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The seasonal relationship between P_Penny and AHT_EQ was found to depend on how far off the equator the ITCZ moves in a series of aquaplanet simulations with varying mixed layer depth. We argue that the asymmetry between the winter and summer Hadley cells is critical to this result due to two processes: 1) AHT_EQ changes more than would be expected from simply translating the annual mean Hadley cell off the equator due to the intensification of the streamfunction in the winter cell as the Hadley cell moves off the equator, and dos) the ITCZ remains equatorward of the zero streamfunction because the maximum meridional divergence of the streamfunction and upward motion gets pushed into the winter cell due to the asymmetry between the winter and summer Hadley cells. As a consequence, the AHT_EQ change required per degree shift in the ITCZ increases as the ITCZ moves farther off the equator. We note that, in the annual average, the ITCZ is relatively close to the equator and the two branches of the Hadley cell are nearly symmetric as compared to the seasonal extremes. This might lead one to believe that the AHT_EQ change required to move the ITCZ 1° would be comparable to that expected from simply translating the Hadley cell meridionally without the concurrent intensification of the winter cell (on the order of 0.1 PW; see red asterisks in Fig. 5) and less than that found over the seasonal cycle (on the order of 0.3 PW; see upper panel of Fig. 3). _Penny and AHT_EQ over the observed seasonal cycle is statistically indistinguishable from the relationship found for the annual mean changes across the ensemble of climate change perturbation experiments. We argue below that the seasonal relationships between P_Penny and AHT_EQ is dictated by the seasonal cycle because the annual average is seldom realized and is better thought of as the average of the seasonal extremes (and the amount of time spent in the extreme) as illustrated in Fig. 7.

The top panel of Fig. 11 shows smoothed histograms (Eilers and Goeman 2004) of monthly mean P_Cent and AHT_EQ for 200 years of the PI simulation in the IPSL model. The annual mean (black cross) is seldom realized and the system rapidly migrates between seasonal extremes of P_Penny in the Northern Hemisphere and southward AHT_EQ in the boreal summer and P_Penny in the Southern Hemisphere and northward AHT_EQ in the austral summer. The linear best fit (dashed black line) connects the seasonal extremes with a slope equal to the regression coefficient between P_Cent and AHT_EQ and nearly passes through the origin. By statistical construction, the annual mean lies on the linear best fit line. In short, the annual mean reflects the average of the two seasonal extremes. 4

(top) Smoothed histogram (colors) in the AHT_EQ–P_Cent plane taken from a 200-yr-long PI simulation in the L’Institut Pierre-Simon Laplace (IPSL) model. The dashed line is the linear best fit to the monthly data for all years of the simulation and the cross is the annual average. (middle) As at top, but the probability density function is contoured [contour interval of 0.75% (° PW) ?1 ] with black contours showing the PI values and red values showing the 2XCO₂ values. The red and black crosses and dashed lines represent the annual average and linear best fits in the 2XCO₂ and PI simulations respectively. (bottom) As in the middle panel except only the 2.5% (° PW) ?1 contour is shown. The PI simulation is shown in black, 2XCO₂ in red, LGM in blue, and the 6Kyr simulation in green.

Signal of your spatial construction off wind gusts (gray arrows), meridional bulk overturning streamfunction (solid and you can dashed gray outlines to the positive and negative streamfunction philosophy, respectively), precipitation (blue contours), and you can vertically integrated atmospheric temperatures transportation (red-colored arrows) from the Hadley telephone getting (top) boreal june and you can (bottom) austral summer. The new equator ‘s the dashed green line.

(left) The global, annual-averaged atmospheric time funds and you can (middle),(right) the latest interhemispheric contrast of your own time funds familiar with obtain the brand new cross-equatorial atmospheric temperature transport. Brand new direction supports mean the new SH inbuilt with no NH inbuilt split by dos and you can OHT + S is the get across-equatorial sea temperatures transport minus shop within the per hemisphere.

(ii) Warm SST gradient

(top) Scatterplot of one’s seasonal years from exotic rain centroid compared to mix-equatorial atmospheric temperature transport. For each and every cross is according to the new month-to-month mediocre together with size of your get across on every axis represents the latest 95% count on interval reviewed throughout the interannual variability. The latest filled package is the yearly average. New dashed range ‘s the linear most useful complement towards month-to-month averages. (bottom) Once the at most readily useful, however for the fresh new tropical precipitation centroid compared to the new interhemispheric difference between exotic SST.

Seasonal cycle of hemispheric contrast in energy fluxes defined as half the difference in spatial integral of fluxes in the SH minus that in the NH. The solid lines are the observations and the shaded region represents ±1 standard deviation about the CMIP3 PI ensemble average. The terms are defined in the legend and discussed in the text in reference to Eq. (5). The first four terms in the legend sum to yield AHT_EQ.

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(top) Scatterplot of AHT_EQ vs the mass overturning streamfunction at 500 hPa over the equator over the seasonal cycle in the observations. Each asterisk is a monthly average and the dashed line is the linear best fit. (bottom) Scatterplot of the location of the 0 mass overturning streamfunction ?_?=0 at 500 hPa vs AHT_EQ (red asterisk and linear best fit dashed line) and P_Penny vs AHT_EQ (blue asterisk and linear best fit dashed line). _?=0 and AHT_EQ from Eq. (9) is shown by the dashed black line.

(top) Seasonal range of precipitation centroid vs atmospheric heat transport at the equator (AHT_EQ) 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.

Histograms of P_Penny in the CMIP3 PI models and observations. The shaded region is the normalized histogram of monthly mean P_Penny and the seasonal range (defined as twice the amplitude of the annual harmonic) of P_Penny is given by the dashed lines attaching the filled dots (representing the climatological northernmost and southernmost extent). The annual average for each model is also shown with the shaded diamond. The models are organized on the y axis and color coded by annual average P_Penny with the same color bar used in Fig. 6. Observations are given by the thick magenta line and the CMIP3 ensemble average is shown in the thick black lines. The vertical dashed black lines are the ensemble average annual mean, northernmost, and southernmost extent P_Penny.