Madden–Julian Oscillation Strength Research Articles

Sensitivity of simulated MJO to model vertical resolution in GAMIL3

AbstractThe impact of model vertical resolutions on simulation of the Madden–Julian Oscillation (MJO) was investigated using five AMIP simulations by the Grid‐point Atmospheric Model of IAP LASG, version 3 (GAMIL3) with different vertical layers. Results showed that higher vertical resolutions produce a stronger and superior eastward propagation, coupled circulation–convection relationship, and MJO strength, as well as other convectively coupled equatorial waves when compared to the lowest vertical resolution. The improvements may be related to a better description of the tropical circulation in the higher vertical resolutions and model top, albeit without the significant improvement of MJO convection and stratospheric quasi‐biennial oscillation in all simulations. Among the four tested high resolutions, the simulations with higher vertical resolutions from the surface to about 850 hPa produced better eastward propagation and larger total explained variance of the MJO, indicating the importance of the lower troposphere in simulating the MJO.

Interdecadal variability of intensity of the Madden–Julian oscillation

AbstractThe interdecadal variability of intensity of the Madden–Julian oscillation (MJO) during boreal winter was investigated based on two reanalysis datasets (NOAA 20CR and ERA‐20C). Both the reanalysis datasets reveal statistically significant power spectrum peaks at 12–20‐year periods. A composite analysis shows that during the active interdecadal phase, eastward propagating MJO activity was strengthened over the equatorial warm pool expanded from the Indian Ocean (IO) to the western Pacific (WP). The cause of the enhanced MJO variability was attributed to the interdecadal change of background mean precipitation in situ. A further diagnosis shows that the increase of the background precipitation over the Maritime Continent and the WP was attributed to interdecadal warm sea surface temperature (SST) anomalies in situ, whereas the increase of the precipitation over the IO resulted from the local convective instability due to the increase of wind induced surface evaporation and moisture. The strengthened background precipitation impacted the MJO strength through the increase of the interdecadal background moisture in the lower and middle troposphere induced by anomalous vertical moisture advection.

Barrier Effect on MJO Propagation by the Maritime Continent in the MJO Task Force/GEWEX Atmospheric System Study Models

ABSTRACTThe simulated Madden–Julian oscillation (MJO) events in 27 general circulation models (GCMs) are identified using an MJO tracking method. The results suggest that the occurrence frequencies of simulated MJO events can represent a model’s ability to simulate several characteristics of the MJO to a certain extent during boreal winter, such as propagation range, strength, and termination longitude. All tracked MJO events are classified into those that propagate through the Maritime Continent (MC) (MJO-C) and those that do not (MJO-B), and the weakening and blocking effects on MJO propagation by the MC in GCMs were quantified. In general, if a GCM shows a stronger weakening effect on MJO strength over the MC, it tends to produce a stronger blocking effect on MJO propagation over the MC during boreal winter. The barrier effect of the MC on MJO propagation is exaggerated in most GCMs, while it can be underestimated in some GCMs, especially the coupled GCMs. Strong lower-tropospheric premoistening is identified ahead of the MJO convection center when it is over the central MC for MJO-C but not for MJO-B in most GCMs. Such strong premoistening is mainly attributed to the zonal gradient of lower-tropospheric easterly anomalies within the front Walker cell, which could be a precursor leading to the eastward propagation of MJO convection. In contrast to the observation, the role of the background sea surface temperature and land–sea precipitation contrast in the barrier effect on MJO propagation by the MC is not well captured by most GCMs.

Role of Madden–Julian Oscillation in Modulating Monsoon Retreat

The Madden–Julian oscillation (MJO) is the major fluctuation in tropical weather on a seasonal scale. The impact of MJO on different epochs, viz., onset, advance and active break is well known. There can be several MJO events in a season and it may enhance/suppress the retreat process. The present study aims to find the MJO-modulated retreat of monsoon. The results suggest that the fastest retreat of monsoon occurred in the years 2007 and 2008, while slowest retreat of monsoon occurred in the year 1979. The retreat features of the Indian summer monsoon (ISM) are investigated with the MJO phase and amplitude variations. The daily MJO indices for the retreat period 1979–2016 are used. The results reveal that the MJO strength decreases during the transition phase (i.e., summer monsoon to winter monsoon transition). The monsoon retreat is most favored by strong MJO phase 4 and phase 5. The fastest retreat of monsoon occurred in the years 2007 and 2008, while the slowest retreat of monsoon occurred in the year 1979. There exists a weak positive correlation between the MJO amplitude and the retreat period of monsoon. The monsoon retreat is most favored by strong MJO phase 4 and phase 5. The MJO amplitude variations during MJO phases 1–8 suggest that the MJO amplitude decreases with increase in retreat period. The MJO-modulated retreat results in slow retreat of monsoon, whereas fast and normal retreat of monsoon is seen on rare occasions. Weak MJO events lead to normal retreat of monsoon.

