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On the unseasonal flooding over the Central United States during December 2015 and January 2016
Atmospheric Research 196 (2017) 23–28
Contents lists available at ScienceDirect
Atmospheric Research journal homepage: www.elsevier.com/locate/atmosres
On the unseasonal flooding over the Central United States during December 2015 and January 2016
MARK
Wei Zhang⁎, Gabriele Villarini IIHR-Hydroscience & Engineering, The University of Iowa, Iowa City, Iowa, USA
A B S T R A C T The unseasonal winter heavy rainfall and flooding that occurred during December 2015–January 2016 had large socio-economic impacts for the central United States. Here we examine the climatic conditions that led to the observed extreme precipitation, and compare and contrast them with the 1982/1983 and 2011/2012 winters. The large precipitation amounts associated with the 1982/1983 and 2015/2016 winter flooding were linked to the strongly positive North Atlantic Oscillation (NAO), with large moisture transported from the Gulf of Mexico. The anomalous upper-level trough in the 1982- and 2015- Decembers over the western United States was also favorable for strong precipitation by leading the cold front over the central United States. In contrast, the extremely positive NAO in December 2011 did not lead to heavy rainfall and flooding because the Azores High center shifted too far westward (like a blocking high) preventing moisture from moving towards the central and southeastern United States.
1. Introduction The unseasonal 2015/2016 winter flooding along the Mississippi River had large societal and economic repercussions, with at least 50 fatalities due to the severe weather and flooding across the central United States and over one billion dollars in losses (NOAA, 2016; USGS, 2016). The flooding resulted from strong precipitation events with more than 500 mm of rain in the period from December 12 to December 31, 2015 (USGS, 2016). Much of the rainfall during this period was concentrated in two pulses (14–17 and 26–29 December 2015), with the second one that was stronger and longer lasting (Fig. 1). The first and second pulses are mainly caused by a stationary front and a cold front, respectively (NOAA, 2017). The daily precipitation during the two pulses was characterized by heavy precipitation over broad regions of the central and southeastern United States (Figs. S1–2). Fig. 2 shows the U.S. Geological Survey (USGS) stream gages which recorded an annual maximum daily discharge during December 2015 and January 2016. There are large areas of the central (Iowa, Missouri, Arkansas, Oklahoma, Illinois, Indiana, and Ohio) and southeastern United States (Alabama, Georgia, South Carolina, and Tennessee) which recorded annual maximum peaks during this period, with discharge values that were close or exceeded the record values in Missouri, Illinois, Arkansas, Tennessee, Mississippi and Louisiana (NOAA, 2016). In Missouri alone, there were 16 flood-related fatalities, and the City of St. Louis (Fig. 2b) and 37 counties were declared Federal Disaster Areas because of this
⁎
flooding (NOAA, 2016; USGS, 2016). On 31 December 2015, due to the second pulse of rainfall (Fig. 1) the discharge for the Mississippi River at St. Louis (USGS 07010000) exceeded the major flood level established by the National Weather Service (Fig. 2b and Table S1) and was above the minor flood level for about two weeks. The goal of this work is to provide an understanding of the large-scale environmental conditions that led to these flood events. Previous studies found that the North Atlantic Oscillation (NAO) and the Pacific-North American pattern (PNA) exerted large influence on the precipitation variability in the United States with regional characteristics, particularly for the winter season (e.g., Archambault et al., 2008; Coleman and Rogers, 2003; Henderson and Robinson, 1994; Hurrell, 2002; Hurrell and Deser, 2009; Leathers et al., 1991). NAO is a major large-scale climate mode of atmospheric variability over the extra-tropical Atlantic Ocean, and is characterized by a north-south dipole of sea-level pressure anomalies between the Icelandic Low and the Azores High (e.g., Barnston and Livezey, 1987; Hurrell et al., 2003; Lamb and Peppler, 1987; Myoung et al., 2015; Wallace and Gutzler, 1981; Wettstein and Mearns, 2002). NAO can modulate the climatology and extreme United States precipitation and temperature at different time scales, especially in the eastern United States (e.g., Archambault et al., 2008; Hurrell, 2002; Hurrell and Deser, 2009; Ning and Bradley, 2014, 2015). PNA consists of a 500-hPa planetary-scale wave train originating from the tropical Pacific and arching towards Florida (e.g., Leathers et al., 1991; Oliver, 2005; Wallace and Gutzler, 1981), and it is
Corresponding author. E-mail address: [email protected] (W. Zhang).
http://dx.doi.org/10.1016/j.atmosres.2017.05.014 Received 30 November 2016; Received in revised form 18 April 2017; Accepted 30 May 2017 Available online 31 May 2017 0169-8095/ © 2017 Published by Elsevier B.V.
