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Cut-off Lows (COLs) in the Southern Hemisphere (SH) and the mechanisms involved in their development are investigated in detail using the eddy kinetic energy (EKE) budget applied to data from the ERA-Interim reanalysis. This approach considers the most important processes that are typical for the evolution of midlatitude disturbances such as the baroclinic (BRC) and barotropic (BRT) conversions, and the ageostrophic flux convergence (AFC), known as downstream development. Composites of the volume-integrated EKE and its components are evaluated based on the 200 most intense SH COLs (> 98th percentile) observed in a 36-yr period. Results show that the AFC together with the BRC conversion are the most important contributor to the EKE growth for the COLs, characterizing the downstream baroclinic development. The AFC plays an important role in genesis and intensification phases of the COLs, while the BRC conversion is important for the system maintenance. The dissipation of the COLs occurs due to dispersive fluxes (ageostrophic flux divergence) together with other processes not directly computed in the EKE equation, such as friction and latent heat release which are problematic in reanalysis datasets. The BRT conversion contributes negatively to the COL development by transferring EKE to the zonal flow kinetic energy, though this is not enough to dampen the intensification. Regional differences were found in the energetics, indicating that COLs originating upstream of the continents are clearly dominated by ageostrophic fluxes, while the systems over the Australian region are mostly driven by baroclinic processes.

Eddy saturation describes the nonlinear mechanism in geophysical flows whereby, when average conditions are considered, direct forcing of the zonal flow increases the eddy kinetic energy, while the energy associated with the zonal flow does not increase. We present a minimal baroclinic model that exhibits complete eddy saturation. Starting from Phillips’ classical quasi-geostrophic two-level model on the beta channel of the mid-latitudes, we derive a reduced order model comprising of six ordinary differential equations including parameterised eddies. This model features two physically realisable steady state solutions, one a purely zonal flow and one where, additionally, finite eddy motions are present. As the baroclinic forcing in the form of diabatic heating is increased, the zonal solution loses stability and the eddy solution becomes attracting. After this bifurcation, the zonal components of the solution are independent of the baroclinic forcing, and the excess of heat in the low latitudes is efficiently transported northwards by finite eddies, in the spirit of baroclinic adjustment.

We study differential rotation in late-stage shell convection in a 3D hydrodynamic simulation of a rapidly rotating $16M_\odot$ helium star with a particular focus on the convective oxygen shell. We find that the oxygen shell develops a quasi-stationary pattern of differential rotation that is neither described by uniform angular velocity as assumed in current stellar evolution models of supernova progenitors, nor by uniform specific angular momentum. Instead, the oxygen shell develops a positive angular velocity gradient with faster rotation at the equator than at the pole by tens of percent. We show that the angular momentum transport inside the convection zone is not adequately captured by a diffusive mixing-length flux proportional to the angular velocity or angular momentum gradient. Zonal flow averages reveal stable large-scale meridional flow and an entropy deficit near the equator that mirrors the patterns in the angular velocity. The structure of the flow is reminiscent of simulations of stellar surface convection zones and the differential rotation of the Sun, suggesting that similar effects are involved; future simulations will need to address in more detail how the interplay of buoyancy, inertial forces, and turbulent stresses shapes differential rotation during late-stage convection in massive stars. If convective regions develop positive angular velocity gradients, angular momentum could be shuffled out of the core region more efficiently, potentially making the formation of millisecond magnetars more difficult. Our findings have implications for neutron star birth spin periods and supernova explosion scenarios that involve rapid core rotation., 13 pages, 6 figures. Published in MNRAS

