Madden–Julian Oscillation Dynamics Research Articles

Vertically Resolved Analysis of the Madden‐Julian Oscillation Highlights the Role of Convective Transport of Moist Static Energy

AbstractWe simulate the Madden‐Julian oscillation (MJO) over an aquaplanet with uniform surface temperature using the multiscale modeling framework (MMF) configuration of the Energy Exascale Earth System Model (E3SM‐MMF). The model produces MJO‐like features that have a similar spatial structure and propagation behavior to the observed MJO. To explore the processes involved in the propagation and maintenance of these MJO‐like features, we perform a vertically resolved moist static energy (MSE) analysis for the MJO (Yao et al., 2022, https://doi.org/10.1175/jas‐d‐20‐0254.1). Unlike the column‐integrated MSE analysis, our method emphasizes the local production of MSE variance and quantifies how individual physical processes amplify and propagate the MJO's characteristic vertical structure. We find that radiation, convection, and boundary layer (BL) processes all contribute to maintaining the MJO, balanced by the large‐scale MSE transport. Furthermore, large‐scale dynamics, convection, and BL processes all contribute to the propagation of the MJO, while radiation slows the propagation. Additionally, we perform mechanism‐denial experiments to examine the role of radiation and associated feedbacks in simulating the MJO. We find that the MJO can still self‐emerge and maintain its characteristic structures without radiative feedbacks. This study highlights the role of convective MSE transport in the MJO dynamics, which was overlooked in the column‐integrated MSE analysis.

The Influence of Cloud Types on Cloud‐Radiative Forcing During DYNAMO/AMIE

AbstractCloud‐radiative feedback is known to be an important process for the Madden‐Julian Oscillation (MJO) and its accurate representation in general circulation models. The MJO is known to have a stronger cloud‐radiative feedback compared to higher frequency convectively coupled equatorial waves (CCEW), for reasons that remain unclear. The objective of this study is to use data from the Dynamics of the MJO/Atmospheric Radiation Measurement MJO Investigation Experiment field campaign to investigate how cloud type evolution and their associated cloud‐radiative forcing (CRF) differ between the MJO and CCEWs and their application to MJO dynamics under moisture mode theory. This study finds that the amplitude of CRF is the largest within the MJO compared to the higher frequency CCEWs, and CRF maximizes after the maximum precipitation (enhanced phase) of the MJO. We suggest this delay in the timing of maximum CRF occurs because most cloud types (congestus, deep, anvil, and cirrus) simultaneously maximize in frequency after the enhanced phase of the MJO, which is not seen for the higher frequency CCEWs. These specific cloud types help maintain the MJO through their radiatively‐driven moistening which acts to prolong the enhanced phase of the MJO. The decomposition of radiatively‐driven moistening by cloud types shows that simultaneous moistening by deep, anvil, and congestus clouds are particularly responsible for the maintenance of the MJO under moisture mode theory. This analysis suggests the importance of capturing the correct evolution of different cloud types and their associated radiative effects to better understand the thermodynamics of the MJO.

Interactions of Large‐Scale Dynamics and Madden‐Julian Oscillation Propagation in Multi‐Model Simulations

AbstractThe underrepresentation of the Madden‐Julian Oscillation (MJO) in climate models remains a challenge, limiting our ability to improve medium‐ to extended‐range atmospheric prediction. Motivated by recent work identifying the importance of the ratio of equatorial Rossby (ER) to Kelvin wave circulations in MJO propagation, this study examines MJO dynamics in 25 climate model simulations. We find that poor MJO models simulate anomalously large ER wave circulations to the west and small Kelvin and ER wave circulations to the east of the convective center. To quantify the role of circulation asymmetries in MJO propagation, we formulate a new west/east (W/E) zonal wind speed ratio. Our W/E ratio differs from other similar metrics in that it implicitly accounts for the ER wave gyres and it can be applied at all levels. Poor model ER wave biases are associated with excessive 700–1000‐hPa convergence, convection, and vertical moisture advection co‐located and west of the convective center while Kelvin and ER wave biases to the east are associated with a weaker dry anomaly and smaller horizontal moisture advection at 450–750‐hPa. Together, these biases help explain the stationary and slight westward MJO propagation in poor models. Space‐time spectral analyses of the zonal wind and precipitation confirm that good models produce realistic power, coherence, and phase for the MJO while poor models vastly underrepresent Kelvin waves and the MJO. Even though Kelvin waves are more realistic in good models, there are still model‐wide biases in circulation‐convection coupling for ER waves and Kelvin waves.

