Vertical shear of the zonal wind across the equatorial tropopause over the Indian Ocean to the Maritime Continent is caused by a combination of the quasi-biennial oscillation (QBO) of the stratosphere and the seasonal cycle and interannual variability of the upper troposphere. The Madden–Julian Oscillation (MJO) has been previously observed to be more active during the easterly than the westerly phase of the QBO. Kelvin waves interacting with the background flow explain most of the propagation characteristics of the MJO in the equatorial upper troposphere. This work assesses the hypothesis that Kelvin wave propagation under conditions of easterly wind in both the upper troposphere and stratosphere maintains the upper tropospheric MJO circulation, but that this signal is disrupted with westerly wind shear that likely includes critical layers that would prevent Kelvin wave energy from passing. Linear regression of reanalysis data against an MJO index shows more coherent downward propagating Kelvin waves during easterly shear and no Kelvin-wave-like signal near the tropopause during conditions expected to include critical layers there. Historical analysis of this vertical shear shows that it is the primary focus of enhanced MJO variance with the easterly QBO, yielding the seasonally enhanced signal December through February and the erratic variability from year to year due to tropospheric contributions to shear. A wavenumber frequency spectrum analysis of lower stratospheric zonal wind shows that power shifts from high to low frequency between QBO westerly to easterly phases, consistent with Kelvin waves propagating at the phase speed range of the MJO during easterly QBO.

This work hypothesizes that the Madden-Julian oscillation (MJO) alters the vertical structure of synoptic convectively coupled Kelvin waves in a manner that allows nonlinear advection by the altered Kelvin waves to feed back onto the MJO. Through wavelet decomposition and linear regression, this work extracts the winds, specific humidity, and temperature signals associated with a Kelvin wave of phase speed 9 m s21 and wavenumber 6 over the east Indian Ocean during the locally enhanced and suppressed convective phases of the MJO. Results support the idea of a feedback cycle, suggesting that the MJO modifies the background state of the Kelvin wave, altering its vertical structure. In turn, Kelvin wave nonlinear advection 1) accelerates MJO westerlies and decelerates MJO easterlies; 2) moistens the MJO in the mid-troposphere but dries the MJO in the lower troposphere; and 3) warms the MJO in the lower and upper troposphere but cools the MJO in the midtroposphere by about 10%. We expect this percent contribution to increase when looking at a wider range of slow-moving Kelvin wave frequencies. This work establishes strong interactions between the MJO and slow-moving embedded Kelvin waves, which calls attention for future works on Kelvin wave behavior in the previously ignored spectral region traditionally separating synoptic Kelvin waves from the MJO.

The Madden-Julian Oscillation (MJO) varies seasonally. Both moist and dry dynamical processes would contribute to this seasonality. Previous results have suggested strong dependence of MJO phase speed on planetary-scale upper tropospheric Kelvin waves interacting with the mean flow. Composites and phase speed spectra assess the association between the Indian Basin MJO circulation and convection with variations in equatorial upper tropospheric background wind patterns, including seasonal variability. Results show that the fastest eastward propagation over the Indian Ocean (>10 ms(-1)) tends to occur during northern spring when background upper tropospheric easterlies are weakest. Northern winter signals typically advance eastward between 4 and 10 ms(-1). Strong easterly background wind conditions during northern summer usually prevent propagation eastward along the equator from the Western Indian Ocean. Results also show relative amplitude variations between the MJO's upper and lower tropospheric zonal wind signals, with the upper tropospheric circulation signals being disproportionately stronger than the lower tropospheric ones over the Western Hemisphere to East Africa. The upper tropospheric easterly wind anomalies grow over the Western Indian Ocean first, as specific humidity increases in lower tropospheric easterly wind to the east. Then, lower tropospheric westerly wind emerges west of the emerging convection, suggesting that lower tropospheric wind change depends more directly on moist processes than the upper tropospheric wind.

Fields of equatorial upper-tropospheric circulation data are lag regressed against wavelet-filtered indices of upper-tropospheric zonal wind anomalies over a range of phase speeds at 50-day periods to study the propagation mechanisms of the upper-tropospheric circulation signal of the Madden-Julian oscillation (MJO) over the Indian Ocean. Results show that the MJO upper-tropospheric zonal wind is accelerated by the geopotential gradient force in quadrature with the existing wind anomaly, yielding its eastward propagation. This effect is offset by Doppler advection of the MJO wind by the easterly background wind. Divergence of mass by the zonal wind propagates the associated geopotential height anomalies in concert with the winds. Results confirm that advection of background wind by the MJO wind amplifies MJO wind anomalies in phase with those anomalies where the background wind is zonally confluent and breaks it down in regions of diffluent background wind. Coincidence of the mass source driven by moist convection and zonally diffluent zonal wind anomalies with falling geopotential height relative to the regions east and west is consistent with planetary-scale Kelvin wave zonal wind signals providing favorable conditions for convection. Circulation data at 100 hPa confirm approximately dry Kelvin wave dynamics driving MJO-associated equatorial circulations eastward.

