Characterization of Moist Processes Associated With Changes in the Propagation of the MJO With Increasing CO₂

Abstract

The processes that lead to changes in the propagation and maintenance of the Madden‐Julian Oscillation (MJO) as a response to increasing CO 2 are examined by analyzing moist static energy budget of the MJO in a series of NASA GISS model simulations. It is found changes in MJO propagation is dominated by several key processes. Horizontal moisture advection, a key process for MJO propagation, is found to enhance predominantly due to an increase in the mean horizontal moisture gradients. The terms that determine the strength of the advecting wind anomalies, the MJO horizontal scale and the dry static stability, are found to exhibit opposing trends that largely cancel out. Furthermore, reduced sensitivity of precipitation to changes in column moisture, i.e., a lengthening in the convective moisture adjustment time scale, also opposes enhanced propagation. The dispersion relationship of Adames and Kim, which accounts for all these processes, predicts an acceleration of the MJO at a rate of ∼3.5% K −1 , which is consistent with the actual phase speed changes in the simulation. For the processes that contribute to MJO maintenance, it is found that damping by vertical MSE advection is reduced due to the increasing vertical moisture gradient. This weaker damping is nearly canceled by weaker maintenance by cloud‐radiative feedbacks, yielding the growth rate from the linear moisture mode theory nearly unchanged with the warming. Furthermore, the estimated growth rates are found to be a small, negative values, suggesting that the MJO in the simulation is a weakly damped mode.

Changes in Madden‐Julian Oscillation Precipitation and Wind Variance Under Global Warming

“…Under global warming, increased lower‐tropospheric water vapor due to increased surface temperature results in an increase of MJO precipitation amplitude of more than 9.5%/K (Arnold et al, ). These precipitation increases are consistent with decreases in gross moist stability of the tropical atmosphere or related quantities (Adames et al, ; Arnold et al, ; Wolding et al, ). However, changes in MJO wind variability do not necessarily change in the same way (Adames et al, ; Wolding et al, ).…”

Trends in Tropical Wave Activity from the 1980s to 2016

“…To further elaborate, considering the MJO band, for instance, there is good agreement between observed trends (Fig. 10a) and modeling efforts using GCMs (e.g., Jones and Carvalho 2006;Arnold et al 2015;Song and Seo 2016;Adames et al 2017) in the increased occurrence of MJO events attributed to global warming. While it is generally argued that MJO intensity will likely increase in a warmer climate, both observations and modeling studies discuss considerable uncertainties with regard to intensity and duration changes in the MJO.…”

Sensitivity of the Madden Julian Oscillation to Ocean Warming in a Lagrangian Atmospheric Model

“…Moreover, the proposed mechanism of MJO intensification with warming oceans, enhanced surface fluxes owing to the non-linear nature of the Clausius-Clapeyron equation, is closely related to the steeper vertical gradient in moisture noted in several earlier climate warming experiments with an intensifying MJO [15,17,38]. The vertical and horizontal structure of the MJO in the LAM is also consistent with observations, both for the mature stage (e.g., Figures 2a,b, 3a,b, and 4a,b), and for the other stages [23].…”

“…However, it is also possible that the causality works the other way, and that the basic state westerlies extend farther eastward in the ocean warming experiments because the MJO is stronger and propagates farther eastward. Similarly, quantitatively explaining the increase in phase speed is not straightforward, and will likely require considering not only changes to surface evaporation, but also changes to the basic state moisture field and their interaction with MJO wind perturbations [24,38,41].…”