Role of cloud radiative feedback in the Madden–Julian oscillation dynamics: a trio-interaction model analysis

The authors expand the original wave dynamic-moisture (WM) model by implementing the cloud radiative feedback (CRF) to study the role of the CRF in the Madden–Julian oscillation (MJO) in comparison with the role of the planetary boundary layer (PBL) process. The linear instability analysis is used to elucidate the reactions of the WM mode, WM-CRF mode, WM-PBL mode, and WM-PBL-CRF mode. Compared with the stationary and damped WM mode, the CRF can present an important instability source for all wavenumbers without the planetary-scale selection and tends to slow down the planetary-scale eastward propagation. On the other hand, the PBL process, with the planetary-scale selection, can destabilize the eastward propagation while accelerate the eastward propagation of the planetary-scale oscillation. When the PBL and the CRF processes are both included, the unstable mode is achieved and period is nearly 20–90 days, consistent with the observations. Both the WM and the WM-CRF modes present unrealistic coupled Kelvin–Rossby wave structure, which disagrees with the observations. These caveats can be remitted in the WM-PBL mode and the WM-PBL-CRF mode. The PBL can couple the Kelvin and Rossby waves and present the observed geopotential low in front of the convective center. The CRF, however, can make the phase relation between the precipitation anomalies and pressure anomalies changed in the presence of PBL process.

Four Theories of the Madden-Julian Oscillation.

Studies of the Madden‐Julian Oscillation (MJO) have progressed considerably during the past decades in observations, numerical modeling, and theoretical understanding. Many theoretical attempts have been made to identify the most essential processes responsible for the existence of the MJO. Criteria are proposed to separate a hypothesis from a theory (based on the first principles with quantitative and testable assumptions, able to predict quantitatively the fundamental scales and eastward propagation of the MJO). Four MJO theories are selected to be summarized and compared in this article: the skeleton theory, moisture‐mode theory, gravity‐wave theory, and trio‐interaction theory of the MJO. These four MJO theories are distinct from each other in their key assumptions, parameterized processes, and, particularly, selection mechanisms for the zonal spatial scale, time scale, and eastward propagation of the MJO. The comparison of the four theories and more recent development in MJO dynamical approaches lead to a realization that theoretical thinking of the MJO is diverse and understanding of MJO dynamics needs to be further advanced.

Identifying and Characterizing Tropical Oceanic Mesoscale Cold Pools using Spaceborne Scatterometer Winds

AbstractConvective cold pools over oceans contribute to the air‐sea exchange of heat and moisture. Beyond cold pools' physical properties, cold pools and their intersections are important mechanisms controlling the life cycle of tropical convective systems. However, few observational techniques are available to routinely observe the properties of cold pools and their association with convection. The primary goal of this study is to demonstrate a new technique using gradients in remotely sensed ocean vector winds as a proxy for the structure and characteristics of tropical oceanic cold pools. Using Advanced Scatterometer (ASCAT) vector wind retrievals with field of view of 12.5 km, regions of enhanced gradients in surface winds are identified as gradient features. A 3‐km Weather Research and Forecasting (WRF) model simulation was carried out to illustrate the technique over the Indian Ocean during the 2011 DYNAmics of Madden‐Julian Oscillation (DYNAMO) field campaign. A forward operator was applied to the model wind fields, allowing direct comparisons of wind gradients and virtual temperature‐defined cold pool thermodynamic and kinematic properties. In situ buoy measurements of air temperature were also used to validate ASCAT‐identified cold pools. Quantitative skill metrics demonstrate that the technique has a low false alarm rate (<10%), high critical success index (>85%), and a low bias (~1). The gradient feature technique was then applied to ASCAT‐retrieved ocean vector winds, revealing global variations in cold pool properties. These properties are shown to be related to the properties of global convective systems observed by the Tropical Rainfall Measurement Mission (TRMM).

