Publication Abstracts

Xian 2008

Xian, P., 2008: Seasonal Migration of the ITCZ and Implications for Aerosol Radiative Impact. Ph.D. thesis. Columbia University.

An axisymmetric tropical circulation model is constructed and shown to exhibit an abrupt seasonal migration of the Intertropical Convergence Zone (ITCZ) between the two hemispheres, similar to observations. The model is similar in dynamical behavior to previous axisymmetric circulation models in which absolute angular momentum is approximately conserved in the upper branch of the Hadley Circulation (HC) in the absence of eddies. However, the model also includes prognostic sea surface temperature (SST), and a more physical representation of diabatic heating, where only solar heating at the top of the atmosphere (TOA) is externally prescribed. The ITCZ is prescribed to coincide with the warmest SST or else the minimum column moist static stability. It is shown that the angular momentum conserving circulation produces a small temperature gradient within the circulation and allows the ITCZ to move off the equator with seasonally varying solar forcing. It is found that there are two factors substantially affecting the abrupt transition of the ITCZ: the nonlinear meridional advection of angular momentum by the circulation and ocean thermal inertia. Angular momentum is well-mixed by nonlinear dynamics, resulting in minimum atmospheric temperature at the equator, and a similar equatorial minimum in SST. This inhibits convection over the equator, while favoring a rapid seasonal transition of the ITCZ between the warmer surface water on either side of the equator. The abrupt transition of the ITCZ also occurs with the inclusion of prognostic boundary layer moisture, although the transition is more gradual as a result of inertia introduced by moisture.

The axisymmetric circulation model with prognostic boundary layer (BL) moisture is shown to have multiple equilibria whereby the ITCZ can have more than one locations with the same solar heating depending upon the initial condition. The off-equatorial equilibrium is found to be associated with a large BL moisture gradient between the convecting and subsiding regions, which is maintained by the large-scale circulation, which converges moisture over the ITCZ and dries other areas via large-scale subsidence. The emergence of the multiple equilibria is determined by the moisture and thermal inertia of the atmosphere and ocean system. The result provides a possible explanation for the observed persistence of the eastern Pacific ITCZ within the northern hemisphere rather than the southern hemisphere, assuming that the axisymmetric model is appropriate for regional study.

The moist axisymmetric model is used to study the radiative impact of aerosols upon the circulation. The total radiative forcing is separated into the TOA forcing and atmospheric adiabatic heating (denoted "ATM"). It is shown that the sum of the response to these two forcings is approximately the response to the total forcing. The response of the tropical circulation to the TOA forcing is shown to be much larger than to the ATM forcing. In response to negative aerosol TOA forcing, evaporation decreases more than required to offset the total forcing at the surface, resulting in an increase in the sensible heat flux and net longwave radiation at the surface, thus changing the partitioning among the surface fluxes. It is demonstrated that evaporation depends upon not only the air-sea surface temperature difference, but also the surface air temperature, which is related to the TOA forcing. Thus how the surface energy balance responds to surface forcing depends additionally upon how the TOA forcing changes the partitioning among the surface fluxes. As a consequence, precipitation is reduced much more by the TOA forcing than by the ATM forcing, even though surface radiative forcing is identical in both experiments. In addition, less precipitation results in a weaker meridional circulation, which lessens the moisture gradient between the ITCZ and the subsiding region and favors a shift of the ITCZ toward the equator, which is apparent with a magnitude of dust aerosol forcing associated with the Last Glacial Maximum.

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BibTeX Citation

@phdthesis{xi04000r,
  author={Xian, P.},
  title={Seasonal Migration of the ITCZ and Implications for Aerosol Radiative Impact},
  year={2008},
  school={Columbia University},
  address={New York, N.Y.},
}

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RIS Citation

TY  - THES
ID  - xi04000r
AU  - Xian, P.
PY  - 2008
BT  - Seasonal Migration of the ITCZ and Implications for Aerosol Radiative Impact
PB  - Columbia University
CY  - New York, N.Y.
ER  -

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