Australia: The Land Where Time Began

A biography of the Australian continent 

Atlantic Ocean - Northeast Circulation Impacted by Mesoscale Polar Storms

Deep water formation in the subpolar North Atlantic Ocean is regulated by atmospheric processes, which means the large-scale circulation of the ocean is influenced by these atmospheric processes (Marshall & Schott, 1999). This climatically sensitive region of the Atlantic Ocean is passed over by thousands of mesoscale storms, aka polar lows, every year, and many of these storms are not picked up by meteorological reanalyses or numerical models as they are either too small or too short-lived (Condron et al., 2006; Kolstad, 2008; Zahn & von Storch, 2010). In this article Condron & Renfrew present the results of simulations that they carried out using global circulation model, that permitted eddies and sea ice, that they ran both with and without parameterisation of polar lows. The high speed winds and heat fluxes that are observed in polar lows, as well as their integrated effects, are reproduced by the parameterisation, which leads to simulated depth increases, and in the Nordic seas, frequency and area of deep convection, which then leads to an increased transport of heat to the north into the region, and deep water being transported southwards through the Denmark Strait. Condron & Renfrew conclude that for the large-scale circulation polar lows are important and suggest they should be included in climate short-term predictions. A decrease in the number of polar lows is predicted over the northeast Atlantic in the 21st century by recent studies (Kolstad & Bracegirdle, 2008; Zahn & von Storch, 2010) and this would imply deep convection would be reduced and a potential weakening of the Atlantic meridional overturning circulation (AMOC).

Polar lows are mesoscale (<1,000 km diameter) low pressure systems that are present throughout the polar seas of the world, but tend to cluster in the North Atlantic over the Nordic Sea (Greenland Sea, Iceland Sea and Irminger Sea) (Zahn & von Storch, 2010; Harold et al., 1999; Bracegirdle & Gray, 2008; Zahn & Storch, 2008). There are minor differences here in atmospheric forcing that are sufficient to alter the density of surface waters, such that the sinking of water to depth is changed (Kolstad & Bracegirdle, 2008). One of the fundamental mechanisms that are responsible for the renewal of North Atlantic Bottom Water (NADW), which is the major deepwater mass that drives the Atlantic meridional overturning circulation (Jungclaus et al., 2008) (AMOC) is the ventilation process, the deep open ocean convection.

In global meteorological analyses, reanalyses or climate models polar lows are generally not well resolved, because of their small scales, which are typically about 250 km, and short lifetimes, which are typically 24-48 hours (Condron et al., 2006; Kolstad & Bracegirdle, 2008; Zahn & von Storch, 2010). And yet, the most intense polar lows are associated with winds that are localised and of gale force, and losses of heat from the ocean, >1,000 W/m², which is sufficient to signify change in the underlying ocean (Shapiro et al., 1987; Condron, et al., 2008; Saetra et al., 2008; Moore, 1996). In this paper, for brevity, the term polar low was used to encompass all polar mesoscale cyclones. It was, however, noted by Condron & Renfrew that the term polar low is usually reserved for polar mesoscale cyclones that are more intense having wind speeds of >15 m/s. It has been predicted by many climate models that the AMOC will be slowed down by anthropogenic increases in CO₂ (IPCC Climate Change, 2007). In omitting polar lows, however, climate models are at present deficient at forcing the ocean over the critical regions of deep water formation in the subpolar seas, which limits confidence in their predictions. In this study a state of the art, global, high resolution (1/6^o, about 18 km) was used, that permits eddies, ocean/sea ice circulation model (MITgcm; (Marshall et al., 1997)) in order to determine the impact on circulation of polar lows. The model represents, at this resolution, the boundary currents and deep water overflows of the subpolar sea that are crucial for the formation of NADW and the AMOC (Schweckendiek & Willebrand, 2005; Dickson et al., 2008); circulation features that had been poorly resolved in an earlier coarse-resolution 2-year pilot study of the impact of polar lows on the ocean (Condron et al., 2008). Perturbation and Control ocean model experiments, with and without polar lows over the northeast Atlantic (50^oN – 80^oN, 50⁰W-50^oE) respectively, were run for 21 years (1978-1998) starting from identical ocean states. Therefore, any differences between the simulations can be attributed to the impact of polar lows on the ocean. About 60,000 polar lows were parameterised into the surface wind fields in the perturbation integration. The mean density of cyclones compared well to existing polar-low climatologies that had been determined by the use of a variety of vortex identification techniques (Harold et al., 1999; Bracegirdle & Gray, 2008; Zahn & Storch, 2008).

