A mystery in the tropical east Pacific: double ITCZ and equatorial asymmetry

The intertropical convergence zone (ITCZ) over the east Pacific Ocean remains 5o-10o in latitude north of the equator most of the year. However, a southern-hemisphere branch of the ITCZ also often emerges from March to April in the east Pacific, resulting in a double ITCZ. We have no satisfactory explanation to date as to what promotes the double ITCZ in austral fall and hampers it in boreal fall. This study is aimed at determining the growth mechanism of the east Pacific double ITCZ in austral fall.

ITCZexperiment Satellite measurements from the Tropical Rainfall Measuring Mission (TRMM) and QuikSCAT are analyzed to derive 8-year climatologies of sea surface temperature (SST), surface convergence, column water vapor (CWV), shallow and deep precipitating clouds, and surface heat fluxes from 2000 to 2007. SST produces the initial signature of the south ITCZ, referred to as the southeast Pacific warm band, as early as January. A pronounced feature of the warm band is a local SST maximum around 90oW, resulting from the sum of different surface heat fluxes having different signs and competing geographical patterns. Surface convergence and CWV gradually develop along the southeast Pacific warm band. The population of shallow cumulus starts growing to form the south ITCZ in February, a month earlier than vigorous deep convection that finally organizes in the south ITCZ in March. The double ITCZ therefore does not emerge abruptly in austral fall, but results from a series of distinctive evolutionary stages in the preceding months, initiated by SST in the preceding austral summer.

Simple experiments are next performed to diagnose the key factors that give rise to the southeast Pacific warm band. In the experiments, the heat budget of the ocean mixed layer is calculated with the surface heat fluxes (longwave, shortwave, latent heat, advection, and upwelling), all derived from satellite observations. Two sets of experiments are performed. The first is an "austral fall experiment," where SST is initialized by the observed climatology of 1 November. In the control run, all of the fluxes are varied daily. Additional runs are conducted with one of the fluxes fixed at a "wrong" season, defined as the July climatology in this case, while the remaining terms are varied daily as done for the control. The second experiment is a "boreal fall experiment," which is identical to the austral fall experiment, except that the initial conditions are initiated to 1 July and the "wrong" month fluxes are fixed at their January climatologies. Excerpts from the experimental results shown above (the boreal fall experiment in the left column and the austral fall experiment in the right) implies that the warm band appears or disappears most distinctly when the shortwave flux (2nd top in the figure) is switched over. It follows that the absorbed shortwave flux primarily drives the southeast Pacific warm band. Insolation remains near the annual maximum from austral summer through late February over the latitudinal range of the south ITCZ, resulting in a peak in tropical southeast Pacific SST and the south ITCZ in March. It is now obvious why the double ITCZ is absent in boreal fall: the incoming shortwave flux is at a minimum in the preceding solstice season and thus incapable of maintaining a warm ocean surface in the southern hemisphere. The ocean mixed layer tends to cool in July, when the shortwave flux is overwhelmed by negative fluxes such as latent heat and longwave fluxes. The real mystery might be not why the south ITCZ is transient but why the ITCZ could persist all the year around in the northeast Pacific.

While theories exist to explain this north-south ITCZ asymmetry, observational verification has yet to be done so the proposed hypotheses are quantitatively examined. To this end, the equatorial asymmetry of the east Pacific ITCZ is next explored on the basis of an ocean surface heat budget analysis carried out with a variety of satellite data products.

Ocean heat budget The annual mean climatology of absorbed shortwave flux (Qsw) exhibits a pronounced meridional asymmetry due to a reduction of insolation by high clouds in the north ITCZ (figure on left). Ocean mixed-layer advection (Qadv) has the largest, if not exclusive, effect of counteracting this shortwave-exerted asymmetry. Other heat fluxes, in particular latent heat flux (Qlh), predominate over the advective heat flux in magnitude but are secondary with respect to equatorial asymmetry. The asymmetry in advective heat flux stems from a warm pool off the Central American coast and, to a lesser extent, the North Equatorial Counter Current, neither of which exist in the southern hemisphere. The irregular continental geography presumably comes into play by generating a warm pool north of the equator and bringing cold waters to the south in the far eastern Pacific. In addition to the annual climatology, the north-south contrast in the seasonal cycle of surface heat flux is instrumental in sustaining the north ITCZ throughout the year. The northeast Pacific is exposed to a seasonal cycle considerably weaker than that in the southeast Pacific, arising from multiple causes including the finite eccentricity of the Earth's orbit and meridional gradient in mixed layer absorptivity. Simple experiments generating synthetic sea surface temperature (SST) illustrate that the muted seasonal cycle of heat flux forcing moderates SST seasonal variability in the northeast Pacific and thus allows the north ITCZ to persist year-around.

Existing theories on the ITCZ asymmetry are briefly examined in light of the present findings: 1) The wind-evaporative-SST (WES) feedback is at work but seems too weak to entirely account for the equatorial asymmetry, 2) the stratus-SST feedback is a localized effect near the South American coast, and 3) the upwelling-SST feedback is limited to the equatorial cold tongue and is unlikely to help maintain the ITCZ.

This study was carried out in collaboration with Tristan L'Ecuyer (Colorado State University), who is developing and providing the radiation budget data product (HERB) analyzed in this work.

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Last modified: Wed May 22 10:27:45 2013