In this dissertation, we present advances in turbulence modeling for magnetically confined plasmas. We investigate the ecology of microscopic drift wave turbulence and the self-generated macroscopic flows in magnetically confined plasmas. We formulate reduced models that self-consistently describe the evolution of turbulence and mean plasma profiles (including flows) and recover trends obtained from the CSDX device and HL-2A tokamak. The dissertation is divided to three parts. The first part presents a reduced model that describes the interplay between drift wave turbulence and zonal and axial flows in the adiabatic plasma of CSDX, where the electron response is Boltzmann. The model explains how free energy released from the density gradient accelerates both axial and azimuthal flows in CSDX. A description of the interactions between the disparate scales of the plasma via the parallel and perpendicular Reynolds stresses $\langle \tilde v_x \tilde v_z \rangle $ and $\langle \tilde v_x \tilde v_y \rangle $ is presented. Expressions for these stresses are decomposed into a diffusive component that relaxes the flow profile, and a residual stress responsible for accelerating the corresponding flow. Moreover, parallel and perpendicular flow dynamics are described using an extended mixing length approach. This accounts for the degree of symmetry breaking in the parallel direction and parametrizes the efficiency of $\nabla n$ in driving the axial flow. In the second part of the dissertation, the relationship between drift waves and zonal flows is examined in depth via a more specific model. Analytical results obtained from this model confirm the published experimental data showing a suppression of turbulence with the increase in magnitude of the magnetic field \textbf{B}. A new criterion for access to enhanced confinement is introduced. This criterion captured by the dimensionless quantity $R_{DT}$, compares the production rate of turbulent enstrophy due to relaxation of the mean profiles, to the corresponding destruction rate via coupling to the mean flow. When $R_{DT} >1$, the profiles steepen and enhanced confinement is accessible. In the third paper, a novel idea for understanding the physics of the density limit problem in low $\beta$ tokamaks is presented. The collapse of the zonal shear flow when the electron response transitions from Boltzmann to hydrodynamic scaling, along with cooling of the edge and the onset of MHD activity is predicted by the observation that the zonal flow drive will drop as the electron parallel diffusion time increases with density. This leads to a simple, verified understanding of the density limit phenomenon in $L$-modes.

We develop a simple model for the generation and amplification of intrinsic axial flow in a linear device, Controlled Shear Decorrelation Experiment (CSDX). This model develops a novel dynamical symmetry breaking mechanism in drift wave turbulence, which does not require complex magnetic field structure, such as shear. Thus, the model is applicable to both tokamaks and linear devices. This mechanism is, essentially, a form of negative viscosity phenomenon. Negative compressibility ITG turbulence can also induce a negative viscosity increment. However, we show that no intrinsic axial flow can be generated by pure ITG turbulence in a straight magnetic field. When the flow gradient is steepened by any drive mechanism, the flow profile saturates at a level close to the value above which parallel shear flow instability (PSFI) becomes dominant over the ITG instability. This saturated flow gradient exceeds the PSFI linear threshold, and grows with $\nabla T_{i0}$ as $|\nabla V_\parallel| / |k_\parallel c_s| \sim|\nabla T_{i0}|^{2/3} / (k_\parallel T_{i0})^{2/3}$.The coupling of azimuthal and axial flows in CSDX--in absence of magnetic shear--is investigated.In particular, we focus on the apportionment of turbulence energy between azimuthal and axial flows, and how the azimuthal flow shear affects axial flow generation and saturation by drift wave turbulence.Detailed measurements of intrinsic axial flow parallel to the magnetic field are performed on CSDX, with no axial momentum input.The results present a direct demonstration that the broken spectral symmetry of drift wave turbulence causes the development of axial mean flows in cylindrical magnetized plasmas.Measurements suggest the axial flow is parasitic to the drift wave--zonal flow system.Besides, we show that consideration of wave--flow resonance resolves the long-standing problem of how zonal flows (ZFs) saturate in the limit of weak or zero frictional drag and also determines the ZF scale directly from analysis. We show that resonant vorticity mixing, which conserves potential enstrophy, enables ZF saturation in the absence of drag, and so is effective at regulating the Dimits up-shift regime. Vorticity mixing is incorporated as a nonlinear, self-regulation effect in an extended 0D predator--prey model of drift--ZF turbulence.

Internal waves with high vertical wavenumber structures on the northern part of the eastern ridge in Luzon Strait were investigated using shipboard observations and a wave propagation model. Detailed repeat observations across the ridge revealed complex undulations of isopycnals near the ridge that were characterized by vertically alternating concave and convex structures. In addition, strong eastward and westward flow areas with horizontal and vertical scales of 3–5 km and 150–250 m, respectively, occurred alternately on the downstream side of the ridge. These high-wavenumber structures in flow and density fields were associated with internal waves generated by interactions between diurnal tidal flow and the meridional submarine ridge in the strait; an acoustic Doppler current profiler recorded no signals of the Kuroshio Current and semi-diurnal tides in the zonal flow on the western slope of the eastern ridge. Many temperature inversions around the ridge exhibited vertical scales of predominantly ~ 30 m. Simple calculations of internal wave motion reveal that each internal wave mode tends to stagnate near the summits of the ridge depending on the direction and strength of the background tidal flow. This suggests that the observed high-wavenumber structure in the flow profiles near the ridge is ascribed to the superposition of transient internal wave modes, although this is not the main cause of the temperature inversions. As a possible mechanism to explain the temperature inversions, we propose that higher-mode internal waves emanating from the unresolved topographic undulations are trapped by the observed high-wavenumber structures, resulting in turbulent mixing.

