Climate variability and trends in Indian monsoon rainfall

Abstract

Disagreement over the late 20th century hiatus in the rise of global mean surface temperature varied over whether the hiatus was present in the historical runs of the CMIP5 models, whether the hiatus resulted from biases in spatial sampling, and if the hiatus had been correctly modeled using linear trends. In my first chapter, I demonstrate that recently observed trends in global-mean temperature are consistent with the CMIP5 ensemble for all 15-year intervals of observation-model divergence since 1970. My second chapter characterizes the effect of a changing weather station network on time-trends in precipitation extremes. Major studies of extreme rainfall have relied upon the gridded daily precipitation observations provided by the India Meteorological Department (IMD), which assimilate observations from a variable network of weather stations. Replicating the IMD’s interpolation method on satellite-based precipitation observations, however, indicates that temporal changes in the observing weather station network introduce a jump in the record toward more extreme rainfall after 1975. Trends evaluated across this jump are suspect, and trends evaluated subsequent to it are insignificant (p>0.1). My third chapter identifies a way to account for the trend biases identified in the second chapter. Interpolation spatially averages observations such that regions that have lower densities of stations must use station observations that are farther away. I determine regions that covary in rainfall timing and amount, and I introduce a nested regression method for assessing the regional time trends in these gridded rainfall data. Results from these methods are less susceptible to biases from changes to the density of station network than time trends from individual point locations. Finally, my fourth chapter examines the observed changes to orographic precipitation over the Western Ghats and how that may be tied to warming. In particular, I identify increases in precipitation over the Deccan Plateau and declines over coastal regions since 1979. This pattern of rainfall coincides with westerlies, which could indicate changes to the spatial pattern of rainfall due to increases in saturation vapor pressure with warming. In order to investigate this, I compare these changes to those realized in the Geophysical Fluid Dynamics Laboratory HiFLOR models.