heat sources, International Journal of Climatology. (In press) [PDF]

Chase T. N., J. A. Knaff, R. A. Pielke, E. Kalnay (2003), Changes in global monsoon circulations since 1950, Natural Hazards 29, 229-254.

Chase, T. N., R. A. Pielke, T. G. F. Kittel, R. Nemani, S. R. Running (1996), The sensitivity of a general circulation model to large scale changes in leaf area index, Journal Geophysical Research 101,7,393-7,408.

Lee, E., T. N. Chase, B. Rajagopalan, R. G. Barry, T. W. Biggs, and P. J. Lawrence, Effects of irrigation and vegetation activity on early Indian summer monsoon variability, Geophysical Research Letter. (In review)