The moisture budget decomposition of this chapter allows the change inP∗−E∗in these two idealized model experiments to be understood in terms of simple thermodynamic changes and changes in lower- tropospheric stationary-eddy vertical motion (ω∗850). It is thus important to understand why ω∗850 changes as it does with climate change. The lower-level vorticity balance implies that most changes inω∗850are due to changes in lower-tropospheric stationary-eddy horizontal motion. We thus discuss changes in the lower-tropospheric manifestation of stationary eddies in these experiments in Part II (Chapters 5 and 6).
Several studies have focused on how barotropic stationary eddies respond to climate change using realistic forcing (Stephenson and Held, 1993; Brandefelt and K¨ornich, 2008; Simpson et al., 2014, 2015). These studies have found robust shifts in stationary-eddy circulations in certain models, but there has been little discussion of the amplitude of stationary waves or the associated vertical veloc- ities. Simulations with linear stationary-wave models or comprehensive climate model simulations with simplified forcing have been used to separate the roles of different forcing mechanisms in the modern stationary wave climatology (Nigam et al., 1986, 1988; Valdes and Hoskins, 1989; Ting, 1994; Held et al., 2002) and in the response of stationary waves to global warming (Stephenson and Held, 1993; Joseph et al., 2004). These studies have given a rough picture of which stationary high- and low-pressure systems result from which forcings and how the stationary wave field changes in response to changes in these forcings or to global climate change. However, stationary-wave models specify zonally anomalous heating and transient-eddy heat and momentum fluxes, all of which are part of the stationary-wave response. The zonally anomalous heating, for example, could respond very differently to climate change when stationary waves are forced by topography or equatorial heating. By using these GCM experiments with simplified geometry, we hope to gain insight into what determines changes in these nonlinear terms as well.
The advantage of the idealized model experiments here is that we can separately analyze the Rossby wave response associated with topographic forcing and equatorial heating. We have already seen how these two types of forcing give different circulation responses to climate change in Fig.
3.13. We would like to understand which changes in the idealized climate with orographic forcing lead to an increase and then decrease with warming of rms ω∗pi
and which changes in the idealized climate with a Walker circulation lead to a decreasing rms ω∗p
i
throughout the range of climates.
Part II will lay out the very different mechanisms governing these changes.
Chapter 4
The amplitude of the zonally anomalous hydrological cycle in CMIP5
This chapter is a draft in preparation with coauthors Michael Byrne and Tapio Schneider.
Abstract
The wet gets wetter, dry gets drier paradigm is a useful starting point for understanding zonal- mean changes in precipitation minus evaporation (P−E). It can explain the expected moistening of the high latitudes and drying of the subtropics in response to global warming. We examine P −E changes over the next century in comprehensive climate models from the Coupled Model Intercomparison Project Phase 5 (CMIP5). We show that wet gets wetter, dry gets drier can not be extended to apply to regional variations about the zonal mean, which account for the majority of the spatial variability ofP−E in the modern climate. Wet and dry zones shift substantially in response to shifts in the stationary-eddy circulations that cause them. The largest changes are in the tropical oceans where wet zones get drier and dry zones get wetter in response to a restructuring and decrease in strength of tropical circulations such as the Walker circulation. Further progress can be made by examining changes in the zonal variance ofP −E. The zonal variance of P −E increases robustly at all latitudes in the Representative Concentration Pathways RCP8.5 scenario, but with a smaller fractional increase than the moisture content of the atmosphere. The variance
change can be split into dynamic and thermodynamic components by relating it to changes in surface specific humidity, stationary-eddy divergent circulations, and transient-eddy fluxes. The modeled sub Clausius-Clapeyron change of zonalP −E variance gives evidence of a decrease in stationary- eddy overturning and in zonally anomalous transient-eddy moisture flux convergence with global warming.
4.1 Introduction
The availability of water will be a crucial issue for society during the next century. This depends on the spatiotemporal variability of net precipitation (precipitation minus evaporation,P−E). In the absence of changes in atmospheric circulations, the change ofP−E with warming can simply be related to the change in moisture content of the atmosphere (Mitchell et al., 1987; Chou and Neelin, 2004; Held and Soden, 2006). Due to energetic constraints on near-surface relative humidity (Held and Soden, 2000, 2006; Schneider et al., 2010), the moisture content of the atmosphere, which is concentrated near the surface, increases substantially with warming approximately following the Clausius-Clapeyron relation. This provides a simple framework for predicting qualitative changes in P−E with warming; wet regions will get wetter and dry regions will get drier, simply because the same circulations transport more moisture. This framework works best for large spatial averages, because circulations shifts such as the expansion of the Hadley cell lead to local departures from this thermodynamic wet gets wetter mechanism (Chou and Neelin, 2004; Held and Soden, 2006; Chou et al., 2009). Weakening of overturning circulations, especially in the tropics, can also offset part of this thermodynamic change (Chou and Neelin, 2004; Held and Soden, 2006; Vecchi and Soden, 2007; Chou et al., 2009; Seager et al., 2010).
Most of the success of the wet gets wetter mechanism comes from its applicability to zonal- meanP−E, because zonal-mean overturning circulations such as the Hadley circulation are subject to angular-momentum constraints limiting their response to climate change (Held and Hou, 1980;
Schneider, 2006; Walker and Schneider, 2006; Schneider et al., 2010). The remaining zonally anoma- lous P−E, obtained by subtracting off the zonal-mean, explains 60% of the total spatial P −E
variance in the modern climate (Wills and Schneider, 2015). We will analyze to what extent the wet gets wetter mechanism is useful for changes in zonally anomalous P −E. The location and strength of regions of stationary-eddy divergence and convergence are only loosely set by zonally anomalies in heating and topography, so we might expect dynamic changes in zonally anomalous P −E to be large. We suggest looking instead at the zonal variance of P−E, which provides a bulk measure of the amplitude of zonal hydrological cycle variations. The zonal variance ofP−E is unaffected by zonal circulation shifts and should change more thermodynamically. To the extent it does not, it implies a zonal-mean slowdown of stationary-eddy overturning circulations and/or decreased moisture fluxes by transient eddies.