The Processes That Drive the Temperature Anomalies of the Pacific–North American Teleconnection Pattern

The Surface Air Temperature (SAT) anomalies associated with the Pacific/North American (PNA) teleconnection pattern are examined using composite analysis with daily data from the ECMWF (European Centre for Medium Range Weather Forecasts) ERA interim reanalysis. Following the finding that the SAT anomaly patterns of the positive and negative PNA phases are different, we examine the positive and negative PNA phases separately.

The processes that drive both the growth and decay of the SAT anomalies associated with the PNA are determined using the thermodynamic energy equation. Terms examined in the thermodynamic energy equation include horizontal temperature advection, vertical temperature advection, adiabatic warming, longwave heating/cooling, latent heat release and vertical mixing. It is found that the growth of individual PNA SAT anomalies arises from either 1) the advection of the climatological temperature by the anomalous wind, 2) the advection of the anomalous temperature by the climatological wind, or 3) vertical mixing. The decay of these anomalies is found to be primarily due to longwave heating/cooling, with vertical mixing and the anomalous advection of the anomalous temperature by the anomalous wind playing a secondary role. This implies that each PNA anomaly is driven by a different mechanism.

For both PNA phases, the two most pronounced SAT anomalies develop over the North American continent. These anomalies are driven by both the advection of the climatological temperature field by the anomalous wind and vertical mixing. When the PNA is in the positive phase, positive SAT anomalies are observed over northwestern North America and negative anomalies are found over southeastern North America, and vice versa for the negative PNA phase. However, west of the North American continent, differences between the SAT anomaly patterns of the two PNA phases are seen, suggesting that the two PNA phases may be excited by different mechanisms. Most notably, the positive phase of the PNA is accompanied by a pronounced positive SAT anomaly over eastern Siberia that is not present during the negative phase of the PNA.

Analysis of the vertical temperature profiles elucidates two key details about the PNA. First, all of the temperature anomalies that accompany the PNA extend throughout the depth of the troposphere. Second, the mechanism underlying the growth of each temperature anomaly is independent of height over all anomalies, except for eastern Siberia, where near the surface (about 10 meters above the ground), growth via vertical mixing dominates, and above the surface the anomaly grows via horizontal advection of the anomalous temperature by the climatological wind. This result indicates that 1) the Siberian temperature anomaly associated with the positive phase of the PNA originates from processes farther upstream, and 2) that the mixing observed near the surface resulted from changes in the vertical temperature gradient caused the horizontal temperature advection aloft. The results of this study lead us to hypothesize that the positive phase of the PNA can be excited by an upstream source that does not excite the negative phase of the PNA.

The SAT anomaly pattern of the PNA, which is driven by both vertical mixing and horizontal temperature advection, is compared to the SAT anomaly pattern of NAO, which is driven solely by horizontal temperature advection. Because vertical mixing plays an important role in the development of the PNA’s SAT anomalies, we conclude that an accurate boundary layer parametrization is important for modeling the PNA. The same conclusion may not hold for the NAO because vertical mixing is not as important for the growth of the NAO’s SAT anomalies.