Synoptic and Climate Attributions of the December 2015 Extreme Flooding in Missouri, USA

Three days of extreme rainfall in late December 2015 in the middle of the Mississippi River led to severe flooding in Missouri. The meteorological context of this event was analyzed through synoptic diagnosis into the atmospheric circulation that contributed to the precipitation event’s severity. The midlatitude synoptic waves that induced the extreme precipitation and ensuing flooding were traced to the Madden Julian Oscillation (MJO), which amplified the trans-Pacific Rossby wave train likely associated with the strong El Niño of December 2015. Though the near-historical El Niño contributed to a quasi-stationary trough over the western U.S. that induced the high precipitation event, an interference between the MJO and El Niño teleconnections resulted in a relatively weak atmospheric signature of the El Niño in comparison to that of the MJO. The influence of anthropogenic climate change on the relationship between ENSO and precipitation across several central U.S. states was also investigated using 17 CMIP5 models from the historical single-forcing experiments. A regime change in ENSO-related precipitation anomalies appears to have occurred, from being negatively correlated before 1950 to positive and significantly correlated after 1970, suggesting a likely effect of anthropogenic warming on the December 2015 extreme precipitation event

Intensification of Premonsoon Tropical Cyclones in the Bay of Bengal and Its Impacts on Myanmar

We analyze multiple global reanalysis and precipitation datasets in order to explain the dynamic mechanisms that lead to an observed intensification of the monsoon trough and associated tropical cyclone (TC) activity over the Bay of Bengal (BOB) during the premonsoon month of May. We find that post-1979 increases in both premonsoon precipitation and TC intensity are a result of enhanced large-scale monsoon circulation, characterized by lower-level cyclonic and upper-level anticyclonic anomalies. Such circulation anomalies are manifest of the tropospheric expansion that is caused by regional warming. The deepened monsoon trough in the BOB not only affects TC frequency and timing, but also acts to direct more cyclones towards Myanmar. We propose that increasing sea surface temperature in the BOB has contributed to an increase in cyclone intensity. Our analyses of the Community Earth System Model single-forcing experiments suggest that tropospheric warming and a deepening of the monsoon trough can be explained by two discreet anthropogenic causes--an increase in absorption due to aerosol loading and an increase in the land-ocean thermal contrast that results from increased greenhouse gases. The ensuing circulation changes provide favorable conditions for TCs to grow and to track eastward towards Myanmar

Megadroughts in the Common Era and the Anthropocene

Exceptional drought events, known as megadroughts, have occurred on every continent outside Antarctica over the past ~2,000 years, causing major ecological and societal disturbances. In this Review, we discuss shared causes and features of Common Era (Year 1–present) and future megadroughts. Decadal variations in sea surface temperatures are the primary driver of megadroughts, with secondary contributions from radiative forcing and land–atmosphere interactions. Anthropogenic climate change has intensified ongoing megadroughts in south-western North America and across Chile and Argentina. Future megadroughts will be substantially warmer than past events, with this warming driving projected increases in megadrought risk and severity across many regions, including western North America, Central America, Europe and the Mediterranean, extratropical South America, and Australia. However, several knowledge gaps currently undermine confidence in understanding past and future megadroughts. These gaps include a paucity of high-resolution palaeoclimate information over Africa, tropical South America and other regions; incomplete representations of internal variability and land surface processes in climate models; and the undetermined capacity of water-resource management systems to mitigate megadrought impacts. Addressing these deficiencies will be crucial for increasing confidence in projections of future megadrought risk and for resiliency planning

Bay of Bengal: Coupling of Pre-Monsoon Tropical Cyclones With the Monsoon Onset in Myanmar