Atmospheric equatorial Kelvin waves-convective disturbances that manipulate tropical wind and rainfall patterns-can propagate eastward at speeds ranging from nearly stationary to 30 m/s, with variability determined by moist processes and advection by the background wind. Current studies on Kelvin waves lack a comprehensive climatology that explains how their structure and propagation speeds change in different background states. Thus, this work builds a variable regression model that uses ERA5 reanalysis data to reconstruct Kelvin waves during different background wind shear conditions and phases of the Madden-Julian Oscillation (MJO) and the El Ni & ntilde;o-Southern Oscillation (ENSO) over the Pacific. Overall, Kelvin waves tend to speed up during background conditions that generate upper-tropospheric westerlies and slow down during upper-tropospheric easterlies. East Pacific Kelvin waves are faster than West Pacific Kelvin waves because of climatological westerly shear in the former and easterly shear in the latter. However, strong westerly shear over the East Pacific allows extratropical Rossby waves to impede on the Kelvin wave, while strong easterly shear over the West Pacific distorts classical Kelvin wave structure. The results provide references for weather prediction models to accurately resolve the interaction between Kelvin waves and background circulation.

Previous research has shown that the equatorial upper tropospheric circulation signal associated with the Madden Julian Oscillation (MJO) over the Indian Ocean behaves like a Kelvin wave, with the eastward-propagation of the associated zonal wind anomaly caused by acceleration by the geopotential gradient force in quadrature with the wind anomaly, with the resultant signal amplified or decayed as the MJO wind advects background zonal wind in regions of background confluent or diffluent flow, with its phase speed adjusted by Doppler shifting by background zonal wind. This paper assesses these previously diagnosed mechanisms in the GEFS V12 forecast model, showing that similar mechanisms occur with the model MJO, but weaker and less organized. Results suggest that, relative to the validation data, the model stalls the MJO upper tropospheric zonal wind anomaly during diffluent background conditions near the Maritime Continent and weakens its amplitude more rapidly than validation data. The stalled propagation leads the model MJO to persist the signals of advection of and by the background wind, but at a reduced rate as the scale of the model MJO wind anomaly diminishes. Results show that beyond the stronger Doppler effect in the model, stalling the model MJO results from filling of the relative geopotential trough collocated with the MJO easterly wind anomaly, leading to breakdown of the geopotential gradient force term that is responsible for propagation of the wind anomaly. Thus, when the Maritime Continent region experiences zonally diffluent flow, the reason for the stronger Maritime Continent barrier effect in the model is that the model does not persist the Kelvin wave propagation mechanism that continues the eastward movement of the easterly wind anomaly in observations.

The variability of the phase speed of the Madden-Julian oscillation (MJO) is poorly understood. The authors assess how the phase speed of the convective signal of the MJO associates with the background states over eastern Africa and the Indian Ocean. Relaxation of the coupling between tropical modes and their circulation has been previously linked to faster propagation; for example, the MJO speeds up over the eastern Pacific where its convective signal decouples from the circulation. In contrast, our results show that fast MJO events happen to exist during periods of wetter background states (>90 days) from East Africa across the Indian Ocean, whereas slow MJO is associated with dry background states. We found that fast MJO exhibits strong active and inactive phases with a structure suggesting more hierarchical convection. Results indicate that the association of the phase speed of the MJO as seen in the integrated fi ltered moist static energy with its tendency is stronger than the association of the phase speed as observed in the dry static energy with its tendency which is consistent with the acceleration of the MJO during wet background states. Also, our results indicate that the MJO may be faster during periods of enhanced low-level moisture because these periods have anomalously weak upper-tropospheric easterly background winds, which reduce the westward advection of the MJO by the background easterly wind, resulting in higher eastward phase speed of the MJO. The acceleration of the MJO by the background zonal wind overwhelms the deceleration associated with the moist-wave dynamics.

