Abstract
Abstract. The 2013–2014 winter averaged sea surface temperature (SST) was anomalously cool in the mid-North Atlantic region. This season was also unusually stormy, with extratropical cyclones passing over the mid-North Atlantic every 3 d. However, the processes by which cyclones contribute towards seasonal SST anomalies are not fully quantified. In this paper a cyclone identification and tracking method is combined with European Centre for Medium-Range Weather Forecasts (ECMWF) atmosphere and ocean reanalysis fields to calculate cyclone-relative net surface heat flux anomalies and resulting SST changes. Anomalously large negative heat flux is located behind the cyclones' cold front, resulting in anomalous cooling up to 0.2 K d−1 when the cyclones are at maximum intensity. This extratropical-cyclone-induced “cold wake” extends along the cyclones' cold front but is small compared to climatological variability in the SSTs. To investigate the potential cumulative effect of the passage of multiple cyclone-induced SST cooling in the same location, we calculate Earth-relative net surface heat flux anomalies and resulting SST changes for the 2013–2014 winter period. Anomalously large winter averaged negative heat flux occurs in a zonally orientated band extending across the North Atlantic between 40 and 60∘ N. The 2013–2014 winter SST cooling anomaly associated with air–sea interactions (ASIs; anomalous heat flux, mixed layer depth and entrainment at the base of the ocean mixed layer) is estimated to be −0.67 K in the mid-North Atlantic (68 % of the total cooling anomaly). The role of cyclones is estimated using a cyclone-masking technique which encompasses each cyclone centre and its cold wake. The environmental flow anomaly in 2013–2014 sets the overall tripole pattern of heat flux anomalies over the North Atlantic. However, the presence of cyclones doubles the magnitude of the negative heat flux anomaly in the mid-North Atlantic. Similarly, the environmental flow anomaly determines the location of the SST cooling anomaly, but the presence of cyclones enhances the SST cooling anomaly. Thus air–sea interactions play a major part in determining the extreme 2013–2014 winter season SST cooling anomaly. The environmental flow anomaly determines where anomalous heat flux and associated SST changes occur, and the presence of cyclones influences the magnitude of those anomalies.
Highlights
The interaction of the ocean and atmosphere has long been recognized as an important element of oceanic cyclogenesis
The latent heat flux (Fig. 3d) is generally negative, with a band of enhanced negative flux extending in a north-eastwards direction from the East Coast of the US towards Iceland, with the values > 200 W m−2 found in the west of the North Atlantic
Since many of the 200 cyclones contributing to the composite QN (Fig. 5e) are generated over the Gulf Stream region it is possible that large negative QN occurring behind of the cyclone centre could be an artefact of their preferential cyclogenesis over a region of climatologically large negwww.weather-clim-dynam.net/1/27/2020/
Summary
The interaction of the ocean and atmosphere has long been recognized as an important element of oceanic cyclogenesis. Dacre et al.: Cyclone SST cyclones, and the eastern parts of the cyclones were associated with decreased fluxes directed from the ocean to the atmosphere These results have been confirmed by many subsequent studies (Persson et al, 2005; Nelson et al, 2014; Schemm and Sprenger, 2015; Dacre et al, 2019) and suggest a close association between the cyclones and surface turbulent fluxes in the midlatitudes. Tanimoto et al (2003) showed that in the central North Pacific, enhanced turbulent fluxes can generate local SST variations, but in regions where ocean dynamics are important, such as the western Pacific, the SST anomalies formed in the early winter determine the mid- and late-winter turbulent heat flux anomalies rather than the passage of cyclones. In this paper we investigate both the SST cooling associated with individual cyclones and the SST cooling associated with the passage of multiple cyclones over the same location in the 2013–2014 season to determine how significant cyclones were in contributing to the observed cooling
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