Network connectivity between the winter Arctic Oscillation and summer sea ice in CMIP6 models and observations

Abstract

Abstract. The indirect effect of winter Arctic Oscillation (AO) events on the following summer Arctic sea ice extent suggests an inherent winter-to-summer mechanism for sea ice predictability. On the other hand, operational regional summer sea ice forecasts in a large number of coupled climate models show a considerable drop in predictive skill for forecasts initialised prior to the date of melt onset in spring, suggesting that some drivers of sea ice variability on longer timescales may not be well represented in these models. To this end, we introduce an unsupervised learning approach based on cluster analysis and complex networks to establish how well the latest generation of coupled climate models participating in phase 6 of the World Climate Research Programme Coupled Model Intercomparison Project (CMIP6) are able to reflect the spatio-temporal patterns of variability in Northern Hemisphere winter sea-level pressure and Arctic summer sea ice concentration over the period 1979–2020, relative to ERA5 atmospheric reanalysis and satellite-derived sea ice observations, respectively. Two specific global metrics are introduced as ways to compare patterns of variability between models and observations/reanalysis: the adjusted Rand index – a method for comparing spatial patterns of variability – and a network distance metric – a method for comparing the degree of connectivity between two geographic regions. We find that CMIP6 models generally reflect the spatial pattern of variability in the AO relatively well, although they overestimate the magnitude of sea-level pressure variability over the north-western Pacific Ocean and underestimate the variability over northern Africa and southern Europe. They also underestimate the importance of regions such as the Beaufort, East Siberian, and Laptev seas in explaining pan-Arctic summer sea ice area variability, which we hypothesise is due to regional biases in sea ice thickness. Finally, observations show that historically, winter AO events (negatively) covary strongly with summer sea ice concentration in the eastern Pacific sector of the Arctic, although now under a thinning ice regime, both the eastern and western Pacific sectors exhibit similar behaviour. CMIP6 models however do not show this transition on average, which may hinder their ability to make skilful seasonal to inter-annual predictions of summer sea ice.

Impact of the Arctic oscillation from March on summertime sea ice

“…The sea ice response to the AO evolves with time, showing that regional sea ice responses are better represented by the March AO than by the seasonally averaged winter AO. The main feature revealed from lag-correlation between sea ice and March AO is that negative correlations are gradually getting stronger, signifying the sea ice decrease with time over the Laptev, East Siberian, Chukchi, and Beaufort Sea (LECB) in the event of positive AO in March (figures 2(a)-(f)) (Gregory et al 2022). The area of sea ice reduction also expands with time, reaching the maximum spatial extent in August.…”

“…Sea ice decrease enhances along the LECB region during the melt season in the event of positive AO in March, whereas the sea ice increases over the region to the northeast of Greenland. The sea ice decrease along the LECB region is found to be more active in the recent 21st century than later part of the 20th century (1980)(1981)(1982)(1983)(1984)(1985)(1986)(1987)(1988)(1989)(1990)(1991)(1992)(1993)(1994)(1995)(1996)(1997)(1998)(1999), due to the fact that sea ice is thinner and more susceptible to surface warming and sea ice motion (Maslanik et al 2007, Williams et al 2016, Gregory et al 2022. Also, different atmospheric circulation response to the March AO between the 21st and 20th century yields the opposite sea ice anomaly between the two periods over the Canada Basin.…”

“…Earlier studies showed that the wintertime AO can have persisting impacts on the surface temperature, pressure, sea ice drift and circulation in the subsequent months. Most importantly, this memory of the wintertime AO can play a profound role in driving variability of the summertime sea ice (Rigor et al 2002, Zhang 2015, Ogi et al 2016, Park et al 2018, Gregory et al 2022. Rigor et al (2002) and Williams et al (2016) found that sea ice motion that responds to the positive phase of the AO modulates the Beaufort Gyre, Transpolar Drift Stream (Mysak 2001) and subsequent ice export through the Fram Strait (Ogi and Wallace 2012), enhancing the summer sea ice loss.…”

“…Advanced understanding is also expected to contribute to the improvement in seasonal prediction skill of the summertime sea ice. Currently, many climate models such as the ones participating in the Coupled Model Intercomparison Project 6 (CMIP6) have underestimated the important connections between the winter AO (average over December through March) and the summer sea ice (Gregory et al 2022) more specifically over the pan-Arctic region such as the Laptev, East Siberian and Beaufort seas, where the strong downward trend of sea ice extent due to climate change (Meredith et al 2019) and increasing role of ocean-air heat exchange (Laptev Sea) (Ivanov et al 2019) was reported. Earlier version of climate models (e.g.…”

“…surface warming/cooling from surface energy budget) and dynamic (low-level circulation and pressure) process to the March AO evolves with time from March through September to drive the sea ice anomaly during the melt season. Finally, sea ice responses to the AO in the 21st century and the late 20th century are compared to quantify their differences (Gregory et al 2022).…”