Modulation of Atlantic Aerosols by the Madden-Julian Oscillation

Much like the better-known EI Nino-Southern Oscillation, the Madden-Julian Oscillation (MJO) is a global-scale atmospheric phenomenon. The MJO involves periodic, systematic changes in the distribution of clouds and precipitation over the western Pacific and Indian oceans, along with differences in wind intensity over even more extensive areas, including the north and subtropical Atlantic Ocean. The lead authors of this paper developed a sophisticated mathematical technique for mapping the spatial and temporal behavior of changes in the atmosphere produced by the MJO. In a previous paper, we applied this technique to search for modulation of airborne particle amount in the eastern hemisphere associated with the "wet" (cloudy) vs. "dry" phases of the MJO. The study used primarily AVHRR, MODIS, and TOMS satellite-retrieved aerosol amount, but concluded that other factors, such as cloud contamination of the satellite signals, probably dominated the observed variations. The current paper looks at MJO modulation of desert dust transport eastward across the Atlantic from northern Africa, a region much less subject to systematic cloud contamination than the eastern hemisphere areas studied previously. In this case, a distinct aerosol signal appears, showing that dust is transported westward much more effectively during the MJO phase that favors westward-flowing wind, and such transport is suppressed when the MJO reduces these winds. Aside form the significant achievement in identifying such an effect, the result implies that an important component of global dust transport can be predicted based on the phase of the MJO. As a consequence, the impact of airborne dust on storm development in the Atlantic, and on dust deposition downwind of the desert sources, can also be predicted and more accurately modeled

Monsoon Intraseasonal Oscillations as simulated by the Superparameterized Community Atmosphere Model

The relative success of the Community Atmosphere Model with superparameterized convection (SP-CAM) in simulating the space-time characteristics of the Madden Julian Oscillation encourages us to examine its simulation of the Indian summer monsoon and monsoon intraseasonal oscillations (MISOs). While the model simulates the onset and withdrawal of the Indian monsoon realistically, it has a significant wet bias in boreal summer precipitation over the Asian monsoon region. The space-time characteristics of the MISOs simulated by the SP-CAM are examined in detail and compared with those of the observed MISO to gain insight into the model's bias in simulating the seasonal mean. During northern summer, the model simulates a 20 day mode and a 60 day mode in place of the observed 15 and 45 day modes, respectively. The simulated 20 day mode appears to have no observed analog with a baroclinic vertical structure and strong northward propagation over Indian longitudes. The simulated 60 day mode seems to be a lower-frequency version of the observed 45 day mode with relatively slower northward propagation. The model's underestimation of light rain events and overestimation of heavy rain events are shown to be responsible for the wet bias of the model. More frequent occurrence of heavy rain events in the model is, in turn, related to the vertical structure of the higher-frequency modes. Northward propagation of the simulated 20 day mode is associated with a strong cyclonic vorticity at low levels north of the heating maximum associated with a smaller meridional scale of the simulated mode. The simulated vertical structure of heating indicates a strong maximum in the upper troposphere between 200 and 300 hPa. Such a heating profile seems to generate a higher-order baroclinic mode response with smaller meridional structure, stronger low-level cyclonic vorticity, enhanced low-level moisture convergence, and higher precipitation. Therefore, the vertical structure of heating simulated by the cloud-resolving model within SP-CAM may hold the key for improving the precipitation bias in the model

Vertical structure of MJO-related subtropical ozone variations from MLS, TES, and SHADOZ data

Tian et al. (2007) found that the MJO-related total column ozone (O_3) anomalies of 10 DU (peak-to-trough) are mainly evident over the subtropics and dynamically driven by the vertical movement of the subtropical tropopause layer. It was then hypothesized that the subtropical total column O_3 anomalies are primarily associated with the O_3 variability in the stratosphere rather the troposphere. In this paper, we investigate the vertical structure of MJO-related subtropical O_3 variations using the vertical O_3 profiles from the Aura Microwave Limb Sounder (MLS) and Tropospheric Emission Spectrometer (TES), as well as in-situ measurements by the Southern Hemisphere Additional Ozonesondes (SHADOZ) project. Our analysis indicates that the subtropical O_3 anomalies maximize approximately in the lower stratosphere (60–100 hPa). Furthermore, the spatial-temporal patterns of the subtropical O_3 anomalies in the lower stratosphere are very similar to that of the total column. In particular, they are both dynamically driven by the vertical movement of subtropical tropopause. The subtropical partial O_3 column anomalies between 30–200 hPa accounts for more than 50 % of the total O_3 column anomalies. TES measurements show that at most 27 % of the total O_3 column anomalies are contributed by the tropospheric components. This indicates that the subtropical total column O_3 anomalies are mostly from the O3 anomalies in the lower stratosphere, which supports the hypothesis of Tian et al. (2007). The strong connection between the intraseasonal subtropical stratospheric O_3 variations and the MJO implies that the stratospheric O_3 variations may be predictable with similar lead times over the subtropics. Future work could involve a similar study or an O_3 budget analysis using a sophisticated chemical transport model in the near-equatorial regions where the observed MJO signals of total column O_3 are weak

Simulation of the intraseasonal variability over the Eastern Pacific ITCZ in climate models

