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1. Moist Thermodynamics of Convectively Coupled Waves over the Western Hemisphere

   https://doi.org/10.1175/JCLI-D-22-0435.1  

   Mayta, Víctor C. ; Adames, Ángel F. ( May 2023 , Journal of Climate)

   Convectively coupled waves (CCWs) over the Western Hemisphere are classified based on their governing thermodynamics. It is found that only the tropical depressions (TDs; TD waves) satisfy the criteria necessary to be considered a moisture mode, as in the Rossby-like wave found in an earlier study. In this wave, water vapor fluctuations play a much greater role in the thermodynamics than temperature fluctuations. Only in the eastward-propagating inertio-gravity (EIG) wave does temperature govern the thermodynamics. Temperature and moisture play comparable roles in all the other waves, including the Madden–Julian oscillation over the Western Hemisphere (MJO-W). The moist static energy (MSE) budget of CCWs is investigated by analyzing ERA5 data and data from the 2014/15 observations and modeling of the Green Ocean Amazon (GoAmazon 2014/15) field campaign. Results reveal that vertical advection of MSE acts as a primary driver of the propagation of column MSE in westward inertio-gravity (WIG) wave, Kelvin wave, and MJO-W, while horizontal advection plays a central role in the mixed Rossby gravity (MRG) and TD wave. Results also suggest that cloud radiative heating and the horizontal MSE advection govern the maintenance of most of the CCWs. Major disagreements are found between ERA5 and GoAmazon. In GoAmazon, convection is more tightly coupled to variations in column MSE, and vertical MSE advection plays a more prominent role in the MSE tendency. These results along with substantial budget residuals found in ERA5 data suggest that CCWs over the tropical Western Hemisphere are not represented adequately in the reanalysis.

   In comparison to other regions of the globe, the weather systems that affect precipitation in the tropical Western Hemisphere have received little attention. In this study, we investigate the structure, propagation, and thermodynamics of convectively coupled waves that impact precipitation in this region. We found that slowly evolving tropical systems are “moisture modes,” i.e., moving regions of high humidity and precipitation that are maintained by interactions between clouds and radiation. The faster waves are systems that exhibit relatively larger fluctuations in temperature. Vertical motions are more important for the movement of rainfall in these waves. Last, we found that reanalysis and observations disagree over the importance of different processes in the waves that occurred over the Amazon region, hinting at potential deficiencies on how the reanalysis represents clouds in this region.

    

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2. Moisture Recharge–Discharge Cycles: A Gross Moist Stability–Based Phase Angle Perspective

   https://doi.org/10.1175/JAS-D-21-0297.1  

   Maithel, Vijit ; Back, Larissa ( September 2022 , Journal of the Atmospheric Sciences)

   Abstract Moist static energy (MSE) budgets and gross moist stability (GMS) have been widely used as a diagnostic tool to study the evolution of moisture and convection at different time scales. However, use of GMS is limited at shorter time scales because many points in the tropics have close-to-zero large-scale vertical motion at a given time. This is particularly true in the case of convective life cycles, which have been shown to exist with noise-like ubiquity throughout the tropics at intraseasonal time scales. This study proposes a novel phase angle–based framework as a process-level diagnostic tool to study the MSE budgets during these cycles. Using the GMS phase plane, a phase angle parameter is defined, which converts the unbound GMS into a finite ranged variable. The study finds that the convective life cycles are closely linked to evolution of moisture and effectively behave as moisture recharge–discharge cycles. Convective cycles in different datasets are studied using TOGA COARE, a mix of different satellite products and ERA-Interim. Analysis of the MSE budget reveals that the cyclic behavior is a result of transitions between wet and dry equilibrium states and is similar across different regions. Further, vertical and horizontal advection of MSE are found to act as the primary drivers behind this variability. In contrast, nonlinearities in the radiative and surface flux feedbacks are found to resist the convective evolution. A linearized model consistent with moisture mode dynamics is able to replicate the recharge–discharge cycle variability in TOGA COARE data. Significance Statement In the tropics, variability of moisture and rainfall are closely linked to each other. Through this study we aim to better understand the evolution of moisture in observed daily time series data. We present a novel phase angle–based diagnostic tool to represent and study the energy budget of the system at this time resolution. Our results suggest that similar processes and mechanisms are relevant across different regions and at different scales in the tropics with moisture dynamics being important for these processes. Further, a key role is played by the energy transport associated with the large-scale circulation that drives moisture evolution in a cyclic pattern. 

