Drivers of exceptionally cold North Atlantic Ocean temperatures and their link to the 2015 European heat wave

Abstract

The North Atlantic and Europe experienced two extreme climate events in 2015: exceptionally cold ocean surface temperatures and a summer heat wave ranked in the top ten over the past 65 years. Here, we show that the cold ocean temperatures were the most extreme in the modern record over much of the mid-high latitude North-East Atlantic. Further, by considering surface heat loss, ocean heat content and wind driven upwelling we explain for the first time the genesis of this cold ocean anomaly. We find that it is primarily due to extreme ocean heat loss driven by atmospheric circulation changes in the preceding two winters combined with the re-emergence of cold ocean water masses. Furthermore, we reveal that a similar cold Atlantic anomaly was also present prior to the most extreme European heat waves since the 1980s indicating that it is a common factor in the development of these events. For the specific case of 2015, we show that the ocean anomaly is linked to a stationary position of the Jet Stream that favours the development of high surface temperatures over Central Europe during the heat wave. Our study calls for an urgent assessment of the impact of ocean drivers on major European summer temperature extremes in order to provide better advance warning measures of these high societal impact events.

The GEOVIDE cruise in May–June 2014 reveals an intense Meridional Overturning Circulation over a cold and fresh subpolar North Atlantic

“…We estimated the accumulated air-sea heat loss from summer 2013 to summer 2014 at 6.8 × 10 21 J, while the accumulated ocean heat loss for the same period amounted to 4.8 × 10 21 J (averaged of ISAS, EN4, and JAMSTEC estimates). This result is also in agreement with the findings of Duchez et al (2016), who argued the 2013-2015 intense air-sea heat fluxes drove water masses transformation, which is an irreversible process. Recently, Frajka-Williams et al (2017) explained that such short-term cooling is mainly caused by the atmospheric forcing since the hypothetical slowdown of the AMOC would take longer to generate a cooling of this magnitude.…”

“…In the Irminger Sea, the WMLD in Fig. 7 reaches 1200 m although deep convection did not exceed 700 m in winter 2014 in the central Irminger Sea (Piron, 2015;Duchez et al, 2016). It most likely results from the advection in the depth range 700-1200 m of high-oxygen intermediate water with densities slightly denser than the water above and possibly formed south of Greenland, as suggested by Fig.…”

Linking the Tropical Northern Hemisphere Pattern to the Pacific Warm Blob and Atlantic Cold Blob

“…This cooling event is referred to as the Atlantic cold blob (Henson 2016) or the Atlantic warming hole (Rahmstorf et al 2015) to emphasize the fact that this is the most prominent area in global oceans that has cooled in the face of a prevailing warming trend in global oceans (Henson 2016;Robson et al 2016). The Atlantic cold blob was also suggested to affect downstream European climate (Duchez et al 2016). Several mechanisms have been proposed to explain the generation of the Atlantic cold blob, including atmospheric forcing associated with the North Atlantic Oscillation (NAO; Delworth et al 2016;Yeager et al 2016), an ocean circulation change linked to the Atlantic meridional overturning circulation (AMOC; Rahmstorf et al 2015;Robson et al 2016;Duchez et al 2016), and the melting of the Greenland ice sheet (Schmittner et al 2016).…”

Historical Trends and Variability in Heat Waves in the United Kingdom

“…The patterns of SSTs in the North Atlantic have changed in recent years, with a cold anomaly in the subpolar gyre and a warm anomaly in the subtropics [35]. A similar cold anomaly was last present during the 1990s when the AMO index was negative and the subtropical SSTs were also cooler than average [36].…”

“…The impact of these SST gradients on UK heat waves is unclear. In 2015, much of central Europe experienced prolonged periods of very high temperatures which have been partly attributed to the strong SST gradient in the north Atlantic [35]. In contrast, the UK only experienced a single very warm day (1 July 2015) when temperatures over 30 • C were recorded much of England.…”