Final answer:
The depth of the thermocline in the eastern tropical Pacific b. decreases due to the arrival of warmer water and thermal expansion during an El Niño event, causing a thicker warm-water layer and a higher sea surface height compared to neutral conditions.
Step-by-step explanation:
In response to changes in surface currents, sea surface heights in the eastern tropical Pacific are higher than during neutral conditions. At the same time, the arrival of the warmer water in the east causes the surface warm-water layer to thicken, which is evidenced by the decreased depth of the thermocline to the east compared with neutral conditions. The thermocline is a layer of water where the temperature changes more drastically with depth than it does in the layers above or below.
During an El Niño event, warm water moves eastward along the equator, and because warm water expands more than colder water due to thermal expansion, the sea surface rises. Consequently, the warm-water layer becomes thicker, and the thermocline is shallower in the eastern Pacific compared to neutral conditions. This shallower thermocline indicates that the warmer surface water extends to a greater depth before reaching the cooler, deeper water below the thermocline.