Myanmar remained largely closed to the world through political instability for several years, when it continued to suffer terribly at the hands of nature that remained largely unknown. Of note is the period between 2008 and 2013, during which the country suffered at least eight major natural calamities that killed more than 141,000 people and affected 3.2 million. The worst of these was Cyclone Nargis in May 2008 that killed more than 130,000. With an estimated $4 billion in damages, Nargis remains the deadliest and most destructive named cyclone ever to have occurred in the North Indian Ocean. Recent studies have shown that, due to increased greenhouse gases and aerosol loading in the atmosphere, more and stronger tropical cyclones (TCs) in the last three decades are tracking eastwards toward the Indochina peninsula. Unfortunately, the Burmese lack the capacity to deal with the impacts of such storms. Myanmar was left behind as the world made significant technological and industrial advancement; but agriculture, which employs at least 65% of the active labor force, has remained the backbone of the Myanmar economy – an industry that is heavily reliant on monsoon rainfall. The pre-monsoon TC season in the Bay of Bengal (BoB) precedes the onset of the Myanmar monsoon but sometimes the two (i.e.TC formation and the monsoon onset) occur in unison. This work studied the mechanism by which the Madden Julian Oscillation (MJO) modulates the Myanmar monsoon onset and TC activity collectively (i.e. ISO-Onset-TC connection). Avoiding TC destruction at the beginning of the planting season is crucial, so is the monsoon onset date critical for planning. Additional understanding of the aforementioned ISO-Onset-TC connection could provide further insight into predicting the Myanmar monsoon onset and aid in disaster planning for TC impact. This research is part of a two-year NASA funded project to study extreme climate and weather events

Towards the Prediction of Climate Extremes with Attribution Analysis Through Climate Diagnostics and Modeling: Cases from Asia to North America

This project summarizes the findings of research organized in two parts. The first involved the characterization of changes in the variability of climate that lead to extreme events. The second focused on the predictability of extreme climate on time-scales ranging from short forecast lead-times to long-lead climate predictions exceeding a year. Initial studies focused on three interrelated, yet regionally unique extreme climate phenomena. First, the relationship between increasing greenhouse gas (GHG) emissions and particulate matter (PM) concentration in basin terrain was investigated. Next, we evaluated changes in large-scale atmospheric circulation associated with two climate phenomena at either extreme side of the water cycle--droughts and floods. In the final analysis, an attempt was made to understand the mechanisms that link two North Pacific ENSO precursor patterns to the ENSO cycle

The missing PDO-ENSO teleconnection explaining weather variability in the United States Intermountain West

Understanding the variability and precipitation regimes in the United States Intermountain West is of utmost importance for agriculture planning and water management. Although the accuracy of weather and climate models in predicting precipitation has significantly improved over the last three decades, overall model performance in the United States Intermountain West (U.S. IMW) is weaker and less accurate when compared to the model-skill for the rest of the country. In this study, we show that the Pacific Decadal Oscillation (PDO) can be used to facilitate forecasting of precipitation regimes in the U.S. IMW. A linear analysis of a twenty-years sliding (SCORR) correlation between the Palmer Drought and Severity Index (PDSI) and the Nino 3.4 index bears a great resemblance to the PDO signal constructed from tree ring data from 1930 to 2000. It was found that positive PDO phases correspond to wet periods in the U.S. IMW whiles the reversed PDO pattern introduces a dry regime. A similar analysis using historical simulation of the Community Earth System Model (CESM), a fully coupled global climate model reveals a pattern consistent with the ones observed in PDO-SCORR analyses. But here, precipitation anomalies are used rather than the PDSI index. A difference in streamfunction composite between El Nino and La Nina years is computed for two historical periods: the PDO cold phase (1784-1864) during which twice as many La Nina’s were observed, and the PDO warm phase (1874-1954) during which twice as many El Nino’s were observed. The result shows a wave train that transports moisture to the entire South-Western U.S. during El Nino years

The 2014/15 Snowpack Drought in Washington State and its Climate Forcing

This fifth edition of explaining extreme events of the previous year (2015) from a climate perspective continues to provide evidence that climate change is altering some extreme event risk. Without exception, all the heat-related events studied in this year’s report were found to have been made more intense or likely due to human-induced climate change, and this was discernible even for those events strongly influenced by the 2015 El Niño. Furthermore, many papers in this year’s report demonstrate that attribution science is capable of separating the effects of natural drivers including the strong 2015 El Niño from the influences of long-term human-induced climate change. Other event types investigated include cold winters, tropical cyclone activity, extreme sunshine in the United Kingdom, tidal flooding, precipitation, drought, reduced snowpack in the U.S. mountain west, arctic sea ice extent, and wildfires in Alaska. Two studies investigated extreme cold waves and monthly-mean cold conditions over eastern North America during 2015, and find these not to have been symptomatic of human-induced climate change. Instead, they find the cold conditions were caused primarily by internally generated natural variability. One of these studies shows winters are becoming warmer, less variable, with no increase in daily temperature extremes over the eastern United States. Tropical cyclone activity was extreme in 2015 in the western North Pacific (WNP) as measured by accumulated cyclone energy (ACE). In this report, a study finds that human-caused climate change largely increased the odds of this extreme cyclone activity season. The 2015 Alaska fire season burned the second largest number of acres since records began in 1940. Investigators find that human-induced climate change has increased the likelihood of a fire season of this severity. Confidence in results and ability to quickly do an attribution analysis depend on the “three pillars” of event attribution: the quality of the observational record, the ability of models to simulate the event, and our understanding of the physical processes that drive the event and how they are being impacted by climate change. A result that does not find a role for climate change may be because one or more of these three elements is insufficient to draw a clear conclusion. As these pillars are strengthened for different event types, confidence in the presence and absence of a climate change influence will increase. This year researchers also link how changes in extreme event risk impact human health and discomfort during heat waves, specifically by looking at the role of climate change on the wet bulb globe temperature during a deadly heat wave in Egypt. This report reflects a growing interest within the attribution community to connect attribution science to societal impacts to inform risk management through “impact attribution.” Many will watch with great interest as this area of research evolves in the coming years

