Identification of an intra-month regime shift over the North Atlantic

The North Atlantic Oscillation (NAO) is generally accepted as being an inter-annual oscillation in the atmospheric mass field. The positive phase of the NAO is associated with a strong Icelandic Low and is the result of storms that track in a northeasterly direction towards Greenland. The negative phase has a substantially weaker Icelandic Low or a blocking high over Greenland and is the result of storms that track in an easterly direction across the North Atlantic.

Our research shows that not only do the flow regimes associated with the NAO appear on shorter timescales, but that the shifts in the regimes occur on intra-month timescales. Making use of the 6-hourly NCEP Reanalysis, we identify these regime shifts for a location in the GIN Seas (just east of Greenland). By triggering on the pressure field at this location, we create 2D composites of the North Atlantic basin in various fields including the pressure, heat flux and precipitation fields. The plots illustrate the coherent spatial structure of the respective flow regimes.