Mixed Rossby Gravity Research Articles

Effects of equatorially-confined shear flow on MRG and Rossby waves

Linear modal stability analysis of a mean zonal shear flow is carried out in the framework of rotating shallow water equations (RSWE), both under the β-plane approximation and in the full spherical coordinate system. Two base flows – equatorial easterly (EE) and westerly (EW) – with Gaussian profiles highly confined to small latitudes are analysed. At low Froude number, mixed Rossby-gravity (MRG) and Rossby waves are found to be particularly affected by shear, with prominent changes at higher wavenumbers. These waves become practically non-dispersive at large wavenumbers in EE. The perturbations are found to be more confined equatorially in EE than in EW with the degree of confinement being more pronounced in the β-plane system compared to the full spherical system. At high Froude number, the phase speeds are significantly larger in the β-plane system for all families of waves. Under the β-plane approximation, exponentially unstable modes can be excited, having negative (positive) phase speed in EE (EW). Strikingly, this flow is always neutrally stable with the full spherical system. This speaks for the importance of studying the whole spherical system even for equatorially confined shear.

The intricacies of identifying equatorial waves

AbstractEquatorial waves (EWs) are synoptic‐ to planetary‐scale propagating disturbances at low latitudes with periods from a few days to several weeks. Here, this term includes Kelvin waves, equatorial Rossby waves, mixed Rossby–gravity waves, and inertio‐gravity waves, which are well described by linear wave theory, but it also other tropical disturbances such as easterly waves and the intraseasonal Madden–Julian Oscillation with more complex dynamics. EWs can couple with deep convection, leading to a substantial modulation of clouds and rainfall. EWs are amongst the dynamic features of the troposphere with the longest intrinsic predictability, and models are beginning to forecast them with an exploitable level of skill. Most of the methods developed to identify and objectively isolate EWs in observations and model fields rely on (or at least refer to) the adiabatic, frictionless linearized primitive equations on the sphere or the shallow‐water system on the equatorial ‐plane. Common ingredients to these methods are zonal wave‐number–frequency filtering (Fourier or wavelet) and/or projections onto predefined empirical or theoretical dynamical patterns. This paper gives an overview of six different methods to isolate EWs and their structures, discusses the underlying assumptions, evaluates the applicability to different problems, and provides a systematic comparison based on a case study (February 20–May 20, 2009) and a climatological analysis (2001–2018). In addition, the influence of different input fields (e.g., winds, geopotential, outgoing long‐wave radiation, rainfall) is investigated. Based on the results, we generally recommend employing a combination of wave‐number–frequency filtering and spatial‐projection methods (and of different input fields) to check for robustness of the identified signal. In cases of disagreement, one needs to carefully investigate which assumptions made for the individual methods are most probably not fulfilled. This will help in choosing an approach optimally suited to a given problem at hand and avoid misinterpretation of the results.

Comparison of forecasting accuracy for Madden Julian Oscillation (MJO) and Convectively Coupled Equatorial Waves (CCEW) using Tropical Rainfall Measuring Mission (TRMM) and ERA-Interim precipitation forecast data for Indonesia

Tropical Rainfall Measuring Mission (TRMM) and ERA-Interim forecast data analyzed using second-order autoregressive AR(2) and space-time-spectra analysis methods (respectively) revealed contrasting results for predicting Madden Julian Oscillation (MJO) and Convectively Coupled Equatorial Waves (CCEW) phenomena over Indonesia. This research used the same 13-year series of daily TRMM 3B42 V7 derived datasets and ERA-Interim reanalysis model datasets from the European Center for Medium-Range Weather Forecasts (ECMWF) for precipitation forecasts. Three years (2016 to 2018) of the filtered 3B42 and ERA-Interim forecast data was then used to evaluate forecast accuracy by looking at correlation coefficients for forecast leads from day +1 through day +7. The results revealed that rainfall estimation data from 3B42 provides better results for the shorter forecast leads, particularly for MJO, equatorial Rossby (ER), mixed Rossby-gravity (MRG), and inertia-gravity phenomena in zonal wavenumber 1 (IG1), but gives poor correlation for Kelvin waves for all forecast leads. A consistent correlation for all waves was achieved from the filtered ERA-Interim precipitation forecast model, and although this was quite weak for the first forecast leads it did not reach a negative correlation in the later forecast leads except for IG1. Furthermore, Root Mean Square Error (RMSE) was also calculated to complement forecasting skills for both data sources, with the result that residual RMSE for the filtered ERA-Interim precipitation forecast was quite small during all forecast leads and for all wave types. These findings prove that the ERA-Interim precipitation forecast model remains an adequate precipitation model in the tropics for MJO and CCEW forecasting, specifically for Indonesia.

