Quantifying the Relationship Between Atmospheric River Origin Conditions and Landfall Temperature

American Geophysical Union (AGU) | October 11th, 2022

The temperature of landfalling atmospheric rivers (ARs) has direct implications for regional water resources. Compared to cool ARs, warm ARs can result in more surface ru

The temperature of landfalling atmospheric rivers (ARs) has direct implications for regional water resources. Compared to cool ARs, warm ARs can result in more surface runoff and flooding, less water availability via low snow accumulations and enhanced snowmelt, and greater challenges for storm forecasting and reservoir operations. Based on case studies, ARs with subtropical origin locations and warm origin conditions are assumed to be associated with warmer landfall temperatures—though, to date, this has not yet been demonstrated systematically. We analyze North Pacific ARs that made landfall along the West Coast of North America from 1980 to 2017. We find that ARs originating over the subtropical Pacific Ocean near Hawaii (“Pineapple Express”-type ARs) are indeed 1.5°C warmer and 2 kg/m2 moister in winter than ARs originating elsewhere. We extend this analysis for the full study population of ARs by quantifying AR origin conditions, including the spatial distribution of origin temperature, moisture content, and integrated vapor transport (IVT). We use fixed effects multivariate regression to quantify the relative influence of the origin conditions on landfall temperature. Our regression models show that AR origins with lower latitudes, longer AR lifetimes, and stronger IVT are associated with warmer landfall temperatures and that ARs that start warm generally stay warm. We also find that the phase of the El Niño/Southern Oscillation does not exhibit a consistent relationship with AR landfall temperature. Overall, our results partially affirm—yet also complicate—common assumptions about the role of different origin conditions in landfalling AR temperature.

Quantifying the role of atmospheric rivers in the interior western United States

Royal Meteorological Society (RMS) | July 13th, 2012

Atmospheric rivers (ARs) have increasingly been recognized for their contribution to high-impact weather and climate variability. A recent investigation based on observat

Atmospheric rivers (ARs) have increasingly been recognized for their contribution to high-impact weather and climate variability. A recent investigation based on observations located primarily in lowland valleys and basins of the western United States suggests that 10 – 50% of the total cool season (November to April) precipitation between water years 1998 and 2008 occurred on the day of and day following AR landfall (hereafter the AR fraction), as identified using  Special Sensor Microwave Imager data. However, these results are based only on ARs crossing the North American west coast between 32.5◦N and 52.5◦N, which excludes those crossing the west coast of the Baja Peninsula. Here, we identify ARs in the ERA-Interim reanalysis and examine the AR fraction at high-elevation observational sites and in the NOAA/CPC Unified Daily Precipitation Analysis. At high-elevation snowpack telemetry sites, we find good agreement with the AR fraction obtained previously for valley and basin locations. We also show that including ARs crossing the west coast of the Baja Peninsula (as far south as 24◦N) substantially increases the AR fraction over the southwestern United States

Recent California Water Year Precipitation Deficits: A 440-Year Perspective

American Meteorological Society (AMS) | June 15th, 2015

An analysis of the October 2013–September 2014 precipitation in the western United States and in par- ticular over the California–Nevada region suggests this

An analysis of the October 2013–September 2014 precipitation in the western United States and in par- ticular over the California–Nevada region suggests this anomalously dry season, while extreme, is not un- precedented in comparison with the approximately 120-yr-long instrumental record of water year (WY; October–September) totals and in comparison with a 407-yr WY precipitation reconstruction dating back to 1571. Over this longer period, nine other years are known or estimated to have been nearly as dry or drier than WY 2014. The 3-yr deficit for WYs 2012–14, which in California exceeded the annual mean precipitation, is more extreme but also not unprecedented, occurring three other times over the past approximate 440 years in the reconstruction. WY precipitation has also been deficient on average for the past 14 years, and such a run of predominantly dry WYs is also a rare occurrence in the authors’ merged reconstructed plus instrumental period record.