Persistent hot–dry or cold–wet summer weather can have significant impacts on agriculture, health, and the environment. For northwestern Europe, these weather regimes are typically linked to, respectively, blocked or zonal jet stream states. The fundamental dynamics underlying these circulation states are still poorly understood. Edward Lorenz postulated that summer circulation may be either fully or almost intransitive, implying that part of the phase space (capturing circulation variability) cannot be reached within one specific summer. If true, this would have major implications for the predictability of summer weather and our understanding of the drivers of interannual variability of summer weather. Here, we test the two Lorenz hypotheses (i.e., fully or almost intransitive) for European summer circulation, capitalizing on a newly available very large ensemble (2000 years) of present-day climate data in the fully coupled global climate model EC-Earth. Using self-organizing maps, we quantify the phase space of summer circulation and the trajectories through phase space in unprecedented detail. We show that, based on Markov assumptions, the summer circulation is strongly dependent on its initial state in early summer with the atmospheric memory ranging from 28 days up to ~45 days. The memory is particularly long if the initial state is either a blocked or a zonal flow state. Furthermore, we identify two groups of summers that are characterized by distinctly different trajectories through phase space, and that prefer either a blocked or zonal circulation state, respectively. These results suggest that intransitivity is indeed a fundamental property of the atmosphere and an important driver of interannual variability.

The main objective of this work is to reveal the forming mechanism of the shoulder driven zone during scroll tool friction stir welding (FSW). Scroll tools are widely used in FSW as they offer extra advantages of eliminating the tilted tool axis that is usually employed during the process. Therefore, it is vital to understand the forming mechanism of the shoulder zone when using an advanced scroll tool, which is important for optimizing welding parameters, analyzing welding defects, and controlling microstructure and properties of welds. In this present study, a series of FSW of thick section 6061 Al-T6 and Cu-AstmB370 (Cu foil as tracer) trials was conducted. A novel marker-insert technique by placing the Cu foil outside the welding path was employed, and the zonal flow patterns were characterized via OM, SEM and EDS. For the first time, the material flow behavior and actual flow patterns in the shoulder zone were detected by observing the distribution of Cu foil fragments in the weld zone. It was found that the thickness of shoulder zone varies along the weld transverse cross section with the thicker portion on the advancing side than the retreating side. The flow pattern of the shoulder driven zone reveals that a simple layer-to-layer banded structure appeared in the bottom portion of shoulder driven zone, whereas a featureless flow pattern showed in the top portion of the shoulder zone. Accordingly, the forming mechanism of shoulder driven zone was revealed based on the identified flow patterns.

Three-dimensional (3D) planetary wave analysis can provide more regionalized information on stratospheric-tropospheric dynamic interactions. The upward wave flux from the troposphere to the stratosphere is maximized above northeastern Eurasia, while the downward flux occurs mainly over the North America and North Atlantic (NANA) region, which is much stronger during mid-to-late winter. This distribution is determined by the wave-wave interactions between the different wavenumbers of planetary waves, especially between wavenumber 1 and wavenumber 2. The upward wave flux anomalies in early winter are negatively correlated with the strength of the stratospheric polar vortex (SPV). During the mid-to-late winter months, the strength of the SPV is positively correlated with the first mode of the 3D wave flux and has a leading relationship of approximately one month. The stronger SPV corresponds to a stronger upward wave flux above northern Eurasia and stronger downward flux over the NANA region. The interannual variations in wave flux during early winter are closely associated with the Scandinavian wave train pattern. In contrast, the wave flux variations are related to the circulation anomaly corresponding to the Arctic Oscillation during mid-to-late winter, which causes climate anomalies across the Northern Hemisphere, especially coherent temperature changes in northern Europe, eastern United States and northeastern China.

