Objective: Determine whether or not changes in the ITCZ across the Pacific are responsible for westerly wind anomalies that kickstart Bjerknes feedback and El Nino conditions. Python code.

OLD PLOTS of Plot of average ITCZ position with time (per basin):



New plot of entire Pacific ITCZ location and varying measures of strength


Latitude of ITCZ using wind convergence algorithm over the Pacific Ocean during nuclear winter.

Value of convergence at the latitude of the ITCZ across the Pacific. A weakening of convergence is observed during the summer months of the 150 Tg nuclear winter run if you use the surface winds. However, the signal is significantly weakened when the 850 hPa level is used.

Amount of precipitation at the latitude of the ITCZ across the Pacific. A clear weakening is observed for 8 years, before the precipitation pattern resumes a maximum during summer, which still is weaker than the control run. 14 years after the injection of soot, the ITCZ strength in terms of precipitation reaches pre-war levels.






Seasonal migration of the ITCZ over the Pacific in CESM-WACCM4
a. Latitude of Maximum 850 hPa Wind Convergence
First, we compare the ITCZ location in CESM-WACCM4 using the latitude of maximum wind convergence at 850 hPa with figures from Schneider et al., (2014) over the same locations. Compared to the reanalysis data used by Schneider, the ITCZ as simulated by CESM-WACCM4 ventures further south of the equator across the Pacific, in terms of the wind convergence algorithm as well as if the maximum precipitation was used. Over southeast Asia, the ITCZ exhibits more meridional variability in the reanalysis compared to CESM-WACCM4. During the summer, the reanalysis data shows the ITCZ as far north as 23N over southeast Asia, while it never gets north of 10N in CESM-WACCM4. However, here there are clear differences between the wind convergence algorithm and the max latitude of precipitation method of determining the location of the ITCZ. To rectify this, we will make these same plots using an alternative method where maximum precipitation is used.
Figure 1. Location of ITCZ in CESM-WACCM4 and reanalysis over the Pacific Ocean (top) and over Southeast Asia (bottom).

Fig 2a. From Schneider et al., 2014 showing the ITCZ location in the Pacific based on the latitude of maximum precipitation. Precipitation data from daily TMPA data averaged over 1998-2012 and winds are from ECMWF interim reanalysis for the same years.


Fig 2b. From Schneider et al., 2014 showing the ITCZ location in Southeast Asia based on the latitude of maximum precipitation. Precipitation data from daily TMPA data averaged over 1998-2012 and winds are from ECMWF interim reanalysis for the same years.

b. Latitude of Maximum 850 hPa Wind Convergence + Precipitation
A comparison of two different methods of ITCZ identification is shown below. The top two images are Pacific Ocean and Southeast Asian ITCZ / Precipitation climatologies by averaging together the location of the ITCZ using wind convergence with the location of the ITCZ using the latitude of maximum precipitation. The bottom images show the result if only wind convergence is used to identify the ITCZ. This simply pulls the ITCZ closer to the area of maximum precipitation, which has the effect of shifting the ITCZ further north during the summer-fall in the Pacific, and shifting the ITCZ further south during the winter/spring in Southeast Asia. Because CESM-WACCM4 has a fundamentally different precipitation pattern compared to this reanalysis, using it does not result in any obvious benefits.






ITCZ changes following the injection of 150 Tg of soot:






Literature Review on Changes in the ITCZ through Various Means:


Yu, S. and Pritchard, M. 2019. A Strong Role for the AMOC In Partitioning Global Energy Transport and Shifting ITCZ Position in Response to Latitudinally Discrete Solar Forcing in CESM1.2. Journal of Climate, Volume 32, DOI: 10.1175/JCLI-D-18-0360.1.

Ocean circulation responses to interhemispheric radiative imbalance can damp north-south migrations of the ITCZ by reducing the burden on atmospheric energy transport.
ITCZ migration entangled with understanding of AMOC response to hemispherically asymmetric radiative forcing.
The partitioning to external forcing has only begun to receive wide attention.
Position of the ITCZ roughly coincides with the ascending branch of the Hadley circulation- hemispheric heating imbalance can be lessened if zonal-mean ITCZ migrates toward a warmer hemisphere since thermally direct Hadley cell transports energy following direction of its upper branch.
[i.e., the energetic framework; see Schneider et al. (2014) and Kang et al. (2018) for reviews]
The oceanic circulation can also alter the position of the zonal-mean ITCZ by reducing the burden of the atmospheric energy transport.
Ocean dynamics can damp ITCZ shift responses to high-latitude forcing by imposed cloud brightness (Kay et al. 2016), ocean albedo (Hawcroft et al. 2017), sea ice cover (Tomas et al. 2016), or stratospheric aerosols (Hawcroft et al. 2018).
More pronounced ITCZ shifts occur when ocean dynamics are left out.



Hawcroft, M., Haywood, J., Collins, M., Jones, A. 2018. The contrasting climate response to tropical and extratropical energy perturbations. Climate Dynamics.

Issues with tropical precipitation in models may be associated with: inadequate convection parameterization, convective-dynamic coupling, ocean-atmosphere coupling, implying the double-ITCZ problem may be of tropical origin.
Might be worth it to perform a meridional energy transport analysis for the nuclear winter case, which could help with quantifying the impact to the ITCZ.
Summary of study conclusions:
1. The response of tropical atmosphere to hemispherically asymmetric energy perturbations in a coupled model is highly dependent on the latitude of the forcing, with ocean + atmosphere circulation responses to extratropical perturbations modulating the tropical response.
2. The relationship between tropical precip assymetry and cross-equatorial atmospheric energy transport is very closely coupled, as is the relationship between both those indices and tropical SST asymmetry.
3. The structural biases in model which lead to biases in cross-equatorial energy transport are resistant to change- takes a forcing equivalent to twice the stratospheric AOD after Pinatubo over southern tropics to correct cross-equatorial energy transport biases.
4. Biases in tropical climate in models may have dependence on the particular latitude of any albedo/energy budget biases or forcing.


Schneider, T,. Bischoff, and G. Haug, 2014: Migrations and dynamics of the intertropical convergence zone.Nature,513, 45–53,https://doi.org/10.1038/nature13636

Different ways to identify the ITCZ: tropical belt of deep convective clouds or as maximum in-time mean precipitation.
Central Atlantic and Pacific: ITCZ migrates between 9N and 2N.
Indian Ocean and adjacent land surfaces: the ITCZ swings more dramatically between avg latitudes of 20N and 8S, prompting seasonal rainfall variations of the South Asian Monsoon.
ITCZ weakened or shifted southward over the Holocene because summer insolation in NH weakened with the precession of Earth's perihelion from NH towards SH.
ITCZ does not follow insolation maximum nor sinusoidal seasonal variations of IH temperature contrast- other mechanisms control position of ITCZ and its rainfall intensity.
ITCZ rainfall fed by warm + moist trade winds
Moist static energy of upper tropospheric air is greater than that of the surface (really? Mostly due to gravity?)
the ITCZ can be expected to lie near the 'energy flux equator.'
The energy flux equator, and approximately the ITCZ position, depend to 1st order on cross-equatorial atmospheric energy flux F0 and on net energy input to the atmosphere at or near the equator:
div F0 = S0 – L0 – O0