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Natalie P. Thomas, Michael G. Bosilovich, Allison B. Marquardt Collow, Randal D. Koster, Siegfried D. Schubert, Amin Dezfuli, and Sarith P. Mahanama

( Meehl and Tebaldi 2004 ; Wang et al. 2020 ). Over the United States, trends in heat-wave frequency have been generally positive in recent decades ( Oswald and Rood 2014 ; Schoof et al. 2017 ; Oswald 2018 ; Shafiei Shiva et al. 2019 ), although regional trends vary based on the index used ( Smith et al. 2013 ). Several studies have noted the greater increase in nighttime (Tmin) versus daytime (Tmax) heat waves over the United States ( Lyon and Barnston 2017 ; Rennie et al. 2019 ), specifically

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Franklin R. Robertson, Michael G. Bosilovich, and Jason B. Roberts

Halpert 1987 ; Dai and Wigley 2000 ; Gu et al. 2007 ; Robertson et al. 2014 ). Midlatitude storm-track changes embodying teleconnections with tropical forcing also have significant variations at higher latitudes. Over longer time scales Pacific decadal variability [PDV or Pacific decadal oscillation (PDO); e.g., Power et al. 1999 ; Dai 2013 ; Lyon et al. 2014 ] and other basin-scale phenomena [e.g., the Atlantic multidecadal oscillation (AMO); Enfield et al. 2001 ; Sutton and Hodson 2005

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Krzysztof Wargan, Gordon Labow, Stacey Frith, Steven Pawson, Nathaniel Livesey, and Gary Partyka

an added value brought to satellite observations of ozone through data assimilation. In particular, work done at the GMAO over the past decade has shown that assimilation of retrieved ozone data from the Microwave Limb Sounder (MLS) along with total ozone observations from the Ozone Monitoring Instrument (OMI), both onboard the Earth Observing System (EOS) Aura satellite, produces realistic global distributions of ozone in the stratosphere and upper troposphere ( Stajner et al. 2008 ; Wargan

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Gloria L. Manney and Michaela I. Hegglin

preclude those changes being related to climate change impacts that may themselves be correlated with ENSO changes. Several other modes of natural variability, such as the North Atlantic Oscillation, Arctic Oscillation, southern annular mode, quasi-biennial oscillation, Pacific decadal oscillation, and Madden–Julian oscillation, may also be associated with changes in the upper-tropospheric jets on decadal or longer time scales (e.g., Thompson et al. 2000 , 2011 ; Overland and Wang 2005 ; Woollings

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Michael G. Bosilovich, Franklin R. Robertson, Lawrence Takacs, Andrea Molod, and David Mocko

(not shown) are less obviously affected by observing system changes. We suspect that short-term climate fluctuations such as Pacific decadal variability ( Power et al. 1999 ; Ogata et al. 2013 ), the Atlantic multidecadal oscillation ( Enfield et al. 2001 ; Zhang and Delworth 2006 ), or other modes of natural climate variability are likely sources of significant hydrologic cycle variations over this period. While spatial patterns of these modes may be present, these signals are masked in global

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Richard I. Cullather and Sophie M. J. Nowicki

and the full time series shown in Fig. 4 are associated with the region to the north of the Canadian Arctic Archipelago. The patterns shown in Fig. 15 suggest a correlation with high pressure in the area, while the regression for the full available time period shown in Fig. 4 indicates regression values near zero and of no significance. These differences may be due to interannual variability and the overall lack of melt events in these basins prior to the most recent decade. Similar Arctic

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Ronald Gelaro, Will McCarty, Max J. Suárez, Ricardo Todling, Andrea Molod, Lawrence Takacs, Cynthia A. Randles, Anton Darmenov, Michael G. Bosilovich, Rolf Reichle, Krzysztof Wargan, Lawrence Coy, Richard Cullather, Clara Draper, Santha Akella, Virginie Buchard, Austin Conaty, Arlindo M. da Silva, Wei Gu, Gi-Kong Kim, Randal Koster, Robert Lucchesi, Dagmar Merkova, Jon Eric Nielsen, Gary Partyka, Steven Pawson, William Putman, Michele Rienecker, Siegfried D. Schubert, Meta Sienkiewicz, and Bin Zhao

late 1990s, banned the release of the main ozone-depleting substances. Because the rate of the springtime polar ozone depletion depends on temperature and the strength of the Antarctic polar vortex in a given year, the size of the ozone hole exhibits a dynamically driven interannual variability superimposed on decadal-scale trends. This dynamical modulation is also evident in Fig. 24 . The extremely small (less than 3 × 10 6 km 2 ) ozone hole in 2002 occurred in conjunction with the only major

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Lawrence Coy, Paul A. Newman, Steven Pawson, and Leslie R. Lait

1. Introduction The quasi-biennial oscillation (QBO) consists of downward descending easterly and westerly zonal wind regimes that dominate the zonal mean wind variability in the tropical lower stratosphere (100–10 hPa, ~18–30 km in altitude) with a varying (~28 month) period [see Baldwin et al. (2001) and references therein]. The QBO has been a persistent characteristic of the tropical lower stratosphere since observations began in 1953. However, a significant disruption of the QBO occurred

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Laura M. Hinkelman

used to investigate a wide range of topics including basic climatology, variability on diurnal to decadal scales, climate trends, and dynamical processes. For example, Urankar et al. (2012) used MERRA to study the feedbacks of aerosol and clouds on the energy budget over India. L’Ecuyer et al. (2015) relied on multiple variables from MERRA in their assessment of the state of the global energy budget in the early twenty-first century, while Trenberth et al. (2015) used it to examine top

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Allison B. Marquardt Collow, Michael G. Bosilovich, and Randal D. Koster

global precipitation and surface evaporation ( Reichle and Liu 2014 ; Takacs et al. 2016 ). MERRA-2 also features numerous developments in the underlying model ( Molod et al. 2015 ), such as in the surface layer and boundary layer parameterizations and in the cumulus convection scheme. The data assimilation has been updated to the latest Gridpoint Statistical Interpolation analysis scheme version and includes global dry mass constraints that help minimize spurious temporal variability effects

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