Interference of ENSO on North American Precipitation Associated with Aleutian Low/PNA Variability

Large-scale atmospheric circulation patterns, particularly those remotely forced from slowly evolving tropical SST, are a key source of predictability for terrestrial climate anomalies over North America. One such pattern is the Aleutian low (AL), as part of the Pacific North American (PNA) pattern, hereafter AL/PNA. AL/PNA variability arises either from internal atmospheric dynamics or is remotely forced from ENSO. How ENSO modifies the North American precipitation response to AL/PNA remains unclear, as teasing out the ENSO signal is difficult. Here, we use carefully designed coupled model experiments to investigate how ENSO modifies the AL/PNA pattern and the related precipitation effects over North America. To do so, we implement a version of CESM in which ENSO is dynamically suppressed, thus removing ENSO and related teleconnections to compare with a control version of CESM. This approach is devoid of typical caveats related to removing ENSO through statistical techniques.

While it is well known that the ENSO forced atmospheric circulation response exhibits distinctions from the typical PNA pattern, we find that these differences in the strength and location of the PNA centers of action produce significant differences in precipitation over the U.S. In particular, +PNA forced by internal atmospheric dynamics is characterized by atmospheric circulation anomalies confined over the North Pacific and North America and results in anomalously dry conditions over the southeast U.S. in wintertime. In contrast, when ENSO forcing contributes to and modifies the +PNA pattern, ENSO forced wet anomalies over the southeast U.S. erode the typical dry anomalies associated with a non-ENSO forced +PNA, thus resulting in a full reversal of the anomaly sign in some areas. A simple partial regression analysis applied to observed precipitation products confirms the ENSO versus non-ENSO PNA-related precipitation differences. Hence, we find that the ENSO teleconnection acts to interfere with the southeast U.S. precipitation signal associated with non-ENSO AL/PNA variability. Finally, our analysis also identifies benchmark atmospheric circulation and precipitation signals associated with non-ENSO and ENSO forced AL/PNA patterns that can be used to assess model fidelity.