Impact of atmospheric rivers on coastal oceanographic conditions off central-southern Chile

Atmospheric rivers (AR) are transient, narrow, and elongated channels in the atmosphere through which poleward transport of water vapor takes place from the tropics. In central-southern Chile, ARs contribute about 50% of the annual precipitation. ARs also produce extreme rainfall events, which may occasionally result in compound events. The water vapor associated with an AR originates from evaporation from the ocean, local mass convergence, and direct poleward moisture transport from the tropics. One way to understand the sources of moisture along an AR is the analysis of column integrated water vapor (IWV) budget, as suggested in previous studies. In this study, we have evaluated moisture and heat budgets for 50 land-falling zonal AR between 1980 and 2023 that caused heavy to extreme rainfall in the Southern Andes. The ARs are identified based on a global AR detection algorithm included in the ARTMIP (Guan and Waliser, 2018). The budget terms for all the ARs are calculated using the ERA5 reanalysis datasets along a backward trajectory obtained from the HYSPLIT trajectory model at 6-hour intervals. The budget analysis suggests that while mass convergence dominates the vertically integrated water vapor transport (IVT), the advection of moisture is significantly enhanced near the coast and the landfalling region. It is shown that the enhanced IWV bands from the tropics and subtropics can merge along the AR channel to enhance moisture convergence. The convergence of the tropical IWV bands occurs in a region between a stationary subtropical anticyclone and a midlatitude trough which migrate towards the continent. This region of convergence further acts as a source of moisture and heat that ends up being transported to the subtropical Andes.  Over the land-falling regions, the IVT convergence and precipitation rate remain tightly associated (R2 =0.79), suggesting stronger IVT convergence (intense AR) may produce heavy rainfall events in Chile. It is also shown that synergistic action between moisture and heat enhances IVT convergence along an AR. An enhanced IVT convergence along AR channels produces intense AR. These intense AR can produce extreme rainfall events when they encounter coastal mountains and the Andes through the orographic precipitation enhancement mechanisms.

We apply the Ralph et al. scaling method to a reanalysis dataset to examine the climatology and variability of landfalling atmospheric rivers (ARs) along the western North American coastline during 1980–2019. The local perspective ranks AR intensity on a scale from 1 (weak) to 5 (strong) at each grid point along the coastline. The object-based perspective analyzes the characteristics of spatially independent and temporally coherent AR objects making landfall. The local perspective shows that the annual AR frequency of weak and strong ARs along the coast is highest in Oregon and Washington and lowest in Southern California. Strong ARs occur less frequently than weak ARs and have a more pronounced seasonal cycle. If those ARs with integrated water vapor transport (IVT) weaker than 250 kg m−1 s−1 are included, there is an enhanced seasonal cycle of AR frequency in Southern California and a seasonal cycle of AR intensity but not AR frequency in Alaska. The object-based analysis additionally indicates that strong ARs at lower latitudes are associated with stronger wind than weak ARs but similar moisture, whereas strong ARs at higher latitudes are associated with greater moisture than weak ARs but similar wind. For strong ARs, IVT at the core is largest for ARs in Oregon and Washington and smaller poleward and equatorward. Both IVT in the AR core and cumulative IVT along the coastline usually decrease after the first day of landfall for weak ARs but increase from the first to second day for strong ARs.

A southerly wind event and precipitation in Ny Ålesund, Arctic

Atmospheric rivers (ARs) are narrow filaments of high water vapor content that have considerable influence on the western United States (US) hydroclimate. ARs provide significant amounts of annual precipitation and snowfall and affect mountain snowpack via snow water equivalent (SWE) accumulation and ablation. With ARs projected to become increasingly key players in western US hydrology, water resource managers will rely progressively more on AR seasonal forecasts to infer flood/drought risks and make informed decisions about water supply allocation. However, precisely how well current seasonal climate prediction systems capture ARs and their associated hydrologic variables is still an open question. Here, we evaluate the ability of high (HR) and low resolution (LR) CCSM4 and CESM1 seasonal global retrospective forecasts to characterize precipitation, snowfall, and SWE changes associated with western US landfalling ARs. HR forecasts more accurately represent hydrologic variables than LR forecasts, however, CCSM4-HR underestimates AR-related snowfall, causing enhanced AR-related SWE ablation. Further investigation reveals amplified onshore positive temperature advection by south-southwesterly biased AR winds causes ARs in CCSM4-HR to be embedded within thicker air columns, yielding increased freezing level heights, reduced snowfall, and increased SWE loss. Results suggest both HR and LR global seasonal forecast models are capable of characterizing AR distribution and frequency, but HR models are needed for proper precipitation, snowfall, and SWE representation. Furthermore, models used to assess AR-related hydrological processes must contain accurate wind fields, as even minor biases can have a profound effect on a model’s ability to simulate AR precipitation and SWE accumulation/ablation rates.

