What could have caused the decrease in ocean temperature near christchurch​

Water moving from the equator is warmer than would be expected based on latitude, and so is warmer than the air it passes.

Explanation:

Im not 100% but I hope It Helps.

relative humidity, temperature and dew point -- are bound together in the mathematical relationship below. relative humidity changes when temperatures change. because warm air can hold more water vapor than cool air, relative humidity falls when the temperature rises if no moisture is added to the air. so i would say a