What does the APO mean?

The Asian–Pacific Oscillation (APO) is a relatively new teleconnection index and a number of peer-reviewed studies have confirmed its’ existence. This study reexamines the concept of the APO teleconnection as currently published in the literature. Most results have found that this pattern could be defined as its own teleconnection pattern within the Pacific Ocean Basin. This work demonstrates that the APO is inevitably associated with an additional action centre located over the North Atlantic Ocean, and the index can be extended. Previous studies have used a variety of methods to represent the APO, and a method is proposed here to standardise the formulation. It is argued that the extended index proposed here provides a more robust result. The APO typically is identified using the eddy temperature rather than geopotential height as the source material for its construction, as geopotential could not adequately represent this teleconnection pattern. This leads to a discussion regarding the basic criterion for defining teleconnective activity within the extratropical regions. We also identify other problems in the current understanding of APO theory that need to be addressed.