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W. Zhang, G. Villarini
Fig. 1. The daily time series of rainfall averaged over the region (35°N–45°N, 100°W–85°W) from 1 December 2015 to 31 January 2016.
December 2015 feature three rainfall bands in the central and southeastern United States (Fig. 3b), consistent with what observed in terms of discharge (Fig. 2a). Panels c and d in Fig. 3 show the precipitation anomalies in December 1982 and 2011 when the NAO indices are extremely positive (Fig. 4), and highlight the differences in the rainfall amounts during those months. To diagnose the physical mechanisms, we investigate the time series of December NAO, PNA and precipitation over the central United States precipitation (100°W–85°W, 35°N–45°N; Fig. 4). Previous studies have shown that the negative PNA is associated with large precipitation amount over the central United States (Leathers and Palecki, 1992; Leathers et al., 1991; Ning and Bradley, 2014). However, the PNA index in December 2015 is positive, suggesting that PNA should not be responsible for these extremely large precipitation amounts. NAO has a significant positive correlation with the precipitation amount in central United States for the period 1950–2015 (correlation coefficient of 0.37 significant at the 0.01 level based on t-test) (Fig. 4). Therefore, stronger (weaker) precipitation over the central United States is expected during the positive (negative) NAO phase. In December 2015, there was a strong NAO event, similar to that in December 1982, suggesting that the strong NAO events in the Decembers of 1982 and 2015 were partially responsible for the extremely heavy precipitation (Fig. 3, panels b and c) that caused the unseasonal flood events. Positive (negative) NAO is associated with a stronger (weaker) Azores High center over the western Atlantic, which tends to transport excessive (deficient) moisture from the Atlantic to the continental United States. Such moisture supply in winter plays a central role in modulating precipitation. In December 2011, there was an extremely strong NAO event, but the precipitation was much weaker than what observed in 1982 and 2015 (Figs. 3d and 4). Here we examine the moisture flux, geopotential height and wind fields anomalies in the Decembers of 1982, 2011 and 2015 to diagnose the physical mechanisms underlying the differences in precipitation over the central United States during these three years with large precipitation and/or NAO values. The climatology of moisture flux in 1979–2010 and the anomalies in the Decembers of 1982, 2011 and 2015 are shown in Fig. 5. The moisture flux is vertically integrated between 1000 hPa and 300 hPa. Climatologically, the moisture transport to the continental United States in December occurs from the eastern Pacific by the subtropical jet stream and from the western Atlantic by the Azores High center (Fig. 5a), consistent with previous studies (e.g., Mo et al., 2005). In
one of the most important climate modes influencing the weather and climate over the United States (e.g., Coleman and Rogers, 2003; Henderson and Robinson, 1994; Leathers et al., 1991). During the negative PNA phase, there are more and stronger rainfall events over the central United States, while the opposite is true for the positive PNA phase (e.g., Lavers and Villarini, 2013; Mallakpour and Villarini, 2016, 2017; Patricola et al., 2015; Nayak and Villarini, 2017). While major flooding was observed along the Mississippi River and the central United States making headlines in numerous media outlets, another very wet winter occurred over this area in December 1982 (Stone and Bingham, 1991). Given these competing climate modes (i.e., NAO, PNA), we want to understand which one is the dominant one during these heavy precipitation and flood events. Improved understanding of the climate conditions associated with the 1982/1983 and 2015/2016 winter events can provide basic information critical to improve the predictability of these extremes. 2. Data We use the daily precipitation data by the Climate Prediction Center (CPC) with has a 0.25° × 0.25° spatial resolution over the continental United States (Chen et al., 2008). NAO and PNA are calculated following the method discussed in Barnston and Livezey (1987) and are available from the Climate Prediction Center (http://www.cpc.ncep. noaa.gov/products/precip/CWlink/pna/nao.shtml; http://www.cpc. ncep.noaa.gov/products/precip/CWlink/pna/pna.shtml). The vertically integrated moisture flux, wind fields and geopotential height data are obtained from the European Centre for Medium-Range Weather Forecasts (ECMWF) ERA–Interim reanalysis available from 1979 to the present (Dee et al., 2011). Anomalies are defined as the departures from a long-term average (climatology). 3. Results We focus our analyses of precipitation and relevant large-scale environmental conditions on December 2015 because it was the month with the largest amount of precipitation causing annual maximum discharge peaks over the central United States (Fig. 1). Fig. 3a shows the climatology of precipitation for the 1979–2010 Decembers, with over 300 mm of total precipitation in the eastern edge of Oklahoma, Alabama and northern Georgia. The precipitation anomalies in 24
