Sort by expression, current month only!

[Chloe19]

I have a pivot table with Region as rows, Months as columns and volumes below it.

I added a measure which shows me if the value is above or below the previous month and now I would like to sort the pivot table by Region where the variance is the largest but ONLY for the current month. 

Variable: vSuarVariance

expression: alt((sum([SUARS]))-after(sum([SUARS])),sum([SUARS]))

Any idea how I could add this under Region, sort by expression but sort only for current month?

Thank you!