During boreal summer, convective activity over the eastern Pacific (EPAC) inter-tropical convergence zone (ITCZ) exhibits vigorous intraseasonal variability (ISV). Previous observational studies identified two dominant ISV modes over the EPAC, i.e., a 40-day mode and a quasi-biweekly mode (QBM). The 40-day ISV mode is generally considered a local expression of the Madden-Julian Oscillation. However, in addition to the eastward propagation, northward propagation of the 40-day mode is also evident. The QBM mode bears a smaller spatial scale than the 40-day mode, and is largely characterized by northward propagation. While the ISV over the EPAC exerts significant influences on regional climate/weather systems, investigation of contemporary model capabilities in representing these ISV modes over the EPAC is limited. In this study, the model fidelity in representing these two dominant ISV modes over the EPAC is assessed by analyzing six atmospheric and three coupled general circulation models (GCMs), including one super-parameterized GCM (SPCAM) and one recently developed high-resolution GCM (GFDL HIRAM) with horizontal resolution of about 50 km. While it remains challenging for GCMs to faithfully represent these two ISV modes including their amplitude, evolution patterns, and periodicities, encouraging simulations are also noted. In general, SPCAM and HIRAM exhibit relatively superior skill in representing the two ISV modes over the EPAC. While the advantage of SPCAM is achieved through explicit representation of the cumulus process by the embedded 2-D cloud resolving models, the improved representation in HIRAM could be ascribed to the employment of a strongly entraining plume cumulus scheme, which inhibits the deep convection, and thus effectively enhances the stratiform rainfall. The sensitivity tests based on HIRAM also suggest that fine horizontal resolution could also be conducive to realistically capture the ISV over the EPAC, particularly for the QBM mode. Further analysis illustrates that the observed 40-day ISV mode over the EPAC is closely linked to the eastward propagating ISV signals from the Indian Ocean/Western Pacific, which is in agreement with the general impression that the 40-day ISV mode over the EPAC could be a local expression of the global Madden-Julian Oscillation (MJO). In contrast, the convective signals associated with the 40-day mode over the EPAC in most of the GCM simulations tend to originate between 150°E and 150°W, suggesting the 40-day ISV mode over the EPAC might be sustained without the forcing by the eastward propagating MJO. Further investigation is warranted towards improved understanding of the origin of the ISV over the EPAC

Ocean temperature and salinity components of the Madden-Julian oscillation observed by Argo floats

New diagnostics of the Madden-Julian Oscillation (MJO) cycle in ocean temperature and, for the first time, salinity are presented. The MJO composites are based on 4 years of gridded Argo float data from 2003 to 2006, and extend from the surface to 1,400 m depth in the tropical Indian and Pacific Oceans. The MJO surface salinity anomalies are consistent with precipitation minus evaporation fluxes in the Indian Ocean, and with anomalous zonal advection in the Pacific. The Argo sea surface temperature and thermocline depth anomalies are consistent with previous studies using other data sets. The near-surface density changes due to salinity are comparable to, and partially offset, those due to temperature, emphasising the importance of including salinity as well as temperature changes in mixed-layer modelling of tropical intraseasonal processes. The MJO-forced equatorial Kelvin wave that propagates along the thermocline in the Pacific extends down into the deep ocean, to at least 1,400 m. Coherent, statistically significant, MJO temperature and salinity anomalies are also present in the deep Indian Ocean

Regional Climate Model Evaluation System powered by Apache Open Climate Workbench v1.3.0: an enabling tool for facilitating regional climate studies

The Regional Climate Model Evaluation System (RCMES) is an enabling tool of the National Aeronautics and Space Administration to support the United States National Climate Assessment. As a comprehensive system for evaluating climate models on regional and continental scales using observational datasets from a variety of sources, RCMES is designed to yield information on the performance of climate models and guide their improvement. Here, we present a user-oriented document describing the latest version of RCMES, its development process, and future plans for improvements. The main objective of RCMES is to facilitate the climate model evaluation process at regional scales. RCMES provides a framework for performing systematic evaluations of climate simulations, such as those from the Coordinated Regional Climate Downscaling Experiment (CORDEX), using in situ observations, as well as satellite and reanalysis data products. The main components of RCMES are (1) a database of observations widely used for climate model evaluation, (2) various data loaders to import climate models and observations on local file systems and Earth System Grid Federation (ESGF) nodes, (3) a versatile processor to subset and regrid the loaded datasets, (4) performance metrics designed to assess and quantify model skill, (5) plotting routines to visualize the performance metrics, (6) a toolkit for statistically downscaling climate model simulations, and (7) two installation packages to maximize convenience of users without Python skills. RCMES website is maintained up to date with a brief explanation of these components. Although there are other open-source software (OSS) toolkits that facilitate analysis and evaluation of climate models, there is a need for climate scientists to participate in the development and customization of OSS to study regional climate change. To establish infrastructure and to ensure software sustainability, development of RCMES is an open, publicly accessible process enabled by leveraging the Apache Software Foundation's OSS library, Apache Open Climate Workbench (OCW). The OCW software that powers RCMES includes a Python OSS library for common climate model evaluation tasks as well as a set of user-friendly interfaces for quickly configuring a model evaluation task. OCW also allows users to build their own climate data analysis tools, such as the statistical downscaling toolkit provided as a part of RCMES.</p