Atmospheric Research 196 (2017) 23–28
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Fig. 2. (a) Map of the USGS stream gages with daily discharge data during the water year 2016 (blue circles), with the sites for which an annual maximum occurred during December 2015–January 2016 identified with red circles (see also Supplementary Table 1). The dark gray boundaries denote the HUC2 basins affected by these events. (b) Daily discharge time series for the Mississippi River at St. Louis (USGS 07010000) from November 1st 2015 to January 31st 2016. The major, moderate and minor flood levels established by the National Weather Service are also presented as horizontal lines. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
December 2015, there were large moisture flux anomalies transported from the North Atlantic to the continental United States through the Gulf of Mexico (Fig. 5b). In addition, there are similar moist flux anomalies moving from the North Atlantic to the central United States in December 1982 (Fig. 5c). However, the moisture flux anomalies in December 2011 (Fig. 5d) are much weaker than those in 1982 and 2015, particularly for the moisture moving northward towards the central United States. The southern flank of the Azores High center plays an important role in transporting the moisture flux from the North Atlantic to the central United States. Therefore, the differences in the moisture flux transport to the study region between 1982/2015 and
2011 can be due to the Azores High centers associated with NAO (see also Smith and Baeck (2015)). The geopotential height and wind fields at lower, middle and upper levels are analyzed as well (Fig. 6). The upper level (200-hPa) geopotential height and wind fields anomalies in 2015 were characterized by an anomalous high pressure system that extended over the central and eastern United States from the Atlantic, and an anomalous upper-level trough in the western continental United States (Fig. 6a). This anomalous upper-level trough was closely related to the intrusion of the polar jet stream and the cold fronts (Fig. 6a), which have also been identified to influence the flood event in December 2015 (USGS, 2016). The 25
Atmospheric Research 196 (2017) 23–28
W. Zhang, G. Villarini
Fig. 3. (a) The climatology of December total precipitation (shading, unit: mm) for the period 1979–2010, (b)–(d) anomalies of December total precipitation (shading, unit: mm) for the years 2015, 1982 and 2011.
represented by anomalous positive geopotential height and wind fields over the eastern United States (Fig. 6c). The Azores High center played a key role in transporting moisture from the North Atlantic to the central United States, as shown in Fig. 5b. The moist and warm air tended to climb up and precipitate upon interacting with the cold dry air from the polar region (frontal weather), causing extreme precipitation over the central United States (Figs. 3b and 6a–c). The
anomalous middle-level (500-hPa) high in December 2015 marched towards the eastern United States while the anomalous low (upper trough) moved further south to Mexico (Fig. 6b). The anomalous low associated with the polar jet stream and the anomalous Azores High center resulted in a confluence zone over the central United States (Fig. 6b). At the lower-level (850 hPa), the anomalous low moved further south and interacted with the anomalous Azores High
Fig. 4. The December NAO index (red), PNA index (blue) and the normalized average precipitation (black) over the central United States (100°W–85°W, 35°N–45°N) during 1950–2015. The black solid circles denote the average precipitation in December 1982, 2011 and 2015. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
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Atmospheric Research 196 (2017) 23–28
W. Zhang, G. Villarini
Fig. 5. (a) The climatology of December vertically integrated moisture flux (vector, unit: kg·m− 1·s− 1) for the period 1979–2010, (b)–(d) the anomalies of December vertically integrated moisture flux (shading, unit: kg·m− 1·s− 1) in the years 2015, 1982 and 2011.
Fig. 6. The 200 hPa, 500 hPa and 850 hPa geopotential height and wind anomalies in December of (a–c) 2015, (d–f) 1982 and (g–i) 2011.
