Although the forcing is spatially uniform, the asymmetric hemispheric response of the cloud radiative forcing results in altered cross-equatorial atmospheric energy transport, which causes the displacement of tropical precipitation. However, the sign of the cross-equatorial atmospheric energy transport is not consistent across models, causing a large spread in the tropical rainfall response. Furthermore, even in the case of small changes in atmospheric energy transport, there is a significant shift in tropical precipitation.
Large uncertainties in ∆ due to the dependence of the vertical structure of moist static energy on convection schemes and cloud modeling require their improvement for better planning of tropical precipitation in the future. Schematic of the mechanism showing the ITCZ located in the energy-excess hemisphere. Global warming, which indicates a rise in the Earth's average temperature, is a major feature of the current climate.
There is greater warming over land in the Northern Hemisphere (NH) and less over the Southern Ocean (Screen and Simmonds 2010). Even in the pre-industrial period, there is a warmer NH, which is thought to be the result of heat transport across the equatorial ocean to the north (Kang et al. 2014). However, our current climate is hemispherically asymmetric with a warmer NH and an ITCZ in the NH.
It is of interest to investigate the robustness of the ITCZ response to increasing CO2 in the asymmetric hemispheric state.
Experiment setup
Background
Robust responses of global warming
Since the tropical tropospheric temperature is flat, the convective instability varies locally and then the precipitation is determined by the relative SST, the deviation of the tropical mean: this is the warmer-wetter mechanism. Both mechanisms are at play in determining the spatial distribution of tropical rainfall (Huang et al. 2013). Although not conclusive in observations, the weakening of the tropical circulation is also one of the most robust changes in global warming simulations (Mitas and Clement 2005; Shon and Park 2010;
The weakening is evident in the zonally asymmetric component, the Walker circulation, while the zonally symmetric component, the Hadley circulation, exhibits large uncertainty ( Ma et al. 2012 , Vecchi et al. 2006 ). Based on a simple moisture budget scaling, the strength of the tropical circulation is predicted to weaken as column-integrated water vapor increases faster than global mean precipitation ( Held and Soden 2006 ). One cause for the weakening of the tropical circulation is proposed to be increased atmospheric stability associated with deepening convection in a warmer climate (Chou and Chen 2010).
The shift of the ITCZ
Energetic framework & limitation
In our experiments, seven out of eight models exhibit the ITCZ between θe and (Fig. 3.3.2), as is the case in Fig. Although their exact locations are different, we can expect them to respond to the heat disturbance in similar ways. For example, as the NH warms relative to the SH, the anomalous energy must be transported southward across the equator, which will be accomplished by moving the ascending branch of the HC northward to strengthen the southern HC.
Subsequently, all three latitudinal indices (θe, ITCZ and ) are expected to be shifted northward. In one extreme case, where changes in ∆ are negligible, δ arises from , so θ can derive, and thus the ITCZ. Previous studies have shown that θe can be shifted by changes in the equatorial region (blue in Fig.
3.3.3; Kang et al. 2008) and the net energy input to the equatorial atmosphere, equal to the divergence of (red in Fig. Regardless of the origin of θeresponse, the ITCZ shift is related to the θeshift, assuming that changes in ∆ are small. In the other extreme case, eq. 2) indicates that δ can only arise from ∆, in which case the θe.
Indeed, Shaw et al. 2015) shows that the cross-equatorial energy flux is influenced by changes in equatorial gross moisture stability. In the case of δ = 0 there is no θe shift, but changes in and thus the ITCZ shift can arise from changes in. Thus, we can expect some cases where the previous energy flux perspective fails to predict the ITCZ shift when the changes in ∆ are substantial.
In this study, it is shown that the response of the ITCZ to a doubling of CO2 cannot be understood without considering changes in. the mass flux maximum changes sign ( ) in the reference. Schematic depicting the qualitative shift of the location of the ITCZ as the northward cross-equatorial atmospheric energy decreases (blue) and as the net energy input to the equatorial atmosphere increases (red), adopted by Bischoff and Schneider (2014).
Results
ITCZ response and energy flux equatorial response with 90% confidence level in columns. The response of the ITCZ is closely related to the response of the low-level mass flux δ at the reference location of the ITCZ (Figure 4.5). Models with a northerly δ at the reference ITCZ show a northward shift of the ITCZ and vice versa.
The relationship between the ITCZ response (in degrees) and the change in lower-level mass flux at the location of the reference ITCZ (δ ; in kg/s). Because the purpose of this study is to explain the displacement of the ITCZ, we performed the analysis in the ITCZ. In all cases except AM2(0), the ITCZ at 1xCO2 lies between and , so ∆ at the ITCZ is negative.
Regardless of the sign of ∆, a positive δ∆ indicates a greater increase in energy transport at the upper level than at the lower level, and vice versa. The diagram explaining the sign of the total gross moisture stability (∆) at the ITCZ in the reference integration (1xCO2). It indicates that the shift of the ITCZ in response to a doubling of CO2 cannot be understood without considering the total change in gross moisture stability in the tropics.
Since the sign of δ∆m is determined by the ratio of the changes in the total energy transport in the upper level to that in the lower level, it will have a bearing on how the convection responds to 2xCO2. In previous studies, where ∆m is held fixed, δ = 0 is thought to cause no shift in energy flux equator, hence the ITCZ. However, if there were significant changes in ∆m, which is the case in our set of experiments, δ = 0 could still result in changes and accompany a shift in the ITCZ.
For example, the AM2(4X) case has a negative ∆m at the reference ITCZ, and δ∆m is negative, so δ is negative, indicating a southward ITCZ shift. This contrast is mostly due to differences in the shortwave component of cloud radiative forcing (Fig. 4.9). In all δ ≠ 0 cases, δ and δ∆m have the same sign, so unless their relative magnitudes are known a priori, the sign of δ cannot be determined from Eq. 2), in contrast to the δ =0 case, where the sign of δ can only be determined from the sign of δ∆m.
Ratio of the anomalous atmospheric energy flux (δ in W) to (a) the anomalous lower-level mass flux (δ in kg/s) and (b) the total gross moisture stability change. The flow diagram summarizing the lower-level sign of mass flux changes can be determined from the sign of changes in atmospheric energy transport and total gross moisture stability.
Conclusion
This is the reason why the effect of Δ was negligible compared to the effect of mass transport changes to realize cross-equatorial transport of atmospheric energy. Alternatively, the uniform radiative forcing causes a rather subtle shift of the Hadley circulation compared to the asymmetric hemispheric forcing. In fact, if the Earth were perfectly hemispherically symmetric, the uniform force would not cause any latitudinal displacement.
It is the hemispheric asymmetry of today's Earth that causes a latitudinal shift in response to the uniform radiative forcing. It turns out that the effect of changes in mass transport due to the displacement of the Hadley circulation is much smaller than the effect of Δ in driving changes in atmospheric energy transport in response to increased CO2. Therefore, the present study addresses the importance of accounting for changes in total gross moisture stability ∆ in order to properly understand the response of tropical precipitation to uniform radiative forcing.
Thus, not only from the point of view of local physics, but also from the point of view of large-scale dynamics, the improvement of convection schemes and cloud modeling is necessary for a more accurate projection of tropical rainfall.
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