• Adler, R. F., and Coauthors, 2003: The version-2 Global Precipitation Climatology Project (GPCP) monthly precipitation analysis (1979–present). J. Hydrometeor., 4, 11471167, https://doi.org/10.1175/1525-7541(2003)004<1147:TVGPCP>2.0.CO;2.

    • Search Google Scholar
    • Export Citation
  • An, W., X. Liu, S. W. Leavitt, G. Xu, X. Zeng, W. Wang, D. Qin, and J. Ren, 2014: Relative humidity history on the Batang–Litang Plateau of western China since 1755 reconstructed from tree-ring δ18O and δD data. Climate Dyn., 42, 26392654, https://doi.org/10.1007/s00382-013-1937-z.

    • Search Google Scholar
    • Export Citation
  • Borges, M. D., and D. L. Hartmann, 1992: Barotropic instability and optimal perturbations of observed nonzonal flows. J. Atmos. Sci., 49, 335354, https://doi.org/10.1175/1520-0469(1992)049<0335:BIAOPO>2.0.CO;2.

    • Search Google Scholar
    • Export Citation
  • Borges, M. D., and P. D. Sardeshmukh, 1995: Barotropic Rossby wave dynamics of zonally varying upper-level flows during northern winter. J. Atmos. Sci., 52, 37793796, https://doi.org/10.1175/1520-0469(1995)052<3779:BRWDOZ>2.0.CO;2.

    • Search Google Scholar
    • Export Citation
  • Branstator, G., 1995: Organization of storm track anomalies by recurring low-frequency circulation anomalies. J. Atmos. Sci., 52, 207226, https://doi.org/10.1175/1520-0469(1995)052<0207:OOSTAB>2.0.CO;2.

    • Search Google Scholar
    • Export Citation
  • Branstator, G., 2002: Circumglobal teleconnections, the jet stream waveguide, and the North Atlantic Oscillation. J. Climate, 15, 18931910, https://doi.org/10.1175/1520-0442(2002)015<1893:CTTJSW>2.0.CO;2.

    • Search Google Scholar
    • Export Citation
  • Bretherton, C. S., C. S. Smith, and J. M. Wallace, 1992: An intercomparison of methods for finding coupled patterns in climate data. J. Climate, 5, 541560, https://doi.org/10.1175/1520-0442(1992)005<0541:AIOMFF>2.0.CO;2.

    • Search Google Scholar
    • Export Citation
  • Brewer, M. C., and C. F. Mass, 2016a: Projected changes in western U.S. large-scale summer synoptic circulations and variability in CMIP5 models. J. Climate, 29, 59655978, https://doi.org/10.1175/JCLI-D-15-0598.1.

    • Search Google Scholar
    • Export Citation
  • Brewer, M. C., and C. F. Mass, 2016b: Projected changes in heat extremes and associated synoptic- and mesoscale conditions over the Northwest United States. J. Climate, 29, 63836400, https://doi.org/10.1175/JCLI-D-15-0641.1.

    • Search Google Scholar
    • Export Citation
  • Chang, E. K. M., C. G. Ma, C. Zheng, and A. M. W. Yau, 2016: Observed and projected decrease in Northern Hemisphere extratropical cyclone activity in summer and its impacts on maximum temperature. Geophys. Res. Lett., 43, 22002208, https://doi.org/10.1002/2016GL068172.

    • Search Google Scholar
    • Export Citation
  • Chang, J.-C., and M. Mak, 1995: Nonmodal barotropic dynamics of the intraseasonal disturbances. J. Atmos. Sci., 52, 896914, https://doi.org/10.1175/1520-0469(1995)052<0896:NBDOTI>2.0.CO;2.

    • Search Google Scholar
    • Export Citation
  • Cohen, J., and Coauthors, 2014: Recent Arctic amplification and extreme mid-latitude weather. Nat. Geosci., 7, 627637, https://doi.org/10.1038/ngeo2234.

    • Search Google Scholar
    • Export Citation
  • Coumou, D., and S. Rahmstorf, 2012: A decade of weather extremes. Nat. Climate Change, 2, 491496, https://doi.org/10.1038/nclimate1452.

    • Search Google Scholar
    • Export Citation
  • Coumou, D., J. Lehmann, and J. Beckmann, 2015: The weakening summer circulation in the Northern Hemisphere mid-latitudes. Science, 348, 324327, https://doi.org/10.1126/science.1261768.