Transient Response of MJO Precipitation and Circulation to Greenhouse Gas Forcing

AbstractRecent studies have shown that Madden‐Julian oscillation (MJO) precipitation anomaly amplitude tends to increase while associated circulations weaken at the end of 21st century in Coupled Model Intercomparison Project phase 5 models under Representative Concentration Pathway 8.5. Transient changes of MJO characteristics earlier in the 21st century have received less attention. In this study, changes of MJO precipitation and circulation amplitude during these interim time periods under Representative Concentration Pathway 8.5 are examined in Coupled Model Intercomparison Project phase 5 models. Multimodel mean changes in MJO precipitation and circulation amplitude are not individually detectable in the early and middle 21st century relative to the historical period (1986–2005). However, robust multimodel mean decreases in the ratio of MJO wind to precipitation anomalies occur even early in the 21st century. This decreased ratio is explained by increasingly large tropical static stability as the climate warms, which under weak temperature gradient balance mandates that a diabatic heating anomaly is balanced by an increasingly weaker circulation anomaly. These results suggest the robustness of weak temperature gradient theory for explaining MJO dynamics, not only in an equilibrium climate but also in the transient response.

Parameter Modulation of Madden-Julian Oscillation Behaviors in BCC_CSM1.2: The Key Role of Moisture-Shallow Convection Feedback

To reveal key parameter-related physical mechanisms in simulating Madden-Julian Oscillation (MJO), seven physical parameters in the convection and cloud parameterization schemes of Beijing Climate Center Climate System Model (BCC_CSM1.2) are perturbed with Latin hypercube sampling method. A new strategy is proposed to select runs with good and poor MJO simulations among 85 generated ones. Outputs and parameter values from good and poor simulations are composited separately for comparison. Among the seven chosen parameters, a decreased value of precipitation efficiency for shallow convection, higher values of relative humidity threshold for low stable clouds and evaporation efficiency for deep convective precipitation are crucial to simulate a better MJO. Changes of the three parameters act together to suppress heavy precipitation and increase the frequency of light rainfall over the Indo-Pacific region, supplying more moisture in low and middle troposphere. As a result of a wetter lower troposphere ahead of the MJO main convection, the low-level moisture preconditioning along with the leading shallow convection tends to be enhanced, favorable for MJO’s further development and eastward propagation. The MJO’s further propagation across the Maritime Continent (MC) in good simulations is accompanied with more land precipitation dominated by shallow convection. Therefore, the above-mentioned three parameters are found to be crucial parameters out of the seven ones for MJO simulation, providing an inspiration for better MJO simulation and prediction with this model. This work is valuable as it highlights the key role of moisture-shallow convection feedback in the MJO dynamics.

Dynamic moisture mode versus moisture mode in MJO dynamics: importance of the wave feedback and boundary layer convergence feedback

The Madden–Julian oscillation (MJO) is an equatorial eastward moving system with a planetary-scale coupled Kelvin–Rossby wave structure. The equatorial waves and their interaction with convection are expected to play an important role in MJO dynamics. Using the trio-interaction model for essential MJO dynamics, this study investigates the importance of dynamic feedback that includes wave feedback (WF) and boundary layer convergence feedback (BLCF), by comparing the moisture mode (MM) that contains only moisture feedback (MF) and cloud-radiative feedback (CRF), with the dynamic moisture mode (DMM) that includes additional WF and BLCF. It is shown that the dynamic feedbacks fundamentally change the properties of the MJO mode. For the MM, the MF alone yields a damping and quasi-stationary mode on wavenumber 2–4. The CRF can destabilize the MM, but it cannot produce planetary wave selection. By including the dynamic feedbacks (WF and BLCF), the resultant DMM is an unstable mode with a preferred planetary scale, which moves eastward slowly, yielding a 30–90-day period. The dynamic feedbacks produce the planetary scale selection of the DMM through generating more eddy available moist static energy on the longer wavelengths. The WF can significantly change the structure of the MM and links the propagation of the DMM to the Kelvin and Rossby wave components, with stronger Kelvin (Rossby) wave favoring faster (slower) propagation. The BLCF enhances the Kelvin wave component on the longer wavelengths, changing the horizontal structures and accelerating the eastward propagation. The WF relates the dispersion feature of the DMM to the properties of the Kelvin and Rossby waves. Since the Kelvin-wave (Rossby-wave) frequency increases (decreases) with increasing wavenumber, their coupling in the DMM yields a quasi-constant frequency at the planetary scales (wavenumber 1–3).

