Explain how thermohaline circulation is influenced by salinity and temperature.

Thermohaline circulation is influenced by salinity and temperature because these are the factors that most greatly influence seawater's density. 
Seawater's density depends on temperature because warmer water is less dense and cooler water is more dense. It depends on salinity because saltier water is more dense, and fresher water is less dense. 
As large masses of water interact, they begin to arrange themselves based on density, and by extension buoyancy. Much like high and low airpressure systems, currents are generated by the movement of less dense water (let's say fresher, warmer water) moving above the more dense water (let's say saltier, colder water). This movement creates a circulation through the oceans.