Investigate northern hemisphere teleconnection patterns in winter (DJF).
The ERA Interim data set is a homogenous re-analysis dataset with global geographic coverage. It is thus ideal for your purposes. Since teleconnection patterns are controlled by the spatial covariance of pressure, you will need sea level pressure (msl) data.
Apply an EOF analysis to winter (DJF) mean sea level pressure anomalies in the northern hemisphere extratropics (20N-90N). Plots the first five resulting EOF time series and loading patterns and think about their meaning.
Hint: Barnston and Livezey (1987) and online resources by NOAA can help with your interpretations.
Optional: Carry out the same analysis for the other seasons.