The rate of change of temperature during 1979-2020 is assessed as a continuous function of specific humidity for the tropical lower troposphere using ERA5 reanalysis data and a modulation regression technique. Statistical stability is assessed through a Monte Carlo experiment and for consistency with bulk data through bivariate histograms that show how the broader distributions of moisture and temperature have evolved together over time. Reanalysis ensemble spread is also assessed. Results show that warming rates exceed 2 K century21 in dry air over land and water at 900 hPa but are significantly smaller in moist air over water and near zero in the moistest air over land. Cooling at 900 hPa is demonstrated for the global tropics and for the tropical oceans near 1000 hPa at specific humidities near a mass fraction of 0.01. These cooling results may be driven by decadal internal variability or a forced response to CO2 in the southeast Pacific basin. Similar cooling does not occur over land.

Analysis of seasonally varying signals of model error biases, as well as the diagnosis of space-time patterns of synoptic error, here is accomplished through the development of a statistical postprocessing algorithm based on time- extended empirical orthogonal functions (EEOFs) built on patterns of variance. This work analyzes GEFSv12 200-hPa geopotential height (Z200) model error against ERA5 reanalysis data. The mean-square error variance between GEFSv12 reforecast and ERA5 grows rapidly after day 7 of a forecast and continues to increase through the end of the 16-day forecast period. At lead time 16, the largest variance occurs in mid- to high-latitude oceanic storm tracks. Variance is highest during hemispheric winter when baroclinic energy is abundant. The seasonal cycle of error shows largest anomalies in the high latitudes during hemispheric winter. After running the titular two-step, space-time EEOF algorithm, a standardized spatial eigenspectrum shows that eigenvalues increase at longer lead times after decreasing in the medium range, demonstrating that the algorithm extracts large-scale signals of systematic error in the long range. Leading wintertime space-time eigenmode pairs include high-latitude blocking structures and Rossby wave trains. Results suggest that the large-scale systematic errors in GEFSv12 Z200 initiate in part from inaccurate representations of phase speeds of atmospheric waves in the polar jet.

Eastward-moving moist deep convection and atmospheric circulation signals associated with the tropical Madden-Julian oscillation (MJO) sometimes break down as they cross the Maritime Continent region, but other times, the signal propagates across the region maintaining amplitude or regaining it over the west Pacific basin. This paper assesses the hypothesis that upper-tropospheric zonal diffluence of the background wind over the Maritime Continent causes much of this Maritime Continent barrier effect and its variation over time, through two mechanisms: 1) by slowing down the MJO as stronger-than-average background upper-tropospheric zonal wind over the Indian Ocean advects the MJO circulation signal westward, slowing its eastward advance, and 2) through the zonal advection of the background wind by subseasonal zonal wind across a region of zonal diffluence of the background wind, which advects the background wind of the opposite sign to the MJO wind. Advection of the opposite-signed background wind counteracts the MJO wind and reduces its associated upper-tropospheric mass divergence, weakening the mechanisms of the upper-tropospheric Kelvin wave component of the MJO circulation. Composites of MJO-associated zonal wind and outgoing longwave radiation signals diminish as they cross the Maritime Continent region when the region's background zonal winds are diffluent, and composites of data reconstructing the relevant advection terms reveal the direct action of the advection mechanisms.

The time extended Empirical Orthogonal Function (EEOF) patterns of error developed in a companion paper are applied in this study for model data postprocessing. Projecting forecast anomalies from the model seasonal cycle from GEFSv12 outputs onto these EEOF patterns reconstructs the error principal components (PCs) in a manner that can be applied in realtime. When these reconstructed error anomalies are compared with original GEFSv12 200 hPa Geopotential Height (Z200) reforecast error anomalies, the projected and validation error signals tend to align temporally. When distributed globally, projection and validation error anomalies have statistically significant positive Pearson Correlation coefficients with each other at most grid points outside of the tropics. Categorical Heidke Skill Score (HSS) analysis reaffirms that by lead day 16 of a forecast, the algorithm skillfully predicts systematic errors for locations outside of the tropics. Hemispheric winter demonstrates the highest magnitude HSS values in the high latitudes, illustrating the underlying seasonality of Z200 behavior and predictability. On average, wintertime HSS values hover around 20%, with high latitude locations exceeding mean values of 40%. Comparatively summertime values generally exceed 10% improvement outside of the tropics. High latitude storm track regions, such as the North Atlantic, North Pacific, and South Pacific contain the most Z200 variance in the globe and the highest HSS values. This algorithm will be applied to realtime forecasts and additional variables in future work, with the end goal of implementation into the NOAA-CPC subseasonal forecasts.

