Here is a bit of a refresher taken from last season's outlook that illustrates what I have alluded to above:
There exist some notable differences between these two disparate types of la nina events. Here are a couple of schematics outlining these differences:
Differences between east-based La Nina's & central-based La Nina's
Figure A is east-based. Figure B is central-based
DJF 500mb for East-based years:
DJF Temps
Ironically in east-based years JFM is colder than Dec. Dec is typically the warmest month in the
east-based years, which runs counter to typical cold ENSO climo.
JFM 500mb east-based years
JFM Temps
Compare all of that to central-based La Nina years:
DJF 500mb
DJF Temps
December is the coldest month for central-based Nina's.
One thing worth noting is not all East-based Nina's are frigid, as there is variability due to the relatively weak
ocean-atmosphere coupling. This is because many east-based events, as is the case with modoki el nino events, are
weaker. You can have warmth, especially if the event is weaker, and one such case is 2005-2006.
Likewise, there were some weak modoki events that were fairly cold across the eastern US, such as 2000-20001.
This is further proof that there does indeed exist a structural mechanism beyond strength that dictates the overall character of la nina events and one of the focal points of this writing is to elucidate that point.
There are some "hybrid" events that share both east-based and modoki traits. You can have a Nina that is a mix of east-based versus central-based, or "basin-wide", such as both this season and last.