    • Search Google Scholar
    • Export Citation
  • Coumou, D., G. Di Capua, S. Vavrus, L. Wang, and S. Wang, 2018: The influence of Arctic amplification on mid-latitude summer circulation. Nat. Commun., 9, 2959, https://doi.org/10.1038/s41467-018-05256-8.

    • Search Google Scholar
    • Export Citation
  • Czaja, A., and C. Frankignoul, 2002: Observed impact of Atlantic SST anomalies on the North Atlantic Oscillation. J. Climate, 15, 606623, https://doi.org/10.1175/1520-0442(2002)015<0606:OIOASA>2.0.CO;2.

    • Search Google Scholar
    • Export Citation
  • Dai, A., 2013: The influence of the inter-decadal Pacific oscillation on U.S. precipitation during 1923–2010. Climate Dyn., 41, 633646, https://doi.org/10.1007/s00382-012-1446-5.

    • Search Google Scholar
    • Export Citation
  • Dee, D. P., and Coauthors, 2011: The ERA-Interim reanalysis: Configuration and performance of the data assimilation system. Quart. J. Roy. Meteor. Soc., 137, 553597, https://doi.org/10.1002/qj.828.

    • Search Google Scholar
    • Export Citation
  • Ding, Q. H., and B. Wang, 2005: Circumglobal teleconnection in the Northern Hemisphere summer. J. Climate, 18, 34833505, https://doi.org/10.1175/JCLI3473.1.

    • Search Google Scholar
    • Export Citation
  • Donat, M., A. Lowry, L. Alexander, P. O’Gorman, and N. Maher, 2016: More extreme precipitation in the world’s dry and wet regions. Nat. Climate Change, 6, 508513, https://doi.org/10.1038/nclimate2941.

    • Search Google Scholar
    • Export Citation
  • Emanuel, K., 2005: Increasing destructiveness of tropical cyclones over the past 30 years. Nature, 436, 686688, https://doi.org/10.1038/nature03906.

    • Search Google Scholar
    • Export Citation
  • Emanuel, K., M. Fantini, and A. J. Thorpe, 1987: Baroclinic instability in an environment of small stability to slantwise moist convection. Part I: Two-dimensional models. J. Atmos. Sci., 44, 15591573, https://doi.org/10.1175/1520-0469(1987)044<1559:BIIAEO>2.0.CO;2.

    • Search Google Scholar
    • Export Citation
  • Farrell, B. F., 1988: Optimal excitation of neutral Rossby waves. J. Atmos. Sci., 45, 163172, https://doi.org/10.1175/1520-0469(1988)045<0163:OEONRW>2.0.CO;2.

    • Search Google Scholar
    • Export Citation
  • Farrell, B. F., 1989: Optimal excitation of baroclinic waves. J. Atmos. Sci., 46, 11931206, https://doi.org/10.1175/1520-0469(1989)046<1193:OEOBW>2.0.CO;2.

    • Search Google Scholar
    • Export Citation
  • Fischer, E. M., and R. Knutti, 2015: Anthropogenic contribution to global occurrence of heavy-precipitation and high-temperature extremes. Nat. Climate Change, 5, 560564, https://doi.org/10.1038/nclimate2617.

    • Search Google Scholar
    • Export Citation
  • Gritsun, A., G. Branstator, and A. Majda, 2008: Climate response of linear and quadratic functionals using the fluctuation–dissipation theorem. J. Atmos. Sci., 65, 28242841, https://doi.org/10.1175/2007JAS2496.1.

    • Search Google Scholar
    • Export Citation
  • Groisman, P. Ya., and Coauthors, 1999: Changes in the probability of heavy precipitation: Important indicators of climatic change. Climatic Change, 42, 243283, https://doi.org/10.1023/A:1005432803188.

    • Search Google Scholar
    • Export Citation
  • Groisman, P. Ya., R. W. Knight, D. Easterling, T. Karl, G. Hegerl, and V. Razuvaev, 2005: Trends in intense precipitation in the climate record. J. Climate, 18, 13261350, https://doi.org/10.1175/JCLI3339.1.

    • Search Google Scholar
    • Export Citation
  • Hakim, G. J., 2000: Role of nonmodal growth and nonlinearity in cyclogenesis initial-value problems. J. Atmos. Sci., 57, 29512967, https://doi.org/10.1175/1520-0469(2000)057<2951:RONGAN>2.0.CO;2.