MJO Initiation Triggered by Amplification of Upper‐Tropospheric Dry Mixed Rossby‐Gravity Waves

AbstractA possibly important dynamical process for the Madden–Julian oscillation (MJO) convective initiation is proposed. An MJO event during the “CINDY2011” field campaign is triggered by eastward‐moving lower‐tropospheric mixed Rossby‐gravity (MRG) wave packets, and its leading precursor is predominance of upper‐tropospheric MRGs in the Indian Ocean (IO). Simple three‐dimensional model experiments reveal that the upper‐tropospheric MRGs in the IO are amplified particularly in the western IO (WIO) by their westward advection and wave accumulation due to the upper‐level convergence in mean easterlies of the Walker circulation. The model also predicts downward dispersion of the amplified upper‐tropospheric MRGs and resultant lower‐tropospheric MRG wave packet formation. This MRG evolution consistently explains the MJO initiation process during CINDY2011, which is further verified by ray tracing for MRGs. Upper‐tropospheric circumnavigating Kelvin waves assist the proposed mechanism by promoting MRG‐wave accumulation (advection) in their westerly (easterly) phases via enhanced zonal convergence and weakened easterlies (enhanced easterlies).

Variation of Radar-Observed Precipitation Characteristics in Relation to the Simultaneous Passages of a Madden–Julian Oscillation Event and Convectively Coupled Equatorial Waves during the Years of the Maritime Continent Pilot Study

AbstractIn this study, we examined the variations of precipitation morphology and rainfall in relation to the simultaneous passages of a Madden–Julian oscillation (MJO) event and convectively coupled equatorial waves (CCEWs) observed during the Years of the Maritime Continent pilot study. We utilized globally merged infrared brightness temperature data and the radiosonde and radar data observed aboard the Research Vessel Mirai at 4°4′S, 101°54′E. As well as the observed MJO event, equatorial Rossby waves (ERWs), Kelvin waves (KWs), and mixed Rossby–gravity waves (MRGWs) were identified. The radar data exhibited high-frequency variation, mainly caused by KWs and MRGWs, and low-frequency variation, mainly caused by the MJO and ERWs. The MRGWs predominantly modulated convective echo areas and both convective and stratiform volumetric rainfall. In contrast, the MJO event had little influence on the variance of convective echoes. Moreover, stratiform echo areas and volumetric rainfall were more strongly modulated by the combined effects of the MJO, ERWs, KWs, and MRGWs than their convective counterparts. The intense development of stratiform echo areas and volumetric rainfall was coherent with the superimposition of the active phases of the MJO event and all the analyzed CCEWs. The strongest development and a significant reduction of convective echo-top heights before and after the peak MJO date, respectively, were coherent with the passages of ERWs and MRGWs, which were the dominant wave types in modulating echo-top heights. Thus, it appears that the superimposition of the CCEWs on the MJO event exerted complex modulations on the convective activities within the MJO event.

Contributions of equatorial waves and small-scale convective gravity waves to the 2019/20 quasi-biennial oscillation (QBO) disruption