The Madden-Julian oscillation (MJO) propagates eastward as a disturbance of mostly zonal wind and precipitation along the equator. The initial diagnosis of the MJO spectral peak at 40-50 -day periods suggests a reduction in amplitude associated with slower MJO events that occur at lower frequencies. If events on the low -frequency side of the spectral peak continued to grow in amplitude with reduced phase speed, the spectrum would just be red. Wavelet regression analysis of slow and fast eastward -propagating MJO signals during northern winter assesses how associated moisture and wind patterns could explain why slow MJO events achieve lower amplitude in tracers of moist convection. Results suggest that slow MJO events favor a ridge anomaly over Europe, which drives cool dry air equatorward over Africa and Arabia as the active convection develops over the Indian Ocean. We hypothesize that dry air tracing back to this source, together with a longer duration of the events, leads to associated convection diminishing along the equator and instead concentrating in the Rossby gyres off the equator.

The time-extended empirical orthogonal function (EEOF) patterns of error developed in a companion paper are applied in this study for model data postprocessing. Projecting forecast anomalies from the model seasonal cycle from GEFSv12 outputs onto these EEOF patterns reconstructs the error principal components (PCs) in a manner that can be applied in real time. When these reconstructed error anomalies are compared with the original GEFSv12 200-hPa geopotential height (Z200) reforecast error anomalies, the projected and validation error signals tend to align temporally. When distributed globally, projection and validation error anomalies have statistically significant positive Pearson correlation coefficients with each other at most grid points outside of the tropics. Categorical Heidke skill score (HSS) analysis reaffirms that by lead day 16 of a forecast, the algorithm skillfully predicts systematic errors for locations outside of the tropics. Hemispheric winter demonstrates that the highest magnitude HSS values are in the high latitudes, illustrating the underlying seasonality of Z200 behavior and predictability. On average, wintertime HSS values hover around 20%, with high-latitude locations exceeding mean values of 40%. Comparatively, summertime values generally exceed 10% improvement outside of the tropics. High-latitude storm-track regions, such as the North Atlantic, North Pacific, and South Pacific, contain the most Z200 variance in the globe and the highest HSS values. This algorithm will be applied to real-time forecasts and additional variables in future work, with the end goal of implementation into the NOAA-CPC subseasonal forecasts.

<p>Stratospheric Kelvin waves are known to be absorbed by the background flow via mechanical and thermal damping and, to less extent, by the critical layer interaction. Critical layer interaction occurs when the Kelvin waves' phase speed approaches the background flow's speed. This study aims to depict the structure of the Kelvin waves while approaching the critical layer, where the phase speed of the wave matches the speed of the background flow. In the time domain, the wavelet filtering technique filters Kelvin waves at a specific location and phase speeds using ERA-I zonal wind. Linear regression yields the pattern of specific phase speed's Kelvin wave. Yet, the critical layer interaction of the Kelvin waves with the environmental flow could be studied by choosing a background environment in which its flow speed matches the wave's phase speed, which could be implemented using the varying-coefficient regression technique. We found that the in-phase relationship between the zonal wind and height, associated with the structure of the Kelvin waves, relaxes with the decreasing of the Doppler-shifted speed; then, at a further reduction of the Doppler-shifted speed, the Gill pattern appears. Furthermore, Kelvin waves were found to be absent under an environment of westerly shear.</p>

Kelvin waves have been theorized to be absorbed near the critical layer, where the phase speed matches the speed of the background flow. Nevertheless, it is not clear through observations how the structure of the Kelvin waves evolves near the critical layer or in response to vertical wind shear with or without a critical layer. A novel varying‐coefficient regression technique that has been used to study evolving relationships across the seasonal cycle is used to capture how the wavelet‐filtered waves with a specific phase speed appear in different background winds or vertical shear regimes or a combination of both. Results show a relaxation of the in‐phase relationship between the zonal wind and geopotential height anomalies at slower Doppler‐shifted speeds, followed by the appearance of the Gill pattern at further reduction of the Doppler‐shifted speed, demonstrating a lack of dominant Kelvin wave signals in those conditions. We define the “observed critical layer” as the layer where the signals consistent with Kelvin waves fade away irrespective of the value of the Doppler‐shifted speed, which reduces to zero at the “theoretical critical layer.” Results show that wave structures consistent with Kelvin waves are not present at the top of layer with westerly vertical shear of the zonal wind, yet they exist during the easterly vertical shear of the zonal wind. The second part of the paper presents the dispersion equation of the Kelvin wave under vertical wind shear. The phase speed of the Kelvin wave proportionally increases with the Richardson number (and hence proportionally decreases with the vertical wind shear). The Richardson number varies with the vertical resolution of the data used, adding uncertainty to the calculated phase speed of the wave in the presence of vertical wind shear.

