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Statistics and Dynamics of Extremes

precipitation extremes

Anomalies (expressed as a percent) in total annual precipitation (top), extreme wet events (middle) and dry days (bottom) in 2010-2039, 2040-2069 and 2070-2099 (columns 2-4) relative to the baseline (1970-1999) values (shown in the left column). Stippling indicates areas that experience a change that is significant at the 5% level.

Deepti leads research on the statistics and dynamics of extreme events in the mid-latitudes and the tropics.

Understanding future changes in the frequency, intensity and duration of extreme events in response to increased greenhouse gas forcing is important for informing adaptation and mitigation strategies that minimize damages to natural and human systems. We use observations, reanalysis data and a suite of climate simulations to understand how daily-scale extremes respond to natural and anthropogenic forcings. We employ advanced statistical techniques that are suitable to capture the complexity of the variables we examine. Our research thus far has primarily focused on the United States and South Asia. Following are brief descriptions of these projects

Precipitation extremes over the continental U.S. in the 21st century.

Using nested, high-resolution climate model simulations for the 21st century, we quantify transient changes in daily-scale extreme precipitation events over the U.S. for each season. Our results imply fewer but heavier precipitation events in the future, leading to more frequent wet and dry extremes in most regions of the U.S in most every season. We find substantial increases in the frequency of extreme wet events over the northwestern U.S. in autumn, winter and spring, and the eastern U.S. in spring and summer. Increases in dry day frequency occur over most parts of the U.S., with the most intense and widespread changes in summer. Though many of these changes are likely to emerge by the mid-21st century, increases in summer season dry events become prominent over a substantial part of the U.S. even within the next 20 years. As part of this study, we also examined associated changes in atmospheric circulation, moisture
convergence and daily-scale precipitation variability that could influence the occurrence of extreme precipitation.

Observed changes in the South Asian summer monsoon

South Asia is home to about 24% of the world's population and is one of the world's most disaster prone regions. The majority of the people in this region depend on agriculture for their livelihood and agriculture in this region is still largely rainfed. Consequently, the region has high vulnerability to rainfall variability. Substantial variability in the South Asian Summer Monsoon occurs on an intraseasonal timescale (30-60 day) during which rainfall fluctuates between spells of heavy (”active spells”) and low rainfall (”breaks  spells“). Such rainfall variability has severe implications on natural and human systems including agriculture and water resource availability. In this study, we assess changes in several characteristics of the summer monsoon precipitation including this intraseasonal variability. Further, we seek to understand the role of anthropogenic forcing including greenhouse gases, aerosols and land-use change on the observed changes in daily precipitation characteristics.

Publications

  • Singh, D.*, M. Tsiang, B. Rajaratnam and N.S. Diffenbaugh, Precipitation extremes over the continental United States in a transient, high-resolution, ensemble experiment of 21st century climate, Journal of Geophysical Research – Atmospheres, 118, doi:10.1002/jgrd.50543, 2013.