Abstract. In January 2020, unexpected easterly winds developed in the downward-propagating westerly quasi-biennial oscillation (QBO) phase. This event corresponds to the second QBO disruption in history, and it occurred 4 years after the first disruption of 2015/16. According to several previous studies, strong midlatitude Rossby waves propagating from the Southern Hemisphere (SH) during the SH winter likely initiated the disruption; nevertheless, the wave forcing that finally led to the disruption has not been investigated. In this study, we examine the role of equatorial waves and small-scale convective gravity waves (CGWs) in the 2019/20 QBO disruption using Modern-Era Retrospective Analysis for Research and Applications version 2 (MERRA-2) global reanalysis data. In June–September 2019, unusually strong Rossby wave forcing originating from the SH decelerated the westerly QBO at 0–5∘ N at ∼50 hPa. In October–November 2019, vertically (horizontally) propagating Rossby waves and mixed Rossby–gravity (MRG) waves began to increase (decrease). From December 2019, the contribution of the MRG wave forcing to the zonal wind deceleration was the largest, followed by the Rossby wave forcing originating from the Northern Hemisphere and the equatorial troposphere. In January 2020, CGWs provided 11 % of the total negative wave forcing at ∼43 hPa. Inertia–gravity (IG) waves exhibited a moderate contribution to the negative forcing throughout. Although the zonal mean precipitation was not significantly larger than the climatology, convectively coupled equatorial wave activities were increased during the 2019/20 disruption. As in the 2015/16 QBO disruption, the increased barotropic instability at the QBO edges generated more MRG waves at 70–90 hPa, and westerly anomalies in the upper troposphere allowed more westward IG waves and CGWs to propagate to the stratosphere. Combining the 2015/16 and 2019/20 disruption cases, Rossby waves and MRG waves can be considered the key factors inducing QBO disruption.

High-Frequency Mixed Rossby-Gravity Waves in the Mid-Troposphere Triggered Kerala Floods of 2018

Kerala, the South-West coastal state of India, was ravaged by a series of floods during the South-West Monsoon of 2018. The season was marked by severely anomalous rainfall trends, with over 100 mm of departures from the mean daily precipitation in the northern districts of the State. Unlike previous works, this study not only documents the extreme event, but also identifies the tropical dynamics responsible for it. The synoptic disturbances of the tropical Pacific triggered high-frequency Mixed Rossby-Gravity (MRG) waves in the mid-troposphere during Boreal Summer, which meddled with the Monsoon trough, as it propagated westward from the east Equatorial Indian Ocean to the east coast of Africa. Interestingly, these equatorially-trapped waves were found to have teamed up with Madden–Julian Oscillations (MJO) to fuel deep convection in the tropics and caused heavy rainfall over Kerala in 2018.

Revisiting the Quasi-Biennial Oscillation as Seen in ERA5. Part II: Evaluation of Waves and Wave Forcing

AbstractThis paper describes stratospheric waves in ERA5 and evaluates the contributions of different types of waves to the driving of the quasi-biennial oscillation (QBO). Because of its higher spatial resolution compared to its predecessors, ERA5 is capable of resolving a broader spectrum of waves. It is shown that the resolved waves contribute to both eastward and westward accelerations near the equator, mainly by the way of the vertical flux of zonal momentum. The eastward accelerations by the resolved waves are mainly due to Kelvin waves and small-scale gravity (SSG) waves with zonal wavelengths smaller than 2000 km, whereas the westward accelerations are forced mainly by SSG waves, with smaller contributions from inertio-gravity and mixed Rossby–gravity waves. Extratropical Rossby waves disperse upward and equatorward into the tropical region and impart a westward acceleration to the zonal flow. They appear to be responsible for at least some of the irregularities in the QBO cycle.

Dynamic and thermodynamic modulations of the convectively coupled equatorial waves by the MJO

The convectively coupled equatorial waves (CCEWs), including the Kelvin, mixed-Rossby gravity (MRG), eastward inertio-gravity (EIG), and westward inertio-gravity (WIG) waves, are dominant synoptic-scale waves in the tropics. In this study, the modulation of the CCEWs by the MJO is examined with observational data from 1985–2005 over the Indian Ocean (IO), Maritime Continent (MC), and Western Pacific (WP). We find that the MRG wave is strengthened (weakened) to the west (east) of the MJO convection. In contrast, the Kelvin, WIG and EIG waves are mostly strengthened over the MJO convective center. As MJO modulates the background vertical wind shear and moisture fields, a further analysis was conducted to reveal the relationship between the background dynamic and thermodynamic field changes and the wave intensity change. A significant negative correlation between the MJO-scale vertical wind shear and the MRG intensity variation suggests that the MRG wave is primarily modulated by the dynamic field. The intensity changes of the WIG and EIG waves are significantly correlated to the MJO moisture field. The Kelvin wave, which has both quasi-geostrophic and gravity wave nature, is modulated by both the MJO-scale vertical wind shear and specific humidity.