Despite the very positive NAO index, the environmental conditions during the December 2011 were different from the other two years. The geopotential height anomalies at the 850-hPa level in December 2011 were characterized by an elongated high pressure system over the United States, which acted as a ‘blocking high’ which suppressed precipitation over the continental United States and prevented moisture from moving northward towards the central United States (Fig. 6). The pattern of the 850-hPa geopotential height anomalies (Fig. 6i) was consistent with weak moisture flux anomalies (Fig. 5d) and weak precipitation anomalies (Fig. 3d) over the central United States in December 2011. The lack of an upper-level trough in 2011 may also have led to weaker updraft over the central United States compared to 1982 and 2015. Although the Azores High center in December 2011 was favorable to the transport of moisture westward to the Gulf of Mexico,
subtropical jet stream (850-hPa wind) blowing from the eastern Pacific appeared to be close to climatology with very weak anomalies over the western United States in December 2015 (Fig. 6c). In December 1982, the upper-, middle- and lower-level geopotential height and wind field anomalies were similar to those in 2015, with the Azores High center that extended over the eastern United States and a low pressure system over the western and central United States (Fig. 6a–f). The high pressure systems measured by positive geopotential height anomalies at 200-, 500- and 850-hPa levels in 1982 were slightly weaker than those in 2015 (Fig. 6a–f), consistent with a lower NAO index in 1982 than in 2015 (Fig. 4). This is also consistent with the precipitation in 1982/ 1983 being slightly smaller than what observed during the 2015/2016 winter (Fig. 3). The 1982 and 2015 Decembers shared numerous similarities. 27
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Balmaseda, M., Balsamo, G., Bauer, P., 2011. The ERA-Interim reanalysis: configuration and performance of the data assimilation system. Q. J. R. Meteorol. Soc. 137 (656), 553–597. Henderson, K.G., Robinson, P.J., 1994. Relationships between the Pacific/North American teleconnection patterns and precipitation events in the south-eastern USA. Int. J. Climatol. 14 (3), 307–323. Hurrell, J.W., 2002. Decadal trends in the North Atlantic oscillation. In: Climate Change: Evaluating Recent and Future Climate Change. 4. pp. 201. Hurrell, J.W., Deser, C., 2009. North Atlantic climate variability: the role of the North Atlantic Oscillation. J. Mar. Syst. 78 (1), 28–41. Hurrell, J.W., Kushnir, Y., Ottersen, G., Visbeck, M., 2003. An Overview of the North Atlantic Oscillation. Wiley Online Library. Lamb, P.J., Peppler, R.A., 1987. North Atlantic oscillation: concept and an application. Bull. Am. Meteorol. Soc. 68 (10), 1218–1225. Lavers, D.A., Villarini, G., 2013. 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Mo, K.C., Chelliah, M., Carrera, M.L., Higgins, R.W., Ebisuzaki, W., 2005. Atmospheric moisture transport over the United States and Mexico as evaluated in the NCEP regional reanalysis. J. Hydrometeorol. 6 (5), 710–728. Myoung, B., Kim, S.H., Kim, J., Kafatos, M.C., 2015. On the relationship between the North Atlantic Oscillation and early warm season temperatures in the Southwestern United States. J. Clim. 28 (14), 5683–5698. Nayak, M.A., Villarini, G., 2017. A long-term perspective of the hydroclimatological impacts of atmospheric rivers over the central United States. Water Resour. Res. 53, 1144–1166. Ning, L., Bradley, R.S., 2014. Winter precipitation variability and corresponding teleconnections over the northeastern United States. J. Geophys. Res. Atmos. 119 (13), 7931–7945. Ning, L., Bradley, R.S., 2015. Winter climate extremes over the northeastern United States and southeastern Canada and teleconnections with large-scale modes of climate variability. J. 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Res. 51 (12), 9964–9994. http://dx.doi.org/ 10.1002/2015WR017927. Stone, Roy B., Bingham, Roy H., 1991. Floods of December 1982 to May 1983 in the central and southern Mississippi River and the Gulf of Mexico basins. In: USGS Numbered Series 2362. USGS, 2016. Preliminary Peak Stage and Streamflow Data at Selected U.S. Geological Survey Streamgages for Flooding in the Central and Southeastern United States During December 2015 and January 2016. (2016–1092). Wallace, J.M., Gutzler, D.S., 1981. Teleconnections in the geopotential height field during the Northern Hemisphere winter. Mon. Weather Rev. 109 (4), 784–812. Wang, S.Y.S., Zhao, L., Gillies, R.R., 2016. Synoptic and quantitative attributions of the extreme precipitation leading to the August 2016 Louisiana flood. Geophys. Res. Lett. 43, 11805–11814. http://dx.doi.org/10.1002/2016GL071460. Wettstein, J.J., Mearns, L.O., 2002. The influence of the North Atlantic-Arctic Oscillation on mean, variance, and extremes of temperature in the northeastern United States and Canada. J. Clim. 15 (24), 3586–3600. van der Wiel, K., Kapnick, S.B., van Oldenborgh, G.J., Whan, K., Philip, S., Vecchi, G.A., Singh, R.K., Arrighi, J., Cullen, H., 2016. Rapid attribution of the August 2016 floodinducing extreme precipitation in south Louisiana to climate change. Hydrol. Earth Syst. Sci. 21 (897–921), 2017.
