Final answer:
Trade winds are the most closely related factor that triggers an ENSO event, influencing the sea surface temperatures and leading to either El Niño or La Niña conditions.
Step-by-step explanation:
The event that triggers an ENSO (El Niño Southern Oscillation) event is actually a complex interplay of several factors, but if we have to choose one from the list provided, b) Trade winds is the most directly related. During normal conditions, these easterly winds push warm surface waters towards the western Pacific, allowing upwelling of cooler, nutrient-rich water in the eastern Pacific. However, when these winds weaken, the suppression of upwelling can lead to El Niño conditions with warmer than average sea surface temperatures (SST) in the eastern Pacific. Conversely, when trade winds are stronger than normal, they enhance upwelling, which can contribute to La Niña conditions, characterized by cooler waters in the eastern Pacific.
El Niño is the warm phase associated with high air pressure in the western Pacific and low air pressure in the eastern Pacific, leading to a band of unusually warm ocean water in the central and east-central equatorial Pacific. La Niña, on the other hand, is the cool phase with an increase in upwelling and correspondingly high pressure in the eastern Pacific complemented by low pressure in the western Pacific. Both represent extreme phases of ENSO, which cause global changes in temperature and rainfall, affecting world climate and consequently global economies, particularly those dependent on farming and fishing.