How does the changing seasons affect the large scale easterly flow of the ITCZ?
I hope I can explain this well! I will focus my answer on the ITCZ in the Pacific Ocean. The first point is that the Coriolis force is zero at the equator and maximum at the poles (1). So, another factor comes into play &quot;the walker circulation which is strongly tied to El Niño–Southern Oscillation (ENSO). ENSO states are 3: normal conditions, La Niña, and El Niño.
Given that Coriolis force is minimal around the equator, other wind drivers are more important. El Niño (La Niña) occurs when the tropical Sea surface Temperature (SST) in the central and eastern Pacific ocean is warmer (cooler) than the multi-decadal climate average. During normal conditions, higher pressure over the southeastern Pacific and lower pressure over the western Pacific drive easterly trade winds along the equator (known as Walker circulation), resulting in cold water upwelling and dry weather in the eastern Pacific. When the easterly trades are exceptionally strong, the eastern Pacific becomes colder and drier, resulting in La Niña conditions. The last case, El Niño occurs if the pressure over the eastern Pacific decreases: then easterly winds weaken or reverse direction, leading to a stronger ocean countercurrent that carries warm water into the eastern Pacific beside wetter weather conditions (Ahrens, 2009; Stull, 2018). So during El Niño events, the winds are not always easterlies( heading west) in the western Pacific they are westerlies, and in the eastern Pacific are easterlies. And keep in mind that the ITCZ is strongly linked to warm SST. That is why the ITCZ reaches the northernmost location in September (same as warm SST), although the Solar insolation peak in mid boreal summer (July).
I hope this answers the question. late better than never ;-)