Changes in global monsoon precipitation over the past 56 years

There is a significant new research contribution concerning an assessment of recent trends in the global monsoon systems. Monsoons are a critical component of the climate system in many parts of the world which includes their role in regional and local water resources.

The new paper by Bin Wang and Qinghua Ding is published in Geophysical Research Letters and is entitled,

“Changes in global monsoon precipitation over the past 56 years”. (Geophys. Res. Lett., 33, L06711, doi:10.1029/2005GL025347).

The abstract of the paper reads,

“Changes in the global monsoon rainfall over land were examined using four sets of rain-gauge precipitation data sets compiled for the period of 1948–2003 by climate diagnostic groups around the world. Here, we define a global monsoon rain domain according to annual precipitation range, using simple objective criteria; then, we propose metrics for quantifying the intensity of the global monsoon precipitation. The results suggest an overall weakening of the global land monsoon precipitation in the last 56 years, primarily due to weakening of the summer monsoon rainfall in the Northern Hemisphere. However, since 1980, the global land monsoon rainfall has seen no significant trend, which contrasts with the rapid intensification of global warming during the same period. Meanwhile the oceanic monsoon precipitation shows an increasing trend after 1980. The results provide a rigorous test for climate models that will be used in future climate-change assessment.”

This new paper is consistent with our earlier study,

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

where we concluded, as summarized in the abstract, that

“We examined changes in several independent intensity indices of four major tropical monsoonal circulations for the period 1950-1998. These intensity indices included observed land surface precipitation and observed ocean surface pressure in the monsoon regions as well as upper level divergence calculated at several standard levels from the NCAR/NCEP reanalysis. These values were averaged seasonally over appropriate regions of southeastern Asian, western Africa, eastern Africa and the Australia/Maritime continent and adjacent ocean areas. As a consistency check we also examined two secondary indices: mean sea level pressure trends and low level convergence both from the NCEP reanalysis.

We find that in each of the four regions examined, a consistent picture
emerges indicating significantly diminished monsoonal circulations over the period of record, evidence of diminished spatial maxima in the global hydrological cycle since 1950. Trends since 1979, the period of strongest reported surface warming, do not indicate any change in monsoon circulations. When strong ENSO years are removed from each of the time series the trends still show a general, significant reduction of monsoon intensity indicating that ENSO variability is not the direct cause for the observed weakening.

Most previously reported model simulations of the effects of rising CO2 show an increase in monsoonal activity with rising global surface temperature. We find no support in these data for an increasing hydrological cycle or increasing extremes as hypothesized by greenhouse warming scenarios.”

The Wang and Ding paper concludes with,

“The tropical atmospheric moisture content, latent heating, and overall hydrological cycle have been hypothesized to enhance with increasing tropospheric temperature [e.g., Intergovernmental Panel on Climate Change, 2001]. The numerical simulations with increasing greenhouse gas content generally show increased intensity of the Asian summer monsoonal circulations [e.g., Meehl and Washington, 1993; Hulme et al., 1998]. The inclusion of aerosols, however, seems to suppress the simulated increasing trends in southeast Asia seen in many general circulation model simulations [e.g., Mitchell and Johns, 1997], but not in all [e.g., Roeckner et al., 1999]. The present results provide a rigorous test for climate models that will be used in future climate change assessments………..

It is conceivable that the trend observed over the last 56 years reflects a transition from a strong phase to a weak phase in the multi-decadal variability. Previous studies have suggested that a rapid change occurred in atmospheric circulation and ENSO in the mid-1970s [e.g., Trenberth and Hurrell, 1994]. The results shown here (Figures 2a and 3b) suggest that changes in the NH monsoon strength reflects this “regime shift,â€? which may be a portion of the Interdecadal Pacific Oscillation, a period of 50 to 70 years [Folland et al., 1999], or caused by changes in tropical/sub-tropical land cover and high latitude snow cover [Meehl, 1994; Chase et al., 1996]. There is much we have yet to learn about the causes of observed trends in the global monsoon.”

There last sentence is very revealing with respect to the current understanding of the role of natural- and human-climate forcings and feedbacks.

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