Evidence that is compelling for the overall veracity of the parameterisation is provided by 1-dimentional power spectra of the wind fields. At scales below about 300 km the Control spectrum has power that is much too low, because of the inability of the atmospheric forcing fields to resolve storms below this scale. Between 50 and 400 km the perturbation spectrum has considerably more power, which brings it into line with what has been observed (Nastrom & Gage, 1985; Patoux & Brown, 2001). Therefore, the correct amount of wind forcing will be received at scales of 50-400 km.

Inspection of the mixed-layer depth, which is a proxy for the depth of the open ocean convection, in the Nordic seas revealed that the localised increases in the surface heat flux provide sufficient cooling to destabilise the water column and trigger open-ocean convection on many more occasions and to greater depths. Condron & Renfrew used a 2-tailed t-test and found that the monthly mean changes to the depth at which open-ocean convection occurs in the Greenland Sea, the Norwegian Sea and the Iceland Sea all differ significantly from the Control (p<0.01), with the mean annul increases  of multi mixed-layer depths of 251, 121 and 160 respectively. Polar lows clearly have a significant impact on ventilating the deep ocean. The number of extra days each year when there is deep convection in the Greenland Sea correlates (r = 0.70, p<0.01) with the number of polar lows that occur each winter (December-March). There were an exceptionally larger number of polar lows in the winter of 1993/1994, and the number of days with deep convection increased from 79 to 138.

i)                   The area of deep convection over the Greenland Sea increases, on average, by 27% (~4,500 km²) and is deeper 97% of the time and by up to 2,000 m when polar lows are resolved.

ii)                In the Norwegian Sea, convection is also deeper 97% of the time and by up to 1,000 m.

iii)              In the Iceland Sea, there are similar changes with the area undergoing deep convection increasing by 41% (~1,050 km²) each year.

The circulation of the Greenland Sea gyre has been significantly influenced by the inclusion of polar lows.

1)    The northwards flow of the Warm, saline water of the Norwegian-Spitsbergen Current,

2)    the southwards flow of cold, fresh water of the East Greenland Current,  and

3)    the eastwards flow of the Jan Mayen Current (Dickson et al., 2008),

make up the gyre. Open-ocean convection is facilitated by the cyclonic circulation allowing deep waters that are weakly stratified to be brought up, by thermal wind balance, where it is exposed to wind forcing (Marshall & Schott, 1999). It is revealed by a barotropic streamfunction that the gyre has, on average, a mean strength over the Greenland Sea of 3.5 Sv and an annual maximum (absolute) strength of 24.9 Sv. The monthly strength of the gyre averaged over the Greenland Sea increases 56% of the time during the 21-yeasr integration, when polar lows are included; a 2-tailed binomial test is significant at the 99% level, which implies that stronger rotation is frequent when polar lows are resolved. Over time the impact of the polar lows is accumulative so that rotation is greater in 60% and 68% of the months during the last 10 and 5 years, respectively, during the last 5 years of the Perturbation integration, the mean (absolute maximum) gyre strength is 8.2% (17.2%) stronger than the control.

The volume of the Greenland Sea Deep Water (GSDW) is also a reflection of the changes in open-ocean deep convection. There is a cumulative increase in GSDW of 4.1 x 10³ km³ (5.3%), after 21 years of forcing with polar lows, though a 2-tailed binomial test indicates that it takes ~10 years for the volume of the GDSW to become statistically greater than the control integration (p <0.01). The volume of the GSDW is higher after 10 years (Marshall et al., 1997), when polar lows are simulated in 61% (97%) of months; it is indicated by a 2-tailed t-test that the last 5 years are statistically different at the 95% confidence interval. It is emphasised by the time lag that both pre-conditioning of the ocean and the spinup of the Greenland Sea gyre, that are due to forcing from polar lows, are key for the formation of deep water (Marshall & Schott, 1999).

Statistically significant increases in the transport of dense water overflowing the Greenland-Iceland-Scotland (GIS) ridge result from the increases in convection and the production of deep water in the Nordic seas. The control integration has a mean transport to the south of Denmark Strait Overflow Water (DSOW) of 3.2 Sv that compares well to the observations of 3.07-3.68 Sv (Macrander et al., 2005). When polar lows are simulated, DSOW transport is higher in 58% of months over the full 21 years and in 74% of months over the last 10 years, which indicates that the effects of polar lows here is also cumulative over time. There is a clear trend towards the volume of deep water that overflows the sill being ~10.3 Sv higher in the last 10 years, though the monthly difference are up to ± 0.5 Sv (±15%) and high variability. A 2-tailed binomial test for whether the DSO transport is statistically different for the 21-year period, i.e., higher volume transport being more frequent with polar lows represented, is significant at the 99% level.