An atmospheric river (AR) is a strong filamentary water vapor transport that plays a critical role in regional hydroclimate systems. While climate conditions can affect wildfire activities, the process by which ARs are associated with wildfire patterns remains unclear. Here, we characterize ARs in 2016 and 2020, and associate them with fire spread and burned areas along with other climate conditions in the western U.S. We found the record-high wildfire activity in 2020 was associated with hotter, drier, and windier conditions, with its peak shifted from July to August, unlike the climatological fire seasonality in the western U.S. It was also linked to satellite-observed low soil moisture during pre- and on fire season but high vegetation greenness, a proxy of fuel load, during the pre-fire season. ARs were more frequent but weaker in the summer, while ARs were less frequent and short-lived in the fall of 2020 than those of 2016. The year 2016 experienced a ‘coupled’ precipitation-wind pattern (i.e. higher wind accompanying high precipitation). In contrast, precipitation was much lower in 2020 than in 2016, showing a ‘decoupled’ precipitation-wind pattern, particularly in the spring and fall. Under ARs, the contrasting precipitation-wind patterns in 2020 (dry-windy) and 2016 (wet-windy) were more evident. For example, the surface wind (precipitation) in the AR cases was higher by 9% (34%) than in the non-AR cases in 2020 (both years) (p < 0.01) over land. The daily fire activity records demonstrate that long-lived, successive, and coastal ocean originated (centered) ARs with high precipitation help suppress fire activity (e.g. September-November 2016), while short-lived or no ARs with strong wind and little precipitation rather yield fire activity (e.g. August and September 2020). This result highlights how ARs can be associated with wildfire activity patterns during the pre-fire and fire seasons in the western U.S.

&lt;p&gt;Atmospheric rivers (ARs) are long and narrow river-like features in the lower troposphere that carry most of the atmospheric water vapor fluxes from the tropics to the midlatitudes. Because of the large amount of moisture transported by these storms, they can cause heavy rainfall and major flooding events, such as the Midwest floods of 1993 and 2008. The overarching theme of this thesis is to understand the impacts of ARs on extreme precipitation and floods over the central United States.&lt;/p&gt; &lt;p&gt;First, to improve the understanding of the mechanisms leading to the development of these storms, three ARs that happened during the summer of 2013 are studied in detail. The work provided insight into the synoptic conditions associated with these storms. Moreover, I found that the source of moisture for ARs over the central United States can be located both in the tropics and subtropics, and evaporation over land can also add water vapor along the AR trajectory.&lt;/p&gt; &lt;p&gt;To understand the characteristics of precipitation during these storms, I focused on a 12-year period and used different high spatial and temporal resolution remote sensing-based precipitation products. These analyses showed that most of the AR-related precipitation is located in a narrow region (approximately 150km) within the area where the strongest moisture transport occurs.&lt;/p&gt; &lt;p&gt;The analysis of multiple long-term atmospheric reanalysis products has led to the development of the climatology of ARs over the central United States. This climatology is used to understand the AR characteristics, their long-term impacts on annual precipitation, precipitation extremes, and flooding over the central United States. AR characteristics (e.g., frequency, duration) are generally robust across the different reanalysis products. These storms exhibit a marked seasonality, with the largest activity in winter (more than ten ARs per season on average), and the lowest in summer (less than two ARs per season on average). Overall, ARs generally last less than three days, but exceptionally persistent ARs (more than six days) are also observed. In terms of their impacts on precipitation, AR-related precipitation is able to explain a large portion of the year-to-year variations in the total annual precipitation over the central United States. Moreover, 40% of the top 1% daily precipitation extremes are associated with ARs, and more than 70% of the annual instantaneous peak discharges and peaks-over-threshold floods are associated with these storms, in particular during winter and spring.&lt;/p&gt; &lt;p&gt;The relationship between the frequency of ARs and three prominent large-scale atmospheric modes [Pacific-North American (PNA) teleconnection, Artic Oscillation (AO), and North Atlantic Oscillation (NAO)] is investigated, and the results are used to statistically model the frequency of ARs at the seasonal scale. PNA and AO indices play a significant role in the winter season, when the AR frequency is the highest. Building on these insights, different spatio-temporal Bayesian hierarchical models are developed to describe the frequency of winter heavy precipitation events based on ARs and the large-scale atmospheric modes. The results suggest that over much of the central United States, PNA and AO can be helpful in describing the frequency of ARs in winter, which in turn can be useful to characterize the frequency of heavy rainfall events over the central United States.&lt;/p&gt; &lt;p&gt;Because of the large impacts that these storms have, their short-term predictability is examined by using outputs from five numerical weather prediction (NWP) models with a lead-time up to 15 days. While there are differences among the five NWP models, the results show that the skill in forecasting the occurrence and location of ARs over the central United States decreases with increasing lead time, and the models have positive skills up to the seven-day lead time.&lt;/p&gt;

