(top) Seasonal range of precipitation centroid vs atmospheric heat transport at the equator (AHT) in individual CMIP preindustrial models (dashed colored lines with filled dots on each end), the model ensemble mean (thick purple line and filled dots), and the observations (thick black line and filled dots). The seasonal range is twice the amplitude of the annual harmonic of each variable and the slope of the line is the regression coefficient of the monthly data. The models are color coded by their annual average P_Penny with the color scale given by the color bar to the right. (bottom) As at top, but for precipitation centroid vs interhemispheric contrast of tropical SST.

(top) Seasonal range of precipitation centroid vs atmospheric heat transport at the equator (AHT) in individual CMIP preindustrial models (dashed colored lines with filled dots on each end), the model ensemble mean (thick purple line and filled dots), and the observations (thick black line and filled dots). The seasonal range is twice the amplitude of the annual harmonic of each variable and the slope of the line is the regression coefficient of the monthly data. The models are color coded by their annual average P_Cent with the color scale given by the color bar to the right. (bottom) As at top, but for precipitation centroid vs interhemispheric contrast of tropical SST.

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We note that the intermodel spread of the annual mean P_Penny and AHT are significantly correlated with each other with a regression coefficient of ?0.79 and a slope that is statistically indistinguishable from the ensemble mean seasonal relationship between P_Cent and AHT. This result suggests that the same physics that determine the seasonal migration of the ITCZ location and AHT (associated with meridional shifts in the Hadley cell) also govern the intermodel spread of ITCZ location in the annual mean. _Penny and AHT between the different models and the observations, the robust relationship between P_Cent and AHT allows one to predict P_Cent from AHT (or vice versa) with the same relationship holding for the seasonal problem and for the annual mean intermodel spread problem.

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Histograms of the Spiritual Sites dating service monthly mean P_Cent over the last 150 years of the simulations are shown in Fig. 7. The P_Penny distribution in each model is bimodal; P_Penny is most often found in one of the two solstitial extremes (Lindzen and Hou 1988) and is rarely found in the vicinity of the annual average. By construction, the annual average is the average of the northernmost and southernmost extent shown by the shaded dots in Fig. 7 (and defined as the annual mean plus or minus the seasonal amplitude). Therefore, in order to shift the annual average P_Cent northward, the ITCZ must either migrate farther into (or stay longer in) the Northern Hemisphere in the boreal summer or migrate less far into (or spend less time in) the Southern Hemisphere during the austral summer. The models with an anomalous (relative to the ensemble average) northward annual mean P_Penny (the models shown in red toward the top of the figure) tend to have a P_Cent that migrates farther north in the boreal summer. In contrast, the models with a southward annual mean P_Penny (the models shown in blue toward the bottom of the figure) tend to have a P_Cent that migrates less far north in the boreal summer. Similarly, the magnitude of the southward migration of the ITCZ during the austral winter is largest in the models with an annual mean ITCZ location south of the ensemble average (blue dots) and smallest in the models with an annual mean ITCZ location north of the ensemble mean (red dots). _Penny and AHT in section 4.

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