Trends in Global Monsoon Circulations: Evidence for a Diminished Hydrological Cycle?

We examined changes in several intensity indices of four major tropical monsoonal circulations in the monthly averaged NCEP reanalysis data over the period from 1950 to 1998. The intensity indices discussed here are low level convergence and upper level divergence at several standard levels averaged seasonally over appropriate regions of southeastern Asian, western Africa, eastern Africa and Australia and adjacent ocean areas. As a consistency check we also examined a third index: mean sea level pressure trends averaged over each monsoon region. We find that in each of the four regions examined, the three indices are consistent in indicating significantly diminished monsoonal circulations over the period of record, evidence of a diminished global hydrological cycle since 1950. Trends since 1978, the period of strongest surface warming, are insignificant and uncorrelated with the surface warming.

Monsoon circulations are expected to be sensitive to a variety of environmental trends such as changes in ENSO regime, globally warming temperatures, SST variability and landcover changes. An examination of reported general circulation model simulations of the effects of rising CO2 most often indicate an increase in monsoonal activity with rising global surface temperature. When the effects of aerosols are included the simulated southeastern Asian summer monsoon is often reduced in intensity, however. In contrast, we find a strong general relation between weakened monsoonal intensity and rising global surface temperature for the period 1950-1998 in the observational data. When strong ENSO years are removed from the time series the trends still show a significant reduction of monsoon intensity indicating that ENSO variability is not the primary cause for the observed weakening.