&amp;lt;p&amp;gt;Regarding arctic amplification, meridional transports of moisture and heat from subpolar regions represent a crucial phenomenon. Among such intrusions, Atmospheric Rivers (ARs) are characterized by narrow and transient moisture flows, which are responsible for up to 90% of vertical integrated water vapour transport (IVT) into the Arctic. Moreover, they are relevant for meridional air mass transformations and precipitation events. To identify local sources and sinks of moisture associated with such AR pathways, the accurate determination of total IVT along the AR cross-sections is indispensable. However, since ARs primarily occur over ocean basins, e.g. the North Atlantic, there is a lack of measurements inside ARs. Spaceborne sensors struggle to profile the interior of AR cores, leading to a blind zone where the majority of water vapour is located.&amp;lt;/p&amp;gt;&amp;lt;p&amp;gt;Conversely, airborne released dropsondes currently provide the most detailed insights on ARs. The frequency of dropsonde releases is, however, technically limited, so that uncertainties in the calculated total IVT of the AR transect may be significant. In particular, when the IVT within the AR core has high lateral variability, unresolved AR-IVT characteristics can constrain the moisture budget analysis. During the North Atlantic Waveguide and Downstream Impact Experiment (NAWDEX), conducted in autumn 2016, the High Altitude and LOng- Range research aircraft (HALO) performed several flight segments along high-latitude AR cross-sections. From these North Atlantic ARs associated with strong meridional water vapour transport, we exemplarily present high-resolution measurements and sounding profiles in the interior of AR cross-sections. We focus on a polar case (research flight RF10, 13&amp;lt;sup&amp;gt;th&amp;lt;/sup&amp;gt; October 2016) and include simulations from the cloud-resolving model ICON-2km, to investigate the lateral AR-IVT variability. &amp;amp;#160;&amp;lt;/p&amp;gt;&amp;lt;p&amp;gt;Comparing dropsonde IVT values with the simulations from ICON-2km, the model shows a valid representation of the AR. Therefore, we use the high-resolution simulations to generate additional synthetic observations. They allow us to identify major sources of error for observational representation of IVT variability in AR cross-sections. Analysing the vertical profile of water vapour transport, we find that specific humidity and wind speed contribute to lateral IVT variability at different heights. With regard to the total cross-section IVT, we derive across-track sounding resolutions required for typical arctic AR-IVT characteristics. The considered AR shows the presence of a low-level jet, a pre-cold-frontal strong wind corridor below 1000 m, resulting from the temperature gradient across the cold front. Since maximum values and increasing lateral variability of IVT appear close to this low-level jet, our results emphasize the need of high-resolution, i.e frequent sonde releases, around the low-level jet to calculate the cross-section total IVT. Our findings aim at optimizing observational airborne strategies for future campaigns, e.g. HALO-AC&amp;amp;#179; in 2022, in order to lower the uncertainties of IVT in high-latitude and arctic ARs.&amp;lt;/p&amp;gt;

Here, we analyze the inter‐relationships between weather types (WTs) and atmospheric rivers (ARs) around Aotearoa New Zealand (ANZ), their respective properties, as well as their combined and separate influence on daily precipitation amounts and extremes. Results show that ARs are often associated with 3–4 WTs, but these WTs change depending on the regions where ARs landfall. The WTs most frequently associated with ARs generally correspond to those favoring anomalously strong westerly wind in the mid‐latitudes, especially for southern regions of ANZ, or northwesterly anomalies favoring moisture export from the lower latitudes, especially for the northern regions. WTs and ARs show strong within‐type and inter‐event diversity. The synoptic patterns of the WTs significantly differ when they are associated with AR occurrences, with atmospheric centers of actions being shifted so that moisture fluxes toward ANZ are enhanced. The location, angle, and persistence of ARs appear strongly driven by the synoptic configurations of the WTs. Although total moisture transport shows weaker WT‐dependency, it appears strongly related to zonal wind speed to the south of ANZ, or the moisture content of the air mass to the north. Finally, WT influence on daily precipitation may completely change depending on their association, or lack thereof, with AR events. WTs traditionally considered as favorable to wet conditions may conceal daily precipitation extremes occurring during AR days, and anomalously dry days or near‐climatological conditions during non‐AR days.