    • Search Google Scholar
    • Export Citation
  • Hall, N. M., and P. D. Sardeshmukh, 1998: Is the time-mean Northern Hemisphere flow baroclinically unstable? J. Atmos. Sci., 55, 4156, https://doi.org/10.1175/1520-0469(1998)055<0041:ITTMNH>2.0.CO;2.

    • Search Google Scholar
    • Export Citation
  • Harding, K. J., and P. K. Snyder, 2015: The relationship between the Pacific–North American teleconnection pattern, the Great Plains low-level jet, and North Central U.S. heavy rainfall events. J. Climate, 28, 67296742, https://doi.org/10.1175/JCLI-D-14-00657.1.

    • Search Google Scholar
    • Export Citation
  • Hartmann, D. L., 2016: Matrix methods for analysis of structure in data sets. Matrix methods: EOF, SVD, ETC., University of Washington Department of Atmospheric Sciences objective analysis course notes, 103 pp, https://atmos.washington.edu/~dennis/552_Notes_4.pdf.

  • Held, I. M., 1993: Large-scale dynamics and global warming. Bull. Amer. Meteor. Soc., 74, 228242, https://doi.org/10.1175/1520-0477(1993)074<0228:LSDAGW>2.0.CO;2.

    • Search Google Scholar
    • Export Citation
  • Held, I. M., and B. J. Soden, 2006: Robust responses of the hydrological cycle to global warming. J. Climate, 19, 56865699, https://doi.org/10.1175/JCLI3990.1.

    • Search Google Scholar
    • Export Citation
  • Hoskins, B. J., and T. Ambrizzi, 1993: Rossby wave propagation on a realistic longitudinally varying flow. J. Atmos. Sci., 50, 16611671, https://doi.org/10.1175/1520-0469(1993)050<1661:RWPOAR>2.0.CO;2.

    • Search Google Scholar
    • Export Citation
  • Huang, H.-P., 1999: Scale-dependent properties of optimal perturbations on a zonally varying barotropic flow. J. Atmos. Sci., 56, 12381247, https://doi.org/10.1175/1520-0469(1999)056<1238:SDPOOP>2.0.CO;2.

    • Search Google Scholar
    • Export Citation
  • Infanti, J. M., and B. P. Kirtman, 2016: North American rainfall and temperature prediction response to the diversity of ENSO. Climate Dyn., 46, 30073023, https://doi.org/10.1007/s00382-015-2749-0.

    • Search Google Scholar
    • Export Citation
  • Jaeger, E. B., and S. I. Seneviratne, 2011: Impact of soil moisture–atmosphere coupling on European climate extremes and trends in a regional climate model. Climate Dyn., 36, 19191939, https://doi.org/10.1007/s00382-010-0780-8.

    • Search Google Scholar
    • Export Citation
  • Kalnay, E., and Coauthors, 1996: The NCEP/NCAR 40-Year Reanalysis Project. Bull. Amer. Meteor. Soc., 77, 437471, https://doi.org/10.1175/1520-0477(1996)077<0437:TNYRP>2.0.CO;2.

    • Search Google Scholar
    • Export Citation
  • Kornhuber, K., S. Osprey, D. Coumou, S. Petri, V. Petoukhov, S. Rahmstorf, and L. Gray, 2019: Extreme weather events in early summer 2018 connected by a recurrent hemispheric wave-7 pattern. Environ. Res. Lett., 14, 054002, https://doi.org/10.1088/1748-9326/ab13bf.

    • Search Google Scholar
    • Export Citation
  • Lee, W., and M. Mak, 1995: Dynamics of storm tracks: A linear instability perspective. J. Atmos. Sci., 52, 697723, https://doi.org/10.1175/1520-0469(1995)052<0697:DOSTAL>2.0.CO;2.

    • Search Google Scholar
    • Export Citation
  • Lehmann, J., and D. Coumou, 2015: The influence of mid-latitude storm tracks on hot, cold, dry and wet extremes. Sci. Rep., 5, 17491, https://doi.org/10.1038/srep17491.

    • Search Google Scholar
    • Export Citation
  • Lehmann, J., D. Coumou, and K. Frieler, 2015: Increased record-breaking precipitation events under global warming. Climatic Change, 132, 501515, https://doi.org/10.1007/s10584-015-1434-y.

    • Search Google Scholar
    • Export Citation
  • Lehmann, J., F. Mempel, and D. Coumou, 2018: Increased occurrence of record-wet and record-dry months reflect changes in mean rainfall. Geophys. Res. Lett., 45, 13 46813 476, https://doi.org/10.1029/2018GL079439.

