The majority of sea temperature measurements available in international data bases between 1941 and 1945 are from US ships. The crews of US ships measured the temperature of the water before it was used to cool the ships engine. Because of warmth coming from the ship, the water was often a little warmer than the true sea temperature. At the end of 1945 the number of US observations in the data base dropped rapidly while the number of UK observations increased. UK ships measured the temperature of water samples collected using special buckets. Wind blowing past the buckets as they were hauled onto the deck often caused these measurements to be cooler than the actual sea temperature. The sudden drop in global-mean temperatures at the end of World War 2 is due to the sudden but uncorrected change from US engine room measurements - which are biased warm - to UK measurements - which are biased cool.
Although the drop in 1945 is large in climate-change terms – abouut 0.3°C – its effect is likely to be largest during the period
immediately after the Second World War and very small by the 1960s, when better-insulated buckets came into use and a there was a more varied mix of measurements from different national merchant shipping fleets.
...Marine temperatures are much more prone to systematic biases arising from changes in the way the measurements are taken and the platforms used than are land air temperatures. For example, since the 1970s, sea surface temperatures have been estimated from satellites, but these need considerable adjustment (sometimes in excess of 2 deg C) to be comparable with ship and buoy measurements. The satellite sees only the top millimetre of the ocean surface, while traditional ship-based sampling sees the top few metres. A change is gradually talking place across the world's oceans in the way sea surface temperature measurements are made during the last ten years: the number of ship- based measurements has reduced slightly, but there is a dramatic increase in the number of measurements coming from automatic measurements taken on fixed and drifting buoys. Work is underway to determine the size of the difference between the ships and buoys, as the bias between the two could be of the same order as that in the 1940s.