Abstract
This study analyzes the spatiotemporal features and physical drivers of cold blobs over the subpolar North Atlantic (NA) operating on interannual timescales based on ocean and atmosphere reanalysis datasets. First, we compute a cold blob index as the normalized monthly sea surface temperature (SST) anomalies averaged over study area of 48–58°N and 35–20°W and identify cold blob events from 1951 to 2020 when the cold blob index is smaller than − 0.75 for at least five consecutive months. We analyze the identified six cases that peak in boreal summer. Their duration ranges from 6 to 19 months, with the 2014–2016 case as the most long-lasting one. When the cold blobs occur, an SST tripole pattern is identified over the NA with the most prominent cooling signal surrounding the study area. Based on a mixed-layer heat budget analysis, we quantify the relative contribution of atmospheric and oceanic processes for the development of the NA cold blobs. The surface heat flux term contributes more than one-third of the total cooling at 1 month before the summer peak. Specifically, the westerly anomalies over the study area enhance the background westerlies, inducing evaporative cooling from the ocean to the atmosphere. However, the major modes of North Atlantic Oscillation and East Atlantic Pattern are not significant at this time. The dominant driver for the intensification of the cold blobs is the vertical entrainment. This study provides insights into the characteristics and mechanisms of the NA cold blobs, facilitating their forecasts and climate projection for downstream Europe.
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Data availability
The HadISST and ERSST v5 data are obtained from https://www.metoffice.gov.uk/hadobs/hadisst/ and https://psl.noaa.gov/data/gridded/data.noaa.ersst.v5.html, respectively. The SODA and GODAS datasets can be downloaded from http://iridl.ldeo.columbia.edu/SOURCES/.CARTON-GIESE/.SODA/.v2p0p2-4/ and https://psl.noaa.gov/data/gridded/data.godas.html, respectively. The atmospheric data is available at https://psl.noaa.gov/data/gridded/data.ncep.reanalysis.html. The monthly NAO and EAP indices are achieved from https://www.cpc.ncep.noaa.gov/data/teledoc/telecontents.shtml.
Code availability
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Funding
F.H. is supported by National Key R&D Program of China (2019YFA0607000). J.S. is supported by National Natural Science Foundation of China (42006013) and Qingdao Postdoctoral Grant.
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Shi, J., Wang, J., Ren, Z. et al. Cold blobs in the subpolar North Atlantic: seasonality, spatial pattern, and driving mechanisms. Ocean Dynamics 73, 267–278 (2023). https://doi.org/10.1007/s10236-023-01553-z
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DOI: https://doi.org/10.1007/s10236-023-01553-z