The Congolese rainforest is a hotspot for convection where thunderstorms and rainfall exhibit a strong diurnal cycle. Previous studies have shown that various modes of variability such as the Madden-Julian Oscillation (MJO), the leading mode of intraseasonal variability in the tropics, can impact the diurnal cycles of precipitation and convection over tropical land areas. Thus, this study analyzes the influence of the MJO on diurnal variations of convection and precipitation over the Congo and explores possible mechanisms leading to the observed changes, using Gridded Satellite (GridSat-B1), Tropical Rainfall Measuring Mission (TRMM), and ERA5 rean-alysis data. Results show that convection and precipitation are increased during the MJO enhanced convective phase (RMM phases 1 and 2) and decreased during the suppressed convective phase (RMM phases 5 and 6), where the differences are found to be largest during the morning hours when convection is weakest. Convection is generally deeper during the enhanced phase and shallower during the suppressed phase, accompanied by a slower decay of convective clouds and anvils during the enhanced phase. Furthermore, the influence of the MJO was found to be stronger on stratiform precipitation compared to convective precipitation. While the diurnal cycles of convective precipitation fractions are similar during both MJO phases, the fraction of stratiform precipitation is higher during the enhanced phase, mainly in the morning hours. Suggested atmospheric drivers contributing to the observed differences between the two MJO phases include enhanced upward air motion in the mid-and upper levels and strong divergence in the upper levels during the enhanced phase, and strong mid-level divergence and upper-to mid-level subsidence during the suppressed phase, mainly during the nighttime and morning hours. Furthermore, decreased mid-level wind speed, increased upper-level wind speed, and enhanced relative humidity may be contributing factors to the increased formation of stratiform precipitation during the enhanced phase. These results enhance our understanding of the MJO's impacts on precipitation and convection variability, ultimately improving predictions of MJO modulated rainfall over the Congo.

Convectively coupled equatorial Rossby waves are the dominant mode of westward-moving subseasonal convection in the tropics. A portion of the variance in these waves has been shown to associate with the tropical intraseasonal oscillation, along with a process often mediated by the extratropical Rossby wave response to tropical convection that yields Rossby waves breaking back into the tropical atmosphere. The potential vorticity anomalies driven by Rossby wave breaking become the equatorial Rossby waves. This work creates an index of planetary scale equatorial Rossby waves and applies the method of seasonally varying regression slope coefficients to diagnose their preferred associations with tropical and extratropical circulation features. Results confirm the already known association between these waves and the extratropical atmosphere and they reveal a pattern of westward-and northward-moving anomalies of tropical convection over the Indian Ocean and Southern Asia during the Northern Hemisphere summer. These patterns are associated with a cycle of suppression and enhancement of convection in which negative anomalies of outgoing longwave radiation are found to be 3-times as likely during the wet than the dry phases of the waves.

The issues of Monthly Weather Review would be mostly empty if it were not for the large number of high-quality submissions each year. To retain our position as one of the leading meteorological journals in the world, we need to attract and retain these valuable contributions. However, of the approximately 400 annual submissions, only 56.7% were eventually published in 2021. The percentage of papers being rejected has slowly increased from 33.7% in 2007 (Schultz 2010a) to 37.6% in 2021, with withdrawals and transfers making up the remaining few percent. Although some of those rejections are because the papers are off topic forMonthly Weather Review, most rejections are because the science does not meet our minimum standards or may need to be explained better. Defending the rejection of papers, the great fluid-mechanics scientist Batchelor (1981, p. 16), the founder and chief editor of the Journal of Fluid Mechanics, once wrote,

Several recent works have demonstrated that the Madden-Julian oscillation (MJO) shows more variance in tropical convection and wind during the easterly phase of stratospheric wind in the quasi-biennial oscillation (QBO). Some of these works have shown that reduced variance during the westerly QBO is associated with fewer repeating cycles of events. This article applies a modulation linear regression algorithm to assess changes in the global atmospheric circulation structure for December-February MJO signals tested when time-smoothed 50 hPa winds are easterly or westerly at 10 m center dot s(-1). Results show pronounced differences in the associated extratropical upper tropospheric Rossby wave response between the two QBO states, along with a comparatively lower fraction of total eastward wave energy being consistent with Kelvin waves approaching the Indian basin from the west during the westerly QBO. Instead, westerly QBO appears to favour stronger equatorward-moving extratropical wave precursors across the Western Hemisphere and Eurasia. The results suggest the hypothesis that more dependence on extratropical waves during the westerly QBO may reduce the reliability of the MJO to form sequential trains during the westerly QBO.