| Summary: | The 120-day Mid-latitude Seasonal Oscillation (MLSO) and its possible connections to surface weather over North America are investigated. Atmospheric modes, such as the North Atlantic Oscillation (NAO), Pacific North American (PNA) teleconnection pattern, and the atmosphere-ocean coupled mode El Niño Southern Oscillation (ENSO), are known to have significant impacts on surface weather, e.g., temperature and precipitation. Understanding how the recently discovered MLSO affects the surface weather over North America will be useful for improving the extended-range forecast. The analysis of frequency ratios such as warm over cold days and wet over dry days and composite analysis reveal the influence of MLSO on surface weather. The impact of MLSO is also investigated in conjunction with other climate modes such as NAO, PNA and ENSO. All investigations are split into boreal summer and winter and the analyses of the MLSO combined with other modes are conducted for two modes of phasing: in phase and out of phase. These analyses reveal that the MLSO is an important factor in understanding the temperature variability over North America. 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 deviation when they are further decomposed to account for the MLSO influence. The MLSO is a vital component to the climate system of North America and its affects elsewhere on the globe need to be investigated.
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