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I am new to climate science and have been learning the basic picture of the intertropical convergence zone (ITCZ) and how this leads to the predominant easterly "trade winds".

The picture that I have learned is the following. As the ITCZ is a belt of low surface pressure along the equator, air is drawn in from the north in the northern hemisphere and from the south in the southern hemisphere. Due to the coriolis effect, the southward winds in the northern hemisphere are deflected westward and the northward winds in the southern hemisphere are also deflected westward, with respect to the earth's reference frame. Thus the winds along the ITCZ blow westwards (i.e. Easterly winds).

However, I also know that the ITCZ is not a perfect circle, and also moves around with the seasons, so that in the northern hemisphere summer it is predominantly in the northern hemisphere, and in the southern hemisphere summer it is predominantly in the southern hemisphere, and on an average day you might find parts of the belt in the northern hemisphere and parts in the southern hemisphere.

But then my basic picture breaks down. Indeed, if the ITCZ lies north of the equator, then the winds approaching from the north are deflected westwards, whilst winds approaching from the south are deflected eastwards. So the trade winds just south of the ITCZ are now westerlies rather than traditional easterlies, and winds are converging at the ITCZ with opposing directions (and presumably rubbing against one another creating friction). A mirror situation holds with the ITCZ south of the equator.

My questions are:

1) Is my above reasoning correct / applicable to the atmosphere?

2) If so, what effects does this have on the properties of the ITCZ, and what are some examples of these effects on weather patterns? I am very happy for explanations as well as references to appropriate texts.

Many thanks!

A.

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  • $\begingroup$ Welcome. Have you had a look at this (for example): earthscience.stackexchange.com/questions/4309/… ? $\endgroup$
    – user20217
    May 13 '20 at 8:52
  • $\begingroup$ Hi @a_donda, that question appears to address why the ITCZ shifts its distrubution with the seasons, whereas my question regards how this seasonal shift affects the trade winds and the predominant circulation pattern within the ITCZ (i.e. easterly vs westerly). You might sum this up heuristically as saying that I am interested in second-order effects of the ITCZ. $\endgroup$ May 13 '20 at 9:14
  • $\begingroup$ @a_donda I have changed the question title from "How does the changing seasons affect the large scale flow of the ITCZ?" to "How does the changing seasons affect the large scale easterly flow of the ITCZ?" to make this more clear :) $\endgroup$ May 13 '20 at 9:21
  • $\begingroup$ @a_donda thank-you! That is an interesting point of which I wasn't aware. But it is not quite what my question referred to. The focus on the northern hemisphere was just for illustrative purposes. My question regards the changing effect of the coriolis parameter when the ITCZ deviates from the equator. I could equally well consider summer in the southern hemisphere, when I expect the ICTZ to lie predominantly south of the equator, so that trade winds approaching the ITCZ from the north will be deviated eastwards, whilst trade winds approaching from the south will be deviated westwards. $\endgroup$ May 13 '20 at 10:30
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    $\begingroup$ Your reasoning is indeed correct. As for the properties, what do you specifically mean by that? Weather patterns that emerge are for example the Indian monsoon phenomenon which predominantly features westerly winds due to the equator crossing as you explained in your question. You can find a nice short explanation in Wallace, Hobbs, "Atmospheric Science", 2nd ed. page 15-16. $\endgroup$
    – J. Fregin
    May 18 '20 at 11:31
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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 "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 ;-)

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