There are different schematics of the same phenomenon in this section. It’s important to note that the Walker Cell Circulation is three-dimensional and encompasses both oceanic and atmospheric components. During a “normal” climate state, assuming the Walker Cell exists independently, the Pacific atmospheric circulation has certain defined features. At the surface, warmer-than-average equatorial sea surface temperatures exist near the dateline. Likewise, cooler than average sea surface temperatures live near the equatorial American coast. This is due to the upwelling of cooler deep subsurface water from the mid-latitudes. The mechanism for this is multifactorial, but it essentially is driven by counterclockwise winds in the Pacific Ocean, culminating in westerly winds along the equator.
These westerly winds are termed “Trade Winds”. The counterclockwise motion of winds leads to winds that run parallel to the American coastline, bringing in cooler water from the middle and higher latitudes. In the Western Pacific, the opposite is true. Warmer water is brought westward and pools along the Continental Maritime and Dateline.
Remember when I said that this is a three-dimensional model. The Walker Cell exists in the upper levels of the atmosphere as well. This, in part, is driven by the pooling warmer water in the West Pacific that leads to increased rain/monsoons. leading to our atmospheric component. Drier air persists along the American coastline and solar radiation warms the water as it continues to be pushed to the Western Pacific.
During La-Nina conditions, stronger trade winds dominate the equatorial Pacific. The Walker Cell is enhanced. As you head towards the mid-latitudes, deep cold water begins to pool along the coastline as winds travel up the coastline of the Americas. Remember, the winds are running counterclockwise in the Pacific, culminating in a direct east-to-west flow along the equator. In the mid-latitudes, these winds run more north to south (termed equatorward). The winds carry the water from Antarctica (or of similar latitude). This water creates a cold pool of the South American coast and is circulated through the E. Pacific leading to cool SSTs in the Nino regions.
Subsequently, this creates a cascade effect in the Indian Ocean. The Indian Ocean can be monitored in many ways, and one of them is the Indian Ocean Dipole (IOD) which measures SST anomalies. During La-Nina phases, the IOD becomes negative, favoring warmer SSTs near the Western Australian Coast and cooler SSTs near Africa
But back to the Walker Cell…
The opposite scenario exists for El-Nino. Weaker trade winds establishes a weaker Walker Cell Circulation. The result is that the cold pool off the SA coast line isn’t able to travel up and around into the equator. In fact, due to low pressure and convection moving from Australia/Indonesia into the Central Pacific, warmer waters from those areas move into the ENSO regions. They have less trade winds to fight against, there is less resistance. The cold pool off SA isn’t there or cannot counterbalance the weakened Walker Circulation.
Sometimes, but not always, these El-Nino effects cascade into the Indian Ocean, leading to a positive IOD.