<p>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 <a href="https://en.wikipedia.org/wiki/Coriolis_force" rel="nofollow noreferrer">(1)</a>. So, another factor comes into play <a href="https://en.wikipedia.org/wiki/Walker_circulation" rel="nofollow noreferrer">&quot;the walker circulation</a> which is strongly tied to <a href="https://en.wikipedia.org/wiki/El_Ni%C3%B1o%E2%80%93Southern_Oscillation" rel="nofollow noreferrer">El Niño–Southern Oscillation (ENSO)</a>. ENSO states are 3: normal conditions, La Niña, and El Niño.</p>
<p>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 (<a href="https://books.google.de/books/about/Meteorology_Today.html?id=8xWQPwAACAAJ&amp;source=kp_book_description&amp;redir_esc=y" rel="nofollow noreferrer">Ahrens, 2009</a>; <a href="https://books.google.de/books?id=sjVfAAAACAAJ&amp;dq=stull%20practical%20meteorology&amp;hl=en&amp;sa=X&amp;ved=2ahUKEwjC4f6g-t7uAhWM0eAKHfvUB-wQ6AEwAHoECAEQAg" rel="nofollow noreferrer">Stull, 2018</a>). 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 <a href="https://journals.ametsoc.org/view/journals/clim/24/1/2010jcli3716.1.xml" rel="nofollow noreferrer">strongly linked to warm SST</a>. 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).</p>
<p>I hope this answers the question. late better than never ;-)</p>

CC BY-SA 4.0
2021-02-10T09:08:40.607Z
2021-02-10T09:08:40.607Z
ahmathelte
<p>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". </p>

<p>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). </p>

<p>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. </p>

<p>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 <em>eastwards</em>. 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.</p>

<p>My questions are:</p>

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

<p>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.</p>

<p>Many thanks!</p>

<p>A. </p>

CC BY-SA 4.0
2020-05-13T09:14:33.707Z
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.
Aerinmund Fagelson
CC BY-SA 4.0
2020-05-13T09:21:38.073Z
@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 :)
Aerinmund Fagelson
CC BY-SA 4.0
2020-05-13T10:30:27.620Z
@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.
Aerinmund Fagelson
CC BY-SA 4.0
2020-05-18T11:31:56.987Z
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.
Joscha Fregin
CC BY-SA 4.0
2020-05-18T14:07:12.160Z
@J.Fregin thank-you for the great example and for the reference! An answer containing examples of phenomena such as westerly winds during the Indian monsoon was exactly the kind of thing I was looking for. More specifically, I was interested to find out whether this effect might drive either turbulence or some "bending" of the ITCZ. Since winds below the ITCZ are in opposing directions to winds above the ITCZ, I wondered if this produced a notable torque, or if it influenced mixing within the ITCZ.
Aerinmund Fagelson
<meteorology><geophysics><climate>
How does the changing seasons affect the large scale easterly flow of the ITCZ?
Aerinmund Fagelson
# How does the changing seasons affect the large scale easterly flow of the ITCZ?

(Asked by: username_0 on 2020-05-13)

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.

**Tags:** meteorology, geophysics, climate

> **Comments:**
> **username_0**: 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.  
> **username_0**: @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 :)  
> **username_0**: @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.  
> **username_1**: 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.  
> **username_0**: @J.username_1 thank-you for the great example and for the reference! An answer containing examples of phenomena such as westerly winds during the Indian monsoon was exactly the kind of thing I was looking for. More specifically, I was interested to find out whether this effect might drive either turbulence or some "bending" of the ITCZ. Since winds below the ITCZ are in opposing directions to winds above the ITCZ, I wondered if this produced a notable torque, or if it influenced mixing within the ITCZ.  

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## 1 answers

### Answer by username_2 on 2021-02-10. Score: 2

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)](https://en.wikipedia.org/wiki/Coriolis_force). So, another factor comes into play ["the walker circulation](https://en.wikipedia.org/wiki/Walker_circulation) which is strongly tied to [El Niño–Southern Oscillation (ENSO)](https://en.wikipedia.org/wiki/El_Ni%C3%B1o%E2%80%93Southern_Oscillation). 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](https://books.google.de/books/about/Meteorology_Today.html?id=8xWQPwAACAAJ&source=kp_book_description&redir_esc=y); [Stull, 2018](https://books.google.de/books?id=sjVfAAAACAAJ&dq=stull%20practical%20meteorology&hl=en&sa=X&ved=2ahUKEwjC4f6g-t7uAhWM0eAKHfvUB-wQ6AEwAHoECAEQAg)). 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](https://journals.ametsoc.org/view/journals/clim/24/1/2010jcli3716.1.xml). 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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