QBO‐MJO Connection

AbstractActivities of the Madden‐Julian Oscillation (MJO) in boreal winter has recently been found to be stronger in easterly phases of the stratospheric quasi‐biennial oscillation (QBO) than its westerly phases. This QBO‐MJO connection was investigated in this study using a method that identifies individual MJO events by tracking their eastward propagating signals in precipitation. Stronger MJO activities in QBO easterly phases are a consequence of more MJO days, not larger amplitudes of individual MJO events as previously thought. More MJO days come from more MJO events initiated over the Indian Ocean and their longer duration because of a weaker barrier effect of the Maritime Continent on MJO propagation. Zonal heterogeneity exists in the connection between QBO, MJO, and tropical total precipitation in general. This poses a challenge to our current understanding of the MJO dynamics, which has yet to fully include upper‐tropospheric and stratospheric processes.

Roles of the Moisture and Wave Feedbacks in Shaping the Madden–Julian Oscillation

This study investigates the moisture and wave feedbacks in the Madden–Julian oscillation (MJO) dynamics by applying the general three-way interaction theoretical model. The three-way interaction model can reproduce observed large-scale characteristics of the MJO in terms of horizontal quadrupole-vortex structure, vertically tilted structure led by planetary boundary layer (PBL) convergence, slow eastward propagation with a period of 30–90 days, and planetary-scale circulation. The moisture feedback effects can be identified in this model by using diagnostic thermodynamic and momentum equations, and the wave feedback effects are investigated by using a diagnostic moisture equation. The moisture feedback is found to be responsible for producing the MJO dispersive modes when the convective adjustment process is slow. The moisture feedback mainly acts to reduce the frequency and growth rate of the short waves, while leaving the planetary waves less affected, so neglecting the moisture feedback is a good approximation for the wavenumber-1 MJO. The wave feedback is shown to slow down the eastward propagation and increase the growth rate of the planetary waves. The wave feedback becomes weak when the convective adjustment time increases, so neglecting the wave feedback is a good approximation for the MJO dynamics during a slow adjustment process. Sensitivities of these two feedbacks to other parameters are also discussed. These theoretical findings suggest that the two feedback processes, and thus the behaviors of the simulated MJO mode, should be sensitive to the parameters used in cumulus parameterizations.

Effect of Spatial Variation of Convective Adjustment Time on the Madden–Julian Oscillation: A Theoretical Model Analysis

The observed convective adjustment time (CAT) associated with Madden–Julian Oscillation (MJO) precipitation is found to vary significantly in space. Here, we investigate the effect of different spatial distributions of CAT on MJO precipitation based on the frictional coupled dynamics moisture (FCDM) model. The results show that a large value of CAT tends to decrease the frequency and growth rate of eastward-propagating MJO-like mode in the FCDM model, delaying the occurrence of MJO deep convection and slowing down its eastward propagation. A large phase lag between circulation and convection decreases convective available potential energy (CAPE). In the observations, a small background vertical moisture gradient (BVMG) tends to increase the frequency associated with cold sea surface temperature (SST), while a large value of CAT tends to decrease the frequency. Due to their competing effect, the simulated frequency and phase speed remain the same when the convection moves from a warm to a cold SST region. The convection is heavily suppressed over the cold SST region due to the decreasing growth rate of unstable wavenumber-one mode with smaller BVMG and longer CAT. This theoretical finding should improve our understanding of MJO dynamics and simulation.