Summer precipitation totals in the Alpine Region do not exhibit a systematic trend over the last 120 years. However, we find significant low frequency periodicity of interannual variability which occurs in synchronization with a dominant two-phase state of the atmospheric circulation over the Alps. Enhanced meridional flow increases precipitation variability through positive soil moisture precipitation feedbacks on the regional scale, whereas enhanced zonal flow results in less variability through constant moisture flow from the Atlantic and suppressed feedbacks with the land surface. The dominant state of the atmospheric circulation over the Alps in these periods appears to be steered by zonal sea surface temperature gradients in the mid-latitude North Atlantic. The strength and the location of the westerlies in the mid-latitude Atlantic play an important role in the physical mechanisms linking atmosphere and oceanic temperature gradients and the meridional/zonal circulation characteristics.

Changes in atmospheric circulation under increasing greenhouse gas concentrations are important because of their implications for weather extremes and associated societal risks. However, uncertainties in models and future projections are still large and drivers behind circulation changes are not well understood. Particularly for Europe, a potential weakening of the Atlantic meridional overturning circulation (AMOC) is considered important as it affects SST patterns and ocean–atmosphere heat fluxes and, subsequently, European climate. Here we detect and characterize changes in atmospheric circulation patterns over the North Atlantic under increasing CO2 concentrations in simulations of a very high-resolution, fully coupled climate model (CM2.6) with a realistic representation of the AMOC. We use an objective clustering technique (self-organizing maps) and validate the model’s clusters against reanalysis data. We compare the frequency of those patterns in a CO2 doubling experiment, characterized by an AMOC decline, with those in a preindustrial run, and find statistically significant changes. The most robust findings are 1) a ~30% increase in zonal flow regimes in February, relevant for flood risk in northwestern Europe, and 2) a ~60% increase in anticyclonic (high pressure) circulation directly west of the United Kingdom in August, relevant for western and central European drought. A robust decrease in the frequency of Scandinavian blocking is also seen across most months and seasons. Despite the uncertainties regarding atmospheric circulation response to climate change, our findings contribute to the increasing evidence for the emergence of robust high-impact changes over Europe.

Linear and nonlinear barotropic vorticity model frameworks are constructed to understand the formation of the monsoon trough in boreal summer over the western North Pacific. The governing equation is written with respect to specified zonal background flows, and a wave perturbation is prescribed in the eastern boundary. Whereas a uniform background mean flow leads no scale contraction, a confluent background zonal flow causes the contraction of zonal wavelength. Under linear dynamics, the wave contraction leads to the development of smaller scale vorticity perturbations. As a result, there is no upscale cascade. Under nonlinear dynamics, cyclonic (anticyclonic) wave disturbances shift northward (southward) away from the central latitude due to the vorticity segregation process. The merging of small-scale cyclonic and anticyclonic perturbations finally leads to the generation of a pair of large-scale cyclonic and anti-cyclonic vorticity gyres, straddling across the central latitude. The large-scale cyclonic circulation due to nonlinear upscale cascade can be further strengthened through a positive convection-circulation feedback.

The Southern Ocean lies beneath a unique region of the global atmosphere with minimal effects of landmasses on the zonal flow. The absence of landmasses also means that in situ observations of precipitation are limited to a few ocean islands. Two reanalyses and two satellite-based gridded datasets are analyzed to estimate the character of the distribution of precipitation across the region. The latitudinal variation is computed across three longitudinal sectors, representing the Pacific, Atlantic, and Indian Oceans. The most recent ECMWF reanalysis (ERA5) is found to produce the most accurate estimate of the mean profile and seasonal cycle of precipitation. However, there is little consistency in the estimates of trends in monthly anomalies of precipitation. A more consistent description of precipitation trends is found by using linear regression of the precipitation anomaly with the local mean sea level pressure anomaly, the southern annular mode, and the Southern Oscillation index. In broad terms, precipitation is found to be decreasing at lower latitudes and increasing at higher latitudes, which is consistent with earlier climate model simulations on the impacts of anthropogenic climate change.

