The shifting NAO and cyclonic Rossby Wave Breaking

Spatial climate patterns such as the North Atlantic Oscillation (NAO) are often assumed to be stationary in winter on interannual time scales and are defined by applying EOF analysis over many winters. However, it has been known for some time that the NAO is not stationary on long time scales. Its northern center of action has been found to shift east/west on decadal time scales. Here we address the connection between the shifting NAO and Rossby wave breaking (RWB).

We find that the frequent cyclonic RWB region (cyclonic surf zone) during 1958-1977 was located south of Greenland, and then shifted eastwards during 1978-1997. The spatial pattern of the eastward shift of the cyclonic surf zone is similar to the eastward shift of the northern node of the NAO. On year-to-year basis, we also find a relationship showing that a northeastward shift in the cyclonic surf zone corresponds well to an increased NAO index and a southwest shift corresponds to a decreased NAO index.

We use composite analyses of 6-hourly geopotential fields to explain the similarity of the shifting in the NAO and cyclonic RWB on the longer time scale. Our composite result shows that the negative NAO-like phase relative to cyclonic RWB depends on where cyclonic RWB occurs. When cyclonic RWB takes place on the margin of the North Atlantic, only a part of the resulting geopotential anomaly is located in the North Atlantic. The rest of the anomaly associated with cyclonic RWB lies outside of the North Atlantic and is not captured by the NAO index because of the assumption that the NAO pattern is stationary. Therefore, the location of cyclonic RWB can affect the variability of the NAO index.

A composite of zonal winds shows an eastward extension of the Atlantic jet as cyclonic RWB occurs to the northeast. This changed jet location resembles the difference in the jets between 1958-1977 and 1978-1997. The result suggests that the eastward shift of the NAO is linked to the shift of cyclonic RWB surf zones responding to the changed jet location on the decadal time scale.