Why is ITCZ mean position north of the equator?

Sen Zhao
Last updated on Aug 10, 2022

Why is the intertropical convergence zone (ITCZ) mean position north of the equator? Two differnt theories were proposed to understand this question in the literatures.

This post was inspired by the talk by graduate student Pocheng Chen in Prof. Fei-Fei Jin’s Group Meeting.
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1. Positive feedbacks of air-sea interaction

The ITCZ problem involves a circular chicken-and-egg argument. The ITCZ stays north of the equator because SST is higher; and the SST is higher north because the ITCZ stays there. The circular argument suggests that air-sea interaction may play a role.

There are several proposed air-sea interaction mechanisms may favor the unstable growth of an antisymmmetric ITCZ mode.

a) Wind-evaporation-SST (WES) feedback (Xie and Philander 1994, Neelin et al. 1987, Emanuel 1987)

**Figure 1. Schematic for wind-evaporation-SST feedback**, Credit: [Shang-Ping Xie](http://iprc.soest.hawaii.edu/users/xie/o-a.pdf)
Figure 1. Schematic for wind-evaporation-SST feedback, Credit: Shang-Ping Xie

Surface evaporation is a function of SST and wind speed. The interaction of surface wind and SST via evaporation yields a positive feedback as follows. Consider a meridional dipole of SST anomalies with positive north and negative south of the equator. The SST-induced pressure anomalies drive a southerly cross-equatorial wind, which gains an easterly south and westerly north of the equator because of the Coriolis force. Superimposed on the prevailing easterly trades, these wind anomalies increase (decrease) the wind speed south (north) of the equator, intensifying (reducing) evaporative cooling there. This dipole of latent heat flux anomalies acts to amplify the initial SST dipole.

The positive WES feedback is at the center of this ITCZ circular argument since it is very effective in adjusting SST. Surface heat flux is the dominant mechanism for SST changes outside the equatorial upwelling zone, because the general downwelling there makes SST insensitive to changes in thermocline depth.

b) Meridional wind-upwelling-SST feedback (Chang and Philander 1994, Mitchell and Wallace 1992, Li 1997)

Consider a initial meridional dipole of SST perturbation with positive north and negative south of the equator, a southerly cross-equatorial wind anomaly is induced. This wind anomaly would drive the oceanic Ekman current with surface convergence (divergence) north (south) of equator. This promotes an asymmetry anomalous vertical velocity at bottom of the oceanic surface. An downwelling (upwelling) appears at north (south) of equator and further strengthens initial dipole of SST perturbation.

**Figure 2. Schematic diagrams showing a) the coupled ocean–atmosphere instability of the first kind, the positive feedback between the meridional wind and SST, proposed by Mitchell and Wallace (1992) and Chang and Philander (1994), and b) the coupled ocean–atmosphere instability of the second kind, the evaporation–wind feedback (Xie and Philander 1994). c) ISCCP monthly mean stratus cloud fields. Contours represent the annual mean field and vectors represent annual harmonics (upward denotes a maximum amplitude in January, rightward in April, and so on)**, Credit: [Li 1997](https://doi.org/10.1175/1520-0469(1997)0542.0.CO;2)
Figure 2. Schematic diagrams showing a) the coupled ocean–atmosphere instability of the first kind, the positive feedback between the meridional wind and SST, proposed by Mitchell and Wallace (1992) and Chang and Philander (1994), and b) the coupled ocean–atmosphere instability of the second kind, the evaporation–wind feedback (Xie and Philander 1994). c) ISCCP monthly mean stratus cloud fields. Contours represent the annual mean field and vectors represent annual harmonics (upward denotes a maximum amplitude in January, rightward in April, and so on), Credit: Li 1997

c) Stratus cloud-radiation-SST feedback (Li and Philander 1996, Philander 1996)

The positive feedback between the low-level marine stratus clouds and SST: the lower the SST, the larger the static stability of the lower troposphere, the stronger the atmospheric inversion, and the thicker the deck of low-level stratus clouds; the increase in the clouds further shields the ocean from shortwave radiation and causes even lower SST.

2. Energy balance and transport dynamics

From perspective of energy transport dynamics, Schneider et al. (2014) argued that the mean position of the ITCZ north of the equator arises primarily because the Atlantic Ocean transports energy northward across the Equator, rendering the Northern Hemisphere warmer than the Southern Hemisphere.