Recent warming of landfalling atmospheric rivers along the west coast of the United States

American Geophysical Union (AGU) | May 30th, 2019

Abstract: Atmospheric rivers (ARs) often generate extreme precipitation, with AR temperature strongly influencing hydrologic impacts by altering the timing and magni

Abstract: Atmospheric rivers (ARs) often generate extreme precipitation, with AR temperature strongly influencing hydrologic impacts by altering the timing and magnitude of runoff. Long‐term changes in AR temperatures therefore have important implications for regional hydroclimate—especially in locations where a shift to more rain‐dominated AR precipitation could affect flood risk and/or water storage in snowpack. In this study, we provide the first climatology of AR temperature across five US West Coast sub‐regions. We then assess trends in landfalling AR temperatures for each sub‐region from 1980 to 2016 using three reanalysis products. We find AR warming at seasonal and monthly scales. Cool‐season warming ranges from 0.69°C to 1.65°C over the study period. We detect monthly‐scale warming of >2°C, with the most widespread warming occurring in November and March. To understand the causes of AR warming, we quantify the density of AR tracks from genesis to landfall, and analyze along‐track AR temperature for each month and landfall region. We investigate three possible influences on AR temperature trends at landfall: along‐track temperatures prior to landfall, background temperatures over the landfall region, and AR temperature over the coastal ocean adjacent to the region of landfall. Generally, AR temperatures at landfall more closely match coastal and background temperature trends than along‐track AR temperature trends. The seasonal asymmetry of the AR warming and the heterogeneity of influences have important implications for regional water storage and flood risk—demonstrating that changes in AR characteristics are complex, and may not be directly inferred from changes in the background climate. Plain Language Summary: Atmospheric river (AR) storms are well known for their ability to accumulate snowpack, provide drought relief, and generate extreme precipitation and flooding along the West Coast of the United States. AR temperature is an important variable for determining the water resource impacts of a given event, such as the ratio of rain to snow delivered by an individual storm. As a result, changes in AR temperature have implications for both water storage and flood risk. We find substantial warming in ARs at both the seasonal and monthly scales, as well as seasonal and regional variations in the amount of warming along the US West Coast. To understand the warming of ARs at the landfall regions, we compare these monthly temperature trends with trends in temperature along the AR tracks, background temperature over the landfall region, and temperature over the coastal ocean adjacent to the landfall region. The most robust warming occurs in November and March, which has important implications for increased regional flood risk and decreased water storage, and motivates further investigation in other AR‐prone regions around the globe.

Reduced Moisture Transport Linked to Drought Propagation Across North America

American Geophysical Union (AGU) | July 24th, 2019

Droughts reduce the availability of water, which affects communities and ecosystems worldwide. Recent studies have shown that droughts may travel up to thousands of kilom

Droughts reduce the availability of water, which affects communities and ecosystems worldwide. Recent studies have shown that droughts may travel up to thousands of kilometers across continents, but it is still unclear what are the reasons behind these observed drought displacements. While there may be several reasons, we study one particular way in which droughts may be able to travel across continents. Our idea is that a drought in one area may decrease evaporation locally, which will reduce water vapor in the air. As the wind blows, it will transport drier air, which may lead to less precipitation downwind. We find links between droughts that take place in different regions across North America, suggesting that droughts may travel in this way, for example, from the U.S. Southwest to the U.S. Midwest. We also show that the lower evaporation that took place over the western United States, likely due to droughts in the region in 2012, increased the severity of the drought in the U.S. Midwest that year. Our study highlights the importance of sustainable land use management and the need for coordination between communities in upwind and downwind regions to reduce drought risks. It may also help improve future drought predictions.