the blocking-like system located over the continental United States did not enable the northward moisture transport over the central United States; moreover, little atmospheric moisture can precipitate due to the lack of an upper-level trough having dry and cold air critical for the lift of the moist and humid air. 4. Discussion and conclusion The unseasonal winter flood events during December 2015–January 2016 had large economic and societal impacts for the central United States. We have examined the climatic conditions related to the occurrence of these flood events, and compared and contrasted them with the 1982/1983 and 2011/2012 winters. The extremely large precipitation amounts responsible for the 1982/1983 and 2015/2016 flood events were linked to the strong NAO, with strong moisture fluxes from the Gulf of Mexico. The anomalous upper-level trough in the Decembers of 2015 and 1982 over the western United States was also favorable for strong precipitation by leading the cold front over the central United States. In contrast, the extremely strong NAO in December 2011 was not associated with heavy precipitation and flooding because the Azores High center shifted far too westward (like a blocking high) preventing moisture moving towards the central and southeastern United States. A better understanding of the impacts of climate modes on extreme precipitation and flood events over the central United States will benefit the preparedness, administration and management of floods. Future studies will examine the association between NAO and winter precipitation over the continental United States in climate model simulations. Considering that the devastating Louisiana flood in August 2016 has been attributed to anthropogenic forcing (van der Wiel et al., 2016; Wang et al., 2016), our future work will examine the possible impacts of anthropogenic forcing on the frequency and magnitude of winter flood events. Acknowledgement This material is based upon work supported by the Broad Agency Announcement (BAA) Program and the Engineer Research and Development Center (ERDC)–Cold Regions Research and Engineering Laboratory (CRREL) under Contract No. W913E5-16-C-0002, IIHRHydroscience & Engineering, the Iowa Flood Center, and the National Science Foundation under CAREER Grant AGS-1349827. Appendix A. Supplementary data Supplementary data to this article can be found online at http://dx. doi.org/10.1016/j.atmosres.2017.05.014. References Archambault, H.M., Bosart, L.F., Keyser, D., Aiyyer, A.R., 2008. Influence of large-scale flow regimes on cool-season precipitation in the northeastern United States. Mon. Weather Rev. 136 (8), 2945–2963. Barnston, A.G., Livezey, R.E., 1987. Classification, seasonality and persistence of lowfrequency atmospheric circulation patterns. Mon. Weather Rev. 115 (6), 1083–1126. Chen, M., Shi, W., Xie, P., Silva, V., Kousky, V.E., Wayne Higgins, R., Janowiak, J.E., 2008. Assessing objective techniques for gauge-based analyses of global daily precipitation. J. Geophys. Res. Atmos. 113 (D4). Coleman, J.S., Rogers, J.C., 2003. Ohio River Valley winter moisture conditions associated with the Pacific–North American teleconnection pattern. J. Clim. 16 (6), 969–981. Dee, D., Uppala, S., Simmons, A., Berrisford, P., Poli, P., Kobayashi, S., Andrae, U.,
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On the unseasonal flooding over the Central United States during December 2015 and January 2016
MARK
Wei Zhang⁎, Gabriele Villarini IIHR-Hydroscience & Engineering, The University of Iowa, Iowa City, Iowa, USA
A B S T R A C T The unseasonal winter heavy rainfall and flooding that occurred during December 2015–January 2016 had large socio-economic impacts for the central United States. Here we examine the climatic conditions that led to the observed extreme precipitation, and compare and contrast them with the 1982/1983 and 2011/2012 winters. The large precipitation amounts associated with the 1982/1983 and 2015/2016 winter flooding were linked to the strongly positive North Atlantic Oscillation (NAO), with large moisture transported from the Gulf of Mexico. The anomalous upper-level trough in the 1982- and 2015- Decembers over the western United States was also favorable for strong precipitation by leading the cold front over the central United States. In contrast, the extremely positive NAO in December 2011 did not lead to heavy rainfall and flooding because the Azores High center shifted too far westward (like a blocking high) preventing moisture from moving towards the central and southeastern United States.
1. Introduction The unseasonal 2015/2016 winter flooding along the Mississippi River had large societal and economic repercussions, with at least 50 fatalities due to the severe weather and flooding across the central United States and over one billion dollars in losses (NOAA, 2016; USGS, 2016). The flooding resulted from strong precipitation events with more than 500 mm of rain in the period from December 12 to December 31, 2015 (USGS, 2016). Much of the rainfall during this period was concentrated in two pulses (14–17 and 26–29 December 2015), with the second one that was stronger and longer lasting (Fig. 1). The first and second pulses are mainly caused by a stationary front and a cold front, respectively (NOAA, 2017). The daily precipitation during the two pulses was characterized by heavy precipitation over broad regions of the central and southeastern United States (Figs. S1–2). Fig. 2 shows the U.S. Geological Survey (USGS) stream gages which recorded an annual maximum daily discharge during December 2015 and January 2016. There are large areas of the central (Iowa, Missouri, Arkansas, Oklahoma, Illinois, Indiana, and Ohio) and southeastern United States (Alabama, Georgia, South Carolina, and Tennessee) which recorded annual maximum peaks during this period, with discharge values that were close or exceeded the record values in Missouri, Illinois, Arkansas, Tennessee, Mississippi and Louisiana (NOAA, 2016). In Missouri alone, there were 16 flood-related fatalities, and the City of St. Louis (Fig. 2b) and 37 counties were declared Federal Disaster Areas because of this
