This is just a hypothesis of mine and I have no evidence whatsoever to back this up but... this recent trend of a colder pattern for the northern US and Canada come springtime. There are studies showing that the stratosphere is becoming colder (and stronger?) on average during the winter as the troposphere warms. Ex: This past February saw the coldest 50mb temps on record in the stratosphere over the high latitudes. I wonder whether the colder SPV during the winter allows it to survive longer (on average) towards spring without a significant disruption until the sun angle increases forcing it to weaken anyway. When it finally does weaken it may affect the troposphere and lead to more high-latitude blocking depending on how dramatic the disruption and/or SPV split is.
Of course, the SPV and TPV are not always connected as seen earlier this winter and many other times in the past. Also, one could argue that there isn't enough data yet to know for sure how climate change is affecting the SPV/TPV. Like I said... just a hypothesis of mine. I have no concrete evidence to back it up.