What meteorological event is primarily responsible for the occurrence of El Niño?

El Niño is primarily driven by a sequence of interrelated atmospheric changes in the Pacific Ocean. These changes include variations in wind patterns, sea surface temperatures, and atmospheric pressure, which collectively disrupt the normal upwelling of cold, nutrient-rich water along the equator. During these events, the trade winds weaken or reverse, leading to a significant warming of the ocean surface in the central and eastern Pacific. This warming alters weather patterns globally, impacting rainfall, temperature, and even storm activities across various regions.

The other options do not capture the complexity and interconnectivity of the atmospheric and oceanic elements involved in El Niño. While consistent eastward winds are a characteristic of normal conditions, it is the changes in these winds that lead to El Niño, not their constancy. Similarly, increased volcanic activity can affect climate but is not a primary driver of El Niño. Lastly, while oceanic currents play a role, El Niño's defining characteristic is the abrupt atmospheric changes rather than just shifts in currents alone.