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Gloria L. Manney, Michaela I. Hegglin, and Zachary D. Lawrence

natural modes of variability ( Lorenz and DeWeaver 2007 ; Scaife et al. 2008 ; McLandress et al. 2011 ; Hudson 2012 ; Lin et al. 2014 , 2015 ; Waugh et al. 2015 , 2018 ; Grise et al. 2018 , and references therein). However, UT jets show complex patterns of seasonal and regional variability ( Manney et al. 2014 ; Manney and Hegglin 2018 ; Spensberger and Spengler 2020 , and references therein), which makes it difficult to disentangle their response to climate change (i.e., radiative forcing

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Gloria L. Manney, Michelle L. Santee, Zachary D. Lawrence, Krzysztof Wargan, and Michael J. Schwartz

rainfall and low-level temperatures or winds (e.g., Kajikawa et al. 2012 ; Preethi et al. 2017 ; Kodera et al. 2019 ; Brönnimann et al. 2016 ; Wu et al. 2020 ; and references therein), and some suggest a trend toward earlier monsoon onset in spring, with possible relationships to anthropogenic forcing (e.g., Kajikawa et al. 2012 ; Bollasina et al. 2013 , 2014 ; Bombardi et al. 2020 ). Near-surface metrics have been linked to the upper-level circulation, with, for example, westward and

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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

Applications, version 2 (MERRA-2; Gelaro et al. 2017 ), is the primary tool used in this analysis. Hourly data from MERRA-2 are available at a spatial resolution of 0.625° longitude by 0.5° latitude starting in January 1980. MERRA-2 is a global atmospheric reanalysis with a variety of updates relative to the original MERRA ( Rienecker et al. 2011 ). Among these is the inclusion of an observation-driven precipitation field to force the land surface ( Reichle et al. 2017a ). An evaluation of the MERRA-2

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

typically observed. Vertically propagating equatorial waves are believed to be the principal forcing mechanism of the QBO ( Lindzen and Holton 1968 ). Selective filtering of vertically propagating waves by the QBO wind distribution coupled with the tendency of the waves to break or thermally dissipate, deposit momentum, and thereby dissipate in regions of the QBO wind shear produce appropriately signed zonal wind accelerations that effectively lower the shear regions by approximately 1 km month −1

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Lawrence Coy, Krzysztof Wargan, Andrea M. Molod, William R. McCarty, and Steven Pawson

(MERRA-2), system. The goal is not only to characterize the basic QBO structure in MERRA-2 but also to determine how the zonal momentum and ozone forcing terms interact with the data assimilation process in producing the reanalysis QBO signature. For comparison we also examine the earlier NASA MERRA system. In particular, Politowicz and Hitchman (1997) explored QBO forcing in terms of a two-dimensional model and found differences in the QBO response depending on which model term applied the forcing

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Rolf H. Reichle, Q. Liu, Randal D. Koster, Clara S. Draper, Sarith P. P. Mahanama, and Gary S. Partyka

-generated precipitation, however, is corrected with precipitation observations before reaching the land surface. Observation-corrected precipitation was also used in the Climate Forecast System Reanalysis (CFSR; Saha et al. 2010 ; Meng et al. 2012 ) and in MERRA-Land ( Reichle et al. 2011 ; Reichle 2012 ). The latter is an offline, land-only reanalysis product. It provides significantly better land surface moisture storage dynamics than the original MERRA product because the MERRA-Land precipitation forcing was

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Rolf H. Reichle, Clara S. Draper, Q. Liu, Manuela Girotto, Sarith P. P. Mahanama, Randal D. Koster, and Gabrielle J. M. De Lannoy

both the MERRA-Land precipitation and the (uncorrected) AGCM-generated precipitation from MERRA-2 and MERRA. Moreover, in MERRA-2 the precipitation is corrected within the coupled atmosphere–land modeling system, allowing the near-surface air temperature and humidity to respond to the improved precipitation forcing. MERRA-2 thus provides more self-consistent surface meteorological data than were used for MERRA-Land ( Reichle et al. 2017 ). This enhanced self-consistency in the forcing data also

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Krzysztof Wargan and Lawrence Coy

convergence of the downward branch of the residual circulation as the leading factor in the TIL formation in the middle latitudes during winter. We note that these mechanisms are not necessarily independent: for example, a dynamical change due to wave forcing will also be manifested in the residual circulation. Sudden stratospheric warming (SSW) events are large-scale disturbances of the wintertime polar vortex, characterized by a rapid increase of polar temperatures with a reversal of the 60°–90°N

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Young-Kwon Lim, Robin M. Kovach, Steven Pawson, and Guillaume Vernieres

anomalies produced by the GEOS Ocean Data Assimilation System (ODAS) are examined for the boreal winter period of three El Niño events. Difference fields in Fig. 4 (right) show that the westerly current anomaly is relatively weaker in 2015/16 along the equatorial Pacific, compared to the earlier two super El Niño events ( Fig. 4 , top and middle right). Details on the near-surface zonal wind anomaly as a forcing to trigger the westerly ocean current anomaly and grow El Niño will be further discussed

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Richard I. Cullather and Sophie M. J. Nowicki

. DiGirolamo , and G. Neumann , 2009 : Evaluation of surface and near-surface melt characteristics on the Greenland ice sheet using MODIS and QuikSCAT data . J. Geophys. Res. , 114 , F04006 , . Hanna , E. , and Coauthors , 2014 : Atmospheric and oceanic climate forcing of the exceptional Greenland ice sheet surface melt in summer 2012 . Int. J. Climatol. , 34 , 1022 – 1037 , . 10.1002/joc.3743 Hanna , E. , T. E

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