The shaded areas show the measured temperatures above and below normal. This graph shows a typical year and one can see that temperatures are rarely exactly 'normal', they are usually a bit above, or a bit below. The warming was typically 2 C and occasionally as much as 3 above normal. This is very large. This illustrates the large effect El Ni os have on the local ocean temperatures.
Pacific salmon are sensitive to temperature and so are heavily affected.
When water changes its temperature, it can have a very large effect on the air over the water. This is because for the same volumes, water carries about 1000 times more heat than does air.