Climate Change Impacts on Precipitation: an Interview with David Neelin

David Neelin is a Professor in UCLA’s Atmospheric Sciences who works on climate modeling, including precipitation predictions in the face of climate change. Dr. Neelin’s work has contributed to understanding and predictability of many aspects of natural climate variation such as El Niño and its impacts on precipitation. His group often uses interdisciplinary tools adapted from mathematics or physics to understand the interplay between very local effects such as rainfall occurring at cloud-scales and global-scale changes. A recent study by Dr. Neelin’s group, in collaboration with Prof. Alex Hall’s group, quantifies California’s water future for researchers, water managers and policymakers to working on California’s water solutions.

by Sharona Sokolow

Part of an occasional series about the work of UCLA water experts.

I recently spoke with Dr. Neelin to get a better understanding for his research, its implications for Los Angeles, and how his work will help UCLA researchers reach the Grand Challenge goal of a water sustainable Los Angeles by 2020.

What prompted this study?

I’ve done a lot of work on changes under natural climate variability and factors that maintain precipitation in current climate and the climate change aspect is just a natural extension of this. It’s hard to work on natural variability without working on climate change because the climate is changing under us!

Explain how this study measures precipitation effects for the Los Angeles area:

In earlier work, our team had noted and coined a term for an effect in projections of climate change: the “rich-get-richer effect,” which explains why the areas that are already getting a lot of rainfall tend to get more, and the areas that already don’t get a lot of rainfall, get less. Essentially, the patterns of moisture transport in current climate are enhanced with increased moisture in future climate.

Regionally, this effect implies that Mexico tends to get reduced rainfall under global warming, as does Central American and the Caribbean. When you get down to the scale of a state, or small country, the predictions tend to have a lot of uncertainty. California happens to be sitting right at the node of the “rich-get-richer” pattern. Down in Mexico, there is a tendency to get drier. Oregon to the north has a high probability to get more rainfall, as if they needed it!

California sits right at the zero line — between the larger scale increasing precipitation tendency to the north and decreasing tendency to the south — which makes it a sensitive area. Remember, this pattern sets up over thousands of kilometers, so if different climate models predict the same large-scale pattern but with a very slight shift, that can make the difference between precipitation increase or decrease in southern and central California. And besides the north-south effect, you can have additional factors for the regional circulation affecting California. One factor that we examined under global warming is the jet stream, which tends to slightly increase in speed and shifts slightly eastward toward the coast as well.

This most recent set of climate model projections show central to northern California getting a slight increase in precipitation and it’s associated with this regional change where the jet stream is blowing more strongly onto the coast in our region. This tends to favor storms and moisture transport onto the California coast. When focusing your attention on regional aspects of climate change, you also have to take into account of what is happening in the larger picture, such as what the jet stream over the Pacific Ocean does to our storms.

When you’re projecting water resources for California, you have several factors going on at once. You have to take into account that changing temperatures are going to change the evapotranspiration from the land surface so even with rainfall input, your land tends to dry out faster. You have to take into account the different timing of the snowpack melt because that’s one of the major effects on storage. And you have to take into account the projection of the precipitation, which has substantial uncertainty for the locations close to this node between drying and wetting regions — like Los Angeles. There’s a tendency for water managers to say “Oh my God, that’s not much help. I’m just going to think about the snowpack because it’s going to melt earlier.” The role of the jet stream adds another factor in estimating precipitation. So the water manager doesn’t just hear there’s uncertainty in California precipitation, but can think about how that physical scenario could affect their particular part of the infrastructure in much more detail than climate scientists who are just looking at precipitation, and they can use this scenario to think through the challenges of the precipitation side.”

How will this study help researchers working on UCLA’s Grand Challenge Initiative?

For Los Angeles water sustainability, a large fraction of our water is coming from outside our immediate region, whether it’s coming from the Sierras or the Colorado River Basin. You need to have a larger scale picture of the uncertainties in the impacts of the water supply. Knowing that there’s a reasonable chance of having a trade-off where the further north you go in California, the more likely you are to have additional rainfall is good background. Knowing that the Southwest, when you take the evapotranspiration into account, is likely to have less water availability coming from the Colorado River Basin is good information. These are major factors that go into a planning process. Finally, having information about the atmospheric dynamics that influence uncertainties and about the natural variability that will continue to influence year-by-year water supply is also useful in thinking through how these changes come about and in developing strategies to adapt to climate change.