4:15 PM - 4:30 PM
[AOS09-10] Interdecadal Variation of Perturbational Feedback Parameter in MIROC6 piControl Simulation
Keywords:Climate sensitivity, Internal variation, ENSO, Decadal variation, Background field
How much the temperature will rise in the future by global warming is a quite fundamental question. The uncertainty of prediction in temperature rise in the future is, however, still large, and the difficulty in estimating climate sensitivity (climate system response towards external forcing, such as greenhouse gas increase) from the past to the future is the main cause of the uncertainty. Climate sensitivity is usually defined by global temperature rise responded to the doubling of CO2 when the earth system has reached the equilibrium state, which is called equilibrium climate sensitivity (ECS: Yoshimori et al., 2012). It is suggested, however, that climate sensitivity before equilibrium state is not constant, but varies on interdecadal time scales. such as, feedback parameter variation in the historical experiment (e.g. Gregory and Andrews, 2016) and global warming hiatus in the beginning of 21st century (e.g. Easterling and Wehner, 2009). It has been shown that the atmospheric circulation and cloud property changes according to spatiotemporal pattern of sea surface temperature (SST) rise are the causes of feedback parameter variation (Mauritsen, 2016, Zhou et al., 2014). Spatiotemporal pattern variation of SST involves factors not only external forcing change that is directly related to global warming, such as greenhouse gas increase, but also internal variability in annual to interdecadal timescales. These factors are both taken into account in the preceding studies, and it is not easy to sort them separately.
Therefore, a new feedback parameter (Perturbational Feedback Parameter, hereafter, PFP) was defined and medium to long term variation was identified in order to investigate the impact of internal variation on climate sensitivity variation. Values of PFP varied on interdecadal time scales and the equatorial SST had positive anomaly in the eastern Pacific and negative anomaly in the western Pacific when PFP was large. On the background field, ENSO amplitude, relative to the global mean surface temperature change by 1K, became large. Consequently, the equatorial high SST more effectively contributes to convective clouds shift towards the equator caused by ITCZ shift and to lower cloud increase in the subtropics caused by strengthening of descending flow and static stability. These cloud responses brought about the net downward radiative flux change through reflection increase and thus enhanced PFP.
Further analysis, to make sure the robustness of ENSO amplitude and property change impact, suggested that Hadley circulation and its following cloud changes are, however, not fully explained by ENSO change. Another longer variation, which dominates over the extratropical North Pacific, also play an important role in controlling low-level cloud cover and thus PFP variation.
Therefore, a new feedback parameter (Perturbational Feedback Parameter, hereafter, PFP) was defined and medium to long term variation was identified in order to investigate the impact of internal variation on climate sensitivity variation. Values of PFP varied on interdecadal time scales and the equatorial SST had positive anomaly in the eastern Pacific and negative anomaly in the western Pacific when PFP was large. On the background field, ENSO amplitude, relative to the global mean surface temperature change by 1K, became large. Consequently, the equatorial high SST more effectively contributes to convective clouds shift towards the equator caused by ITCZ shift and to lower cloud increase in the subtropics caused by strengthening of descending flow and static stability. These cloud responses brought about the net downward radiative flux change through reflection increase and thus enhanced PFP.
Further analysis, to make sure the robustness of ENSO amplitude and property change impact, suggested that Hadley circulation and its following cloud changes are, however, not fully explained by ENSO change. Another longer variation, which dominates over the extratropical North Pacific, also play an important role in controlling low-level cloud cover and thus PFP variation.