Australia: The Land Where Time Began

A biography of the Australian continent 

Polynyas in the Open Ocean and Southern Ocean Deep Convection

An open ocean polynya is a large area that is ice free and is surrounded by sea ice. In the Southern Ocean the Maud Rise Polynya occurs occasionally in the austral winter and spring in the vicinity of Maud Rise near the Greenwich Meridian. In the mid-1970s the Maud Rise Polynya served as a precursor to the Weddell Polynya that is larger and more persistent and is associated to deep convection in the open ocean. The Maud Rise Polynya, however, does not generally lead to the Weddell Polynya, as occurred in the September to November 2017 occurrence of a strong Maud Rise Polynya. Diverse, long term observations and reanalysis data were used to find that the combination of ocean that was stratified weakly near Maud Rise, and the a spin-up of the cyclonic Weddell Gyre that was wind induced had a crucial role in generating the Maud Rise Polynya in 2017. More specifically, eddy activity, which weakened and raised the pycnocline, was intensified by the enhanced flow over the southwestern flank of Maud Rise. In 2018, however, a Weddell Polynya formation was hindered by surface salinity that was relatively low associated with the positive Southern Annular Mode, which contrasts with the condition in the 1970s of a prolonged, negative Southern Annular Mode that induced a saltier surface layer and weaker pycnocline.

There are 2 modes in the Southern Ocean in which water can attain sufficient density to descend into the deep ocean (Gordon, 1991). The mode that is more common stems from the dense shelf water that is formed within the coastal polynyas mainly in the Weddell and Ross seas (Curry & Webster, 1999; Wadhams, 2000), which then descends down the continental slope as gravity currents or plumes (Legg et al., 2009). In the climate system of the present, the continental margin mode, aka near-boundary convection (Wadhams, 20003), is the dominant contributor to Antarctic Bottom Water formation that spreads across the global ocean, is Another mode for the Southern Ocean surface water masses to descend into the deep ocean.

A persistent larger-scale open ocean polynya that was observed only once in the Weddell Sea since the first satellite observations of the Antarctica winter sea ice cover was available in 1972. This polynya, the Weddell Polynya (WP) (Carsey, 1980), remained open for 3 consecutive winters from 1974-1976 and its average size was about 250 x 10³ km². The Weddell Deep Water (WDW) was cooled significantly and freshened down to a depth of 2,700 m by open ocean deep convection (Gordon, 1978; Gordon, 1982). The total heat lost by the WDW during the 3 years of the WP was estimated to be 12.6 x 10²⁰ Joules (Gordon, 1982).

Open ocean polynyas that are smaller and less persistent are also observed in the vicinity of Maud Rise that is located in the Weddell Sea during the winter and early spring (Comiso & Gordon, 1987). In October-November 1973 a Maud Rise Polynya (MRP) occurred, 1 year before the Weddell Polynya in the mid-1970s occurred, so was therefore presumed to be a precursor (Holland, 2001). A decrease in the heat content of WDW in the vicinity where they occurred (Gordon, 1982; Holland, 2001) was led to by the occurrence of the Weddell Polynya and the Maud Rise Polynya. A longer lived Maud Rise Polynya was observed from September to November 1973 (Swart et al., 2018) following the development of the Maud Rise Polynya in 2016 (Masloff et al., 2018). There was a decline in the sea ice concentration to below 10%, almost ice free, and its size extended over about 50 x 10³ km². The 2017 MRP (MRP2017) was the largest event since 1980. The MRP did not, however, generate a WP in the austral winter of 2018, as might have been expected, and there wasn’t a repeat of the MRP in 2018.

South of the Antarctic Circumpolar Current (ACC) and within the Southern Ocean the thermohaline stratification is characterised by surface water that is cold and of low salinity that is separated by a rather weak pycnocline from saline deep water that is relatively warm. Within the larger circulation feature of the Weddell Sea, which is referred to as the Weddell Gyre, the pycnocline is shallowed. Within the Weddell Gyre before and after the WP measurements of the thermohaline stratification reveal active winter open ocean deep convection, cooling of the WDW, and by injecting warm water into the surface layer, acting to prevent overlying sea ice cover, inducing the WP (Legg et al., 2009). Many studies have been performed for this very unusual natural phenomenon, in order to find the mechanism of occurrence of the WP event. Since the 1970s WP and the previous MRPs, however, with the exception of MRP2017 occurred during the time when the observation of the Weddell Sea were still not active, these studies were based on only numerical models from simple models to (Mertinson, Killworth & Gordon, 1981; Ou & Gordon, 1986; Ou, 1991) to climate models that were fully coupled (Martin, Park & Latif, 2013; de Lavergne et al., 2014; Zanowski, Hallberg   & Sarmiento, 2015; Dufour et al., 2017; Wang et al., 2017; Weijer et al., 2017; Zanowski & Hallberg, 2017; Kurtakoti et al., 2018), and very limited, short term observation data measured in the Weddell Sea (Gordon & Huber, 1984; Gordon & Huber, 1990; Gordon & Huber, 1995; Gordon, Visbeck & Comiso, 2007), in which the models did not include a process of data assimilation. In this paper Cheon & Gordon investigate the conditions that govern the generation of the MRP2017 by the use of diverse, long-term observation and reanalysis data and prove the hitherto hypotheses that had been proposed by previous studies. The Sea Surface Height (SSH) data provided by AVISO and high resolution (1/12^o x 1/12^o in horizontal direction reanalysis data from the Hybrid Coordinate Ocean Model (HYCOM) of the Naval research Laboratory are in this study used in order to estimate the variation of the intensity of the Weddell Gyre since the mid-1990s, as well as in situ observational data of the thermohaline stratification. This study focussed on 4 questions:

1)    How had the thermohaline stratification of the upper ocean been weakened within the Weddell Gyre prior to the occurrence of MRP2017?