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3. Barotropic and Moisture–Vortex Growth of Monsoon Low Pressure Systems

   https://doi.org/10.1175/JAS-D-22-0252.1  

   Luo, Haochang ; Adames Corraliza, Ángel F. ; Rood, Richard B. ( December 2023 , Journal of the Atmospheric Sciences)

   As one of the most prominent weather systems over the Indian subcontinent, the Indian summer monsoon low pressure systems (MLPSs) have been studied extensively over the past decades. However, the processes that govern the growth of the MLPSs are not well understood. To better understand these processes, we created an MLPS index using bandpass-filtered precipitation data. Lag regression maps and vertical cross sections are used to document the distribution of moisture, moist static energy (MSE), geopotential, and horizontal and vertical motions in these systems. It is shown that moisture governs the distribution of MSE and is in phase with precipitation, vertical motion, and geopotential during the MLPS cycle. Examination of the MSE budget reveals that longwave radiative heating maintains the MSE anomalies against dissipation from vertical MSE advection. These processes nearly cancel one another, and it is variations in horizontal MSE advection that are found to explain the growth and decay of the MSE anomalies. Horizontal MSE advection contributes to the growth of the MSE anomalies in MLPSs prior to the system attaining a maximum amplitude and contributes to decay thereafter. The horizontal MSE advection is largely due to meridional advection of mean state MSE by the anomalous winds, suggesting that the MSE anomalies undergo a moisture–vortex instability (MVI)-like growth. In contrast, perturbation kinetic energy (PKE) is generated through barotropic conversion. The structure, propagation, and energetics of the regressed MLPSs are consistent with both barotropic and moisture–vortex growth.

    

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4. Easterly Waves in the East Pacific during the OTREC 2019 Field Campaign

   https://doi.org/10.1175/JAS-D-21-0128.1  

   ( November 2021 , Journal of the Atmospheric Sciences)

   Easterly waves (EWs) are off-equatorial tropical synoptic disturbances with a westward phase speed between 11 and 14 m s⁻¹. Over the east Pacific in boreal summer, the combination of EWs and other synoptic disturbances, plus local mechanisms associated with sea surface temperature (SST) gradients, define the climatological structure of the intertropical convergence zone (ITCZ). The east Pacific ITCZ has both deep and shallow convection that is linked to deep and shallow meridional circulations, respectively. The deep convection is located around 9°N over warm SSTs. The shallow convection is located around 6°N and is driven by the meridional SST gradient south of the ITCZ. This study aims to document the interaction between east Pacific EWs and the deep and shallow meridional circulations during the Organization of Tropical East Pacific Convection (OTREC) field campaign in 2019 using field campaign observations, ERA5, and satellite precipitation. We identified three EWs during the OTREC period using precipitation and dynamical fields. Composite analysis shows that the convectively active part of the EW enhances ITCZ deep convection and is associated with an export of column-integrated moist static energy (MSE) by vertical advection. The subsequent convectively suppressed, anticyclonic part of the EW produces an increase of moisture and column-integrated MSE by horizontal advection that likely enhances shallow convection and the shallow overturning flow at 850 hPa over the southern part of the ITCZ. Therefore, EWs appear to strongly modulate shallow and deep circulations in the east Pacific ITCZ.

    

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5. Intermodel Spread in Walker Circulation Responses Linked to Spread in Moist Stability and Radiation Responses

   https://doi.org/10.1029/2022JD037382  

   Duffy, Margaret L. ; O’Gorman, Paul A. ( January 2023 , Journal of Geophysical Research: Atmospheres)

   The response of the Pacific Walker circulation (WC) to long‐term warming remains uncertain. Here, we diagnose contributions to the WC response in comprehensive and idealized general circulation model (GCM) simulations. We find that the spread in WC response is substantial across both the Coupled Model Intercomparison Project (CMIP6) and the Atmospheric Model Intercomparison Project (AMIP) models, implicating differences in atmospheric models in the spread in projected WC strength. Using a moist static energy (MSE) budget, we evaluate the contributions to changes in the WC strength related to changes in gross moist stability (GMS), horizontal MSE advection, radiation, and surface fluxes. We find that the multimodel mean WC weakening is mostly related to changes in GMS and radiation. Furthermore, thespreadin WC response is related to the spread in GMS and radiation responses. The GMS response is potentially sensitive to parameterized convective entrainment which can affect lapse rates and the depth of convection. We thus investigate the role of entrainment in setting the GMS response by varying the entrainment rate in an idealized GCM. The idealized GCM is run with a simplified Betts‐Miller convection scheme, modified to represent entrainment. The weakening of the WC with warming in the idealized GCM is dampened when higher entrainment rates are used. However, the spread in GMS responses due to differing entrainment rates is much smaller than the spread in GMS responses across CMIP6 models. Therefore, further work is needed to understand the large spread in GMS responses across CMIP6 and AMIP models.

    

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