The deadly himalayan snowstorm of October 2014: Synoptic conditions and associated trends

The Himalayan snowstorm of October 2014 resulted from the unusual merger of a tropical cyclone with an upper trough, and their collective changes under climate warming have increased the odds for similar events. © 2015 American Meteorological Society

The feasibility of reconstructing hydroclimate over West Africa using tree-ring chronologies in the Mediterranean region

Dendrochronology in West Africa has not yet been developed despite encouraging reports suggesting the potential for long tree-ring reconstructions of hydroclimate in the tropics. This paper shows that even in the absence of local tree chronologies, it is possible to reconstruct the hydroclimate of a region using remote tree rings. We present the West Sub-Saharan Drought Atlas (WSDA), a new paleoclimatic reconstruction of West African hydroclimate based on tree-ring chronologies from the Mediterranean Region, made possible by the teleconnected climate relationship between the West African Monsoon and Mediterranean Sea surface temperatures. The WSDA is a one-half degree gridded reconstruction of summer Palmer Drought Severity indices from 1500 to 2018 CE, produced using ensemble point-by-point regression. Calibration and verification statistics of the WSDA indicate that it has significant skill over most of its domain. The three leading modes of hydroclimate variability in West Africa are accurately reproduced by the WSDA, demonstrating strong skill compared to regional instrumental precipitation and drought indices. The WSDA can be used to study the hydroclimate of West Africa outside the limit of the longest observed record and for integration and comparison with other proxy and archaeological data. It is also an essential first step toward developing and using local tree-ring chronologies to reconstruct West Africa’s hydroclimate

Assessing Future Tropical Cyclone Risk Using Downscaled CMIP6 Projections

The authors employ the Columbia Hazard model (CHAZ) to characterise future tropical cyclone (TC) activity under the shared socioeconomic pathways (SSP) SSP2-4.5, SSP3-7.0, and SSP5-8.5 by downscaling 12 models that participated in the Coupled Climate Model Intercomparison Project’s sixth generation (CMIP6), focusing on the Western North Pacific (WNP) and North Atlantic (ATL) basins. Results from CHAZ are also used in conjunction with the Impact Forecasting Atlantic/Caribbean Tropical Cyclone Wind Model, an industry catastrophe model, to project future changes in financial losses in the ATL. As with previous downscaling of CMIP5 models in CHAZ, projections of TC frequency depend on the choice of moisture variable used in the tropical cyclone genesis index (TCGI), despite similar trends when the two are applied in the historical period. Simulations using column relative humidity (CRH) project an increasing TC frequency trend in the future, while those using saturation deficit (SD) project a decrease. In the WNP, TC frequency scales linearly with the rise in global mean surface temperature, highlighting a direct link with anthropogenic greenhouse gas (GHG) radiative forcing. While ATL TC frequency in the SD experiments exhibits the same trend, the CRH response is complex and nonlinear, probably due to the higher sensitivity of the response of TC potential intensity to aerosol versus GHG forcing. Our projections of financial losses are equally uncertain, consistent with the corresponding bifurcation in TC frequency between the CRH and SD experiments. These projections, despite their inherent uncertainties, can still be useful if viewed as placing bounds on future changes in risk, since not very large increases (i.e., much greater than around 10%) are projected in any of the ATL loss results.1. Introductio