Impact of stochastic physics on tropical precipitation in the coupled ECMWF model

Uncertainties in parametrized processes in general circulation models can be represented as stochastic perturbations to the model formulation. The European Centre for Medium‐Range Weather Forecasts (ECMWF) has pioneered approaches to represent these model errors in forecasting systems. In particular, the stochastically perturbed physical tendency (SPPT) scheme for the atmosphere is used in their operational ensemble system for medium‐ and long‐range predictions. Recent studies have shown that these stochastic approaches can both increase the reliability of the probabilistic forecasts and reduce long‐term mean biases of the model climate. Towards developing a seamless prediction system in the future, these benefits of stochastic parametrization for both short‐term and long‐term forecasts make it an essential component of the next generation Earth System models. We present results of the impact of different configurations of the SPPT scheme in ECMWF's seasonal forecasting System 4 on the mean and variability in tropical precipitation. Small‐scale perturbations in the SPPT scheme play a significant role in reducing the mean biases in tropical precipitation. The stochastic physics also nonlinearly rectify the convection and precipitation during different phases of El Niño Southern Oscillation events and improve the reliability of the ensemble forecasts for the Madden–Julian Oscillation (MJO). They impact the MJO dynamics by modulating the convective and suppressed phases of the MJO. Finally, we discuss some of the caveats to this analysis and some future prospects.

A trio-interaction theory for Madden–Julian oscillation

The Madden–Julian oscillation (MJO) is the dominant mode of tropical atmospheric intraseasonal variability and a primary source of predictability for global sub-seasonal prediction. Understanding the origin and perpetuation of the MJO has eluded scientists for decades. The present paper starts with a brief review of progresses in theoretical studies of the MJO and a discussion of the essential MJO characteristics that a theory should explain. A general theoretical model framework is then described in an attempt to integrate the major existing theoretical models: the frictionally coupled Kelvin–Rossby wave, the moisture mode, the frictionally coupled dynamic moisture mode, the MJO skeleton, and the gravity wave interference, which are shown to be special cases of the general MJO model. The last part of the present paper focuses on a special form of trio-interaction theory in terms of the general model with a simplified Betts–Miller (B-M) cumulus parameterization scheme. This trio-interaction theory extends the Matsuno–Gill theory by incorporating a trio-interaction among convection, moisture, and wave-boundary layer (BL) dynamics. The model is shown to produce robust large-scale characteristics of the observed MJO, including the coupled Kelvin–Rossby wave structure, slow eastward propagation (~5 m/s) over warm pool, the planetary (zonal) scale circulation, the BL low-pressure and moisture convergence preceding major convection, and amplification/decay over warm/cold sea surface temperature (SST) regions. The BL moisture convergence feedback plays a central role in coupling equatorial Kelvin and Rossby waves with convective heating, selecting a preferred eastward propagation, and generating instability. The moisture feedback can enhance Rossby wave component, thereby substantially slowing down eastward propagation. With the trio-interaction theory, a number of fundamental issues of MJO dynamics are addressed: why the MJO possesses a mixed Kelvin–Rossby wave structure and how the Kelvin and Rossby waves, which propagate in opposite directions, could couple together with convection and select eastward propagation; what makes the MJO move eastward slowly in the eastern hemisphere, resulting in the 30–60-day periodicity; why MJO amplifies over the warm pool ocean and decays rapidly across the dateline. Limitation and ramifications of the model results to general circulation modeling of MJO are discussed.