2)    How did the basin-scale atmospheric circulation above the Weddell Sea intensify the underlying cyclonic Weddell Gyre circulation?

3)    What events occurred in the vicinity of Maud Rise and led to the MRP2017?

4)    Why didn’t the MRP2017 lead to a larger-scale WP event that was more persistent, as materialised in the 1970s?

Discussion

There are 6 stages that categorise the life cycle of the MRP, from preconditioning to its eventual cessation:

1)    There is a weakening of the upper ocean stratification, which provides the precondition for the open-ocean polynya (Gordon, Visbeck & Comiso, 2007)

2)    Small scale convection in the upper ocean when the water column becomes sufficiently destabilised to trigger this convection, which causes the relatively warm WDW below the pycnocline depth to rise to the surface.

3)    The warm deep water that is upwelled melts the overlying sea ice or prevents it forming. The MRP serves as a precursor to the WP, as in the 1970s.

4)    The relatively warm surface water (~-1.8^oC) within the polynya is exposed to the cold winter atmosphere, once the MRP occurs.

5)    If insufficient freshwater introduction by the melting of regional sea ice, oceanic deep convection ensues, and the corresponding ocean-to-atmosphere heat loss is enormous. The warmth of the WDW is essential for maintenance of an open-ocean polynya; therefore the drastic shortage of deep ocean heat content in the deep water can be a crucial factor to eventually attenuate the open-ocean polynya.

6)    The open-ocean polynya disappears in the final stage, as does the deep-ocean convection, though in the Weddell Sea the remnant impact of convection persists until the WDW heat is restored by advection from the circumpolar ocean (Smedsrud, 2005; Cheon et al., 2018).

The development of a subsequent WP in 2018was hindered by the lack of regional conditioning on a larger scale, though in 2017 conditions were right for the development of the MRP2017 event. In the Weddell Sea the surface salinity correlates inversely with the SAM index (Gordon, Visbeck & Comiso, 2007). In the 1970s the WP followed a prolonged, negative SAM which induces the atmosphere to be drier than normal over the Weddell Sea, which increases the salinity of the cold surface layer, which results in the weakening of the pycnocline that separates it from the warmer deep water (Gordon & Huber, 1990; Gordon & Huber, 1995; Gordon, Visbeck & Comiso, 2007). The pycnocline that had been weakened acted to transfer the heat of the ocean to the surface, and this affected adversely the cover of sea ice. Since the 1990s the SAM has been in the positive mode, and the sea surface salinity remained relatively fresh, which hindered the ability of the MRP2017 to spark the larger scale open-ocean WP. Also, between 2015 and 2016 the negative wind stress curl over the Weddell Sea reached its peak and began to weaken, and so did the Weddell Gyre. In 2015, the activity of Maud Rise eddy reached its peak and began to decline. The WP might have occurred if these had continued to intensify. Finally, the fact that by April of 2018, the deep water in the vicinity of Maud Rise had already lost a huge amount of heat, which is an indication that the deep water that was upwelled may not have been warm enough to melt sea ice or to prevent it forming during the austral winter of 2018.

The earlier study that had analysed the historical observations and model simulations of the 5^th Coupled Model Intercomparison Project (CMIP5) expected that in the Southern Ocean, open-ocean deep convection would weaken and cease in association with the upper ocean stratification proceeded by the freshening of the surface of the Antarctic ocean since the 1950s (de Lavergne et al., 2014). According to Cheon & Gordon surface freshening of the southern polar ocean may act to hinder open-ocean polynya occurrence in the future as well. According to the 5^th Assessment report (AR5) of the IPCC, however, the Southern Annular Mode index has been increasing gradually since the late 1950s. It is indicated by the increasing, positive SAM index of the westerly winds in the Southern Hemisphere shifted polewards and intensified, which implies a wind stress curl over the Weddell Sea that is increasingly negative. As has been illustrated in this paper, it activates the mesoscale WDW eddies in the vicinity of Maud Rise, which increases the possibility that an open-ocean polynya will occur and this conflicts with the prediction of the aforementioned study (de Lavergne et al., 2014). It was concluded by Cheon & Gordon that the combined effect of a weakened thermohaline stratification with wind-induced Maud Rise eddy activity that has increased have a crucial role in the generation of open-ocean polynyas in the Southern Ocean. Therefore, between the freshening of the surface, a positive SAM index, both being predicted by CMIP5 models and an important question is which factor is the more crucial factor for predicting the occurrence of open-ocean polynyas.

Sources & Further reading

Cheon, W. G. and A. L. Gordon (2019). "Open-ocean polynyas and deep convection in the Southern Ocean." Scientific Reports 9(1): 6935.