⁎
flooding (NOAA, 2016; USGS, 2016). On 31 December 2015, due to the second pulse of rainfall (Fig. 1) the discharge for the Mississippi River at St. Louis (USGS 07010000) exceeded the major flood level established by the National Weather Service (Fig. 2b and Table S1) and was above the minor flood level for about two weeks. The goal of this work is to provide an understanding of the large-scale environmental conditions that led to these flood events. Previous studies found that the North Atlantic Oscillation (NAO) and the Pacific-North American pattern (PNA) exerted large influence on the precipitation variability in the United States with regional characteristics, particularly for the winter season (e.g., Archambault et al., 2008; Coleman and Rogers, 2003; Henderson and Robinson, 1994; Hurrell, 2002; Hurrell and Deser, 2009; Leathers et al., 1991). NAO is a major large-scale climate mode of atmospheric variability over the extra-tropical Atlantic Ocean, and is characterized by a north-south dipole of sea-level pressure anomalies between the Icelandic Low and the Azores High (e.g., Barnston and Livezey, 1987; Hurrell et al., 2003; Lamb and Peppler, 1987; Myoung et al., 2015; Wallace and Gutzler, 1981; Wettstein and Mearns, 2002). NAO can modulate the climatology and extreme United States precipitation and temperature at different time scales, especially in the eastern United States (e.g., Archambault et al., 2008; Hurrell, 2002; Hurrell and Deser, 2009; Ning and Bradley, 2014, 2015). PNA consists of a 500-hPa planetary-scale wave train originating from the tropical Pacific and arching towards Florida (e.g., Leathers et al., 1991; Oliver, 2005; Wallace and Gutzler, 1981), and it is
Corresponding author. E-mail address: [email protected] (W. Zhang).
http://dx.doi.org/10.1016/j.atmosres.2017.05.014 Received 30 November 2016; Received in revised form 18 April 2017; Accepted 30 May 2017 Available online 31 May 2017 0169-8095/ © 2017 Published by Elsevier B.V.
Atmospheric Research 196 (2017) 23–28
W. Zhang, G. Villarini
Fig. 1. The daily time series of rainfall averaged over the region (35°N–45°N, 100°W–85°W) from 1 December 2015 to 31 January 2016.
December 2015 feature three rainfall bands in the central and southeastern United States (Fig. 3b), consistent with what observed in terms of discharge (Fig. 2a). Panels c and d in Fig. 3 show the precipitation anomalies in December 1982 and 2011 when the NAO indices are extremely positive (Fig. 4), and highlight the differences in the rainfall amounts during those months. To diagnose the physical mechanisms, we investigate the time series of December NAO, PNA and precipitation over the central United States precipitation (100°W–85°W, 35°N–45°N; Fig. 4). Previous studies have shown that the negative PNA is associated with large precipitation amount over the central United States (Leathers and Palecki, 1992; Leathers et al., 1991; Ning and Bradley, 2014). However, the PNA index in December 2015 is positive, suggesting that PNA should not be responsible for these extremely large precipitation amounts. NAO has a significant positive correlation with the precipitation amount in central United States for the period 1950–2015 (correlation coefficient of 0.37 significant at the 0.01 level based on t-test) (Fig. 4). Therefore, stronger (weaker) precipitation over the central United States is expected during the positive (negative) NAO phase. In December 2015, there was a strong NAO event, similar to that in December 1982, suggesting that the strong NAO events in the Decembers of 1982 and 2015 were partially responsible for the extremely heavy precipitation (Fig. 3, panels b and c) that caused the unseasonal flood events. Positive (negative) NAO is associated with a stronger (weaker) Azores High center over the western Atlantic, which tends to transport excessive (deficient) moisture from the Atlantic to the continental United States. Such moisture supply in winter plays a central role in modulating precipitation. In December 2011, there was an extremely strong NAO event, but the precipitation was much weaker than what observed in 1982 and 2015 (Figs. 3d and 4). Here we examine the moisture flux, geopotential height and wind fields anomalies in the Decembers of 1982, 2011 and 2015 to diagnose the physical mechanisms underlying the differences in precipitation over the central United States during these three years with large precipitation and/or NAO values. The climatology of moisture flux in 1979–2010 and the anomalies in the Decembers of 1982, 2011 and 2015 are shown in Fig. 5. The moisture flux is vertically integrated between 1000 hPa and 300 hPa. Climatologically, the moisture transport to the continental United States in December occurs from the eastern Pacific by the subtropical jet stream and from the western Atlantic by the Azores High center (Fig. 5a), consistent with previous studies (e.g., Mo et al., 2005). In
