Atmospheric Rivers in an Ever-Changing Climate

Atmospheric Rivers in An Ever-Changing Climate

Ash Bray
College of Social and Behavioral Sciences
The University of Arizona, Tucson, AZ 85721
Email: ashebray@email.arizona.edu

Abstract

Atmospheric rivers are large, intermittent circulation features along the mid-latitude regions of the globe, capable of causing horrific floods if they collide with mountainous terrain. Though they were discovered by meteorologists and deeply feared by hydrologists, they have only recently come to immediate attention of climatologists. In a letter published in Environmental Research Letters [1], Lavers and others researched atmospheric rivers affecting the British Isles in the context of climate change. They considered these potentially calamitous meteorological features in then-present and future climate model situations, putting them through hypothetical structures that could cause the rivers to strengthen. This work was a breath of fresh air that estimated extreme events in future climates with an impact-driven approach.
In their well-known paper written over a decade ago, Newell and Zhu [2], with the use of global meteorological data, recognized atmospheric structures within the lower troposphere that consisted of narrow ‘rivers’ comprised of filament bands that seemed to transport vast amounts of moisture with peak winds of more than 10 m s−1, towards the poles and across subtropical boundaries. Atmospheric rivers are mainly observed in the wintertime across the mid-latitudes of the northern hemisphere. They’ve also been reported within the southern hemisphere when they pulverize the Andes of South America [3]. Meteorologists partner atmospheric rivers with winter storms’ warm sector precipitation. Along the North American Pacific Coast, these rivers are referred to as a ‘pineapple express’ – due to their originating from the atmosphere around Hawaii. It’s important to note that not all atmospheric rivers that are identified over the oceans near landmasses cause major flooding. The directional flow and topography of a region play a critical role when gauging the potential danger.

Hydrologists have found that the most severe floods occur when these rivers hit coastlines perpendicular to mountain ranges, raining out above the snowline [4]. Extreme precipitation events aforementioned, teamed up with the lifted snowline, tend to cause detrimental flooding along major riverbeds (e.g. Oregon’s Willamette River). Warner and others [5] estimated that the top 50 observed floods in the US Pacific Northwest were caused by atmospheric rivers, including the severe floods of 2017. However, atmospheric rivers are not limited to just the Eastern Pacific coastal regions. It has been shown that along the North Atlantic in Western Europe atmospheric rivers are responsible for eight of the top ten severe precipitation events, seen as far inland as Germany and Poland [6]. Despite their potential, atmospheric rivers have just recently been studied within Europe and South America. Meanwhile, off the Pacific Coast of North America, hydrologists have been studying this phenomenon for years [4], going as far as developing meteorological forecasting routines for these events.

In order to properly understand atmospheric rivers, they need to be correctly identified. Atmospheric rivers are restricted to only small sections of any given longitude. Subsequently, rather than separating atmospheric moisture transports using the Reynolds eddy representation, in Newell and Zhu’s formulation [2] of atmospheric rivers, the filament structure is estimated by deriving maximum moisture fluxes from broad flux. Due to their length and intensity, atmospheric rivers play a critical role in the total southerly transport of latent heat as well as the general circulation. Actually, for southerly transports and mid-latitudes, atmospheric rivers critically impact the total flux of moisture, accounting for nearly all of the transport across the subtropical boundary. Atmospheric rivers develop in the warm regions of cyclonic systems in storm track regions. These rivers are necessary for the eddy southerly moisture transport, controlled by the interactions between stationary subtropical anti-cyclones, as well as transient eastward-propagating baroclinic waves [7].

Generally speaking, the hydrological and meteorological impacts of atmospheric rivers are accepted. However, not much effort has been enforced on the understanding of atmospheric rivers in the terms of climate change. This is shocking seeing as after a cataclysmic flooding event, numerous questions are raised as to whether or not said event was influenced by climate change. However, extreme precipitation events are infamously challenging to predict and often difficult to estimate with climate models. Though there are published climate studies that correlate climate change to a global increase in extreme precipitation events, as well as the poleward shift of storm tracks [8], there is still a lot of uncertainty with these modeling results, which are often not applicable to these events. Aforementioned precipitation events influenced by topography are particularly uncertain in climate models due to various model resolutions.

Rather, studying future behaviors of atmospheric rivers with climatological model outputs provides attention to critical details and helps prevent various model deficiencies. Filament structures of maximum vertically combined moisture fluxes can be promptly calculated from daily outputs of climatological models with the formulation of Newell and Zhu [2]. This process gives us an interesting way of studying one specifically dangerous kind of extreme precipitation event along the coastline of the region of interest. For example, Dettinger [9] pored over atmospheric rivers in California. He expects an increase in severe and more frequent flooding events, as well as a prolonged flood season due to the intensification of atmospheric rivers in the 21^st century. This stormy climate isn’t unusual. In southern California, Paleoclimatologists classified several stormy periods within the last 10,000 years that were influenced by continual episodes of intensified atmospheric rivers. In the Lavers [1] study, the frequency of atmospheric rivers off the coast of the British Isles are expected to double by the end of the 21^st century due to climate change. In this study, they state that the higher moisture contents in a warmer world, rather than the changes in winds, will to lead to the intensification of atmospheric rivers. Nonetheless, the central unanswered question remains: Will the foreseen enhanced atmospheric moisture shift towards land in the 21^st century [10] come from the increased broad fluxes or from the increase in devastating extreme events such as atmospheric rivers?