In the paper the seasonal trends in the zonal asymmetry in the quasi-stationary wave pattern of total ozone column (TOC) at southern polar latitudes have been investigated. We evaluated and compared seasonal trends in the zonal TOC asymmetry from modern era satellite measurements using the Total Ozone Mapping Spectrometer data and the second Chemistry Climate Model Validation (CCMVal-2) assessment. The model longitude phase shifts in asymmetry are in general consistent with the eastward phase shifts observed in historical period 1979–2005, however, there are underestimated values in individual seasons. Future trends in zonal asymmetry from the eleven CCMVal-2 models up to 2100 are presented. They demonstrate the appearance of reverse (westward) future phase shifts, mainly in austral summer. The results are in agreement with previous study and highlight that the general eastward/westward phase shift is caused by both greenhouse gases changes and ozone depletion/recovery. The greenhouse gases change drives a basic long-term eastward shift, which is enhanced (decelerates or reverses) in austral spring and summer by ozone depletion (recovery). The trends in the TOC asymmetry are forced by a general strengthening of the stratospheric zonal flow, which is interacting with the asymmetry of the Antarctic continent to displace the quasi-stationary wave-1 pattern and thus influences the TOC distribution. The results will be useful in prediction of seasonal anomalies in ozone hole and long-term changes in the local TOC trends, in ultraviolet radiation influence on the Southern Ocean biological productivity and in regional surface climate affected by the zonally asymmetric ozone hole.

The characteristics of the wintertime Arctic Oscillation (AO) and North Atlantic Oscillation (NAO) and their impacts on climate variability over the Northern Hemisphere are important metrics for evaluating a climate system model. Observational analyses reveal that the horizontal and vertical structures in the AO and NAO exhibit a meridional dipole and a large-scale barotropic pattern between the Arctic and mid-latitudes. Historical model simulations from the Energy Exascale Earth System Model (E3SM-HIST) are used to identify how well it captures these major climate modes. It is found that the simulated AO and NAO modes have spatial structures similar to the observed features. In addition, the observed frequency bands in the AO and NAO-related time variability are captured well in the E3SM-HIST simulation. Associated with the positive phase in wintertime AO and NAO, zonal flow and warm advection in mid-latitude continents are enhanced, along with stronger cold flow from enhanced northerly winds over high latitudes. These features are linked to the atmospheric circulation pattern reflected by lower SLP anomalies over the Arctic and higher SLP anomalies over the mid‐latitudes. In E3SM-HIST, these spatial associations and main structural features are analogous to those in observations. In the time-height evolution related to winter AO and NAO modes, it can also be seen that the simulations reproduce the downward propagating patterns in observations. Nevertheless, the vertical structures associated with AO and NAO in E3SM-HIST exhibit substantial biases in the lower stratosphere. The cause of these stratospheric biases is investigated using the strength of climatological stratospheric polar vortex (SPV) and wave activity fluxes. The results herein suggest that E3SM-HIST has a reasonable skill in reproducing the observed characteristics related to the winter AO and NAO, although there exist systematic biases in the associated climate variability.

To study the forced variability of atmospheric circulation regimes, the use of model ensembles is often necessary for identifying statistically significant signals as the observed data constitute a small sample and are thus strongly affected by the noise associated with sampling uncertainty. However, the regime representation is itself affected by noise within the atmosphere, which can make it difficult to detect robust signals. To this end we employ a regularised k-means clustering algorithm to better identify the signal in a model ensemble. The approach allows for the identification of six regimes for the wintertime Euro-Atlantic sector and leads to more pronounced regime dynamics, compared to results without regularisation, both overall and on sub-seasonal and interannual time-scales. We find that sub-seasonal variability in the regime occurrence rates is mainly explained by changes in the seasonal cycle of the mean climatology. On interannual time-scales relations between the occurrence rates of the regimes and the El Niño Southern Oscillation (ENSO) are identified. The use of six regimes captures a more detailed response of the circulation to ENSO compared to the common use of four regimes. Predictable signals in occurrence rate on interannual time-scales are found for the two zonal flow regimes, namely a regime consisting of a negative geopotential height anomaly over the Norwegian Sea and Scandinavia, and the positive phase of the NAO. The signal strength for these regimes is comparable between observations and model, in contrast to that of the NAO-index where the signal strength in the observations is underestimated by a factor of 2 in the model. Our regime analysis suggests that this signal-to-noise problem for the NAO-index is primarily related to those atmospheric flow patterns associated with the negative NAO-index as we find poor predictability for the corresponding NAO (Formula presented.) regime.

