I need to replace my LP pool heater and I am considering a heat pump pool heater. I hope southern Wisconsin is warm enough to allow a heat pump to operate effectively.

Hi there.  I found  a couple of heat pump pool heaters for you to check out.  Please take a look at what I found. 

Let me start with the Insta Matik Manual Heat Pump.  The Insta Matik heaters are the most popular.  You need to have an above-ground pool or a small inground pool.  These are unique by their design, high performance, long lasting reliability and operating cost. 

If not  that one how about a Hayward EasyTemp Heat Pump Pool Heater.  Have titanium heat exhangers.  It utilizes an electronic temperature control which will ensure constant temperature and energy savings.  The thermostic expansion valve protects the compressor and keeps the refrigerant amount for better performance. 

Now if you have an above ground pool, you may also want to look into solar.  It's just a thought.  I hope one of the above will work out for you.  Thank you for your question.