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Drivers of exceptional coastal warming in the northeastern United States

Abstract

The northeastern United States (NEUS) and the adjacent Northwest Atlantic Shelf (NWS) have emerged as warming hotspots, but the connection between them remains unexplored. Here we use gridded observational and reanalysis datasets to show that the twentieth-century surface air temperature increase along the coastal NEUS is exceptional on the continental and hemispheric scale and is induced by a combination of two factors: the sea surface temperature (SST) increase in the NWS associated with a weakening Atlantic Meridional Overturning Circulation (AMOC), and atmospheric circulation changes associated with a more persistent positive North Atlantic Oscillation. These connections are important because AMOC slowdown and NWS warming are projected to continue. A survey of climate model simulations indicates that realistic SST representation at high spatial resolution might be a minimum requirement to capture the observed pattern of coastal warming, suggesting that prior projection-based assessments may not have captured key features in this populous region.

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Fig. 1: Linear trends in surface air temperature between 1902 and 2018.
Fig. 2: Connection between SST and regional SAT.
Fig. 3: Co-variability between SST and 500 hPa geopotential heights.
Fig. 4: Regressions of atmospheric field anomalies onto the SST-Z500-MCA2 time series.
Fig. 5: Ensemble mean SAT trends in CMIP6 experiments and CESM-LE.

Data availability

All underlying raw observational and model data are publically available online. For temperature data, see https://crudata.uea.ac.uk/cru/data/hrg/ for CRU TS v4 data; http://berkeleyearth.org/data/ for Berkeley Earth data; https://www.ncei.noaa.gov/access/metadata/landing-page/bin/iso?id=gov.noaa.ncdc:C00332 for nClimGrid; and https://psl.noaa.gov/data/gridded/data.20thC_ReanV3.html for 20CRv3 data. For SST data, see https://www.metoffice.gov.uk/hadobs/hadisst/ for HadISST; and https://www.ncdc.noaa.gov/data-access/marineocean-data/extended-reconstructed-sea-surface-temperature-ersst-v5 for ERSST V5.

CMIP6 model output is available from the Earth System Grid Federation (ESGF; https://esgf-node.llnl.gov/projects/cmip6). CESM-LENS data are available from the UCAR website (https://www.cesm.ucar.edu/projects/community-projects/LENS/data-sets.html). The AMOC index time series are available at http://www.pik-potsdam.de/~caesar/AMOC_slowdown/ and http://www.pik-potsdam.de/~stefan/amoc_index_data.html.

Code availability

The code used in the analyses described in this study is available in a GitHub repository: https://github.com/avkarmalkar/KH2021_NCC. More information about the code can be obtained from the corresponding author upon reasonable request.

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Acknowledgements

We thank the World Climate Research Programme’s Working Group on Coupled Modelling, which is responsible for CMIP, and the climate modelling groups for producing and making available their model output. A.V.K. was supported by Cooperative Agreement No. G19AC00091 from the United States Geological Survey (USGS). The contents of this publication are solely the responsibility of the authors and do not necessarily represent the views of the Northeast Climate Adaptation Science Center or the USGS. This manuscript was submitted for publication with the understanding that the United States Government is authorized to reproduce and distribute reprints for governmental purposes. The funders had no role in study design, data collection and analysis, decision to publish or preparation of the manuscript. R.M.H. received no specific funding for this work.

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A.V.K. designed the study and conducted the analyses. R.M.H. helped with interpretation. A.V.K. led the writing of the manuscript, with R.M.H. providing extensive comments and feedback throughout the process.

Corresponding author

Correspondence to Ambarish V. Karmalkar.

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The authors declare no competing interests.

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Peer review information Nature Climate Change thanks Vincent Saba, Sang-Ik Shin and the other, anonymous, reviewer(s) for their contribution to the peer review of this work.

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Extended data

Extended Data Fig. 1 Spatial patterns of linear surface air temperature trends.

Trends are calculated for CRU dataset over the period 1902-2018 for a, annual (ANN), b, summer (JJA), and c, winter (DJF) mean SAT and are shown as the total change in °C. Yellow color indicates regions with warming over 2 °C between 1902-2018. The insets show trends for the northeastern United States.

Extended Data Fig. 2 Tropospheric temperature trends.

Linear trends in free air temperature anomalies from the 20CRv3 dataset averaged over two cross sections. The trends are presented as the total change in °C over the period 1902-2015. The triangles indicate latitudes and longitudes of Boston and New York City.

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Supplementary Information

Supplementary Figs. 1–11 and Table 1.

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Karmalkar, A.V., Horton, R.M. Drivers of exceptional coastal warming in the northeastern United States. Nat. Clim. Chang. 11, 854–860 (2021). https://doi.org/10.1038/s41558-021-01159-7

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