14 Feb 2022
14 Feb 2022
Status: a revised version of this preprint is currently under review for the journal OS.

Causes of the 2015 North Atlantic cold anomaly in the ECCOv4 state estimates

Rachael N. C. Sanders1, Daniel C. Jones1, Simon A. Josey2, Bablu Sinha2, and Gael Forget3 Rachael N. C. Sanders et al.
  • 1British Antarctic Survey, NERC, UKRI, Cambridge, UK
  • 2National Oceanography Centre, Southampton, UK
  • 3EAPS, MIT, Cambridge, MA, USA

Abstract. The subpolar North Atlantic is an important part of the global ocean and climate system, with SST variability in the region influencing the climate of Europe and North America. While the majority of the global ocean exhibited higher than average surface temperatures in 2015, the subpolar North Atlantic experienced record low temperatures. This interannual cold anomaly is thought to have been driven by surface forcing, but detailed questions remain about how the anomaly was created and maintained. To better quantify and understand the processes responsible for the cold anomaly, we computed mixed layer temperature budgets in two releases of the ECCO Version 4 global ocean state estimate. These state estimates have been brought into consistency with a large suite of observations without using artificial sources or sinks of heat, making them ideal for temperature budget studies. We found that strong surface forcing drove approximately 75 % of the initial anomalies in the cooling of the mixed layer in December 2013, after which the cold anomaly was sequestered beneath the mixed layer. Re-emergence of the cold anomaly during the summer/autumn of 2014 was primarily the result of a strong temperature gradient across the base of the mixed layer, with vertical diffusion accounting for approximately 70 % of the re-emergence. Weaker surface warming of the mixed layer during the summer of 2015 enhanced the anomaly, causing a temperature minimum. Spatial patterns in the budgets also show large differences between the north and south of the anomaly region, with particularly strong initial surface cooling in the south related to the positive phase of the East Atlantic Pattern. It is important to note that this interannual cold anomaly, which is thought to be primarily driven by surface forcing, is distinct from the multi-decadal North Atlantic "warming hole", which is thought to be primarily driven by changes in advection.

Rachael N. C. Sanders et al.

Status: final response (author comments only)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on os-2022-10', Dr. Nicholas Foukal, 20 Mar 2022
    • AC1: 'Reply on RC1', Rachael Sanders, 20 May 2022
  • RC2: 'Comment on os-2022-10', Anonymous Referee #2, 22 Apr 2022
    • AC2: 'Reply on RC2', Rachael Sanders, 20 May 2022

Rachael N. C. Sanders et al.

Rachael N. C. Sanders et al.


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Short summary
In 2015, record low temperatures were observed in the North Atlantic. Using two ocean models, we show that surface heat loss in December 2013 caused 75 % of the initial cooling, before this "cold blob" was trapped deeper in the ocean. The following summer, the "cold blob" re-emerged at the surface due to a strong temperature difference between the surface ocean and below, driving vertical diffusion of heat. Lower than average surface warming then led to the coldest temperatures in August 2015.