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Precipitation trends

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bligh8:
Position and orientation of the westerly jet determined Holocene rainfall patterns in China

Abstract .. Open access

Proxy-based reconstructions and modeling of Holocene spatiotemporal precipitation patterns for China and Mongolia have hitherto yielded contradictory results indicating that the basic mechanisms behind the East Asian Summer Monsoon and its interaction with the westerly jet stream remain poorly understood. We present quantitative reconstructions of Holocene precipitation derived from 101 fossil pollen records and analyse them with the help of a minimal empirical model. We show that the westerly jet-stream axis shifted gradually southward and became less tilted since the middle Holocene. This was tracked by the summer monsoon rain band resulting in an early-Holocene precipitation maximum over most of western China, a mid-Holocene maximum in north-central and northeastern China, and a late-Holocene maximum in southeastern China. Our results suggest that a correct simulation of the orientation and position of the westerly jet stream is crucial to the reliable prediction of precipitation patterns in China and Mongolia.

https://www.nature.com/articles/s41467-019-09866-8#Fig1

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Larger Increases in More Extreme Local Precipitation Events as Climate Warms

 05 June 2019 https://doi.org/10.1029/2019GL082908

Abstract
Climate models project that extreme precipitation events will intensify in proportion to their intensity during the 21st century at large spatial scales. The identification of the causes of this phenomenon nevertheless remains tenuous. Using a large ensemble of North American regional climate simulations, we show that the more rapid intensification of more extreme events also appears as a robust feature at finer regional scales. The larger increases in more extreme events than in less extreme events are found to be primarily due to atmospheric circulation changes. Thermodynamically induced changes have relatively uniform effects across extreme events and regions. In contrast, circulation changes weaken moderate events over western interior regions of North America and enhance them elsewhere. The weakening effect decreases and even reverses for more extreme events, whereas there is further intensification over other parts of North America, creating an “intense gets intenser” pattern over most of the continent.

The above is something were watching unfold across the central US

Plain Language Summary
Climate models project that extreme precipitation events will intensify during the 21st century at large spatial scales, with several studies suggesting that the most extreme events will exhibit the highest rate of intensification. Identification of the causes of this phenomenon nevertheless remains tenuous, partly because estimating long‐term changes in precipitation extremes is difficult, particularly for precipitation extremes at impact‐relevant spatial scales. Robustly estimated changes in precipitation extremes at small spatial scales can only be obtained from large extreme precipitation data sets from large ensemble simulations. We employ a large ensemble regional climate simulation experiment performed for North America. The large volume of output from this experiment allows us to confidently obtain statistical evidence that precipitation intensification occurs more rapidly with warming for more extreme events at impact‐relevant spatial scales, and secondly, to determine the causes for this phenomenon. The effect of atmospheric moisture increases caused by greenhouse gas warming is found to be similar for extreme precipitation events of different intensities, ranging from 2‐ to 50‐year events. In contrast, atmospheric circulation change due to greenhouse gas warming tends to reduce the effect of the atmospheric moisture increases on less intense events rather than intensifying the effect on more extreme events.

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bligh8:
U.S. hydrologic design standards insufficient due to large increases in frequency of rainfall extremes

Daniel B. Wright  Christopher D. Bosma  Tania Lopez‐Cantu
First published: 12 July 2019 https://doi.org/10.1029/2019GL083235

Abstract
Evidence for intensifying rainfall extremes has not translated into “actionable” information needed by engineers and risk analysts, who are often concerned with very rare events such as “100‐year storms.” Low signal‐to‐noise associated with such events makes trend detection nearly impossible using conventional methods. We use a regional aggregation approach to boost this signal‐to‐noise, showing that such storms have increased in frequency over much of the conterminous United States since 1950, a period characterized by widespread hydrologic infrastructure development. Most of these increases can be attributed to secular climate change rather than climate variability, and we demonstrate potentially serious implications for the reliability of existing and planned hydrologic infrastructure and analyses. Though trends in rainfall extremes have not yet translated into observable increases in flood risks, these results nonetheless point to the need for prompt updating of hydrologic design standards, taking into consideration recent changes in extreme rainfall properties.

Plain Language Summary
Numerous studies have shown that heavy rainfall in the United States and elsewhere is becoming more common and more severe in a warming climate. These studies have not shown, however, how the most extreme rainfall events are changing, since these storms are so rare that they are difficult to assess using conventional techniques, which generally focus on changes at individual geographic locations. We instead use a simple aggregation technique to “pool” multiple observations within a region. This “pooling” allows us to show that rainfall events that exceed common engineering design criteria, including 100‐year storms, have increased in frequency in most parts of the United States since 1950—a period of widespread infrastructure construction. We show that in most locations, these increases are likely due to climate warming. We also show that much of the existing and planned hydrologic infrastructure in the U.S. based on published rainfall design standards is and will continue to underperform its intended reliability due to these rainfall changes.