    • Search Google Scholar
    • Export Citation
  • Li, R. C., W. Zhou, and T. Li, 2014: Influences of the Pacific–Japan teleconnection pattern on synoptic-scale variability in the western North Pacific. J. Climate, 27, 140154, https://doi.org/10.1175/JCLI-D-13-00183.1.

    • Search Google Scholar
    • Export Citation
  • Li, Y., D. W. J. Thompson, and S. Bony, 2015: The influence of atmospheric cloud radiative effects on the large-scale atmospheric circulation. J. Climate, 28, 72637278, https://doi.org/10.1175/JCLI-D-14-00825.1.

    • Search Google Scholar
    • Export Citation
  • Lin, M., and P. Huybers, 2016: Revisiting whether recent surface temperature trends agree with the CMIP5 ensemble. J. Climate, 29, 86738687, https://doi.org/10.1175/JCLI-D-16-0123.1.

    • Search Google Scholar
    • Export Citation
  • Lorenz, R., E. B. Jaeger, and S. I. Seneviratne, 2010: Persistence of heat waves and its link to soil moisture memory. Geophys. Res. Lett., 37, L09703, https://doi.org/10.1029/2010GL042764.

    • Search Google Scholar
    • Export Citation
  • Mak, M., 2011a: Chapter 6.8: Influences of the Earth’s sphericity in the QG theory. Atmospheric Dynamics, Cambridge Press, 182–186.

  • Mak, M., 2011b: Chapter 8B.3: Optimal growth of barotropic disturbance. Atmospheric Dynamics, Cambridge Press, 235–240.

  • Mak, M., and M. Cai, 1989: Local barotropic instability. J. Atmos. Sci., 46, 32893311, https://doi.org/10.1175/1520-0469(1989)046<3289:LBI>2.0.CO;2.

    • Search Google Scholar
    • Export Citation
  • McCrary, R. R., D. A. Randall, and C. Stan, 2014: Simulations of the West African monsoon with a superparameterized climate model. Part II: African easterly waves. J. Climate, 27, 83238341, https://doi.org/10.1175/JCLI-D-13-00677.1.

    • Search Google Scholar
    • Export Citation
  • Mills, C. M., and J. E. Walsh, 2013: Seasonal variation and spatial patterns of the atmospheric component of the Pacific decadal oscillation. J. Climate, 26, 15751594, https://doi.org/10.1175/JCLI-D-12-00264.1.

    • Search Google Scholar
    • Export Citation
  • Mitas, C. M., and W. A. Robinson, 2005: Atmospheric stability in a generalized barotropic model. J. Atmos. Sci., 62, 476491, https://doi.org/10.1175/JAS-3375.1.

    • Search Google Scholar
    • Export Citation
  • Myoung, B., and J. W. Nielsen-Gammon, 2010: The convective instability pathway to warm season drought in Texas. Part I: The role of convective inhibition and its modulation by soil moisture. J. Climate, 23, 44614473, https://doi.org/10.1175/2010JCLI2946.1.

    • Search Google Scholar
    • Export Citation
  • Newman, M., M. A. Alexander, and J. D. Scott, 2011: An empirical model of tropical ocean dynamics. Climate Dyn., 37, 18231841, https://doi.org/10.1007/s00382-011-1034-0.

    • Search Google Scholar
    • Export Citation
  • Nishii, K., and H. Nakamura, 2004: Lower-stratospheric Rossby wave trains in the Southern Hemisphere: A case-study for late winter of 1997. Quart. J. Roy. Meteor. Soc., 130, 325345, https://doi.org/10.1256/qj.02.156.

    • Search Google Scholar
    • Export Citation
  • Petoukhov, V., S. Petri, S. Rahmstorf, D. Coumou, K. Kornhuber, and H. J. Schellnhuber, 2016: Role of quasi-resonant planetary wave dynamics in recent boreal spring-to-autumn extreme events. Proc. Natl. Acad. Sci. USA, 113, 68626867, https://doi.org/10.1073/pnas.1606300113.

    • Search Google Scholar
    • Export Citation
  • Pfleiderer, P., and D. Coumou, 2018: Quantification of temperature persistence over the Northern Hemisphere land-area. Climate Dyn., 51, 627637, https://doi.org/10.1007/s00382-017-3945-x.