A general theoretical framework for understanding essential dynamics of Madden\u2013Julian oscillation

Motivated by observed structure of Madden–Julian oscillation (MJO), a general theoretical model framework is advanced for understanding fundamental aspects of MJO dynamics. The model extends the Matsuno–Gill theory by incorporating (a) moisture feedback to precipitation, (b) a trio-interaction among equatorial waves, boundary layer (BL) dynamics, and precipitation, and (c) a simplified Betts–Miller (B–M) cumulus parameterization. The general model with B–M scheme yields a frictionally coupled dynamic moisture mode, which produces an equatorial planetary-scale, unstable system moving eastward slowly with coupled Kelvin–Rossby wave structure and BL moisture convergence leading major convection. The moisture feedback in B–M scheme reinforces the coupling between precipitation heating and Rossby waves and enhances the Rossby wave component in the MJO mode, thereby slowing down eastward propagation and resulting in a more realistic horizontal structure. It is, however, the BL frictional convergence feedback that couples equatorial Kelvin and Rossby waves with convective heating and selects a preferred eastward propagation. The eastward propagation speed in the model is inversely related to the relative intensity of the equatorial “Rossby” westerly versus “Kelvin” easterly associated with the MJO. The cumulus parameterization scheme may affect propagation speed through changing MJO horizontal structure. The SST or basic-state moist static energy has a fundamental control on MJO propagation speed and intensification/decay. Model sensitivity to BL and cumulus scheme parameters and ramifications of the model results to general circulation modeling are discussed.

Response of the Superparameterized Madden–Julian Oscillation to Extreme Climate and Basic-State Variation Challenges a Moisture Mode View

Abstract The climate sensitivity of the Madden–Julian oscillation (MJO) is measured across a broad range of temperatures (1°–35°C) using a convection-permitting global climate model with homogenous sea surface temperatures. An MJO-like signal is found to be resilient in all simulations. These results are used to investigate two ideas related to the modern “moisture mode” view of MJO dynamics. The first hypothesis is that the MJO has dynamics analogous to a form of radiative convective self-aggregation in which longwave energy maintenance mechanisms shut down for SST ≪ 25°C. Inconsistent with this hypothesis, the explicitly simulated MJO survives cooling and retains leading moist static energy (MSE) budget terms associated with longwave destabilization even at SST < 10°C. Thus, if the MJO is a form of longwave-assisted self-aggregation, it is not one that is temperature critical, as is observed in some cases of radiative–convective equilibrium (RCE) self-aggregation. The second hypothesis is that the MJO is propagated by horizontal advection of column MSE. Inconsistent with this view, the simulated MJO survives reversal of meridional moisture gradients in the basic state and a striking role for horizontal MSE advection in its propagation energy budget cannot be detected. Rather, its eastward motion is balanced by vertical MSE advection reminiscent of gravity or Kelvin wave dynamics. These findings could suggest a tight relation between the MJO and classic equatorial waves, which would tend to challenge moisture mode views of MJO dynamics that assume horizontal moisture advection as the MJO’s propagator. The simulation suite provides new opportunities for testing predictions from MJO theory across a broad climate regime.

Effects of moisture feedback in a frictional coupled Kelvin–Rossby wave model and implication in the Madden–Julian oscillation dynamics

The authors extend the original frictional wave dynamics and implement the moisture feedback (MF) to explore the effects of planetary boundary layer (PBL) process and the MF on the Madden–Julian Oscillation (MJO). This new system develops the original frictional wave dynamics by including the moisture tendency term (or the MF mode), along with a parameterized precipitation based on the Betts–Miller scheme. The linear instability analysis of this model provides solutions to elucidate the behaviors of the “pure” frictional convergence (FC) mode and the “pure” MF mode, respectively, as well as the behaviors of the combined FC–MF mode or the dynamical moisture mode. These results show that without the PBL frictional moisture convergence, the MF mode is nearly stationary and damped. Not only does the PBL frictional feedback make the damping MF mode grow with preferred planetary scale but it also enables the nearly stationary MF mode to move eastward slowly, resulting in an oscillation with a period of 30–90 days. This finding suggests the important role of the frictional feedback in generating eastward propagating unstable modes and selecting the preferred planetary scales. The MF process slows down the eastward-propagating short-wave FC mode by delaying the occurrence of deep convection and by enhancing the Rossby wave component. However, the longest wave (wavenumber one) is insensitive to the MF or the convective adjustment time, indicating that the unstable longest wave is primarily controlled by PBL frictional feedback process. Implications of these theoretical results in MJO simulation in general circulation models are discussed.