Relative impacts of mitigation, temperature, and precipitation on 21st-century megadrought risk in the American Southwest

American Association for the Advancement of Science (AAAS) | October 5th, 2016

Megadroughts are comparable in severity to the worst droughts of the 20th century but are of much longer duration. A megadrought in the American Southwest would

Megadroughts are comparable in severity to the worst droughts of the 20th century but are of much longer duration. A megadrought in the American Southwest would impose unprecedented stress on the limited water resources of the area, making it critical to evaluate future risks not only under different climate change mitigation scenarios but also for different aspects of regional hydroclimate. We find that changes in the mean hydroclimate state, rather than its variability, determine megadrought risk in the American Southwest. Estimates of megadrought probabilities based on precipitation alone tend to underestimate risk. Furthermore, business-as-usual emissions of greenhouse gases will drive regional warming and drying, regardless of large precipitation uncertainties. We find that regional temperature increases alone push megadrought risk above 70, 90, or 99% by the end of the century, even if precipitation increases moderately, does not change, or decreases, respectively. Although each possibility is supported by some climate model simulations, the latter is the most common outcome for the American Southwest in Coupled Model Intercomparison 5 generation models. An aggressive reduction in global greenhouse gas emissions cuts megadrought risks nearly in half.

Remote linkages to anomalous winter atmospheric ridging over the Northeastern Pacific

American Geophysical Union (AGU) | October 24th, 2017

Severe drought in California between 2013 and 2016 has previously been linked to the persistence of atmospheric high atmospheric pressure over the Pacific Ocean (nickname

Severe drought in California between 2013 and 2016 has previously been linked to the persistence of atmospheric high atmospheric pressure over the Pacific Ocean (nicknamed the “Ridiculously Resilient Ridge”), which prevented winter storms from reaching the coast over several consecutive years. There has been much discussion regarding why this high‐pressure system has been so persistent—and some scientists have previously suggested that unusual ocean temperature patterns in the Pacific, reductions in Arctic sea ice, or random weather variations may have played a role. In this study, we investigate relationships between atmospheric high pressure over the North Pacific and possible links to ocean conditions using both real‐world observations and climate model simulations. Our results suggest that persistent atmospheric high pressure similar to that which occurred during California's 2013–2016 drought can be partially linked to unusual Pacific Ocean temperatures and that knowledge of such ocean conditions may offer foresight regarding the potential for future droughts in this region.

Responses and impacts of atmospheric rivers to climate change

Nature Portfolio (Springer Nature) | March 9th, 2020

Atmospheric rivers (ARs) are characterized by intense moisture transport, which, on landfall, produce precipitation which can be both beneficial and destructive. ARs in C

Atmospheric rivers (ARs) are characterized by intense moisture transport, which, on landfall, produce precipitation which can be both beneficial and destructive. ARs in California, for example, are known to have ended drought conditions but also to have caused substantial socio-economic damage from landslides and flooding linked to extreme precipitation. Understanding how AR characteristics will respond to a warming climate is, therefore, vital to the resilience of communities affected by them, such as the western USA, Europe, East Asia and South Africa. In this Review, we use a theoretical framework to synthesize understanding of the dynamic and thermodynamic responses of ARs to anthropogenic warming and connect them to observed and projected changes and impacts revealed by observations and complex models. Evidence suggests that increased atmospheric moisture (governed by Clausius–Clapeyron scaling) will enhance the intensity of AR-related precipitation — and related hydrological extremes — but with changes that are ultimately linked to topographic barriers. However, due to their dependency on both weather and climate-scale processes, which themselves are often poorly constrained, projections are uncertain. To build confidence and improve resilience, future work must focus efforts on characterizing the multiscale development of ARs and in obtaining observations from understudied regions, including the West Pacific, South Pacific and South Atlantic.

Review of Science Used to Estimate Water Supply Reliability Involving the Delta

Delta Stewardship Council (Delta Council) | April 3rd, 2018

Review of Water Supply Reliability Estimation Related to the Sacramento-San Joaquin Delta

Delta Stewardship Council (Delta Council) | June 16th, 2022