By Albert Ossό
Forecasts of summer weather patterns months in advance would be of great value for a wide range of applications. However, the current seasonal dynamic model forecasts for European summers have very little skill (Arribas et al. 2011). In a recent work, Ossó et al. 2018 analyse atmospheric and ocean observations and show evidence that a specific pattern of summer atmospheric circulation—the summer East Atlantic pattern (SEA)—is predictable from the previous spring. In particular, the results suggest that a specific pattern of North Atlantic SST anomalies persists until summer influencing the atmospheric circulation and the position of the jet stream in July-August (JA) by changing the background baroclinicity. An index representing this North Atlantic SST pattern in March–April (MA) can predict the SEA pattern in JA with a cross-validated correlation skill above 0.6. Moreover, the SEA pattern has a strong influence on rainfall in the UK, which can also be predicted months ahead with significant skill (~0.56).
An important question is if the current dynamical models are able to simulate this observed spring SST summer circulation relationship. Here, we analyze the representation of this relation in a 120 years long pre-industrial simulation from the Met Office Global Coupled model 2.0 (GC2) (Williams et al. 2015). Figure 1 shows lagged linear regressions between monthly sea level pressure (SLP) and SST anomalies and the preceding MA SST Index in the ERA-Interim reanalysis (left column) and in the GC2 model output (right column). Note that the GC2 boxes used to calculate the SST index have been optimized to capture the SST dipole anomalies in the model (see Ossó et al. 2018). However the results are roughly the same using the same boxes as in the observations.
Figure 1 shows that the SLP anomalies in GC2 are consistent with a significant July atmosphere response to MA SSTs. The spatial pattern of SLP anomalies resembles that of the observations but their magnitude is about half of the size. In contrast with the observations, the simulated anomalies in August are weak and not statistically significant.
The monthly evolution of atmospheric and ocean anomaly observations suggests that the SEA pattern growth in August is driven by a positive feedback between atmospheric circulation and SSTs. Figure 1 (left column) shows that anticyclonic SEA pattern anomalies in July and August are accompanied by warming of SSTs, which increase the amplitude of the SST dipole that forces the SEA pattern, therefore generating a positive feedback. The lack of this mechanism on the model (Figure 1 right column) could explain why the atmosphere response is limited in July. Similar results have been found for large ensembles using the Met Ofﬁce Decadal Climate Prediction System (DePreSys3) (Nick Dunstone personal communication).