Atmospheric rivers and their relationship to active winter storm seasons

Bermuda-based ILS fund manager Aeolus provides insight on the science of “atmospheric river” episodes and their relationship to active winter storm seasons.

Atmospheric rivers (ARs) are relatively long bands of the atmosphere aloft that span several hundred miles wide. ARs transport water vapour from the tropics to the mid-latitudes. These dynamic columns of vapour move with upper-level winds and carry with them a vigorous combination of water in vapour and condensed (water) form equivalent to multiples of the average flow at the mouth of the Mississippi River. When ARs “make landfall”, they typically release moisture in the form of heavy rain, and where the air is sufficiently cold, heavy snowfall.

The term was originally coined in the early 1990s by researchers Reginald Newell and Yong Zhu at the Massachusetts Institute of Technology to reflect the narrowness of the focused moisture plumes associated with the phenomenon. The graphic below provides a schematic view of a landfalling AR event. Note, the US West Coast is particularly susceptible to AR moisture flows due to the substantial elevation rise occurring along the immediate coastline. Along much of the California coast, including the highly exposed Los Angeles and San Francisco metroplexes, coastal elevation rise and inland ranges like the Sierras provide immediate uplift to an incoming AR band of moisture in vapour form, which in turn causes rising motion, then condensation of water vapour to rain droplets, eventually leading to more clouds and heavier precipitation.

ARs affecting California are not unusual. In fact, the National Oceanic and Atmospheric Administration points out that up to half of the US West Coast’s annual precipitation has its source in just a few AR events. Hence, ARs are part of Western US climatology and they routinely make a significant contribution to California’s annual water supply. Such events can be helpful in drought years and critical to maintaining water resources. On the other hand, a string of intense AR events can overwhelm water management infrastructure and lead to flooding and significant damage and loss of life.

Although ARs come in many shapes and sizes, those containing voluminous water vapour combined with strong winds can lead to extreme rain accumulations and subsequent flooding. To exacerbate the problem, some ARs stall over highly exposed urban watersheds, vulnerable to inland riverine and flash flooding. These heavy rain events can induce mudslides and rockslides and can cause catastrophic loss of life and property.

A well-known example, known as the “Pineapple Express”, is capable of bringing moisture from the Pacific tropical ocean waters near Hawaii to the US West Coast. AR events tend to be more active in the winter season when upper-level winds, known as the “jet stream”, are most intense since they are fuelled by the contrast between temperatures at the poles and the equator.

AR events can also contribute to beneficial increases in snowpack. Unlike rainfall, excessive snowpack rarely translates to major damage, but provides water storage for California’s dry season. For instance, a series of AR events in 2010 fuelled a string of strong winter storms that battered the entire US West Coast from 10-22 December, producing 20+ inches of rain in certain areas. These AR events led to a snowpack in the Sierras corresponding to 75 percent of its normal annual total.

Location and seasonality

The figure to the right (Dettinger 2018) shows the relative frequency of five-year and 20-year return period AR events by latitude and month.

Note that the more extreme AR episodes occur between 35º and 45º latitude, which roughly corresponds to the span of the central to northern California coast to the Pacific Northwest. ARs are most common between the months of October and January.

The ‘hot spot’ for season and location is centred along the northern California coastline in December, which closely corresponds to the activity witnessed in 2022-23. This tells us that the events of this winter season, whilst quite active, are in line with expectations for where and when we would expect these episodes to have their greatest impact.

Implications for Aeolus

As this article lays out, AR episodes are a natural part of the global climate cycle. Historically, the moisture that most ARs provide has been an important and beneficial component of water resource management for the Western US. Even in cases when they are more intense and persistent, AR events have not typically been major insurance events. That said, some aspects of AR risk, such as their damaging winds, are explicitly included in the major catastrophe models, while AR flood potential is included implicitly as a part of the modelled loss calibration process. Therefore, Aeolus Research continues to track the developing science around AR risk, in particular as it relates to the evolving science around climate change. The recent AR episodes in California will provide more data and ground truth to advance our scientific understanding and, as with all key perils, Aeolus is careful to leverage historical experience when setting price.

Dr Peter Dailey serves as head of research at Aeolus. He also acts as a liaison with the research and scientific communities, and serves as a company spokesperson on important topics such as climate change. Dr Dailey holds a Bachelor of Arts in Economics and a Bachelor of Applied Science from the University of Pennsylvania. Dr Dailey also holds master’s and PhD degrees from the University of California, Los Angeles.