    • Search Google Scholar
    • Export Citation
  • Rayner, N. A., D. E. Parker, E. B. Horton, C. K. Folland, L. V. Alexander, D. P. Rowell, E. C. Kent, and A. Kaplan, 2003: Global analyses of sea surface temperature, sea ice, and night marine air temperature since the late nineteenth century. J. Geophys. Res., 108, 4407, https://doi.org/10.1029/2002JD002670.

    • Search Google Scholar
    • Export Citation
  • Russell, D. R., 2006: Development of a time-domain, variable-period surface-wave magnitude measurement procedure for application at regional and teleseismic distances. Part I: Theory. Bull. Seismol. Soc. Amer., 96, 665677, https://doi.org/10.1785/0120050055.

    • Search Google Scholar
    • Export Citation
  • Seager, R., N. Naik, M. Ting, M. A. Cane, N. Harnik, and Y. Kushnir, 2010: Adjustment of the atmospheric circulation to tropical Pacific SST anomalies: Variability of transient eddy propagation in the Pacific–North America sector. Quart. J. Roy. Meteor. Soc., 136, 277296, https://doi.org/10.1002/qj.588.

    • Search Google Scholar
    • Export Citation
  • Simmons, A. J., M. Wallace, and G. W. Branstator, 1983: Barotropic wave propagation and instability, and atmospheric teleconnection patterns. J. Atmos. Sci., 40, 13631392, https://doi.org/10.1175/1520-0469(1983)040<1363:BWPAIA>2.0.CO;2.

    • Search Google Scholar
    • Export Citation
  • Stevens, M. R., and G. J. Hakim, 2005: Perturbation growth in baroclinic waves. J. Atmos. Sci., 62, 28472863, https://doi.org/10.1175/JAS3502.1.

    • Search Google Scholar
    • Export Citation
  • Stolpe, M. B., I. Medhaug, and R. Knutti, 2017: Contribution of Atlantic and Pacific multidecadal variability to twentieth-century temperature changes. J. Climate, 30, 62796295, https://doi.org/10.1175/JCLI-D-16-0803.1.

    • Search Google Scholar
    • Export Citation
  • Teng, H., G. Branstator, H. Wang, G. Meehl, and W. Washington, 2013: Probability of US heat waves affected by a subseasonal planetary wave pattern. Nat. Geosci., 6, 10561061, https://doi.org/10.1038/ngeo1988.

    • Search Google Scholar
    • Export Citation
  • Thorne, P., S. Outten, I. Bethke, and O. Seland, 2015: Investigating the recent apparent hiatus in surface temperature increases: 2. Comparison of model ensembles to observations. J. Geophys. Res. Atmos., 120, 85978620, https://doi.org/10.1002/2014JD022805.

    • Search Google Scholar
    • Export Citation
  • Vimont, D. J., M. A. Alexander, and M. Newman, 2014: Optimal growth of central and east Pacific ENSO events. Geophys. Res. Lett., 41, 40274034, https://doi.org/10.1002/2014GL059997.

    • Search Google Scholar
    • Export Citation
  • Wang, H., B. Wang, F. Huang, Q. Ding, and J.-Y. Lee, 2012: Interdecadal changes of the boreal summer circumglobal teleconnection (1958–2010). Geophys. Res. Lett., 39, L12704, https://doi.org/10.1029/2012GL052371.

    • Search Google Scholar
    • Export Citation
  • Webster, P. J., G. J. Holland, J. A. Curry, and H.-R. Chang, 2005: Changes in tropical cyclone number, duration, and intensity in a warming environment. Science, 309, 18441846, https://doi.org/10.1126/science.1116448.

    • Search Google Scholar
    • Export Citation
  • Westra, S., L. V. Alexander, and F. W. Zwiers, 2013: Global increasing trends in annual maximum daily precipitation. J. Climate, 26, 39043918, https://doi.org/10.1175/JCLI-D-12-00502.1.

    • Search Google Scholar
    • Export Citation
  • Williams, A. P., R. Seager, J. T. Abatzoglou, B. I. Cook, J. E. Smerdon, and E. R. Cook, 2015: Contribution of anthropogenic warming to California drought during 2012–2014. Geophys. Res. Lett., 42, 68196828, https://doi.org/10.1002/2015GL064924.

    • Search Google Scholar
    • Export Citation
  • Wolfe, C. L., and R. M. Samelson, 2008: Singular vectors and time-dependent normal modes of a baroclinic wave-mean oscillation. J. Atmos. Sci., 65, 875894, https://doi.org/10.1175/2007JAS2364.1.

