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Vertically Resolved Weak Temperature Gradient Analysis of the Madden-Julian Oscillation

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December 5, 2016
Brandon Wolding
Hosted by Eric Maloney (advisor), Dave Randall, Sue van den Heever, George Kiladis (affiliate), Jay Ham (Soil and Crop Sciences)

Abstract

Interactions between moisture, convection, and large-scale circulations are thought to play an important role in destabilizing the Madden-Julian Oscillation (MJO). A simplified framework for understanding such interactions is developed, building upon the work of Chikira (2014). Tropical weak temperature gradient (WTG) balance is used to diagnose intraseasonal variations in large-scale vertical velocity from variations in apparent heating, allowing intraseasonal variations in large-scale vertical moisture advection to be decomposed into contributions from various apparent heating processes (e.g. radiative heating, microphysical processes). The WTG diagnosis captures the vertical structure and magnitude of large-scale vertical velocity and vertical moisture advection with exceptional accuracy throughout the free troposphere.

Moisture and moisture variance budgets are used to investigate the MJO in ERA-interim (ERAi) reanalysis and the Superparameterized Community Earth System Model (SP-CESM). Moisture budgets indicate that, during the enhanced phase of the MJO, anomalous moistening by large-scale vertical moisture advection exceeds anomalous drying by microphysical processes and sub-grid scale (SGS) eddy fluxes, such that the net effect of these large and opposing processes (hereafter the column process) is to further moisten regions that are anomalously moist. Moisture variance budgets indicate that the column process helps grow moisture variance, acting to destabilize the MJO. Horizontal advective damping of moisture variance, associated with the modulation of higher frequency convective variability on intraseasonal timescales, acts to stabilize the MJO.

The vertically resolved WTG balance framework is used to assess the contribution various apparent heating processes make to the column process, and its ability to destabilize the MJO. Intraseasonal variations in longwave radiative heating enhance variations in large-scale vertical moisture advection at low and mid levels, strongly supporting destabilization of the MJO in both ERAi and SP-CESM. The effect of convection alone (i.e. without radiative and surface flux feedbacks) is to weakly grow moisture variance in SP-CESM, and weakly damp moisture variance in ERAi, suggesting that the MJO is unrealistically unstable in the former. Surface flux feedbacks appear to play a more important role in destabilizing the real world MJO. Moisture variance budget analysis of periods of weak, moderate, and strong MJO activity suggests that changes in the vertical structure of apparent heating do not play a dominant role in limiting the amplitude of the MJO in SP-CESM in the current climate.

WTG balance provides a useful framework for investigating how the MJO, and its impacts, may change as the climate system warms. Two simulations of SP-CESM, one at pre-industrial levels of CO2 (280 ppm, hereafter PI) and one where CO2 levels have been quadrupled (1120 ppm, hereafter 4xCO2), were analyzed. MJO convective variability increases considerably in the 4xCO2 simulation, a consequence of more favorable mean state moist thermodynamic conditions. A steepened mean state vertical moisture gradient allows MJO convective heating to drive stronger variations in large-scale vertical moisture advection, helping to support enhanced MJO convective variability in the 4xCO2 simulation. The dynamical response to MJO convective heating weakens in the warmer climate, a result of increased tropical static stability. One consequence of this weakened dynamical response is that the MJO’s ability to influence the extratopics, which is closely tied to the strength of its associated divergence, is reduced considerably in the 4xCO2 simulation.