Graduate School of Environmental Earth Science, Sapporo, Japan
It is a long-standing mystery that the intertropical convergence zone (ITCZ) stays north of the equator over the Atlantic and eastern Pacific Oceans despite that the annual-mean solar radiation at the top of the atmosphere is symmetric with respect to the equator. This article reviews recent progresses that have shed new light on this old puzzle.
While latitudinal asymmetries of land-sea distribution certainly are the ultimate cause of the perennial Northern Hemisphere (NH) ITCZ, the chain of causality is poorly understood. Although the Indian Ocean sees largest latitudinal asymmetry in land mass distribution and has a northwestward slanted eastern boundary--a condition thought to favor a perennial NH ITCZ, the ITCZ there moves back and forth between the hemispheres following the seasonal march of the sun. The case of the Indian Ocean illustrates that factors other than continental geometry are also important.
Key to maintaining the perennial Pacific ITCZ is a collocated band of high sea surface temperature (SST). The SSTs at the ITCZ are a few degrees higher than those at the same latitudes south of the equator. The configuration of oceanic ITCZ becomes symmetric about the equator if this latitudinal SST asymmetry is removed in an atmospheric GCM (Philander et al. 1996). The atmospheric response to an eastern Pacific-type SST distribution--with an equatorial minimum and two off-equatorial maxima--is quite straightforward: It forms its ITCZ in the warmer hemisphere (Ishiwatari 1996, personal communication). How the atmosphere forms its ITCZ over a western Pacific-type SST distribution--with a broad equatorial maximum--is a matter of controversy (Pike 1971; Numaguti and Hayashi 1991).
Now the problem is reduced to why the SST is higher in the NH? The ITCZ is also known as the Doldrums for winds are generally weak there. The contrast between weak winds north and strong winds south of the equator is a direct consequence of southerly winds blowing onto the ITCZ. The Coriolis force cause these southerlies to veer, enhancing (weakening) the prevailing easterly trades before (after) they cross the equator. SSTs over the weak (strong) wind zone to the north (south) of the equator thus must increase (decrease) to release the right amount of evaporation and offset the incoming solar radiation. This wind- evaporation-SST (WES) feedback is very effective in adjusting SST: a wind speed difference of a factor of two translates into a SST difference of 11 C (Xie and Philander 1994). 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, an important distinction from El Nino.
The ITCZ problem thus 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 positive WES feedback is at the center of this circular argument. In a coupled ocean-atmosphere model, the WES feedback destabilizes the symmetric climate, leading to an asymmetric steady state with a single ITCZ on only one side of the equator (Xie and Philander 1994). A condition for this spontaneous development of latitudinal asymmetry is the equatorial upwelling that prevents the ITCZ from forming at the equator. This necessary condition thus explain why climatic asymmetry only develops over the eastern Pacific and Atlantic where the equatorial upwelling is observed.
The above WES feedback does not favor either hemisphere, however. The force that keeps the Pacific ITCZ to the north of the equator is likely to come from the American continents. A symmetry-breaking forcing by Americas will excite coupled WES waves, which eliminate deep convection in the Southern Hemisphere on its way to the west. Thus a latitudinal asymmetry is established over a large zonal extent to the west of the continents (Xie 1996). The WES waves propagate westward because SST-forced atmospheric Rossby waves shift wind anomalies to the west of SST perturbation. These WES waves are detected in coupled GCMs, responsible for maintaining the perennial NH ITCZ over the Pacific (Ma et al. 1996; Kimoto and Shen 1997).
Off the west coast of South America, cold SSTs increase the static stability of the lower atmosphere and dense stratus clouds form as a result. These stratus clouds shield solar radiation, cooling sea surface even more (Philander et al. 1996). This positive stratus-SST feedback is most effective in the far eastern Pacific. Over most of the eastern Pacific (west of 100W), the amount of deep convective clouds within the NH ITCZ exceeds that of stratus and the net cloud- SST feedback is negative. Overcoming this negative cloud-SST feedback, the WES feedback sustains the perennial NH ITCZ over a distance of 10,000 km west of the American coast. See Xie and Seki (1997) for an examination of these feedbacks from observations.
Recently the concept of the WES feedback is applied to studying decadal variability over the tropical Atlantic (Chang et al. 1997; Xie and Tanimoto 1998), an oscillation often characterized as a north-south seesaw anti- symmetric with respect to the ITCZ. Now it seems that the tropical ocean- atmosphere system contains two major feedback mechanisms: The Bjerkness feedback is responsible for equatorially centered phenomena like El Nino/ Southern Oscillation, whereas the WES feedback generates features anti- symmetric about the equator. Upwelling dominates SST variations on the equator, while surface heat flux becomes important off the equator where prevailing Ekman downwelling decouples SST from oceanic variability beneath.
"Solar radiation" as the answer to "what drives ocean-atmospheric motion" would be considered too vague today. Only a few years ago, our answer--"land-sea distribution"--to "what keeps the ITCZ north of the equator" was as vague. Considerable details about the chain of causality have now emerged. We now know that the NH ITCZ is coupled with the equatorially asymmetric SST distribution. We also know the key ocean-atmospheric feedbacks involved. Yet there are still gaps to fill: Which continental features are the trigger that puts the oceanic ITCZ to the north of the equator?
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