Nonlinear Interaction between the Drivers of the Monsoon and Summertime Stationary Waves

<p>An intermediate complexity moist General Circulation Model is used to investigate the forcing of the Asian monsoon and the associated upper level anticyclone by land-sea contrast, net horizontal heat transport by the ocean, and topography. The monsoonal pattern is not simply the linear additive sum of the response to each forcing; only when all three forcings are included simultaneously does the monsoonal circulation extend westward to India. This nonadditivity impacts the location of the upper level anticyclone, which is shifted eastward and weaker if the forcings are imposed individually. Sahelian precipitation, and also austral summer precipitation over Australia, southern Africa, and South America, are likewise stronger if all forcings are imposed simultaneously. The source of the nonlinearity can be diagnosed using gross moist stability, but cannot be accounted for using the land-sea breeze paradigm. This non-additivity implies that the question of which forcing is most important is ill-posed.</p>

Quasi-Equilibrium and Weak Temperature Gradient Balances in an Equatorial Beta-Plane Model

Convective quasi-equilibrium (QE) and weak temperature gradient (WTG) balances are frequently employed to study the tropical atmosphere. This study uses linearized equatorial beta-plane solutions to examine the relevant regimes for these balances. Wave solutions are characterized by moisture–temperature ratio (q–T ratio) and dominant thermodynamic balances. An empirically constrained precipitation closure assigns different sensitivities of convection to temperature (εt) and moisture (εq). Longwave equatorial Kelvin and Rossby waves tend toward the QE balance with q–T ratios of εt:εq that can be ~1–3. Departures from strict QE, essential to both precipitation and wave dynamics, grow with wavenumber. The propagating QE modes have reduced phase speeds because of the effective gross moist stability meff, with a further reduction when εt > 0. Moisture modes obeying the WTG balance and with large q–T ratios (>10) emerge in the shortwave regime; these modes exist with both Kelvin and Rossby wave meridional structures. In the υ = 0 case, long propagating gravity waves are absent and only emerge beyond a cutoff wavenumber. Two bifurcations in the wave solutions are identified and used to locate the spatial scales for QE–WTG transition and gravity wave emergence. These scales are governed by the competition between the convection and gravity wave adjustment times and are modulated by meff. Near-zero values of meff shift the QE–WTG transition wavenumber toward zero. Continuous transitions replace the bifurcations when meff < 0 or moisture advection/WISHE mechanisms are included, but the wavenumber-dependent QE and WTG balances remain qualitatively unaltered. Rapidly decaying convective/gravity wave modes adjust to the slowly evolving QE/WTG state in the longwave/shortwave regimes, respectively.

Evidence that horizontal moisture advection regulates the ubiquitous amplification of rainfall variability over tropical oceans

The column moist static energy (MSE) budget equation approximates the processes associated with column moistening and drying in the tropics, and is therefore predictive of precipitation amplification and decay. We use ERA-I and TRMM 3B42 data to investigate day-to-day convective variability and distinguish the roles of horizontal MSE (or moisture) advection versus vertical advection, sources, and sinks. Over tropical convergence zones, results suggest that horizontal moisture advection is a primary driver of day-to-day precipitation fluctuations; when drying via horizontal moisture advection is smaller (greater) than Chikira’s “column process,” precipitation tends to amplify (decay). In the absence of horizontal moisture advection, precipitation tends to increase spontaneously almost universally through a positive column process feedback. This bulk positive feedback is characterized by negative effective gross moist stability (GMS), which is maintained throughout the tropical convergence zones. How this positive feedback is achieved varies geographically, depending on the shape of vertical velocity (omega) profiles. In regions where omega profiles are top-heavy, the effective GMS is negative primarily owing to strong feedbacks between convection and diabatic MSE sources (radiative and surface fluxes). In these regions, vertical MSE advection stabilizes the atmosphere (positive vertical GMS). Where omega profiles are bottom-heavy, by contrast, a positive feedback is primarily driven by import of MSE through a shallow circulation (negative vertical GMS). The diabatic feedback and vertical GMS are in a see-saw balance, offsetting one another. Our results suggest that ubiquitous convective variability is amplified by the same mechanism as moisture-mode instability.

