1 Introduction

As the hydrological cycle accelerates in a warming climate, we expect increasing precipitation at high latitudes and increasing runoff to the Arctic Ocean. Observations show a 7% increase in Eurasian runoff from 1936 to 1999 [Peterson et al., 2002] that may well already have influenced Arctic Ocean circulation and stratification, and therefore also the sea ice cover. By the end of the century, climate model projections show a 30% increase in Arctic runoff (Figure 1) with an indication of an increase in both the freshwater storage in the Arctic Ocean and freshwater export to the North Atlantic [Lehner et al., 2012].

Figure 1 Open in figure viewer PowerPoint Expected annual mean runoff to the Arctic Ocean based on CMIP5 models using Representative Concentration Pathway (RCP) 8.5 (see supporting information Table S1 for models included in the ensemble). The red line shows the ensemble median, and the gray shading shows the interquartile range. The black dotted lines show the runoff values used in the idealized perturbation experiments (see Figure 3). The vertical line at year 2100 separates the period 2005–2100 with a large multimodel ensemble (19 models and total of 49 members) from the period 2100–2300 with a small ensemble (4 models and total of 6 members).

The Arctic Ocean (Figure 2) is strongly stratified, largely ice covered, and receives anomalously large freshwater input per unit area compared to the other world oceans [Rudels, 2015; Rawlins et al., 2010]. The Arctic Ocean stratification is characterized by a cold and fresh surface, a relatively warm and salty Atlantic Water layer at depth, and an intermediate layer of cold but gradually saltier water often termed the cold halocline [Rudels et al., 1996; Steele and Boyd, 1998; Rudels et al., 2004]. The fresh surface waters are derived from river runoff, positive net precipitation, relatively fresh Pacific inflow, and seasonal ice melt. The surface circulation is dominated by the transpolar drift crossing the Arctic Basin from the East Siberian and Laptev Seas to the Fram Strait, and the anticyclonic Beaufort Gyre in the Canada Basin. The cold halocline, derived from brine‐enriched shelf waters and local winter convection, is most evident in the Canada Basin and largely absent in the Nansen Basin [Rudels et al., 1996; Rudels, 2015]. The warm Pacific inflow enters the upper halocline through Bering Strait, affecting the stratification mainly in the Canada Basin. Atlantic Water (AW) enters the Arctic Ocean in two branches, one through the Fram Strait and the other through the Barents Sea. The AW advects cyclonically around the basin creating an AW layer at depth through most of the Arctic Ocean [Rudels, 2015; Jones, 2001].

Figure 2 Open in figure viewer PowerPoint Arctic Ocean bathymetry. Main basins and connecting gateways are labeled. Abbreviations are: BSO, Barents Sea Opening; BN, Barents North; BSE, Barents Sea Exit; KS, Kara Strait; LS, Lancaster Sound; NS, Nares Strait. Labels XS1 and XS2 correspond to the cross sections shown in Figures 8 and 9.

The variability in the pathways of Eurasian river runoff is largely governed by the Arctic Oscillation (AO), the leading mode of the atmospheric variability in the extratropical Northern Hemisphere. The AO influences the transpolar drift and the intensity of Ekman convergence to the Beaufort Gyre [Morison et al., 2012], both of which affect where freshwater tends to accumulate in the Arctic Basin. The bulk of the river runoff enters the vast Eurasian shelves and is transported mainly into the Canada Basin for a high AO index, but toward Fram Strait by the transpolar drift for a low AO index [Morison et al., 2012; Alkire et al., 2015]. Observations also suggest a linkage between the AO and the North American (mainly Mackenzie River) runoff pathways [Yamamoto‐Kawai et al., 2009; Fichot et al., 2013]: there has been a shift from a rather direct outflow via the CAA in early 2000s to a northward pathway into the Beaufort Gyre around 2006 coinciding with a change to a strongly positive AO.

On the large scale, models respond to high‐latitude freshwater perturbations with a slowdown of the oceanic circulation. In numerous hosing experiments, large amounts of freshwater are released over a 50°N–70°N latitude band in the subpolar North Atlantic Ocean. Such a freshwater perturbation reduces convection in the North Atlantic, slows down the surface circulation and Atlantic Meridional Overturning Circulation (AMOC), and reduces the northward ocean heat transport [Manabe and Stouffer, 1995; Stouffer et al., 2006; Stocker et al., 2007], as well as leading to a subsurface warming in the North Atlantic and Arctic Oceans [Mignot et al., 2007]. Similar results are achieved using more realistic perturbations with both Greenland meltwater [Gerdes et al., 2006; Swingedouw et al., 2014] and Arctic river runoff [Rennermalm et al., 2006, 2007]. In fact the large‐scale ocean and climate response is found to be similar to that described above whenever the freshwater forcing originates upstream of the North Atlantic convection sites [Roche et al., 2010] while a qualitatively different response is found if the forcing is applied downstream of the convection sites [Mignot et al., 2007].

The local effects of Arctic river runoff have been studied in more detail using both observations and a variety of models. Idealized regional modeling work links surface freshening to stronger currents inside the Arctic Ocean [Spall, 2013], and high‐resolution modeling work finds that increasing runoff induces stronger currents close to river mouths [Whitefield et al., 2015]. Runoff also affects the sea ice cover and dense water production in the shelf seas as well as the large‐scale hydrography inside the Arctic Basin. More river runoff has been linked to more summer melt, but also to earlier freezing in both observations [Bauch et al., 2013; Nghiem et al., 2014] and modeling studies [Whitefield et al., 2015]. Observations also indicate that less river water on the shelf can increase local bottom water production [Dmitrenko et al., 2010]. Finally, Nummelin et al. [2015] showed that the density and temperature stratification of the Arctic Ocean are tightly linked: under stronger freshwater forcing, the large‐scale hydrography approaches a steady state with a warmer subsurface, but stronger density stratification that together balance the vertical heat flux.

The overall effect of increasing runoff on the climate system requires a combined understanding of the local Arctic processes and those linking the Arctic to the surrounding oceans. While the large‐scale response of the climate system to high‐latitude freshwater perturbation in the Atlantic is rather well documented, the regional response of circulation and hydrography in the Arctic Ocean has not been previously studied outside idealized settings. The conditions in the Arctic are strongly influenced by lower latitudes and vice versa, creating important oceanic connections that are as yet unexplored.

In this study, we use a coupled ocean‐sea ice general circulation model to show two distinct responses to increasing Arctic runoff: (a) a spin‐up of the circulation and warming in the Arctic Mediterranean, and (b) a slowdown and cooling south of the Greenland‐Scotland ridge in the North Atlantic. Inside the Arctic Ocean, neither the dynamic nor thermodynamic response can be understood simply by a reduction in AMOC. For the North Atlantic, our results are largely consistent with previous freshwater hosing studies [Stouffer et al., 2006; Stocker et al., 2007; Mignot et al., 2007; Roche et al., 2010].

The paper is structured as follows: we describe the modeling strategy along with the control simulation in section 2; analyze the Arctic and large‐scale oceanic responses to increasing runoff in section 3; discuss our results in the context of earlier work in section 4; and present concluding remarks in section 5.