Factors controlling the seasonality of the Madden-Julian Oscillation

By analyzing observational data, previous studies have indicated that the tropical Madden-Julian Oscillation (MJO) is active during the boreal winter but relatively weak during the boreal summer. However, the factors that control seasonal MJO variation are not clear. To quantitatively understand the relative contributions of the occurrence frequency of enhanced MJO events and their averaged strength and lifespan to seasonal MJO amplitude, we defined the MJO events of 1979–2014 and analyzed their features in different seasons by using the Real-time Multivariate MJO (RMM) index and the newly proposed RMM-r index. The results indicate that the MJO events show a higher frequency of occurrence, a stronger intensity and a longer duration during the boreal winter (Dec.–Feb.) and spring (Mar.–May). However, the frequency, strength and lifespan of MJO events are all reduced during the boreal summer (Jun.–Aug.) and autumn (Sep.–Nov.). The enhanced MJO events in winter–spring also show a large ratio of variance for eastward to westward components. To elucidate how large-scale background fields affect seasonal MJO variation, a series of sensitivity experiments was conducted by using a 2.5-layer model that can simulate MJO-like features. It is found that the variation in low-level moisture (vertical wind shear) is the key large-scale factor affecting the seasonal variation in MJO strength (in propagation). In comparison with the summer–autumn seasons when the MJO is relatively weakened, the relatively abundant low-level moisture near the equator during boreal winter–spring may strengthen the development of MJO convection and circulation, whereas the relatively weak easterly shear (or the westerly shear anomaly) is conducive to the enhancement of an eastward-propagating MJO component.

Factors controlling the interannual variations of MJO intensity

The interannual variations of intensity of the Madden-Julian Oscillation (MJO) during boreal winter are investigated by using the observed outgoing longwave radiation (OLR) and the reanalysis data of ECMWF and NCEP. The standard deviation of 20-80-day filtered OLR anomaly is used to measure the MJO intensity. The dominant spatial structure of the interannual variability is revealed by an EOF analysis of the MJO intensity field. It is found that the leading mode is associated with eastern Pacific type ENSO, whereas the second mode is related to central Pacific type ENSO. A simple atmospheric model is used to investigate the relative roles of background moisture and wind changes in affecting the overall strength of MJO. The numerical experiments indicate that the background moisture effect is dominant while the background wind change has a minor effect.

The Effects of Explicit versus Parameterized Convection on the MJO in a Large-Domain High-Resolution Tropical Case Study. Part II: Processes Leading to Differences in MJO Development

High-resolution simulations over a large tropical domain (~20°S–20°N, 42°E–180°) using both explicit and parameterized convection are analyzed and compared during a 10-day case study of an active Madden–Julian oscillation (MJO) event. In this paper, Part II of this study, the moisture budgets and moist entropy budgets are analyzed. Vertical subgrid diabatic heating profiles and vertical velocity profiles are also compared; these are related to the horizontal and vertical advective components of the moist entropy budget, which contribute to gross moist stability (GMS) and normalized GMS (NGMS). The 4-km model with explicit convection and good MJO performance has a vertical heating structure that increases with height in the lower troposphere in regions of strong convection (like observations), whereas the 12-km model with parameterized convection and a poor MJO does not show this relationship. The 4-km explicit convection model also has a more top-heavy heating profile for the troposphere as a whole near and to the west of the active MJO-related convection, unlike the 12-km parameterized convection model. The dependence of entropy advection components on moisture convergence is fairly weak in all models, and differences between models are not always related to MJO performance, making comparisons to previous work somewhat inconclusive. However, models with relatively good MJO strength and propagation have a slightly larger increase of the vertical advective component with increasing moisture convergence, and their NGMS vertical terms have more variability in time and longitude, with total NGMS that is comparatively larger to the west and smaller to the east.

