Estimated Impacts of Climate Change on Eddy Meridional Moisture Transport in the Atmosphere

Abstract

Research findings suggest that water (hydrological) cycle of the earth intensifies in response to climate change, since the amount of water that evaporates from the ocean and land to the atmosphere and the total water content in the air will increase with temperature. In addition, climate change affects the large-scale atmospheric circulation by, for example, altering the characteristics of extratropical transient eddies (cyclones), which play a dominant role in the meridional transport of heat, moisture, and momentum from tropical to polar latitudes. Thus, climate change also affects the planetary hydrological cycle by redistributing atmospheric moisture around the globe. Baroclinic instability, a specific type of dynamical instability of the zonal atmospheric flow, is the principal mechanism by which extratropical cyclones form and evolve. It is expected that, due to global warming, the two most fundamental dynamical quantities that control the development of baroclinic instability and the overall global atmospheric dynamics—the parameter of static stability and the meridional temperature gradient (MTG)—will undergo certain changes. As a result, climate change can affect the formation and evolution of transient extratropical eddies and, therefore, macro-exchange of heat and moisture between low and high latitudes and the global water cycle as a whole. In this paper, we explore the effect of changes in the static stability parameter and MTG caused by climate change on the annual-mean eddy meridional moisture flux (AMEMF), using the two classical atmospheric models: the mid-latitude f-plane model and the two-layer β-plane model. These models are represented in two versions: “dry,” which considers the static stability of dry air alone, and “moist,” in which effective static stability is considered as a combination of stability of dry and moist air together. Sensitivity functions were derived for these models that enable estimating the influence of infinitesimal perturbations in the parameter of static stability and MTG on the AMEMF and on large-scale eddy dynamics characterized by the growth rate of unstable baroclinic waves of various wavelengths. For the base climate change scenario, in which the surface temperature increases by 1 °C and warming of the upper troposphere outpaces warming of the lower troposphere by 2 °C (this scenario corresponds to the observed warming trend), the response of the mass-weighted vertically averaged annual mean MTG is -0.2 ℃ per 1000 km. The dry static stability increases insignificantly relative to the reference climate state, while on the other hand, the effective static stability decreases by more than 5.4%. Assuming that static stability of the atmosphere and the MTG are independent of each other (using One-factor-at-a-time approach), we estimate that the increase in AMEMF caused by change in MTG is about 4%. Change in dry static stability has little effect on AMEMF, while change in effective static stability leads to an increase in AMEMF of about 5%. Thus, neglecting atmospheric moisture in calculations of the atmospheric static stability leads to tangible differences between the results obtained using the dry and moist models. Moist models predict ~9% increase in AMEMF due to global warming. Dry models predict ~4% increase in AMEMF solely because of the change in MTG. For the base climate change scenario, the average temperature of the lower troposphere (up to ~4 km), in which the atmospheric moisture is concentrated, increases by ~1.5 ℃. This leads to an increase in specific humidity of about 10.5%. Thus, since both AMEMF and atmospheric water vapor content increase due to the influence of climate change, a rather noticeable restructuring of the global water cycle is expected.

Effects of Global Warming on the Poleward Heat Transport by Non-Stationary Large-Scale Atmospheric Eddies, and Feedbacks Affecting the Formation of the Arctic Climate

“…In this case, the baroclinic instability is considered as an eigenvalue problem that is usually solved using the perturbation method, which admits a plane-wave solution. Then, it can be shown that the meridional heat flux due to transient eddies depends exponentially on the increments of unstable perturbations [62]. Let us explain this in more detail.…”

“…Substituting the normal mode solution ( 7) into the linearized model equations, one can derive the system of algebraic equations that can be solved analytically with respect to phase velocity c. The increments of unstable waves, represented by χ k , are defined as χ k = kc i , where c i = m(c) is the imaginary part of the phase velocity. For the β-plane (Phillips) model, which is the classic model of baroclinic instability, the growth rate χ k is calculated as follows [57,62]:…”

“…The sensitivity functions and , with respect to parameters and , allow the determination of the effect of small perturbations in and on the growth rates of unstable waves. The sensitivity functions represented as partial derivatives of the instability increment , with respect to and , are as follows [62]: To assess the impact of climate change on the increments of unstable waves, we use conventional sensitivity analysis, which involves the calculation of the corresponding sensitivity functions [68]. The sensitivity functions S σ and S u T , with respect to parameters σ 0 and u T , allow the determination of the effect of small perturbations in σ 0 and u T on the growth rates of unstable waves.…”

“…The sensitivity functions S σ and S u T , with respect to parameters σ 0 and u T , allow the determination of the effect of small perturbations in σ 0 and u T on the growth rates of unstable waves. The sensitivity functions represented as partial derivatives of the instability increment χ k , with respect to σ 0 and u T , are as follows [62]:…”

“…Let us now consider how the change in MTG caused by global warming can affect the behavior of unstable baroclinic waves. As shown in [62], global warming causes changes in MTG, which lead to corresponding changes in the parameter u T such that δu T = 0.8 ms −1 . Using sensitivity function S u T , one can estimate the impact of global warming on χ k caused only by δu T .…”