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a climate scientist at the National Center for Atmospheric Research in
Boulder, Colorado. My main interest is in interannual to centennial
climate variability, how it imprints on regional weather and climate,
and how it influences our ability to detect and attribute changes in
climate. I focus mainly on hydroclimate dynamics, ranging from
large-scale atmospheric circulation and precipitation patterns to
ocean-land-atmosphere interactions via moisture and energy fluxes.|
Recently, I have been framing an increasing number of my papers around the question “how do hydroclimate dynamics affect perceivable climate impacts, such as heat records, drought frequency, or water resource management?”
Improving streamflow forecasting in light of climate change
Seasonal streamflow predictions provide a critical management tool for water managers in the American Southwest. In recent decades, persistent prediction errors for spring and summer runoff volumes have been observed in a number of watersheds in the American Southwest. While mostly driven by decadal precipitation trends, these errors also relate to the influence of increasing temperature on streamflow in these basins.
Here we show that incorporating seasonal temperature forecasts from operational global climate prediction models into streamflow forecasting models adds prediction skill for watersheds in the headwaters of the Colorado and Rio Grande River basins. Current dynamical seasonal temperature forecasts now show sufficient skill to reduce streamflow forecast errors in snowmelt-driven regions. Such predictions can increase the resilience of streamflow forecasting and water management systems in the face of continuing warming as well as decadal-scale temperature variability, and thus help to mitigate the impacts of climate non-stationarity on streamflow predictability.
|Projected drought risk in 1.5°C and 2°C warmer climates
The large socio-economic costs of droughts make them a crucial target for impact assessments of climate change scenarios. Using multiple drought metrics and a set of simulations with a climate model targeting 1.5°C and 2°C above preindustrial global-mean temperatures, we investigate changes in aridity and the risk of consecutive drought years.
If warming is limited to 2°C, these simulations suggest little change in drought risk for the US Southwest and Central Plains compared to present day. In the Mediterranean and Central Europe, however, drought risk increases significantly for both 1.5°C and 2°C warming targets, and the additional 0.5°C of the 2°C climate leads to significantly higher drought risk.
Our study suggests that limiting anthropogenic warming to 1.5°C rather than 2°C, as aspired to by the Paris Climate Agreement, may have benefits for future drought risk but that such benefits may be regional and in some cases highly uncertain.
|On the role of atmospheric circulation variability in estimates of "time of emergence"|
Time of emergence of anthropogenic climate change is a crucial metric in risk assessments surrounding future climate predictions. However, internal climate variability impairs our ability to make accurate statements about when climate change emerges from a background reference state. None of the existing efforts to explore uncertainties in time of emergence has explicitly explored the role of internal atmospheric circulation variability. Here we use a dynamical adjustment method based on constructed circulation analogues to provide new estimates of time of emergence of anthropogenic warming over North America and Europe.
influence of temperature on runoff ratio in the Upper Rio Grande|
Recent decades have seen strong trends in hydroclimate over the American Southwest, with major river basins such as the Rio Grande exhibiting intermittent drought and declining runoff. The extent to which these observed trends are exceptional has implications for current water management and seasonal streamflow forecasting practices.
Using observations, paleoclimate reconstructions, and model simulations, we show that runoff ratio - a critical metric expressing how much water from precipitation ends up in the river - varies primarily in proportion to precipitation, but that temperature excerpts a secondary influence: in years of low precipitation, very low runoff ratios are made 2.5-3 times more likely by high temperatures.
This temperature sensitivity appears to have strengthened in recent decades, implying future water management vulnerability should recent warming trends in the region continue.