Modulation of Daily Precipitation over East Africa by the Madden–Julian Oscillation*

Abstract Spatiotemporal variability in East African precipitation affects the livelihood of tens of millions of people. From the perspective of floods, flash droughts, and agriculture, variability on intraseasonal time scales is a critical component of total variability. The principal objective of this study is to explore subseasonal impacts of the Madden–Julian oscillation (MJO) on tropospheric circulations affecting East Africa (EA) during the long (March–May) and short (October–December) rains and associated variability in precipitation. Analyses are performed for 1979–2012 for dynamics and 1998–2012 for precipitation. Consistent with previous studies, significant MJO influence is found on wet and dry spells during the long and short rains. This influence, however, is found to vary within each season. Specifically, indices of MJO convection at 70°–80°E and 120°W are strongly associated with precipitation variability across much of EA in the early (March) and late (May) long rainy season and in the middle and late (November–December) short rainy season. In the early short rains (October) a different pattern emerges, in which MJO strength at 120°E (10°W) is associated with dry (wet) spells in coastal EA but not the interior. In April the MJO influence on precipitation is obscured but can be diagnosed in lead time associations. This diversity of influences reflects a diversity of mechanisms of MJO influence, including dynamic and thermodynamic mechanisms tied to large-scale atmospheric circulations and localized dynamics associated with MJO modulation of the Somali low-level jet. These differences are relevant to problems of subseasonal weather forecasts and climate projections for EA.

A comparative analysis of intraseasonal atmospheric variability in OLR and 1DD GPCP rainfall data over Central Africa

The spatial and temporal structures of the leading modes of intraseasonal atmospheric variability over Central Africa using outgoing longwave radiation (OLR) and one-degree daily Global Precipitation Climatology Project (1DD GPCP) data are compared. A spectral analysis indicates that in the two datasets, the intraseasonal variability is dominated by 20–70-day period bands with center near 40–50 days. Results from empirical orthogonal function (EOF) analysis have shown that three main spatial structures characterize the leading modes of 20–70-day intraseasonal oscillations (ISO), but they differ in the two data by the amount of variance explained by each mode. For both time series, the power spectra of all the three principal components peak around 40–50 days, indicating Madden–Julian oscillation (MJO) signal. Moreover, the cross correlation computed among the principal components indicates that there exists a relatively high positive correlation between the EOFs of similar spatial loadings. For the two datasets, an index of MJO strength was built by averaging the 30–50-day power for each day and each EOF mode. A plot of the two indices revealed a coherent onset, peak, and decay of ISO activity but with different amplitudes. The correlation coefficients computed between the ISO indices corresponding to each mode revealed that they are highly correlated, especially for the annual mean time series where the correlation coefficient reaches up to 0.88 for the Tanzanian mode. Overall, the analysis showed that the leading modes are similar, but the 1DD GPCP loadings have better spatial localization, when compared with those of OLR datasets.

The Effects of Explicit versus Parameterized Convection on the MJO in a Large-Domain High-Resolution Tropical Case Study. Part I: Characterization of Large-Scale Organization and Propagation*

Abstract High-resolution simulations over a large tropical domain (~20°S–20°N, 42°E–180°) using both explicit and parameterized convection are analyzed and compared to observations during a 10-day case study of an active Madden–Julian oscillation (MJO) event. The parameterized convection model simulations at both 40- and 12-km grid spacing have a very weak MJO signal and little eastward propagation. A 4-km explicit convection simulation using Smagorinsky subgrid mixing in the vertical and horizontal dimensions exhibits the best MJO strength and propagation speed. Explicit convection simulations at 12 km also perform much better than the 12-km parameterized convection run, suggesting that the convection scheme, rather than horizontal resolution, is key for these MJO simulations. Interestingly, a 4-km explicit convection simulation using the conventional boundary layer scheme for vertical subgrid mixing (but still using Smagorinsky horizontal mixing) completely loses the large-scale MJO organization, showing that relatively high resolution with explicit convection does not guarantee a good MJO simulation. Models with a good MJO representation have a more realistic relationship between lower-free-tropospheric moisture and precipitation, supporting the idea that the moisture–convection feedback is a key process for MJO propagation. There is also increased generation of available potential energy and conversion of that energy into kinetic energy in models with a more realistic MJO, which is related to larger zonal variance in convective heating and vertical velocity, larger zonal temperature variance around 200 hPa, and larger correlations between temperature and ascent (and between temperature and diabatic heating) between 500 and 400 hPa.