one of the most important climate modes influencing the weather and climate over the United States (e.g., Coleman and Rogers, 2003; Henderson and Robinson, 1994; Leathers et al., 1991). During the negative PNA phase, there are more and stronger rainfall events over the central United States, while the opposite is true for the positive PNA phase (e.g., Lavers and Villarini, 2013; Mallakpour and Villarini, 2016, 2017; Patricola et al., 2015; Nayak and Villarini, 2017). While major flooding was observed along the Mississippi River and the central United States making headlines in numerous media outlets, another very wet winter occurred over this area in December 1982 (Stone and Bingham, 1991). Given these competing climate modes (i.e., NAO, PNA), we want to understand which one is the dominant one during these heavy precipitation and flood events. Improved understanding of the climate conditions associated with the 1982/1983 and 2015/2016 winter events can provide basic information critical to improve the predictability of these extremes. 2. Data We use the daily precipitation data by the Climate Prediction Center (CPC) with has a 0.25° × 0.25° spatial resolution over the continental United States (Chen et al., 2008). NAO and PNA are calculated following the method discussed in Barnston and Livezey (1987) and are available from the Climate Prediction Center (http://www.cpc.ncep. noaa.gov/products/precip/CWlink/pna/nao.shtml; http://www.cpc. ncep.noaa.gov/products/precip/CWlink/pna/pna.shtml). The vertically integrated moisture flux, wind fields and geopotential height data are obtained from the European Centre for Medium-Range Weather Forecasts (ECMWF) ERA–Interim reanalysis available from 1979 to the present (Dee et al., 2011). Anomalies are defined as the departures from a long-term average (climatology). 3. Results We focus our analyses of precipitation and relevant large-scale environmental conditions on December 2015 because it was the month with the largest amount of precipitation causing annual maximum discharge peaks over the central United States (Fig. 1). Fig. 3a shows the climatology of precipitation for the 1979–2010 Decembers, with over 300 mm of total precipitation in the eastern edge of Oklahoma, Alabama and northern Georgia. The precipitation anomalies in 24
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Fig. 2. (a) Map of the USGS stream gages with daily discharge data during the water year 2016 (blue circles), with the sites for which an annual maximum occurred during December 2015–January 2016 identified with red circles (see also Supplementary Table 1). The dark gray boundaries denote the HUC2 basins affected by these events. (b) Daily discharge time series for the Mississippi River at St. Louis (USGS 07010000) from November 1st 2015 to January 31st 2016. The major, moderate and minor flood levels established by the National Weather Service are also presented as horizontal lines. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
December 2015, there were large moisture flux anomalies transported from the North Atlantic to the continental United States through the Gulf of Mexico (Fig. 5b). In addition, there are similar moist flux anomalies moving from the North Atlantic to the central United States in December 1982 (Fig. 5c). However, the moisture flux anomalies in December 2011 (Fig. 5d) are much weaker than those in 1982 and 2015, particularly for the moisture moving northward towards the central United States. The southern flank of the Azores High center plays an important role in transporting the moisture flux from the North Atlantic to the central United States. Therefore, the differences in the moisture flux transport to the study region between 1982/2015 and
2011 can be due to the Azores High centers associated with NAO (see also Smith and Baeck (2015)). The geopotential height and wind fields at lower, middle and upper levels are analyzed as well (Fig. 6). The upper level (200-hPa) geopotential height and wind fields anomalies in 2015 were characterized by an anomalous high pressure system that extended over the central and eastern United States from the Atlantic, and an anomalous upper-level trough in the western continental United States (Fig. 6a). This anomalous upper-level trough was closely related to the intrusion of the polar jet stream and the cold fronts (Fig. 6a), which have also been identified to influence the flood event in December 2015 (USGS, 2016). The 25
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Fig. 3. (a) The climatology of December total precipitation (shading, unit: mm) for the period 1979–2010, (b)–(d) anomalies of December total precipitation (shading, unit: mm) for the years 2015, 1982 and 2011.
represented by anomalous positive geopotential height and wind fields over the eastern United States (Fig. 6c). The Azores High center played a key role in transporting moisture from the North Atlantic to the central United States, as shown in Fig. 5b. The moist and warm air tended to climb up and precipitate upon interacting with the cold dry air from the polar region (frontal weather), causing extreme precipitation over the central United States (Figs. 3b and 6a–c). The
anomalous middle-level (500-hPa) high in December 2015 marched towards the eastern United States while the anomalous low (upper trough) moved further south to Mexico (Fig. 6b). The anomalous low associated with the polar jet stream and the anomalous Azores High center resulted in a confluence zone over the central United States (Fig. 6b). At the lower-level (850 hPa), the anomalous low moved further south and interacted with the anomalous Azores High
Fig. 4. The December NAO index (red), PNA index (blue) and the normalized average precipitation (black) over the central United States (100°W–85°W, 35°N–45°N) during 1950–2015. The black solid circles denote the average precipitation in December 1982, 2011 and 2015. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
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Fig. 5. (a) The climatology of December vertically integrated moisture flux (vector, unit: kg·m− 1·s− 1) for the period 1979–2010, (b)–(d) the anomalies of December vertically integrated moisture flux (shading, unit: kg·m− 1·s− 1) in the years 2015, 1982 and 2011.