    • Search Google Scholar
    • Export Citation
  • Xie, S.-P., and Coauthors, 2015: Towards predictive understanding of regional climate change. Nat. Climate Change, 5, 921930, https://doi.org/10.1038/nclimate2689.

    • Search Google Scholar
    • Export Citation
  • Zhang, Y., and I. M. Held, 1999: A linear stochastic model of a GCM’s midlatitude storm tracks. J. Atmos. Sci., 56, 34163435, https://doi.org/10.1175/1520-0469(1999)056<3416:ALSMOA>2.0.CO;2.

    • Search Google Scholar
    • Export Citation
  • Zhao, S., Y. Deng, and R. X. Black, 2016: Warm season dry spells in the central and eastern United States: Diverging skill in climate model representation. J. Climate, 29, 56175624, https://doi.org/10.1175/JCLI-D-16-0321.1.

    • Search Google Scholar
    • Export Citation
  • Zhao, S., Y. Deng, and R. X. Black, 2017a: A dynamical and statistical characterization of U.S. extreme precipitation events and their associated large-scale meteorological patterns. J. Climate, 30, 13071326, https://doi.org/10.1175/JCLI-D-15-0910.1.

    • Search Google Scholar
    • Export Citation
  • Zhao, S., Y. Deng, and R. X. Black, 2017b: Observed and simulated spring and summer dryness in the United States: The impact of the Pacific sea surface temperature and beyond. J. Geophys. Res. Atmos., 122, 12 71312 731, https://doi.org/10.1002/2017JD027279.

    • Search Google Scholar
    • Export Citation
  • Zhao, S., Y. Deng, and R. X. Black, 2018: An intraseasonal mode of atmospheric variability relevant to the U.S. hydroclimate in boreal summer: Dynamic origin and East Asia connection. J. Climate, 31, 98559868, https://doi.org/10.1175/JCLI-D-18-0206.1.

    • Search Google Scholar
    • Export Citation
  • Zhu, Z., and T. Li, 2016: A new paradigm for continental U.S. summer rainfall variability: Asia–North America teleconnection. J. Climate, 29, 73137327, https://doi.org/10.1175/JCLI-D-16-0137.1.

    • Search Google Scholar
    • Export Citation
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A Nonmodal Instability Perspective of the Declining Northern Midlatitude Synoptic Variability in Boreal Summer

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  • 1 School of Earth and Atmospheric Sciences, Georgia Institute of Technology, Atlanta, Georgia
  • 2 Earth and Ocean Sciences, Nicholas School, Duke University, Durham, North Carolina
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Abstract

The Pacific–North America–North Atlantic sector in general experienced a dryer and warmer climate in summer during the past 40 years. These changes are partly associated with declining midlatitude synoptic variability in boreal summer, especially over the two ocean basins. A nonmodal instability analysis of the boreal summer background flow is conducted for two periods, 1979–94 and 2000–15, to understand dynamical processes potentially responsible for the observed decline of synoptic variability. The synoptic variability associated with fast, nonmodal growth of atmospheric disturbances shows a decline over northern midlatitudes in the later period, in both a barotropic model and a two-level quasigeostrophic model. These results highlight the importance of the changing summer background flow in contributing to the observed changes in synoptic variability. Also discussed are factors likely associated with background flow changes including sea surface temperature and sea ice change.

© 2020 American Meteorological Society. For information regarding reuse of this content and general copyright information, consult the AMS Copyright Policy (www.ametsoc.org/PUBSReuseLicenses).

Corresponding author: Yi Deng, yi.deng@eas.gatech.edu

Abstract

The Pacific–North America–North Atlantic sector in general experienced a dryer and warmer climate in summer during the past 40 years. These changes are partly associated with declining midlatitude synoptic variability in boreal summer, especially over the two ocean basins. A nonmodal instability analysis of the boreal summer background flow is conducted for two periods, 1979–94 and 2000–15, to understand dynamical processes potentially responsible for the observed decline of synoptic variability. The synoptic variability associated with fast, nonmodal growth of atmospheric disturbances shows a decline over northern midlatitudes in the later period, in both a barotropic model and a two-level quasigeostrophic model. These results highlight the importance of the changing summer background flow in contributing to the observed changes in synoptic variability. Also discussed are factors likely associated with background flow changes including sea surface temperature and sea ice change.

© 2020 American Meteorological Society. For information regarding reuse of this content and general copyright information, consult the AMS Copyright Policy (www.ametsoc.org/PUBSReuseLicenses).

Corresponding author: Yi Deng, yi.deng@eas.gatech.edu
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