Atmospheric circulation anomalies over the Ural Mountains are crucial indicators of the anomalous downstream weather and climate over East Asia. Here, we provide a new perspective on the mechanism of Ural circulation anomalies. We use a simple theoretical model to determine that the relationship between the solar forcing and three Ural circulation patterns, namely, neutral type, trough anomaly and ridge anomaly, is a nonlinear relationship following the supercritical pitchfork bifurcation theory. The theory predicts that when the total solar irradiance (TSI) is below a critical value, trough and ridge anomalies represent duplex equilibria and are equally likely to occur at the same TSI. Based on 180 winter months record, we have estimated the bidimensional probability density of TSI and the monthly mean geopotential height at 500 hPa or zonal wind at 850 hPa over the Ural Mountains. Results show that Sc = 1360.9 W m−2 is a critical value of TSI, the neutral type pattern is the single circulation regime when TSI > Sc, whereas trough and ridge anomaly patterns are duplex circulation regimes when TSI

The high-precision Juno gravitational measurements allow us to infer the structure of Jupiter's deep atmospheric zonal flow. Since this inference is nonunique, it is important to explore the space of possible solutions. In this paper, we consider a model in which Jupiter's deep atmospheric zonal flow is barotropic, or invariant along the direction of the rotation axis, until it is truncated at depth by some dynamical process (e.g., Reynolds stress, Lorentz or viscous force). We calculate the density perturbation produced by the $z$-invariant part of the flow using the thermal wind equation and compare the associated odd zonal gravitational harmonics ($J_{3}$, $J_{5}$, $J_{7}$, $J_{9}$) to the Juno-derived values. Most of the antisymmetric gravitational signal measured by Juno can be explained by extending observed winds between $20.9^{\circ}\rm{S}-26.4^{\circ}\rm{N}$ to depths of $\sim 1000$ km. Because the small-scale features of the mid/high latitude zonal flow may not persist to depth, we allow the zonal flow in this region to differ from the observed surface winds. We find that the Juno odd zonal gravitational harmonics can be fully explained by $\sim 1000$ km deep barotropic zonal flows involving the observed winds between $20.9^{\circ}\rm{S}-26.4^{\circ}$N and a few broad mid/high latitude jets., Published in JGR: Planets

Planetary waves in the mesosphere are studied using observational data and models to establish their origin, as there are indications of their generation independently of waves in the stratosphere. The quantitative relationships between zonal wave 1 and wave 2 were studied with a focus on the mid-latitude mesosphere at 50°N latitude. Aura Microwave Limb Sounder measurements were used to estimate wave amplitudes in geopotential height during sudden stratospheric warmings in recent boreal winters. The moving correlation between the wave amplitudes shows that, in comparison with the anticorrelation in the stratosphere, wave 2 positively correlates with wave 1 and propagates ahead of it in the mesosphere. A positive correlation r = 0.5–0.6, statistically significant at the 95% confidence level, is observed at 1–5-day time lag and in the 75–91 km altitude range, which is the upper mesosphere–mesopause region. Wavelet analysis shows a clear 8-day period in waves 1 and 2 in the mesosphere at 0.01 hPa (80 km), while in the stratosphere–lower mesosphere, the period is twice as long at 16 days; this is statistically significant only in wave 2. Possible sources of mesospheric planetary waves associated with zonal flow instabilities and breaking or dissipation of gravity waves are discussed.

A global model with superparameterized physics is used to shed light on the observed regionality of convectively coupled Kelvin waves and the Madden‐Julian Oscillation (MJO). A series of aquaplanet simulations over zonally uniform sea‐surface temperatures is performed, in which the axisymmetric structure of the background zonal flow [u¯] is altered through nudging, while maintaining a quasi‐fixed rainfall climatology. Results show that nudging [u¯] at the equator to match profiles typical of the Indo‐Pacific or eastern Pacific sectors yields eastward‐moving tropical rain spectra typical of those sectors. Two different mechanistic pathways are identified as being responsible for this mean‐flow dependence, in addition to Doppler shifting effects. The first is through shifts of the Rossby wave critical line in the subtropical upper troposphere that affect the lateral forcing of Kelvin‐mode circulations at the equator by eastward and equatorward‐propagating eddies impinging on the tropics from higher latitudes. The second is through changes in the strength of the mean cyclonic shear in the lower tropical troposphere that affect the degree to which intraseasonal fluctuations in Kelvin‐mode zonal winds modulate the activity of higher‐frequency equatorial Rossby‐type eddies. In cases where the mean low‐level cyclonic shear is enhanced, the strength of this modulation, referred to as “shear‐induced eddy modulation” or SIEM, is also seen to be enhanced, such that MJO‐like modes of variability are rendered either unstable or near neutral, depending on the strength of the shear.