Fig. 6. The 200 hPa, 500 hPa and 850 hPa geopotential height and wind anomalies in December of (a–c) 2015, (d–f) 1982 and (g–i) 2011.
Despite the very positive NAO index, the environmental conditions during the December 2011 were different from the other two years. The geopotential height anomalies at the 850-hPa level in December 2011 were characterized by an elongated high pressure system over the United States, which acted as a ‘blocking high’ which suppressed precipitation over the continental United States and prevented moisture from moving northward towards the central United States (Fig. 6). The pattern of the 850-hPa geopotential height anomalies (Fig. 6i) was consistent with weak moisture flux anomalies (Fig. 5d) and weak precipitation anomalies (Fig. 3d) over the central United States in December 2011. The lack of an upper-level trough in 2011 may also have led to weaker updraft over the central United States compared to 1982 and 2015. Although the Azores High center in December 2011 was favorable to the transport of moisture westward to the Gulf of Mexico,
subtropical jet stream (850-hPa wind) blowing from the eastern Pacific appeared to be close to climatology with very weak anomalies over the western United States in December 2015 (Fig. 6c). In December 1982, the upper-, middle- and lower-level geopotential height and wind field anomalies were similar to those in 2015, with the Azores High center that extended over the eastern United States and a low pressure system over the western and central United States (Fig. 6a–f). The high pressure systems measured by positive geopotential height anomalies at 200-, 500- and 850-hPa levels in 1982 were slightly weaker than those in 2015 (Fig. 6a–f), consistent with a lower NAO index in 1982 than in 2015 (Fig. 4). This is also consistent with the precipitation in 1982/ 1983 being slightly smaller than what observed during the 2015/2016 winter (Fig. 3). The 1982 and 2015 Decembers shared numerous similarities. 27
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the blocking-like system located over the continental United States did not enable the northward moisture transport over the central United States; moreover, little atmospheric moisture can precipitate due to the lack of an upper-level trough having dry and cold air critical for the lift of the moist and humid air. 4. Discussion and conclusion The unseasonal winter flood events during December 2015–January 2016 had large economic and societal impacts for the central United States. We have examined the climatic conditions related to the occurrence of these flood events, and compared and contrasted them with the 1982/1983 and 2011/2012 winters. The extremely large precipitation amounts responsible for the 1982/1983 and 2015/2016 flood events were linked to the strong NAO, with strong moisture fluxes from the Gulf of Mexico. The anomalous upper-level trough in the Decembers of 2015 and 1982 over the western United States was also favorable for strong precipitation by leading the cold front over the central United States. In contrast, the extremely strong NAO in December 2011 was not associated with heavy precipitation and flooding because the Azores High center shifted far too westward (like a blocking high) preventing moisture moving towards the central and southeastern United States. A better understanding of the impacts of climate modes on extreme precipitation and flood events over the central United States will benefit the preparedness, administration and management of floods. Future studies will examine the association between NAO and winter precipitation over the continental United States in climate model simulations. Considering that the devastating Louisiana flood in August 2016 has been attributed to anthropogenic forcing (van der Wiel et al., 2016; Wang et al., 2016), our future work will examine the possible impacts of anthropogenic forcing on the frequency and magnitude of winter flood events. Acknowledgement This material is based upon work supported by the Broad Agency Announcement (BAA) Program and the Engineer Research and Development Center (ERDC)–Cold Regions Research and Engineering Laboratory (CRREL) under Contract No. W913E5-16-C-0002, IIHRHydroscience & Engineering, the Iowa Flood Center, and the National Science Foundation under CAREER Grant AGS-1349827. Appendix A. Supplementary data Supplementary data to this article can be found online at http://dx. doi.org/10.1016/j.atmosres.2017.05.014. References Archambault, H.M., Bosart, L.F., Keyser, D., Aiyyer, A.R., 2008. Influence of large-scale flow regimes on cool-season precipitation in the northeastern United States. Mon. Weather Rev. 136 (8), 2945–2963. Barnston, A.G., Livezey, R.E., 1987. Classification, seasonality and persistence of lowfrequency atmospheric circulation patterns. Mon. Weather Rev. 115 (6), 1083–1126. Chen, M., Shi, W., Xie, P., Silva, V., Kousky, V.E., Wayne Higgins, R., Janowiak, J.E., 2008. Assessing objective techniques for gauge-based analyses of global daily precipitation. J. Geophys. Res. Atmos. 113 (D4). Coleman, J.S., Rogers, J.C., 2003. Ohio River Valley winter moisture conditions associated with the Pacific–North American teleconnection pattern. J. Clim. 16 (6), 969–981. Dee, D., Uppala, S., Simmons, A., Berrisford, P., Poli, P., Kobayashi, S., Andrae, U.,
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