Potential Predictability of the Spring Bloom in the Southern Ocean Sea Ice Zone

Every austral spring when Antarctic sea ice melts, favorable growing conditions lead to an intense phytoplankton bloom, which supports much of the local marine ecosystem. Recent studies have found that Antarctic sea ice is predictable several years in advance, suggesting that the spring bloom might exhibit similar predictability. Using a suite of perfect model predictability experiments, we find that November net primary production (NPP) is potentially predictable 7 to 10 years in advance in many Southern Ocean regions. Sea ice extent predictability peaks in late winter, followed by absorbed shortwave radiation and NPP with a 2 to 3 months lag. This seasonal progression of predictability supports our hypothesis that sea ice and light limitation control the inherent predictability of the spring bloom. Our results suggest skillful interannual predictions of NPP may be achievable, with implications for managing fisheries and the marine ecosystem, and guiding conservation policy in the Southern Ocean. Plain Language Summary In very much the same way as we do for the weather, we can make forecasts of many aspects of the earth system. For example, rather than trying to predict how much rain will fall next Tuesday, we can explore how much algal growth might take place in the oceans around Antarctica in several months time. Such predictions could be extremely useful for managing the fragile ecosystems of these regions, for example, informing fishing quotas in an upcoming season. However, just like for weather forecasts, there are upper limits for how far into the future we can expect to accurately make such predictions. It's this upper limit that we try to understand in this theoretical modeling study. We find that the upper limit is actually rather long (as much as 10 years!), and show that this is because of the close relationship between algal growth and sea ice (ice formed at the ocean surface) in this cold polar region. In turn, the extent of the sea ice can be predicted a long time in advance because there is a lot "memory" in this component of the earth system.