Why is the surface waters in the Northwest Atlantic Ocean colder than the surface waters of the Northeast Atlantic Ocean?
The main reason is that the Gulf Stream transports warm surface water from the tropics, driven by the thermohaline circulation. The tropical trade winds push surface water towards Western Atlantic and build up stress. Further north the dominant wind direction is towards the east. The Coriolois force also helps to move the warm water masses towards the east.
The Labrador Current, on the other hand, brings cold water down West Atlantic Ocean. The thermohaline turnover is further south in West Atlantic Ocean.