&amp;lt;p&amp;gt;&amp;lt;span&amp;gt;Atmospheric rivers (ARs) are generally considered to be transient and concurrent with an extratropical cyclone (Ralph et al. 2018). However, this is not necessarily the case for the ARs in the East Asian summer monsoon (EASM). Despite several climatological surveys on the EASM ARs in recent years (e.g., Park et al. 2021a), through what processes they develop is still unclear because of the complex interplay between monsoonal and extratropical circulations in the region (Horinouchi 2014; Park et al. 2021b).&amp;lt;/span&amp;gt;&amp;lt;/p&amp;gt;&amp;lt;p&amp;gt;&amp;lt;span&amp;gt;In this talk, we demonstrate that the EASM ARs have different &amp;amp;#8220;flavors&amp;amp;#8221; in terms of moisture transport characteristics. By quantifying the relative contribution of high- and low-frequency components of the integrated water vapor transport anomaly (IVTA) for each AR, it is found that both components are important in East Asia summer, in contrast to the ARs in the U.S. west coast where the high-frequency component is predominant.&amp;lt;/span&amp;gt;&amp;lt;/p&amp;gt;&amp;lt;p&amp;gt;&amp;lt;span&amp;gt;To investigate the synoptic condition governing the high- and low-frequency IVTA, the EASM ARs are classified into the three categories&amp;amp;#8212;1) transient, 2) quasi-stationary and 3) intermediate ARs&amp;amp;#8212;depending on the fractional contribution of high-frequency IVTA to total IVTA. While the transient ARs are driven by an extratropical cyclone in an analogy of classical ARs, the quasi-stationary ARs are associated with an anomalously enhanced monsoon flow. The intermediate ARs, which are a majority of summertime ARs in East Asia, show the confounding features of the two types. We suggest that the concept of &amp;amp;#8220;transient&amp;amp;#8221; and &amp;amp;#8220;quasi-stationary&amp;amp;#8221; AR flavors offer an important foundation in understanding the EASM ARs with a variety of underlying dynamics. Further implications and possible future works will be also discussed.&amp;lt;/span&amp;gt;&amp;lt;/p&amp;gt;&amp;lt;p&amp;gt;

Atmospheric rivers (ARs) are key drivers of extreme precipitation in the Western U.S. Using regionally downscaled thermodynamic global warming (TGW) simulations, we examine how ARs with varying wind and moisture characteristics respond to warming. We classified 812 historical AR events into Gusty‐Wet, Gusty‐Dry, Calm‐Wet, and Calm‐Dry groups to evaluate differences in precipitation behavior. ARs with stronger winds and higher moisture content exhibit higher precipitation efficiency (PE) and greater integrated water vapor (IWV). Regionally, Calm ARs show higher IWV accumulation due to slower inland transport and reduced PE. Projections indicate increases in storm‐total (sub‐Clausius‐Clapeyron (CC) scaling) and maximum 3‐hourly precipitation (super‐CC scaling) across all groups, with the most pronounced changes in Gusty‐Wet and Calm‐Wet ARs. Spatial differences in surface runoff, PE, and inland reach highlight the importance of AR subtype in shaping future flood hazards. These results offer insights into event‐level and regional‐scale precipitation changes under evolving environmental conditions.

Previous work has demonstrated the strong ocean response to atmospheric rivers (ARs) in the northeast Pacific including coastal currents along the west coast of North America, because of strong surface winds associated with ARs. A recent study on the global distribution of ARs also suggests that the southeast Indian Ocean is one of the areas of relatively strong AR activity. This study investigates the influence of ARs on the Leeuwin Current system, which is one of the major boundary currents in the Indian Ocean. It is demonstrated that winds associated with typical ARs in the southeast Indian Ocean can generate strong poleward coastal currents and sea level rise along the west coast of Australia using a high-resolution ocean reanalysis (0.08° HYCOM). The composite of upper ocean currents and sea surface height (SSH) associated with landfalling ARs along the west coast of Australia is constructed using the HYCOM reanalysis, long-term AR data set, and tide gauge data. The enhancement of the poleward currents generated by ARs is found in the composite, and the magnitude of the enhancement is comparable to the strength of the Leeuwin Current itself. The results also indicate that the fluctuation of SSH and coastal currents along the west coast propagates along the southern coast all the way to the southeast coast (Pacific side) of Australia. The SSH propagation along the coasts is also detected in the tide gauge data in the west and southern coasts of Australia.

