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Historical reanalyses that span more than a century are needed for a wide range of studies, from understanding large‐scale climate trends to diagnosing the impacts of individual historical extreme weather events. The Twentieth Century Reanalysis (20CR) Project is an effort to fill this need. It is supported by the National Oceanic and Atmospheric Administration (NOAA), the Cooperative Institute for Research in Environmental Sciences (CIRES), and the U.S. Department of Energy (DOE), and is facilitated by collaboration with the international Atmospheric Circulation Reconstructions over the Earth initiative. 20CR is the first ensemble of sub‐daily global atmospheric conditions spanning over 100 years. This provides a best estimate of the weather at any given place and time as well as an estimate of its confidence and uncertainty. While extremely useful, version 2c of this dataset (20CRv2c) has several significant issues, including inaccurate estimates of confidence and a global sea level pressure bias in the mid‐19th century. These and other issues can reduce its effectiveness for studies at many spatial and temporal scales. Therefore, the 20CR system underwent a series of developments to generate a significant new version of the reanalysis. The version 3 system (NOAA‐CIRES‐DOE 20CRv3) uses upgraded data assimilation methods including an adaptive inflation algorithm; has a newer, higher‐resolution forecast model that specifies dry air mass; and assimilates a larger set of pressure observations. These changes have improved the ensemble‐based estimates of confidence, removed spin‐up effects in the precipitation fields, and diminished the sea‐level pressure bias. Other improvements include more accurate representations of storm intensity, smaller errors, and large‐scale reductions in model bias. The 20CRv3 system is comprehensively reviewed, focusing on the aspects that have ameliorated issues in 20CRv2c. Despite the many improvements, some challenges remain, including a

Historical reanalyses that span more than a century are needed for a wide rangeof studies, from understanding large-scale climate trends to diagnosing the impactsof individual historical extreme weather events. The Twentieth Century Reanalysis(20CR) Project is an effort to fill this need. It is supported by the National Oceanicand Atmospheric Administration (NOAA), the Cooperative Institute for Research inEnvironmental Sciences (CIRES), and the U.S. Department of Energy (DOE), and isfacilitated by collaboration with the international Atmospheric Circulation Recon-structions over the Earth initiative. 20CR is the first ensemble of sub-daily globalatmospheric conditions spanning over 100 years. This provides a best estimate ofthe weather at any given place and time as well as an estimate of its confidence anduncertainty. While extremely useful, version 2c of this dataset (20CRv2c) has sev-eral significant issues, including inaccurate estimates of confidence and a global sealevel pressure bias in the mid-19th century. These and other issues can reduce itseffectiveness for studies at many spatial and temporal scales. Therefore, the 20CRsystem underwent a series of developments to generate a significant new version ofthe reanalysis. The version 3 system (NOAA-CIRES-DOE 20CRv3) uses upgradeddata assimilation methods including an adaptive inflation algorithm; has a newer,higher-resolution forecast model that specifies dry air mass; and assimilates a largerset of pressure observations. These changes have improved the ensemble-basedestimates of confidence, removed spin-up effects in the precipitation fields, anddiminished the sea-level pressure bias. Other improvements include more accuraterepresentations of storm intensity, smaller errors, and large-scale reductions in model bias. The 20CRv3 system is comprehensively reviewed, focusing on the aspects thathave ameliorated issues in 20CRv2c. Despite the many improvements, some chal-lenges remain, including a systematic bias i

The annual mean heat budget of the upper ocean beneath the stratocumulus/stratus cloud deck in the southeast Pacific is estimated using Simple Ocean Data Assimilation (SODA) and an eddy-resolving Hybrid Coordinate Ocean Model (HYCOM). Both are compared with estimates based on Woods Hole Oceanographic Institution (WHOI) Improved Meteorological (IMET) buoy observations at 208S, 858W. Net surface heat fluxes are positive (warming) over most of the area under the stratus cloud deck. Upper-ocean processes responsible for balancing the surface heat flux are examined by estimating each term in the heat equation. In contrast to surface heat fluxes, geostrophic transport in the upper 50 meters causes net cooling in most of the stratus cloud deck region. Ekman transport provides net warming north of the IMET site and net cooling south of the IMET site. Although the eddy heat flux divergence term can be comparable to other terms at a particular location, such as the IMET mooring site, it is negligible for the entire stratus region when area averaged because it is not spatially coherent in the open ocean. Although cold-core eddies are often generated near the coast in the eddy-resolving model, they do not significantly impact the heat budget in the open ocean in the southeast Pacific., Published in the Journal of Physical Oceanography, v40 p103-120, January 2010. Sponsored in part by the National Oceanic and Atmospheric Administration (NOAA) under Grant NA06AR4310146; and National Science Foundation (NSF) Grants NSF-ATM-0745872 and NSF-OCE-0351802.

The annual mean heat budget of the upper ocean beneath the stratocumulus/stratus cloud deck in the southeast Pacific is estimated using Simple Ocean Data Assimilation (SODA) and an eddy-resolving Hybrid Coordinate Ocean Model (HYCOM). Both are compared with estimates based on Woods Hole Oceanographic Institution (WHOI) Improved Meteorological (IMET) buoy observations at 20 deg S, 85 deg W. Net surface heat fluxes are positive (warming) over most of the area under the stratus cloud deck. Upper-ocean processes responsible for balancing the surface heat flux are examined by estimating each term in the heat equation. In contrast to surface heat fluxes, geostrophic transport in the upper 50 m causes net cooling in most of the stratus cloud deck region. Ekman transport provides net warming north of the IMET site and net cooling south of the IMET site. Although the eddy heat flux divergence term can be comparable to other terms at a particular location, such as the IMET mooring site, it is negligible for the entire stratus region when area averaged because it is not spatially coherent in the open ocean. Although cold-core eddies are often generated near the coast in the eddy-resolving model, they do not significantly impact the heat budget in the open ocean in the southeast Pacific., The original document contains color images. All DTIC reproductions will be in black and white. Pub. in Journal of Physical Oceanography, v40 p103-120, January 2010. Supported in part by National Science of Foundation.

A new version of SODA, which covers the time period 1958–2005, is used to analyze decadal variability of the Pacific Subtropical Cell (STC) circulation. The analysis is based on transport time series across 9°S and 9°N. At the interannual time scale, STC convergence anomalies decrease during El Niños and increase during La Niñas through Sverdrup transport convergence changes. At decadal time scales, the assimilation shows a reduction of interior STC convergence of about 8 Sv from the 1960s to the 1990s and a subsequent rebound into the early 2000s by a similar amount, in agreement with the STC tendencies reported earlier from geostrophic section analysis, and associated with the occurrence and intensity of ENSO events among the decades analyzed. The results are compared with, and differ significantly from, those obtained by the German ECCO (GECCO) assimilation.