4a: The Bjerknes hypothesis
The diagram assumes you are looking down on the ocean from somewhere over the Chilean Andes. In the Bjerknes hypothesis, positive feedback in the interaction between the ocean and atmosphere in the eastern equatorial Pacific leads to a warm ENSO event. The prevailing trade winds are from east to west (right to left). Warming in the central/eastern Pacific region (shown in orange) is associated with reduced trade winds (westerly anomalies), which deepen the thermocline (red line) in the east and reduce the entrainment of cold subsurface water. This leads to further warming in the east, so the trade winds become weaker and the positive feedback continues (see figure 4b).