It was found by this study that there were similar differences, and month-to-month variability, at the Faroe-Shetland Channel, with the volume of southwards flowing water, the Iceland-Scotland Overflow Water (ISOW) greater when polar lows are simulated 59% (66%) of the time in the 21- (last -10) year period. Again it is indicated by a 2-tailed binomial test that the increase in the transport of ISOW is statistically significant at the 99% level. At the GIS ridge the increased outflow is consistent with a lagged and nonlinear response to an increase in the hydraulic height of the dense water reservoir, deep water upstream in the Nordic seas (Jungclaus et al., Springer, 2008). The response in the volume transport of DSOW agree with the results of idealised modelling studies showing that an intensification of the wind forcing increases the volume transported at Denmark Strait, with hydraulic control, as well as wind stress forcing being important (Jungclaus et al., Springer, 2008; Biastoch et al., 2003).

The North Atlantic subpolar gyre (SPG) to the south of the GIS ridge represents an important part of the AMOC and is composed of the warm, saline northwards flow of the Gulf Stream and Irminger Current, the cold, fresh westwards flow of the East and West Greenland Currents, and the cold, fresh, southwards flow of the Labrador Current. Buoyancy contrasts, wind forcing, and overflows of dense water from the Nordic Seas and the Labrador Sea (Langehaus et al., 2012; Treguier et al., 2005) maintain the circulation of the gyre. Dense water from the Labrador Sea (Labrador Sea Water) is also formed by deep convection in the Irminger Sea (Dickson et al., 2008; Pickart et al., 2003); and here including polar lows increases the mean annual monthly mixed-layer depth by 237 m. Averaging the monthly mean barotropic streamfunction over the entire gyre is used to calculate the strength of the SPG. The gyre is caused to spinup by 0.47 Sv (3.9%) on average over the full 21 years and to be stronger 80 % of the time, by including the polar lows. There is a more or less sustained increase in the strength of the gyre after 5 years, so that the gyre is stronger 90% of the time by an average of 0.67% Sv (5.5%) for the last15 years. The eastern part of the gyre (east of 50^oW) i.e., the part that is subject to direct polar low forcing, speeds up by 0.56 Sv (4.6%) over the full 21 years and by 0.79 Sv (6.8%) over the last 15 years. Following an increase in mixed-layer depth in the Nordic seas, the lagged increase in SPG strength, has previously been modelled (Treguier et al., 2005) and attributed to coupling of baroclinicity mode. A 2-tailed t-test for the entire gyre and the eastern part, show that the 21-year averages are significantly different (p<0.02 and p<0.01, respectively). Also a 2-tailed binomial test is significant at the 99% level, confirming a key role is played by resolving polar lows in setting the pace of the SPG. An increase in the northwards transport of heat accompanies the spinup of the SPG; at 55^oN this is greater in 68% of the months in the 21-year integration (statistically significant at the 95% confidence level in a 2-tailed t-test and at the 99% level in a 2-tailed binomial test). For the last 15, 10 and 5 years the transport of heat was greater than 70%, 74% and 75%, respectively, the impact of polar low being again found to be cumulative. Resulting from this, the average increase of the transport of heat over the last 5 years peaks at 0.025 PW (4%), which in a 2-tailed t-test at the 95% level (p<0.02) is statistically significant. The persistent increase in the northwards transport of heat during the 21-year integration leads to an extra 8.97 x10⁶ PJ of heat that was transported polewards.

A state of the art ocean model that was capable of resolving narrow boundary currents, sills and overflows was used that found that mesoscale atmospheric forcing over the northeast Atlantic, in the form of parameterised polar lows, plays an important role in the circulation of the Nordic Seas and the sub polar North Atlantic. The analysis of Condron & Renfrew of the surface and deep basins of the North Atlantic shows that polar lows are having an influence on the Greenland Sea gyre and the frequency, depth and area of deep convection. This, in turn, increases the formation of GSDW and the volume of DSOW that flows southwards at the ridge. Finally, a spinup of the SPG leads to a significant increase in the transport northwards of heat to Europe and North America. The findings of Condron & Renfrew, on the basis of this scenario, point towards polar lows as an important, yet absent, forcing in the ocean, climate and seasonal models.

The number of polar lows is predicted to decrease over the Nordic Seas and move northwards (Kolstad & Bracegirdle, 2008; Zahn & von Storch, 2010) under present IPCC future climate scenarios (IPCC Climate Change, 2007). It was surmised by Condron & Renfrew that the influence of the polar lows on the open-ocean deep convection in the Nordic seas, and thereby decrease the rate of formation of GSDW and the volume of DSOW that flows into the North Atlantic. Condron & Renfrew suggest this could have a considerable effect on deep waters of the North Atlantic, and potentially weaken the return branch of the AMOC.

Sources & Further reading

1. Condron, Alan, and Ian A. Renfrew. "The Impact of Polar Mesoscale Storms on Northeast Atlantic Ocean Circulation." Nature Geosci 6, no. 1 (01//print 2013): 34-37.