The COWL Pattern Identified with a Large AO Index and Its Impact on Annular Surface Temperature Anomalies

In this study, the cold ocean/warm land (COWL) pattern was identified from the leading empirical orthogonal function (EOF) of the monthly 1000-hPa geopotential height field poleward of 20°N. Traditionally, the leading EOF has been recognized as the Arctic Oscillation (AO), or Northern Annular Mode (NAM), which causes annular surface air temperature (SAT) anomalies over high-latitude regions of the Northern Hemisphere. A new finding of the present study is that the total AO events defined by the large AO index actually include a distinct type of events that are characterized by a less-annular spatial structure, i.e., the COWL pattern, which shows an NAO-like distribution in the Atlantic sector and a center of action over the North Pacific with the same sign as that over the Arctic. In addition, unlike canonical AO events, the COWL events also show a less-annular pattern in the stratosphere. Statistically, at least one-third of the AO events can be categorized as the COWL events. The SAT anomalies associated with the COWL pattern have an annular distribution over the high-latitude region of the two continents in the Northern Hemisphere. In contrast, if the COWL events are removed from the total AO events, the remainder shows less annular SAT anomalies. Thus, the typical annular SAT anomalies associated with AO events are in large part due to the contribution of the COWL pattern. Furthermore, the monthly variability and the interannual variability of all the AO events are equally important.