Local predictability of the atmosphere and ocean using Granger causality

The atmosphere and ocean are coupled through complex interactions. Thus, information about the ocean helps to better predict the future of the atmosphere, and information about the atmosphere helps to better predict the ocean. Here, we investigate the spatial and temporal nature of this predictability: where, at what timescales, and for how long does the ocean provide significant predictability to the atmosphere, and vice-versa?

We apply Granger causality, a statistical test of predictability, to time-series of sea-surface temperature and atmospheric vorticity at daily, 5-, and 15-day average resolutions. By introducing time delays, we also determine the persistence of the predictability.

We find that in the deep tropics the ocean provides more predictability to the atmosphere than vice-versa, while the opposite is true in the extratropics. This is consistent with previous work that has shown that in the tropics, the atmospheric state is highly predictable from sea-surface temperature anomalies (e.g. Shukla, 1998). We find that the predictability of the ocean due to the atmosphere persists for a shorter time than the predictability of the atmosphere due to the ocean. Furthermore, with anomalies averaged over 15 days, only the ocean can provide predictability to the atmosphere.

The patterns we observe generally agree with the results of the Kalnay dynamical rule (Ruiz-Barradas et al., 2017), which predicts the directional forcing between the atmosphere and ocean by considering the local phase relationship between simultaneous sea-surface temperature and vorticity anomaly signals.