Contrasting Southern Hemisphere Monsoon Response: MidHolocene Orbital Forcing versus Future Greenhouse Gas–Induced Global Warming

Past changes of Southern Hemisphere (SH) monsoons are less investigated than their northern counterpart because of relatively scarce paleodata. In addition, projections of SH monsoons are less robust than in the Northern Hemisphere. Here, we use an energetic framework to shed lights on the mechanisms determining SH monsoonal response to external forcing: precession change at the mid-Holocene versus future greenhouse gas increase (RCP8.5). Mechanisms explaining the monsoon response are investigated by decomposing the moisture budget in thermodynamic and dynamic components. SH monsoons weaken and contract in the multimodel mean of midHolocene simulations as a result of decreased net energy input and weakening of the dynamic component. In contrast, SH monsoons strengthen and expand in the RCP8.5 multimodel mean, as a result of increased net energy input and strengthening of the thermodynamic component. However, important regional differences on monsoonal precipitation emerge from the local response of Hadley and Walker circulations. In the midHolocene, the combined effect of Walker–Hadley changes explains the land–ocean precipitation contrast. Conversely, the increased local gross moist stability explains the increased local precipitation and net energy input under circulation weakening in RCP8.5.

Vertical Velocity Profiles in Convectively Coupled Equatorial Waves and MJO: New Diagnoses of Vertical Velocity Profiles in the Wavenumber–Frequency Domain

A new diagnostic framework is developed and applied to ERA-Interim to quantitatively assess vertical velocity (omega) profiles in the wavenumber–frequency domain. Two quantities are defined with the first two EOF–PC pairs of omega profiles in the tropical ocean: a top-heaviness ratio and a tilt ratio. The top-heaviness and tilt ratios are defined, respectively, as the cospectrum and quadrature spectrum of PC1 and PC2 divided by the power spectrum of PC1. They represent how top-heavy an omega profile is at the convective maximum, and how much tilt omega profiles contain in the spatiotemporal evolution of a wave. The top-heaviness ratio reveals that omega profiles become more top-heavy as the time scale (spatial scale) becomes longer (larger). The MJO has the most top-heavy profile while the eastward inertio-gravity (EIG) and westward inertio-gravity (WIG) waves have the most bottom-heavy profiles. The tilt ratio reveals that the Kelvin, WIG, EIG, and mixed Rossby–gravity (MRG) waves, categorized as convectively coupled gravity waves, have significant tilt in the omega profiles, while the equatorial Rossby (ER) wave and MJO, categorized as slow-moving moisture modes, have less tilt. The gross moist stability (GMS), cloud–radiation feedback, and effective GMS were also computed for each wave. The MJO with the most top-heavy omega profile exhibits high GMS, but has negative effective GMS due to strong cloud–radiation feedbacks. Similarly, the ER wave also exhibits negative effective GMS with a top-heavy omega profile. These results may indicate that top-heavy omega profiles build up more moist static energy via strong cloud–radiation feedbacks, and as a result, are more preferable for the moisture mode instability.

Cloud-Radiative Impacts On Tropical Circulation Change in GFDL AM4.1

&lt;pre&gt;We use the Geophysical Fluid Dynamics Laboratory (GFDL) state-of-the-art AM4.1 atmospheric model to assess the impact of clouds on the change in tropical circulation. Slab-ocean experiments where cloud microphysical properties are locked to either the pre-industrial or 4xCO&lt;sub&gt;2&lt;/sub&gt; conditions allow us to cleanly separate the circulation changes into a part caused by the cloud radiative effects (CREs), and to a part caused by the CO&lt;sub&gt;2&lt;/sub&gt; changes. The CO&lt;sub&gt;2&lt;/sub&gt;-induced SST changes are shown to dominate the response in the boundary layer, but are rivaled by the impacts of CREs in the mid to upper troposphere. The reduction in the east-to-west sea level pressure difference over the Pacific is solely caused by the increasing CO&lt;sub&gt;2&lt;/sub&gt; and SST, but they only account for about half of the change in the mid-tropospheric Walker circulation. The weakening of the free-tropospheric circulation is shown to be mostly caused by the near-equal contributions the CO&lt;sub&gt;2&lt;/sub&gt; and CREs make to the changes in dry-static and gross moist stability. Also, concerning the &lt;span&gt;meridional&lt;/span&gt; circulation, we show that the response in the strength of the southern branch of the Hadley cell is largely due to CREs, while they have a much smaller impact in the north.&lt;/pre&gt;

The Efficiency of the Hadley Cell Response to Wide Variations in Ocean Heat Transport

