Progress 10/01/17 to 09/30/18
Outputs
Target Audience:We have had conversations with staff in the Northeast Regional Climate Center (NRCC) to determine if the results of our analysis can be incorporated into drought information provided to regional stakeholders ranging from individual growers to Ag and Markets. The NRCC has a partnership with the Cornell Institute for Climate Smart Solutions to develop a New York State / New England Drought Atlas, which includes an effort to explore long-lead drought forecasting. This information is summarized in a newsletter distributed to regional agricultural communities about existing drought conditions and seasonal projections. We have had conversations regarding whether our forecasting tool is sufficiently skillful to be included in thisnewsletter. However, due to a lack of a clear drought predictability signal (as we discuss in "Accomplishments"), it is unclear whether these results will ultimately be appropriate for a drought newsletter. However, our results have indicated thepotential to produce a seasonal forecast for wet conditions during the summer (also discussed in "Accomplishments"). As such, we have also been in conversations with an extension specialist from New York Sea Grant (NYSG) to determine whether this information could be of use to coastal communities. In particular, communities along the shoreline of Lake Ontario suffered from a historic flood in 2017, and there has been great interest in the predictability of wet conditions in that region. Our results have shown promise for this, and we are currently engaged with NYSG to determine how we could best provide relevant climate information to shoreline stakeholders. Changes/Problems:We were generally unable to identify a clear signal of seasonal predictability of drought. This prohibited the creation of outreach products we had envisioned for this work (e.g., a newletter through the NRCC that includes drought forecasts). However, we did identify a potential forecast signal for wet conditions in the region, including in the Great Lakes region, and we have pivoted our outreach efforts to include some of this information in outreach materials provided to Lake Ontario shoreline communities. We have been working with an extension specialist at New York Sea Grant to forward this outreach approach. This work is ongoing. What opportunities for training and professional development has the project provided?The postdoctoral researcher working on this project was afforded two primary sources of professional development. First, through weekly, one-on-one meetings with the project PI, the postdoctoral researcher was taught about large-scale climate processes and their influence on hydroclimate variability over land. These topics were outside of the postdoc's original expertise, and so represent a significant advance in their skillset and knowledge. In addition, the postdoc was provided with the opportunity to present on this work in both study groups and a larger seminar in the department, giving them the chance to develop communication skills regarding the science of drought forecasting. How have the results been disseminated to communities of interest?The results have been presented to members of the Northeast Regional Climate Center, in the hopes of having seasonal forecast information that has emerged out of this work be included in a newsletter provided to regional stakeholders (e.g., farmers, municipalities). These discussions are ongoing. However, due to a lack of a clear drought predictability signal, it is unclear whether these results will ultimately be appropriate for a drought newsletter. Instead, other outreach materials could be created to inform key stateholders aboutthe potential for wet conditions during the summer, which presented a stronger forecasting signal in our results. We have had initial conversations with an extension specialist at New York Sea Grant about how this information could be provided to coastal communities on the shoreline of Lake Ontario. This work is ongoing. What do you plan to do during the next reporting period to accomplish the goals?
Nothing Reported
Impacts
What was accomplished under these goals?
IMPACT Precipitation in the Northeast United States is generally less variable than other regions of the country. Consequently, the impacts of precipitation variability and change on key economic sectors like agriculture have received less attention than in regions like California, the Midwest, and the Great Plains. However, precipitation and streamflow in the Northeast have been increasing over the last several decades, particularly in the summer and in the frequency of extreme events. These trends can lead to increased flooding and exacerbate waterlogged conditions that are detrimental to agricultural production. In addition, short-term droughts can cause significant damage because stakeholders in the Northeast are ill prepared to manage their impacts. For example, the recent drought in 2016 caused crop failures near 30% across large areas of rainfed farmland and significant monetary losses in the agricultural community. Such droughts are projected to become more common with climate change. As climate in the Northeast US continues to change and become more variable, there is a need to better understand the pathways that link large-scale modes of atmospheric circulation to inter-annual variability in regional precipitation and hydrology. Such knowledge would be particularly helpful if tailored for the summer, to help develop long-lead (> 1 month) climate forecasts that could help promote adaptive management in the agricultural and water sectors. Based on the above need, the postdoc supported by this work (under the mentorship of the PI)studiedthe space-time patterns of summer daily rainfall across the Northeast United States, with a focus on historical trends and the potential for long-lead predictability. Wedefined six weather states that represent distinct patterns of rainfall across the region, and examined atmospheric circulation during each state. The states represent the occurrence of region-wide dry and wet conditions associated with large-scale pressure patternsover the Northeast. We