Combined Impacts of PDO and Two Types of La Nina on Climate Anomalies in Europe

Combined impacts of the Pacific decadal oscillation (PDO) and two types of La Nina on climate anomalies in Europe are studied. Particularly, the conjunction of the negative PDO phase and two different types of La Nina events favors strong and significant North Atlantic Oscillation (NAO) pattern anomalies with opposite polarity. For the central Pacific (CP) La Nina, a clear positive NAO signal can be detected, which is accompanied by positive surface air temperature (SAT) anomaly and a dipolar structure of precipitation anomalies in Europe. In addition, a typical negative Pacific-North America (PNA) teleconnection pattern forms, including a high pressure anomaly over the southeastern United States, which may contribute to the development and maintenance of the NAO anomaly by strengthening the baroclinicity and the local eddy-mean flow interaction. However, for the eastern Pacific (EP) La Nina, a zonal wave train in the high latitudes can be observed, which is quite different from the typical PNA structure. Here, an anomalous anticyclone over southern Greenland supports a negative NAO pattern through the local eddy-mean flow interaction and the associated vorticity advection. Hence, reversed SAT and precipitation anomalies occur over Europe. Further analyses indicate that the wave trains emanating from the North Pacific and the synoptic eddy-mean flow interaction play essential roles in forming the anomalous NAO phases. The different wave trains for the CP and EP La Nina events may be attributed to the differences in the location and intensity of anomalous convection induced by different types of SST anomaly as well as by the corresponding background westerly wind anomalies in the upper troposphere.