5.8 Discussion and conclusions
5.8.2 Conclusions
In this chapter, we set out to understand why the the zonal variance of stationary-eddy vertical velocity increased with warming until about 300 K global-mean surface temperature, then decreased
with further warming. These changes are related to a global change in stationary-eddy kinetic energy (sEKE). This change is sensitive to the latitude of topography. Idealized GCM experiment with topography at different latitudes create qualitatively different sEKE responses. The reason for this sensitivity is that the zonal winds at the latitude of the mountain are a key factor in forcing orographic stationary waves, through their contribution to orographic ascent / descent and adiabatic cooling / heating. The stationary-eddy meridional winds needed to balance this heating / cooling depend on the isentropic slope, which decreases with warming, leading to an increase in stationary- wave amplitude. Latent heating, which damps orographic stationary-eddies throughout the range of climates studied, becomes important in warm climates, leading to a reduction in stationary- wave amplitude. The amplification of subsidence downstream of topography by the stationary-eddy vorticity balance and transient-eddy heat fluxes, both tend to increase the amplitude of orographic stationary waves, but do not have a large response to climate change.
To summarize, we group the most important influences on orographic stationary Rossby waves by whether they increase of decrease the stationary-wave amplitude with warming.
Non-monotonic change with warming:
• Change in zonal surface winds at the latitude of the mountain
Decreased stationary-wave amplitude with warming:
• Decrease in top-of-mountain surface pressure due to increased surface temperature
• Increased damping by latent heating
Increased stationary-wave amplitude with warming:
• Decreased isentropic slope due to increased extratropical static stability and reduced meridional temperature gradients
Amplify changes due to other factors:
• Amplified stationary-wave subsidence downstream of topography
• Transient-eddy heat fluxes into cold region downstream of topography
Chapter 6
Circulation changes in the
equatorial heating experiment
Abstract
This chapter considers changes in zonally anomalous tropical circulations with global warming un- der fixed ocean heat-flux forcing. A decrease in strength of such circulations has been identified as a robust response to climate change, though there are still open questions on the mechanisms behind this response. Variation of the aspect ratio of ocean heating in an idealized GCM reveals that the Walker circulation strength is constant across a wide range of vorticity budget regimes.
This suggests that there are independent energetic constraints on the Walker circulation strength.
These energetic constraints take the form of an increase in gross moist stability (GMS) with warm- ing across a wide range of climates, which increases the energy expended per unit mass of deep convection. The increase in GMS with warming results primarily from the increasing depth of deep convection and the corresponding increase in tropopause height. These mechanisms are used to derive quantitative scalings for the change in Walker circulation strength with warming. By using baroclinic mode theory, the Walker circulation strength can be estimated solely from the profile of tropical-mean temperature and the strength of zonally anomalous heating. This chapter also ex- amines the consequences of these Walker circulation strength on the extratropical circulation. The extratropical response is determined primarily by the wavenumber spectrum of tropical forcing and the existence of turning latitudes.
6.1 Introduction
The slowdown of the tropical overturning circulations, such as Walker circulations, with global warming is robust across many types of models and observations (Held and Soden, 2006; Vecchi et al., 2006; Vecchi and Soden, 2007; Merlis and Schneider, 2011). Global energetic constraints on evaporation/precipitation lead to a reduction of the strength of overturning circulations globally in response to increases in the surface specific humidity or the lapse rate of specific humidity (Betts, 1998; Held and Soden, 2006; Vecchi and Soden, 2007; Schneider et al., 2010). Zonally anomalous circulations in the tropics make up 40% of the total overturning of the atmosphere in idealized GCM experiments with zonally anomalous ocean heating (experiments presented herein; calculated from the upward portion of ω500), so the naive assumption would be that they account for 40%
of the decrease in overturning resulting from energetic limitations. As pointed out by Held and Soden (2006) and Vecchi and Soden (2007), it is not unreasonable to think that even more of the decrease in overturning will be concentrated in the zonally anomalous circulations, since zonal-mean circulations (e.g., the Hadley cell) have to obey angular momentum constraints as well as energetic constraints.
However, the global energetic constraints do not provide a quantitative theory for the strength of zonal overturning circulations in particular. We need to look elsewhere to explain the decrease in tropical rms ω∗pi
shown in Fig. 3.13. Merlis and Schneider (2011) observe that ifP∗ is constant across a range of climates, the Walker circulation strength is constrained to decrease rapidly with warming according to
ω↑∗∼ − P∗
∆vqs
, (6.1)
whereω↑ is the upward portion of vertical velocity and ∆vqsis the tropospheric vertical difference of saturation specific humidity. The strength of the upwelling branch of the Walker circulation thus scales with (∆vqs)−1 or approximately inversely with surface specific humidity. We can obtain a
similar result using Eq. 3.12 where
ω∗850 hPa∼ −gP∗−E∗
[qsfc] . (6.2)
Thus the strength of the Walker circulation scales with [qsfc]−1as long asP∗−E∗is fixed across the range of climates. Neither of these explanations is a closed theory because we have not yet explained whyP∗−E∗is constant across the range of climates; an independent theory of the Walker circulation strength is required.
The vorticity budget provides some additional constraints on vertical motion, as was seen already in Chapters 2 and 3. This analysis can be further modified to focus on mid-tropospheric vertical velocities and the strength of overturning in particular. An alternative perspective comes from integrating over the upper troposphere instead of the lower troposphere, eliminating the importance of surface drag. A number of studies have used the moist static energy (MSE) equation and baroclinic mode theory to further constrain tropical circulations (Neelin and Held, 1987; Neelin and Zeng, 2000;
Chou and Neelin, 2004). In particular, Chou and Neelin (2004) propose theanomalous gross moist stability mechanism that explains some aspects of the slowdown of tropical overturning circulations with warming based on the increase of gross moist stability (GMS), an effective static stability felt by tropical circulations. The increase is due to an increasing depth of deep convection and is partially cancelled by an increase in lower-tropospheric specific humidity. This increases the atmospheric energy requirements for deep convection. This is closely related to the warm gets wetter mechanism, where regions with enhanced SST warming experience locally reduced GMS and enhanced precipitation (Xie et al., 2010; Huang et al., 2013). It has not been examined whether GMS continues to increase with warming beyond next-century or doubled-CO2 climates. We will examine the MSE equation in idealized GCM simulations with zonally anomalous ocean heating to see how theanomalous gross moist stability mechanism plays out across a wide range of climates.
The idealized GCM experiments used are the equatorial heating experiments of Chapter 3, with several additional configurations introduced in Section 6.2. The vorticity budget controls on
mid-tropospheric vertical velocities are analyzed in Section 6.3, uncovering a consistent circulation strength across a wide range of vorticity budget regimes. In Section 6.4, we analyze energetic controls on the Walker circulation strength, as diagnosed from the MSE equation and gross moist stability.
The influence Walker circulation changes have on extratropical stationary eddies is then considered in Section 6.5. Section 6.6 provides some discussion and conclusions, including a synthesis with other parts of this thesis.