Sensitivity of MJO propagation to a robust positive Indian Ocean dipole event in the superparameterized CAM

AbstractThe superparameterized Community Atmosphere Model (SPCAM) is used to investigate the impact and geographic sensitivity of positive Indian Ocean Dipole (+IOD) sea‐surface temperatures (SSTs) on Madden‐Julian oscillation (MJO) propagation. The goal is to clarify potentially appreciable +IOD effects on MJO dynamics detected in prior studies by using a global model with explicit convection representation. Prescribed climatological October SSTs and variants of the SST distribution from October 2006, a +IOD event, force the model. Modest MJO convection weakening over the Maritime Continent occurs when either climatological SSTs, or +IOD SST anomalies restricted to the Indian Ocean, are applied. However, severe MJO weakening occurs when either +IOD SST anomalies are applied globally or restricted to the equatorial Pacific. MJO disruption is associated with time‐mean changes in the zonal wind profile and lower moist static energy (MSE) in subsiding air masses imported from the Subtropics by Rossby‐like gyres. On intraseasonal scales, MJO disruption arises from significantly smaller MSE accumulation, weaker meridional advective moistening, and overactive submonthly eddies that mix drier subtropical air into the path of MJO convection. These results (1) demonstrate that SPCAM reproduces observed time‐mean and intraseasonal changes during +IOD episodes, (2) reaffirm the role that submonthly eddies play in MJO propagation and show that such multiscale interactions are sensitive to interannual SST states, and (3) suggest that boreal fall +IOD SSTs local to the Indian Ocean have a significantly smaller impact on Maritime Continent MJO propagation compared to contemporaneous Pacific SST anomalies which, for October 2006, resemble El Niño‐like conditions.

Correction of Depth Bias in Upper-Ocean Temperature and Salinity Profiling Measurements from Airborne Expendable Probes

AbstractDuring the Dynamics of Madden–Julian Oscillation (DYNAMO) Experiment in 2011, airborne expendable conductivity–temperature–depth (AXCTD) probes and airborne expendable bathythermographs (AXBTs) were deployed using NOAA’s WP-3D Orion aircraft over the southern tropical Indian Ocean. From initial analysis of the AXCTD data, about 95% of profiles exhibit double mixed layer structures. The presence of a mixed layer from some of these profiles were erroneous and were introduced because of the AXCTD processing software not being able to correctly identify the starting point of the probe descent. This work reveals the impact of these errors in data processing and presents an objective method to remove such erroneous data from the profiles using spectrograms from raw audio files. Reconstructed AXCTD/AXBT profiles are compared with collocated shipborne conductivity–temperature–depth (CTD) and expendable bathythermograph (XBT) profiles and are found to be in good agreement.

Regression Analysis of Zonally Narrow Components of the MJO

Abstract Recent works have demonstrated that eastward-propagating features smaller than zonal wavenumber 3 but with spatial structures similar to those of the Madden–Julian oscillation (MJO) frequently develop over the Indo-Pacific warm pool. These signals are characterized by periods shorter than 4 weeks, but since they occur as part of a spectral peak of the MJO, they might be characterized by similar physics. These zonally narrow features occur at any phase of traditionally defined 30–60-day MJO events, but they occur most frequently in its active convective phase. This work presents a linear regression analysis based on filtering in the wavenumber–frequency domain to compare such signals with traditionally defined MJOs and 15–30 m s−1, convectively coupled Kelvin waves. Results show that the trough collocated with the easterly wind anomaly extends westward into the region of lower-tropospheric westerly wind and deep convection in the zonally narrow slow signals and MJOs. The fast Kelvin waves have a ridge anomaly collocated with the westerly wind anomaly. The zonally narrow slow signals and MJOs include a warm anomaly in the boundary layer west of the deep convection that is absent in fast Kelvin waves. Results suggest that MJO dynamics are not confined to the 30–60-day band and that time scales as short as 2 weeks could be considered in wavenumber–frequency diagnostics for the MJO.