Landslides are particularly costly disasters, causing about 4,500 fatalities and US$20 billion in damages worldwide each year. In Western North America, where intense and frequent precipitation events interact with complex topography and steep slopes, precipitation-induced landslides (PILs) are a serious geological hazard. Recently, it has been revealed that the majority of PILs in the region are triggered by precipitation from atmospheric rivers (ARs), transient channels of intense water vapor flux in the troposphere. However, the synoptic conditions differentiating landslide-triggering and non-triggering ARs remain unknown. In this study, we explore opportunities for improved landslide forecasting in Western North America using catalogs of land-falling ARs and PILs, along with ERA5 climatological data, from 1996 to 2018. First, we employ event synchronization, a non-linear measure specially tailored for event series analysis, to identify landslide-triggering ARs. Based on the AR-strength scale, which ranks ARs in levels from 1 to 5, we further characterize landslide-triggering ARs in terms of intensity and persistence. Subsequently, we spatially resolve the conditional probability of PIL occurrence given the detection of AR-attributed precipitation in the antecedent week, revealing the contribution of each AR level. Lastly, using hourly estimates of integrated water vapour transport, geopotential height, and precipitation at 0.25&amp;#176; spatial resolution, we differentiate the spatio-temporal evolution of synoptic conditions preceding landslide-triggering and non-landslide triggering ARs. Our results constitute a first, fundamental, and necessary step toward AR-based landslide forecasts, contributing crucial insights to improve forecasting accuracy at the short and early medium-range (1&amp;#8211;7 days).

Chapter 12 - Clouds and Precipitation Associated with Hills and Mountains

Research on Atmospheric Rivers (ARs) has focused primarily on AR (thermo)dynamics and hydrological impacts over land. However, the evolution and potential role of nearshore air‐sea fluxes during landfalling ARs are not well documented. Here, we examine synoptic evolutions of nearshore latent heat flux (LHF) during strong late‐winter landfalling ARs (1979–2017) using 138 overshelf buoys along the U. S. west coast. Composite evolutions show that ARs typically receive upward (absolute) LHF from the coastal ocean. LHF is small during landfall due to weak air‐sea humidity gradients but is strongest (30–50 W/m2 along the coast) 1–3 days before/after landfall. During El Niño winters, southern‐coastal LHF strengthens, coincident with stronger ARs. A decomposition of LHF reveals that sea surface temperature (SST) anomalies modulated by the El Niño Southern Oscillation dominate interannual LHF variations under ARs, suggesting a potential role for nearshore SST and LHF influencing the intensity of landfalling ARs.

[1] The plume structure of Perdido Bay Estuary (PBE), a typical bay on the Florida-Alabama coast along the Gulf of Mexico, was simulated using an existing calibrated model. To better understand plume dynamics in the PBE and similar bay systems, idealized sensitivity experiments were conducted to examine the influence of wind stress on the 3-D plume signature: the results indicate that wind speed and direction significantly influence plume orientation, area, width, length, and depth. The plume size was reduced under the effect of wind and increased wind forcing. Among wind-forced cases, the plume is largest for northerly (offshore) winds and smallest for southerly (onshore) winds. Bay-shelf salt flux and water flux were also investigated, since they are important for the formation of a 3-D plume structure. Model simulations show that water outflow to the coastal ocean is strongest under northerly winds and can be stopped by southerly winds. For moderately strong winds, the outflow and plume size are larger for easterly downwelling-favorable winds than for westerly upwelling-favorable winds; the opposite is true for outflow and plume size for these two wind directions under stronger winds. For all wind directions, the ratio of salt flux and water flux at the bay mouth increases with wind speed. This ratio trend is consistent with higher outflow salinities, and this decreased buoyancy signature, along with more energetic vertical mixing, reduces plume size. A detailed understanding of this water and salt flux is essential to the plume dynamics studied here and for other plumes. Additional particle transport analysis using variable wind forcing was conducted to determine the influence of the plume on particle movement. The results showed a consistency between the surface plume, salt flux, and particle transport and illustrate the strong effects that winds have on particle fate and dispersion.