**Figure 3. Annual-mean precipitation, surface winds, and atmospheric energy balance**, Credit: [Schneider et al. 2014](https://www.nature.com/articles/nature13636)
Figure 3. Annual-mean precipitation, surface winds, and atmospheric energy balance, Credit: Schneider et al. 2014
**Figure 4. Processes controlling zonal-mean ITCZ position**, Credit: [Schneider et al. 2014](https://www.nature.com/articles/nature13636)
Figure 4. Processes controlling zonal-mean ITCZ position, Credit: Schneider et al. 2014

The lower branches of the Hadley circulation (grey arrows) bring warm and moist air masses towards the ITCZ, where they converge, rise and diverge as cooler and drier air masses aloft. Because the moist static energy aloft is greater than near the surface, the Hadley circulation transports energy away from the ITCZ. Eddies transport that energy farther into the extratropics (red wavy arrows). Hemispheric asymmetries in the energy export out of the tropics generally lead to an energy flux that crosses the Equator. Currently, the energy export into the extratropics in the south exceeds that in the north, leading to a southward cross-equatorial energy flux (Figure 5). This implies an ITCZ in the Northern Hemisphere. Coupled to the Hadley circulation are mean zonal winds (red arrows at the sea surface), which are easterly where the near-surface mass flux is equatorward, and westerly where it is poleward. In the oceans, these zonal winds drive subtropical cells, with near-surface mass flux to the right of zonal winds in the Northern Hemisphere, and to the left in the Southern Hemisphere. Water masses cool and sink along their way towards the Hadley circulation termini and return below the sea surface (red and blue arrows). With mean easterlies in the tropics, the returning cool water masses upwell at the Equator, and the subtropical cells transport energy away from the Equator. But the upwelling location can migrate with the ITCZ away from the Equator and can dampen the ITCZ migration.

The atmospheric energy balance (Schneider et al. 2014)

$$ div F = S - L - O $$

  • $F$ the atmospheric energy flux
  • $S$ the net downward shortwave radiation
  • $L$ the outgoing longwave radiation
  • $O$ the ocean energy transport or storge in the ocean.

By expanding the meridional energy flux $F$ to first order in the latitude $\delta$ of the energy flux equator, we obtain $0 = F\delta \approx F0 + (div F_0)a\delta$, where the subscripts $\delta$ and 0 indicate latitude, and $a$ is Earth’s radius. Solving for the energy flux equator gives

$$ \delta \approx - \frac{1}{a} \frac{F_0}{S_0 - L_0 - O_0} $$

The ITCZ position depend on to the first order on the cross-equatorial atmospheric energy flux $F_0$ and on the net energy input to the atmosphere at the equator.

**Figure 5. Atmospheric meridional energy flux and energy flux equator**, Credit: [Schneider et al. 2014](https://www.nature.com/articles/nature13636)
Figure 5. Atmospheric meridional energy flux and energy flux equator, Credit: Schneider et al. 2014

The atmospheric moist static energy flux $F$ in the zonal and annual mean in the present climate (red line) is generally poleward, but it has a small southward component $F_0$ at the Equator. The energy flux equator is the zero of the energy flux, which currently lies around $\delta \approx 2.5$°. Given the equatorial values of the energy flux $F_0$ and of its ‘slope’ with latitude $div F_0$, the energy flux equator $\delta$ can be determined from $F_0 \approx –a \delta div F_0$, where $a$ is Earth’s radius. For example, if $F_0$ increases (indicated schematically by the blue line), the energy flux equator $\delta$ moves southward. Similarly, if $div F_0$ increases, the energy flux equator moves towards the Equator.

The NH is warmer primarily because the Atlantic’s meridional overturning circulation (AMOC) transports energy northward, up the mean temperature gradient. It dominates the cross-equatorial ocean energy transport; the resulting net northward transport across the Equator amounts to about 0.5 PW in the zonal mean.

Summary and discussion

Given an initial asymmetric SST pattern, these aformetioned postive feedbacks would favor the unstable growth of the antisymmmetric ITCZ mode. However, the problem remains unknown that how does the asymmetric land mass and coastal geometry generate the initial asymmetric SST patterns? How does the AMOC formulate?

ITCZ
Assistant Researcher

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