Australia: The Land Where Time Began

A biography of the Australian continent 

Abyssal Ocean Warming and Salinification Following Weddell Polynyas in GDFL CM2G Coupled Climate Models

In this paper Zanowski et al. report the results of their study using the NOAA Geophysical Fluid Dynamics Laboratory’s (GFDL) coupled climate model CM2G to explore the role of the Weddell Sea polynyas in the establishment of deep-ocean properties. Zanowski et al. used statistical composite analysis of more than 30 polynya events that occur in a preindustrial control run covering a period of 2,000 years to quantify the temperature, salinity and water mass changes associated with a composite event. Warming of between 0.002^oC and 0.019^oC per decade occurs at a depth of below 4,200 m in the basins of the Southern Ocean for the time period following the cessation of the composite polynya, which has been termed the “recovery”. Temperature and salinity changes were found to be strongest near the region of polynya formation in the Southern Ocean and the South Atlantic Ocean. It was revealed by comparison of the model results with observations of abyssal temperature that the Weddell Polynya recovery in the 1970s could account for 10% ± 8% of the recent warming in the abyssal Southern Ocean. This percentage is as little as 6% ± 11% or as much as 34% ± 13% for individual Southern Ocean basins.

Conclusions

Changes in the abyssal ocean temperature and salinity that followed the cessation of a composite Weddell Sea polynya in GFDL’s coupled climate model CM2G were analysed.  In order to quantify warming of the deep ocean that occurred during the recovery period 2 composites were constructed. Zanowski et al. found patterns of warming of deep basin below 4,200m of similar, but not smaller than, the magnitude that has been reported previously (Purkey & Johnson, 2010). In the South Atlantic and Southern Ocean basins the strongest warming signals were found to occur which tended to decay with distance from the Weddell Sea. Warming rates of between 0.002^oC and 0.019^oC per decade occurred in the basins of the abyssal Southern Ocean during a period of 19 years in the recovery. It is suggested by comparison with previously published results (Purkey & Johnson, 2010) that 10% ± 8% of the warming observed in the abyssal Southern Ocean could be explained by the recovery of the Weddell Polynya. According to Zanowski et al., however, in the Australian-Antarctic basin warming does not appear to result from the current polynya cycle. When the contribution of the Australian-Antarctic basin is excluded, the Weddell Polynya could explain 6% ± 8% of the warming that has been observed in the abyssal Southern Ocean. The percentage contribution varies between basins and is as high as 34% ± 13% for the Australian-Antarctic basin and 33% ± 14% for the Argentine basin. Percentages for the Weddell-Enderby basin is 6% ± 11% and for the Amundsen-Bellingshausen basin7% ± 15%.

It is suggested by cooling in the Brazil and Angola basins that the current polynya signal reached these basins, though the recovery had yet to begin. Therefore Zanowski et al. do not expect signals from the Weddell Polynya in these basins to have contributed to warming between the mid-1990s and the mid-2,000s, though could do so after this period. It is suggested it is possible that some warming that has been observed in the Brazil basin from 2005 onward (Johnson et al., 2014) results from recovery of the Weddell Polynya.

Analysis of basins in the Northern Hemisphere was not carried out because of the lack of a discernible polynya signal from internal variability. According to Zanowski et al. it is possible for polynya signals to reach these remote basins in less than 50 years by planetary waves (Kawase, 1987; Nakano & Suginohara, 2002; Purkey & Johnson, 2010; Hirabara et al., 2012). It is indicated by recent studies that deep convection and temperature changes of the deep ocean around Antarctica could impact the Northern Hemisphere, the North Atlantic in particular, via changes to the Atlantic meridional overturning circulation (AMOC) (Martin et al., 2013, 2015; Patara & Böning, 2014). An explanation of changes in the basins of the Northern Hemisphere may be helped by further exploration of the propagation of the polynya signal.

It is clear that the abyssal ocean was impacted by the Weddell Polynya of the 1970s. Zanowski et al. say the importance of understanding these transient features is emphasised by their model Analysis, in particularly their spatial structure and the time scales which they act on. It has been proven that composite analysis is a useful tool in the study of the polynyas, though even with more than 30 polynyas variability between individual events complicated analysis of the signal. The robustness of the composite signal was lowered by differences in the length of events, magnitude, and the spatiotemporal influence of the polynyas which often combined. As a result the analysis by Zanowski et al. was limited to broad, mean changes on a large scale. Zanowski et al. plan to investigate the effects of variability between polynyas by regularly forcing large polynyas in the Weddell Sea in order to overcome these issues. The differentiation between changes that are caused by the current polynya cycle and those caused by previous polynya cycles could be helped by control over the timing of polynyas.

Distinguishing between the polynya signal and that of anthropogenic climate change requires the modelling of polynya dynamics be accurate, as the recovery of the polynya is manifested as a warming of the deep and bottom waters. The study by Zanowski et al. employed a single model to investigate open-ocean polynyas, so the results are tied to the particular realisation of ocean dynamics within the model. The effects of open-ocean (?ocean-open) polynyas across CMIP5 models by examination of bottom property changes that result from reduced open-ocean convection under anthropogenic climate change has been indirectly investigated (Heuzé et al., 2015). However, the study of Zanowski et al. aims at understanding impacts that result from models creating AABW through open-ocean convection, rather than through shelf processes (Heuzé et al., 2913) instead of a comparison of open-ocean polynya dynamics and effects. The only comprehensive studies in which impacts related to open-ocean convection are compared across models that could be found by Zanowski et al. was the study by Heuzé et al. (2015). The understanding of the sensitivity of polynya signals to model characteristics such as resolution, background diffusivity, and overflow parameterisation, could benefit from multimodel studies that investigate polynya effects.

Sources & Further reading

1. Zanowski, H., R. Hallberg and J. L. Sarmiento (2015). "Abyssal Ocean Warming and Salinification after Weddell Polynyas in the GFDL CM2G Coupled Climate Model." Journal of Physical Oceanography 45(11): 2755-2772.