Why tropical water produces less upthrust than water in winter in North Atlantic?

tropical fresh water produces less upthrust in winter than the North Atlantic because tropical water is warmer than the cold water of the North Atlantic. i need one more reson too though!Its also becasue salt water is denser than "fresh water" therefore it will provide a greater upthrust

like with mercuary that will provide even more up thrust becase it has an even greater density