identifiedthe existence ofkey inland and coastal storm tracks, and founda positive trend in the frequency of the weather state associated with heavy, region-wide rainfall, which is mirrored by a decreasing trend in the frequency of region-wide dry conditions. Two primary drought types were found that are characterized by region-wide dry conditions linked to a high pressure system over the Northeastand an eastward-shifted storm track associated with light precipitation along the coastline. Finally, we assessed the long-lead predictability of the different weather states based on sea surface temperatures in the Atlantic Ocean from the prior season. We found thatspringtime sea surface temperatures, particularly those in the Caribbean Sea and tropical North Atlantic Ocean, provide some predictability for summers with above-average precipitation and low flows, but theyprovide little information on the occurrence of drought conditions across the Northeast. From a practical perspective, this indicates that we may be able to provide some long-lead warning that wet summer conditions might occur, which could help farmers prepare for waterlogged conditions. However, we will not be able to provide an early warning for damaging drought conditions, like those experienced in 2016. A more detailed description of our activities, results, and key accomplishments for each objective are described below. Objective 1:Characterize the large-scale climate anomalies associated with droughts and pluvials over the Northeast Data: Daily precipitation data for 93 meteorological stations between 1958 and 2017 were used to develop a Hidden Markov Model (HMM) of weather states over the Northeast. We also collected daily streamflow data at 52 streamflow gages throughout the Northeast that are relatively unaltered by anthropogenic activity. To explore large-scale atmospheric circulation during each weather state, we collected daily geopotential heights and vertical and meridional wind speed anomalies from the NCAR/NCEP Reanalysis 1 project. We also calculated vertically integrated water vapor transportto characterize moisture availability. Methods: We developed an HMM for daily summer rainfall over the Northeast. HMMs are a doubly stochastic model in which the daily probability of rainfall occurrences across a network of gages is conditioned on a small number of latent (i.e., unobservable) states that are modeled as a Markovian process. The probabilities of rainfall occurrence within each state, daily sequence of states, and the transition probabilities between states were inferred using the historical record of daily rainfall occurrences. The inferred sequence of weather states was then analyzed to examine how spatial patterns of precipitation occurrence have varied over time, including their average progression during the summer season, trends across the entire 60-year record, and observed frequencies during major droughts. Results and key Outcomes: Based on the analysis above, six weather states were identified that describe the temporal variability of inland and coastal patterns of rainfall that vary in intensity. The frequency of very dry conditions showed significant declines throughout the 60-year record, particularly in the earlier part of the summer. Concurrently, very wet conditions exhibited upwards trends in frequency across the period of record, also in the early months of the summer. The weather states were shown to vary considerably across drought events, suggesting that different climatic mechanisms can lead to dry summer conditions over the Northeast region. Atmospheric compositesrevealed distinct patterns of atmospheric flow associated with a blocking ridge during very dry conditions and two sets of storm tracks associated with a shift between inland and coastal rainfall. These results present a key advance in our knowledge of the types of large-scale climate patterns that drive weather extremes overthe Northeast. Objective 2: Develop a modeling framework to assess the 1-3 month predictability of multiple hydroclimate variables relevant to summertime crop stress and water supplies Methods: Composites were used to characterize the relationship between large-scale atmospheric-oceanic circulation patterns and the HMM states to identify underlying teleconnections. Detrended May sea surface temperature anomalies (SSTAs) were then composited over the 10 years with the highest and lowest frequencies of occurrence for each state, and statistical significance was tested using a bootstrapping approach. We also explored whether SDLFs in the Northeast has similar spring forecasting signals to those for the precipitation states. To eliminate the effects of spring hydrologic memory on summer low flows, SDLFs at each gage were first regressed against proxies for antecedent moisture, and then the space-time variability of the regression residuals were summarized using rotated empirical orthogonal functionanalysis. Similar to the analysis for the weather states, May SSTAs were composited over the 10 years with the highest and lowest values of the rotated principal components. These composites were compared against those for the weather states to determine the level of agreement between the affects of springtime SSTAs on patterns of summertime low flows and precipitation. Results and Key Outcomes: We found an asymmetric relationship between weather states and antecedent SSTAs, particular those in the tropical and subtropical Atlantic, that suggests that dry conditions in the Northeast associated with a blocking ridge are poorly related to boundary forcing, but wet conditions may be predictable based on a springtime signal in the tropical Atlantic. These relationships were similar for low flows across the Northeast, and provides a key advance in knowledge that helps to reconcile a past debate aboutdrought predictability in the Northeast.
Publications
- Type:
Journal Articles
Status:
Under Review
Year Published:
2019
Citation:
Ahn, K.-H., and Steinschneider, S., Seasonal predictability and change of large-scale summer precipitation patterns over the Northeast United States, NY, Journal of Hydrometeorology, Under Review.
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