“…However, the sea ice in the Iceland Sea in our ICON2.5 simulation extends too far east compared to present‐day conditions (see Figure 1). Therefore, the WMT occurs too far in the central Iceland Sea and not in its northwestern part (Spall et al., 2021; Våge et al., 2018, 2022), as has been observed in connection with a retreating sea ice edge toward Greenland (Moore et al., 2022). Furthermore, we note that our simulation period is rather short and hence longer simulations of a similar resolution are required to quantify the effect of PLs on the climate scale.…”
Section: Polar Low In the Iceland And Greenland Seasmentioning
confidence: 99%
“…CAOs typically occur every 1–2 weeks in winter, lasting for 2.5 days on average (Harden et al., 2015). The Iceland and Greenland Seas are both important areas for the formation of dense water (Brakstad et al., 2023; Våge et al., 2022) contributing to the Denmark Strait Overflow Water (DSOW) with a delimiting density of 27.8 kg m −3 (Dickson & Brown, 1994). The DSOW leaves the Nordic Seas and becomes part of the deep return branch of the Atlantic Meridional Overturning Circulation (AMOC, Buckley & Marshall, 2016; Renfrew et al., 2019).…”
Using two case studies, we analyze the effects of explicitly resolving polar lows in a global climate model (ICON‐Sapphire) with a high resolution of 2.5 km on the upper ocean and sea ice. We aim to understand the mechanism of how polar lows form in a global coupled model and how they interact with the upper ocean and sea ice. When polar lows form at the sea ice edge, they induce marine cold air outbreaks that lead to large heat loss from the ocean. This heat loss contributes to dense water formation in the Iceland and Greenland Seas, which replenishes the climatically important Denmark Strait Overflow Water (DSOW). The high wind speeds of polar lows open leads and polynyas in the sea ice cover, such as the Sirius Water Polynya in northeastern Greenland. Heat loss in polynyas is compensated for by the formation of new ice, and the rejected brine densifies the water on the Greenland shelf. In the Labrador Sea, polar lows intensify cold air outbreaks from the sea ice and rapidly deepen the ocean mixed layer. Resolving polar lows and kinematic features in the sea ice improves the realism of climate models, in particular the surface heat loss and the dense water formation in (sub)polar oceans.
“…However, the sea ice in the Iceland Sea in our ICON2.5 simulation extends too far east compared to present‐day conditions (see Figure 1). Therefore, the WMT occurs too far in the central Iceland Sea and not in its northwestern part (Spall et al., 2021; Våge et al., 2018, 2022), as has been observed in connection with a retreating sea ice edge toward Greenland (Moore et al., 2022). Furthermore, we note that our simulation period is rather short and hence longer simulations of a similar resolution are required to quantify the effect of PLs on the climate scale.…”
Section: Polar Low In the Iceland And Greenland Seasmentioning
confidence: 99%
“…CAOs typically occur every 1–2 weeks in winter, lasting for 2.5 days on average (Harden et al., 2015). The Iceland and Greenland Seas are both important areas for the formation of dense water (Brakstad et al., 2023; Våge et al., 2022) contributing to the Denmark Strait Overflow Water (DSOW) with a delimiting density of 27.8 kg m −3 (Dickson & Brown, 1994). The DSOW leaves the Nordic Seas and becomes part of the deep return branch of the Atlantic Meridional Overturning Circulation (AMOC, Buckley & Marshall, 2016; Renfrew et al., 2019).…”
Using two case studies, we analyze the effects of explicitly resolving polar lows in a global climate model (ICON‐Sapphire) with a high resolution of 2.5 km on the upper ocean and sea ice. We aim to understand the mechanism of how polar lows form in a global coupled model and how they interact with the upper ocean and sea ice. When polar lows form at the sea ice edge, they induce marine cold air outbreaks that lead to large heat loss from the ocean. This heat loss contributes to dense water formation in the Iceland and Greenland Seas, which replenishes the climatically important Denmark Strait Overflow Water (DSOW). The high wind speeds of polar lows open leads and polynyas in the sea ice cover, such as the Sirius Water Polynya in northeastern Greenland. Heat loss in polynyas is compensated for by the formation of new ice, and the rejected brine densifies the water on the Greenland shelf. In the Labrador Sea, polar lows intensify cold air outbreaks from the sea ice and rapidly deepen the ocean mixed layer. Resolving polar lows and kinematic features in the sea ice improves the realism of climate models, in particular the surface heat loss and the dense water formation in (sub)polar oceans.
“…For example, despite strong changes in deep convection over observational periods, the DWBC at 53∘N remained relatively steady [4,106,134,135]; modest changes in export from the Labrador Sea do not bear an obvious relationship to changes in deep convection (figure 6 a ). Similarly, the volume transport of OW at the Greenland–Scotland Ridge is observed to be quite steady [127,128], despite variations in convection [136] and water mass properties [137] in the Nordic Seas. Figure 6Time series of transport out of the Labrador Sea at 53∘N for ( a ) UNADW and ( b ) LNADW.…”
The North Atlantic meridional overturning circulation and its variability are examined in terms of the overturning in density space and diapycnal water mass transformation. The magnitude of the mean overturning is similar to the surface water mass transformation, but the density and properties of these waters are modified by diapycnal mixing. Surface waters are progressively densified while circulating cyclonically around the subpolar gyre, with the densest waters and deepest convection occurring in the Labrador Sea and Nordic Seas. The eddy-driven interaction between the convective interior and boundary currents is a key to the export of dense waters from marginal seas. Due to the multitude of pathways of dense waters within the subpolar gyre, as well as mixing with older waters, waters exiting the subpolar gyre have a wide range of ages, with a mean age on the order of a decade. As a result, interannual changes in water mass transformation are mostly balanced locally and do not result in changes in export to the subtropics. Only persistent changes in water mass transformation result in changes in export to the subtropics. The dilution of signals from upstream water mass transformation suggests that variability in export of dense waters to the subtropics may be controlled by other processes, including interaction of dense waters with the energetic upper ocean.
This article is part of a discussion meeting issue ‘Atlantic overturning: new observations and challenges’.
“…These changes in recent decades have had pronounced impacts on the water mass transformation in this region 20 . For example, studies have argued that the reduction of ocean heat loss has weakened deep convection in the Greenland and Iceland Seas 19 , 21 , 22 . In addition, the retreat of wintertime sea ice has led to enhanced ventilation of AW as it flows southward in the East Greenland Current 17 .…”
The warm-to-cold densification of Atlantic Water (AW) around the perimeter of the Nordic Seas is a critical component of the Atlantic Meridional Overturning Circulation (AMOC). However, it remains unclear how ongoing changes in air-sea heat flux impact this transformation. Here we use observational data, and a one-dimensional mixing model following the flow, to investigate the role of air-sea heat flux on the cooling of AW. We focus on the Norwegian Atlantic Slope Current (NwASC) and Front Current (NwAFC), where the primary transformation of AW occurs. We find that air-sea heat flux accounts almost entirely for the net cooling of AW along the NwAFC, while oceanic lateral heat transfer appears to dominate the temperature change along the NwASC. Such differing impacts of air-sea interaction, which explain the contrasting long-term changes in the net cooling along two AW branches since the 1990s, need to be considered when understanding the AMOC variability.
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