This paper is paywalled.... an frankly supports what we already know.

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Recent increase in catastrophic tropical cyclone flooding in coastal North Carolina, USA: Long-term observations suggest a regime shift
https://www.nature.com/articles/s41598-019-46928-9

Hans W. Paerl, Nathan S. Hall, Alexandria G. Hounshell, Richard A. Luettich Jr., Karen L. Rossignol, Christopher L. Osburn & Jerad Bales
Scientific Reports volume 9, Article number: 10620 (2019)...open access

 Abstract

Coastal North Carolina, USA, has experienced three extreme tropical cyclone-driven flood events since 1999, causing catastrophic human impacts from flooding and leading to major alterations of water quality, biogeochemistry, and ecological conditions. The apparent increased frequency and magnitudes of such events led us to question whether this is just coincidence or whether we are witnessing a regime shift in tropical cyclone flooding and associated ecosystem impacts. Examination of continuous rainfall records for coastal NC since 1898 reveals a period of unprecedentedly high precipitation since the late-1990’s, and a trend toward increasingly high precipitation associated with tropical cyclones over the last 120 years. We posit that this trend, which is consistent with observations elsewhere, represents a recent regime shift with major ramifications for hydrology, carbon and nutrient cycling, water and habitat quality and resourcefulness of Mid-Atlantic and possibly other USA coastal regions.

" In addition to their devastating societal and economic impacts, storms associated with this increased frequency are having major ramifications for carbon and nutrient cycling in coastal estuaries and thus represent “hot moments” in coastal biogeochemistry7. In fact, recent work shows that these extreme events caused unprecedented nutrient- and organic matter-laden freshwater discharges to nutrient-sensitive receiving coastal waters, including the USA’s 2nd largest estuarine complex and a key fishery and recreational resource, the Albemarle-Pamlico Sound (APS) (Fig. 1), which drains ~ 40% of North Carolina’s and 10% of Virginia’s coastal plain regions via 5 major rivers8,9."

"Overall, our analysis indicates that; 1) we are experiencing a regime shift in the intensity and quantity of rainfall associated with these events, and 2) this shift has led to unprecedented large loads of nutrients and orgenic matter with major implications for biogeochemical cycling, primary production and overall water quality conditions in the receiving APS and adjacent coastal waters. Furthermore, our observations are consistent with similar observations elsewhere and with predicted hydrologic, nutrient and carbon flux changes taking place in a warming climate1,2,3,4,5,6."

"The receiving waters of the APS have a surface area of 5,335 km2 and drain five major watersheds (Neuse, Tar-Pamlico, Roanoke, Chowan, and Pasquotank Rivers). These watersheds cover an area ~80,000 km2, total freshwater discharge of ∼21 km3 yr−1, and drain about 40% of North Carolina’s and 10% of Virginia’s surface area. Because tidal exchange with the coastal Atlantic Ocean is restricted to three narrow inlets, the APS has a relatively long water residence time of ~1 yr14; this provides suspended algae (phytoplankton) and vascular plants ample time to assimilate nutrients, resulting in high productivity per unit nutrient input. These characteristics are key to the PS serving as a highly productive nursery, supporting ~80% of US mid-Atlantic commercial and recreationally caught finfish and shellfish species11. However, it also makes the system sensitive to nutrient-over enrichment, resultant eutrophication and nuisance algal blooms12,15. The long residence time also enables ample time for photochemical and/or microbial degradation of organic matter16."

"With less than a 2% chance of three such events occurring in a twenty-year period23, either North Carolina has been very unlucky, or the historical record used to define the storm statistics is no longer representative of the present climatic regime. This analysis suggests that the occurrence of three extreme floods resulting from high rainfall tropical cyclone events in the past 20 years is a consequence of the increased moisture carrying capacity of tropical cyclones due to the warming climate

"Thus, evidence is accumulating that we may also be seeing changes to the “system state” of coastal waters in terms of their ability to capture or release CO2 37,38. Such changes caused by an increased frequency of extreme storm events are ostensibly reorganizing coastal carbon cycles

"Considering these extreme precipitation events and their hydrologic and biogeochemical consequences in totality, it is clear that they are unparalleled in the past 120+ years of recorded tropical cyclones in coastal North Carolina (Fig. 3). The potential exists for receiving waters globally to undergo unprecedented perturbations to nutrient and carbon cycling, fisheries habitat and sustainability due to increasing frequency of extreme precipitation events; all of which are still to be determined. With roughly 40% of the world’s population within 100 km of the coast, development inland, as well as along the coastline, will exacerbate the perturbations caused by this type of regime shift.....more within the paper

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