Mechanisms of decadal changes in sea surface height and heat content in the eastern Nordic Seas

Sara Broomé,Léon Chafik,Johan Nilsson

doi:10.5194/os-16-715-2020

Sara Broomé, Léon Chafik + Show 1 more

Open Access

https://doi.org/10.5194/os-16-715-2020

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Abstract

Abstract. The Nordic Seas constitute the main ocean conveyor of heat between the North Atlantic Ocean and the Arctic Ocean. Although the decadal variability in the subpolar North Atlantic has been given significant attention lately, especially regarding the cooling trend since the mid-2000s, less is known about the potential connection downstream in the northern basins. Using sea surface heights from satellite altimetry over the past 25 years (1993–2017), we find significant variability on multiyear to decadal timescales in the Nordic Seas. In particular, the regional trends in sea surface height show signs of a weakening since the mid-2000s, as compared to the rapid increase in the preceding decade since the early 1990s. This change is most prominent in the Atlantic origin waters in the eastern Nordic Seas and is closely linked, as estimated from hydrography, to heat content. Furthermore, we formulate a simple heat budget for the eastern Nordic Seas to discuss the relative importance of local and remote sources of variability; advection of temperature anomalies in the Atlantic inflow is found to be the main mechanism. A conceptual model of ocean heat convergence, with only upstream temperature measurements at the inflow to the Nordic Seas as input, is able to reproduce key aspects of the decadal variability in the heat content of the Nordic Seas. Based on these results, we argue that there is a strong connection with the upstream subpolar North Atlantic. However, although the shift in trends in the mid-2000s is coincident in the Nordic Seas and the subpolar North Atlantic, the eastern Nordic Seas have not seen a reversal of trends but instead maintain elevated sea surface heights and heat content in the recent decade considered here.

Highlights

The Nordic Seas, a collective name for the Greenland– Iceland–Norwegian seas, are the link between the Atlantic and the Arctic oceans and are recognized to play an important role in the global climate system (Drange et al, 2005)
We show that the decadal variability in sea surface height (SSH) is linked to heat content, and through a heat budget and conceptual model in Sect. 3.2 we argue that the variation in temperature of the inflowing Atlantic water in the south is the main contributor to the variability
We identify a shift in the trend of SSH in the Atlantic Water (AW), from a decade with a strong positive trend to a more stagnant decade

Summary

Introduction

The Nordic Seas, a collective name for the Greenland– Iceland–Norwegian seas, are the link between the Atlantic and the Arctic oceans and are recognized to play an important role in the global climate system (Drange et al, 2005). Warm and saline waters of Atlantic origin cross the Greenland– Scotland Ridge (Fig. 1) and flow northward through the eastern part of the Nordic Seas before entering into the Arctic Ocean (Mauritzen, 1996; Orvik and Niiler, 2002; Skagseth et al, 2008; Furevik et al, 2007), affecting the local sea ice and atmosphere on its way. Since the Nordic Seas make up a major source of heat for the Arctic Ocean it is important to understand the thermal variability and the mechanisms behind it. Segtnan et al (2011) used reanalysis to examine the heat and freshwater budgets and find the largest water mass modifications to occur in the eastern part of the Nordic Seas.

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