Persistent SST anomaly vs. dynamical ocean model in winter weather forecasts: Global Ensemble Prediction System versions 5 and 6 over the North Pacific and North Atlantic

Abstract

The impact of coupling an atmosphere model to a dynamical ocean model, rather than using persistent SST anomalies, is assessed for wintertime medium-range forecasts over the North Pacific and North Atlantic. This assessment is based on 20 years (1998–2017) of hindcasts produced by the Global Ensemble Prediction System (GEPS) of Environment and Climate Change Canada (ECCC). We compare an uncoupled atmospheric model (versions 5, GEPS5) with an atmosphere–ocean coupled model (version 6, GEPS6) alongside European Centre for Medium-Range Weather Forecasts Reanalysis v5 (ERA5) as the verification dataset. We find that by the third pentad, or days 11–15, coupling to a dynamic ocean model weakens the Aleutian Low, the Icelandic Low, and the Atlantic Subtropical High. This produces less integrated vapor transport (IVT) over the Pacific and Atlantic Oceans, whose spatial patterns are modulated by phases of Madden–Julian Oscillation (MJO). Coupling also results in colder sea surface temperature (SST) over the Kuroshio Current Extension region and produces a weaker Aleutian Low due to less upward latent heat fluxes. The weaker Aleutian Low further reinforces its weakening through a positive feedback loop. Lastly, the coupling to a dynamical ocean reduces the latent heat flux bias variance by 10 %–20 %, thus improving the IVT.