The madden-julian oscillation as simulated by the mpi earth system model: Over the last and into the next millennium

[1] The Madden-Julian oscillation (MJO), as represented by the Max Planck Institute for Meteorology Earth System Model (MPI-ESM), is analyzed for the first time over time periods ranging from decades to more than a millennium. Particular attention is paid to the behavior of the MJO index as calculated from the leading pair of empirical orthogonal functions (EOFs) derived from a multivariate EOF analysis. The analysis of 1000 year simulations with the MPI-ESM and its predecessor reveals significant interannual (2–6 years) to interdecadal (10–20 years) internal variability of the MJO but relatively little evidence of significant variability at longer timescales in unforced runs. A 1200 year experiment forced by the best estimates of solar variability, volcanism, and changing atmospheric composition indicates that the MJO simulated in the twentieth century is very similar to the MJO simulated since AD 800. The analysis of sensitivity experiments shows the influence of different external forcings: solar variability may contribute to MJO variability on 11 and 22 year periods, but this is difficult to separate from internal variability; and there is a hint of enhanced decadal variability associated with volcanic forcing. Land use change and changes associated with anthropogenic forcing over the twentieth century have no detectable effect on the simulated MJO. An increase of the CO2 concentrations by 1% per year starting in AD 1850 leads to an increase in the MJO strength in the twenty-first century, as does the warming associated with an abrupt quadrupling of the atmospheric CO2 concentration, suggesting that the MJO may intensify with warming.

Intraseasonal atmospheric variability and its interannual modulation in Central Africa

The spatial and temporal structures of the intraseasonal atmospheric variability over central Africa is investigated using 2.5° × 2.5° daily outgoing longwave radiation (OLR) and National Centers for Environmental Prediction (NCEP) Reanalysis zonal winds for the period 1980–2010. The study begins with an overview of the Central African rainfall regime, noting in particular the contrast amongst Western and Eastern parts, with different topography and surface conditions features. The annual mean rainfall and OLR over the region revealed a zone of intense convective activity centered on the equator near 30°E, which extends southward and covers almost all the Congo forest. The annual cycle of rainfall reflects the classical bi-annual shift of Inter-Tropical Convergence Zone across the equatorial belt, between 10°S and 10°N. The result of the empirical orthogonal functions (EOFs) analysis has shown that the three leading EOF modes explain about 45 % of total intraseasonal variability. The power spectra of all the three corresponding principal components (PCs) peak around 45–50 days, indicating a Madden–Julian Oscillation (MJO) signal. The first mode exhibits high positive loadings over Northern Congo, the second over Southern Ethiopia and the third over Southwestern Tanzania. The PCs time series revealed less interannual modulation of intraseasonal oscillations for the Congo mode, while Ethiopian and Tanzanian modes exhibit strong interannual variations. Hovmoller plots of OLR, 200 and 850 hPa NCEP zonal winds found the eastward propagating features to be the dominant pattern in all the three times series, but this propagation is less pronounced in the OLR than in the 850 and 200 hpa zonal wind anomalies. An index of MJO strength was built by averaging the 30–50 day power for each day. A plot of MJO indices and El Nino Southern Oscillation (ENSO) cycle confirm a strong interannual modulation of MJO over Eastern central Africa partially linked with the ENSO events (El Nino and La Nina). Strong MJO activity is observed during La Nina years or during ENSO-neutral years, while weak or absent MJO activity is typically associated with strong El Nino episodes.

Kelvin wave time scale propagation features of the Madden-Julian Oscillation (MJO) as measured by the Chen-MJO index

[1] Kelvin wave time scale propagation features in an indexed measure of the strength of the Madden-Julian Oscillation (MJO) are investigated. Examination of reanalysis data and Chen-MJO index data for three 14 May to 21 September periods shows that recurring events near Sumatra frequently associated with convectively coupled (CC) Kelvin waves tend to correspond in a time-lagged manner with increases in indexed MJO strength at various longitudes. The events, characterized by low-level divergence within a cyclonic circulation off the west coast of Sumatra, combined with northward advances in the dynamic equator, tend to lag increases in indexed MJO strength by several days in the Indian Ocean and Maritime Continent and to lead such increases farther to the east. Maximum time-lagged correlation coefficients between increases in indexed MJO strength and a measure of the strength of the divergence events near Sumatra are consistent with a disturbance propagating eastward at Kelvin wave phase speeds. A similar signal is demonstrated in a 32 year data set of the Chen-MJO index by examining time-lagged correlations between increases in indexed MJO strength at various longitudes and corresponding increases for selected events at 80°E and 160°E. The results suggest an enhanced rate of increase in MJO strength as the apparent Kelvin wave signal passes, roughly marking the beginning of an extended period of MJO growth. Potential limitations of the current analysis are discussed. A look at a particular MJO episode provides evidence consistent with the statistical results, while supporting a role of sequential CC Kelvin wave passages in the propagation of the MJO.