The Hadley cell (HC) plays a key role in the climate response to variations in ocean heat transport (OHT). Increased OHT is characterized by both a robust slowdown of this overturning circulation, with consequent changes in cloudiness driving the climate response, and a compensating reduction in the atmospheric heat transport (AHT). Here a suite of slab-ocean aquaplanet GCM simulations is used to study the robustness of mechanisms driving changes in HC mass and energy transport across a wide range of idealized spatial patterns of OHT. The HC response is intrinsically related to both the spatial pattern of OHT and the dynamical mechanisms driving the slowdown of the cell. The reduced energy flux of the HC is associated with reductions in both the mass flux and the gross moist stability (GMS) of the cell in all cases. However, when OHT convergence patterns are confined to the subtropics and equatorward thereof (i.e., subtropical overturning cells), the circulation response is largely momentum-conserving in nature when compared to OHT convergence patterns that extend into the midlatitudes, resulting in a deformation of the anomalous streamfunction following angular momentum contours. The effects of this deformation are quantified through a simple, yet novel approach of splitting the streamfunction anomalies into their “speed” and “shape” components. The tilt of the outer branch of the streamfunction anomaly dampens the direct climate effects of the slowdown of the cell while enhancing the change in GMS, effectively decoupling the change in the energy flux from the slowdown.

Probing the Response of Tropical Deep Convection to Aerosol Perturbations Using Idealized Cloud-Resolving Simulations with Parameterized Large-Scale Dynamics

A framework is introduced to investigate the indirect effect of aerosol loading on tropical deep convection using three-dimensional limited-domain idealized cloud-system-resolving model simulations coupled with large-scale dynamics over fixed sea surface temperature. The large-scale circulation is parameterized using the spectral weak temperature gradient (WTG) approximation that utilizes the dominant balance between adiabatic cooling and diabatic heating in the tropics. The aerosol loading effect is examined by varying the number of cloud condensation nuclei (CCN) available to form cloud droplets in the two-moment bulk microphysics scheme over a wide range of environments from 30 to 5000 cm−3. The radiative heating is held at a constant prescribed rate in order to isolate the microphysical effects. Analyses are performed over the period after equilibrium is achieved between convection and the large-scale environment. Mean precipitation is found to decrease modestly and monotonically when the aerosol number concentration increases as convection gets weaker, despite the increase in cloud liquid water in the warm-rain region and ice crystals aloft. This reduction is traced down to the reduction in surface enthalpy fluxes as an energy source to the atmospheric column induced by the coupling of the large-scale motion, though the gross moist stability remains constant. Increasing CCN concentration leads to 1) a cooler free troposphere because of a reduction in the diabatic heating and 2) a warmer boundary layer because of suppressed evaporative cooling. This dipole temperature structure is associated with anomalously descending large-scale vertical motion above the boundary layer and ascending motion at lower levels. Sensitivity tests suggest that changes in convection and mean precipitation are unlikely to be caused by the impact of aerosols on cloud droplets and microphysical properties but rather by accounting for the feedback from convective adjustment with the large-scale dynamics. Furthermore, a simple scaling argument is derived based on the vertically integrated moist static energy budget, which enables estimation of changes in precipitation given known changes in surfaces enthalpy fluxes and the constant gross moist stability. The impact on cloud hydrometeors and microphysical properties is also examined, and it is consistent with the macrophysical picture.

Inferences from Simple Models of Slow, Convectively Coupled Processes

A framework for conceptual understanding of slow, convectively coupled disturbances is developed and applied to several canonical tropical problems, including the water vapor content of an atmosphere in radiative–convective equilibrium, the relationship between convective precipitation and column water vapor, Walker-like circulations, self-aggregation of convection, and the Madden–Julian oscillation. The framework is a synthesis of previous work that developed four key approximations: boundary layer energy quasi equilibrium, conservation of free-tropospheric moist and dry static energies, and the weak temperature gradient approximation. It is demonstrated that essential features of slow, convectively coupled processes can be understood without reference to complex turbulent and microphysical processes, even though accounting for such complexity is essential to quantitatively accurate modeling. In particular, we demonstrate that the robust relationship between column water vapor and precipitation observed over tropical oceans does not necessarily imply direct sensitivity of convection to free-tropospheric moisture. We also show that to destabilize the radiative–convective equilibrium state, feedbacks between radiation and clouds and water vapor must be sufficiently strong relative to the gross moist stability.