Extremwetter kann nur im klimahistorischen Kontext verstanden werden. Hier muss ein Automatismus entstehen: Beim Auftreten einer Dürre heute, sollte sofort die Frage gestellt werden, wie sich dies in den hydroklimatischen Verlauf der letzten Jahrhunderte und Jahrtausende einordnet. Wir wollen heute über den großen Teich in die USA blicken, zunächst nach Florida. Ein Forscherteam um Grant Harley rekonstruierte die die Wasserführung des zweitgrößten Flusses in Florida. Die Ergebnisse erschienen im Januar 2017 im Journal of Hydrology. Überraschenderweise ereigneten sich die schlimmsten Dürren im 16. bis 19. Jahrhundert. Am schlimmsten war es wohl in den 1560er Jahren als der Suwannee River über längere Zeit nur 17% der Normalwassermenge führte. Harley und Kollegen fanden zudem Zyklen in den historischen Niederschlägen, die auf eine Beeinflussung durch Ozeanzyklen hinweisen. Hier der Abstract:
Suwannee River flow variability 1550–2005 CE reconstructed from a multispecies tree-ring network
Understanding the long-term natural flow regime of rivers enables resource managers to more accurately model water level variability. Models for managing water resources are important in Florida where population increase is escalating demand on water resources and infrastructure. The Suwannee River is the second largest river system in Florida and the least impacted by anthropogenic disturbance. We used new and existing tree-ring chronologies from multiple species to reconstruct mean March–October discharge for the Suwannee River during the period 1550–2005 CE and place the short period of instrumental flows (since 1927 CE) into historical context. We used a nested principal components regression method to maximize the use of chronologies with varying time coverage in the network. Modeled streamflow estimates indicated that instrumental period flow conditions do not adequately capture the full range of Suwannee River flow variability beyond the observational period. Although extreme dry and wet events occurred in the gage record, pluvials and droughts that eclipse the intensity and duration of instrumental events occurred during the 16–19th centuries. The most prolonged and severe dry conditions during the past 450 years occurred during the 1560s CE. In this prolonged drought period mean flow was estimated at 17% of the mean instrumental period flow. Significant peaks in spectral density at 2–7, 10, 45, and 85-year periodicities indicated the important influence of coupled oceanic-atmospheric processes on Suwannee River streamflow over the past four centuries, though the strength of these periodicities varied over time. Future water planning based on current flow expectations could prove devastating to natural and human systems if a prolonged and severe drought mirroring the 16th and 18th century events occurred. Future work in the region will focus on updating existing tree-ring chronologies and developing new collections from moisture-sensitive sites to improve understandings of past hydroclimate in the region.
Weiter in Texas, wo David Rupp und Kollegen keinen Trend zu verminderten Regenfällen in ihren Modellen feststellen konnten (Rupp et al. 2015):
However, no simulated increase in the frequency of large precipitation deficits, or of soil moisture deficits, was detected from preindustrial to year 2011 conditions.
Nun zum Walker Lake an der Grenze von Nevada und Kalifornien. Hatchett und Kollegen rekonstruktierten die Niederschläge für die letzten 1000 Jahre und fanden einen charakteristischen Wechsel von Trocken- und Feuchtphasen (Hatchett et al. 2015):
Placing the 2012–2015 California-Nevada drought into a paleoclimatic context: Insights from Walker Lake, California-Nevada, USA
Assessing regional hydrologic responses to past climate changes can offer a guide for how water resources might respond to ongoing and future climate change. Here we employed a coupled water balance and lake evaporation model to examine Walker Lake behaviors during the Medieval Climate Anomaly (MCA), a time of documented hydroclimatic extremes. Together, a 14C-based shoreline elevation chronology, submerged subfossil tree stumps in the West Walker River, and regional paleoproxy evidence indicate a ~50 year pluvial episode that bridged two 140+ year droughts. We developed estimates of MCA climates to examine the transient lake behavior and evaluate watershed responses to climate change. Our findings suggest the importance of decadal climate persistence to elicit large lake-level fluctuations. We also simulated the current 2012–2015 California-Nevada drought and found that the current drought exceeds MCA droughts in mean severity but not duration.
Coats et al. 2016 untersuchten ebenfalls die Dürregeschichte in den westlichen USA während des letzten Millenniums und konnten Ozeanzyklen als wichtigen Treiber ausmachen. Allerdings können die gängigen Klimamodelle diesen Zusammenhang nicht nachvollziehen, was die Prognosleistung der Simulationen stark beeinträchtigt. Hier der Abstract aus de Geophysical Research Letters:
Internal ocean-atmosphere variability drives megadroughts in Western North America
Multidecadal droughts that occurred during the Medieval Climate Anomaly represent an important target for validating the ability of climate models to adequately characterize drought risk over the near-term future. A prominent hypothesis is that these megadroughts were driven by a centuries-long radiatively forced shift in the mean state of the tropical Pacific Ocean. Here we use a novel combination of spatiotemporal tree ring reconstructions of Northern Hemisphere hydroclimate to infer the atmosphere-ocean dynamics that coincide with megadroughts over the American West and find that these features are consistently associated with 10–30 year periods of frequent cold El Niño–Southern Oscillation conditions and not a centuries-long shift in the mean of the tropical Pacific Ocean. These results suggest an important role for internal variability in driving past megadroughts. State-of-the-art climate models from the Coupled Model Intercomparison Project Phase 5, however, do not simulate a consistent association between megadroughts and internal variability of the tropical Pacific Ocean, with implications for our confidence in megadrought risk projections.
Erika Wise präsentierte in den Geophysical Research Letters 2016 eine Dürre-Rekonstruktion für die US-Westküste. In den letzten 500 Jahren sind immer wieder Dürren aufgetreten, die denen der Heutezeit ähnelten.
Five centuries of U.S. West Coast drought: Occurrence, spatial distribution, and associated atmospheric circulation patterns
The U.S. West Coast drought commencing in 2012 developed in association with a large, persistent high-pressure ridge linked to internal atmospheric variability. This study places the occurrence, spatial patterns, and associated circulation features of West Coast drought into a paleoclimate context through a synoptic dendroclimatology approach linking atmospheric circulation to surface hydroclimate patterns. Spatial reconstructions of upper atmosphere pressure patterns and cool-season drought show that West Coast-wide drought, although relatively rare compared to north-south dipole drought, has occurred periodically since 1500 Common Era and is consistently associated with a strong ridge centered along the Pacific Northwest coast. Atmospheric blocking is also linked to north-dry dipole droughts, while south-dry and wider Western droughts indicate La Niña-type patterns. The transition latitude between the northern and southern sides of the western precipitation dipole, important for California hydroclimate patterns, has had frequent year-to-year fluctuations but remained centered on 40°N over the past five centuries.
Eine Trendstudie von Ge et al. 2016 zu den US-Westküstendürren dokumentiert Schwankungen und Trends.