The relationship between surface weather over North America and the Mid-Latitude Seasonal Oscillation

The influence of the 120-day Mid-Latitude Seasonal Oscillation (MLSO) on near surface circulation, temperature and precipitation over North America is investigated as an independent mode and in combination with the El Niño-Southern Oscillation (ENSO), as well as the North Atlantic Oscillation (NAO) and Pacific North American (PNA) teleconnection patterns. Empirical evidence based on frequency ratios such as anomalous warm over cold days and wet over dry days and composite analysis reveal an influence of the MLSO during both boreal winter and summer. During winter, above (below) normal MLSO conditions can cause below (above) average temperatures across the US. During summer, the below normal MLSO conditions can cause strong above average temperatures across the US, whereas the below normal MLSO conditions can cause weak temperature anomalies of both signs. The influence of MLSO on the surface air temperature anomalies is more significant and stronger than the effect on precipitation. It is also found that the MLSO plays a role in exciting variability when interacting with other modes, exerting influence over the same region. Expected patterns of temperature and precipitation associated with well-known climate modes show alterations when they are further decomposed to account for the MLSO influence.