Real-Time Identification of Equatorial Waves and Evaluation of Waves in Global Forecasts

AbstractA novel technique is developed to identify equatorial waves in analyses and forecasts. In a real-time operational context, it is not possible to apply a frequency filter based on a wide centered time window due to the lack of future data. Therefore, equatorial wave identification is performed based primarily on spatial projection onto wave mode horizontal structures. Spatial projection alone cannot distinguish eastward- from westward-moving waves, so a broadband frequency filter is also applied. The novelty in the real-time technique is to off-center the time window needed for frequency filtering, using forecasts to extend the window beyond the current analysis. The quality of this equatorial wave diagnosis is evaluated. First, the “edge effect” arising because the analysis is near the end of the filter time window is assessed. Second, the impact of using forecasts to extend the window beyond the current date is quantified. Both impacts are shown to be small referenced to wave diagnosis based on a centered time window of reanalysis data. The technique is used to evaluate the skill of the Met Office forecast system in 2015–18. Global forecasts exhibit substantial skill (correlation > 0.6) in equatorial waves, to at least day 4 for Kelvin waves and day 6 for westward mixed Rossby–gravity (WMRG), and meridional mode number n = 1 and n = 2 Rossby waves. A local wave phase diagram is introduced that is useful to visualize and validate wave forecasts. It shows that in the model Kelvin waves systematically propagate too fast, and there is a 25% underestimate of amplitude in Kelvin and WMRG waves over the central Pacific.

Impacts of convectively coupled equatorial waves on rainfall extremes in Java, Indonesia

AbstractRainfall extremes cause significant socioeconomic impacts in Indonesia, as they are often followed by disastrous events, such as floods and landslides. Of particular interest is Java Island, the most populated region in Indonesia, which is prone to damaging flooding as a result of heavy rainfall. The prediction of rainfall extremes in this region has mainly been focused on the effects of seasonal and intraseasonal variability, such as monsoons and the Madden–Julian Oscillation. Here, using an extensive station database from 1987 to 2017 and the gridded Asian Precipitation‐Highly Resolved Observational Data Integration Toward Evaluation of Water Resources (APHRODITE) product from 1980 to 2007, we show that severe weather conditions associated with rainfall extremes in Java during the rainy season (November to April) can also be attributed to convectively coupled equatorial waves (CCEWs) that occur on a shorter time scale.Evidence is presented that CCEWs, including Kelvin, equatorial Rossby (ER), and mixed Rossby‐gravity (MRG) waves, significantly modulate daily rainfall extremes over Java Island. Of these three types, the Kelvin waves have the greatest influence on heavy rainfall over Java Island. The convectively active (suppressed) phases of Kelvin waves increase (decrease) the probability of extreme rain events over land regions by up to 60% (50%) of the baseline probability. On the other hand, the convectively active phases of ER (MRG) waves increase the probability by up to 45% (40%), while the suppressed phases decrease this by up to 40% (30%). In terms of the mechanism of rainfall extremes, CCEWs modulate moisture flux convergence, leading to the enhancement of local convection over the region. In addition, the analysis of multiple wave events indicates that positive (negative) interferences of the CCEWs lead to an amplification (suppression) of extreme rainfall probability. Overall, the results suggest that equatorial waves provide an important source of the predictability for daily extreme rainfall events over Java Island.

Role of equatorial waves and convective gravity waves in the 2015/16 quasi-biennial oscillation disruption

Abstract. In February 2016, the descent of the westerly phase of the quasi-biennial oscillation (QBO) was unprecedentedly disrupted by the development of easterly winds. Previous studies have shown that extratropical Rossby waves propagating into the deep tropics were the major cause of the 2015/16 QBO disruption. However, a large portion of the negative momentum forcing associated with the disruption still stems from equatorial planetary and small-scale gravity waves, which calls for detailed analyses by separating each wave mode compared with climatological QBO cases. Here, the contributions of resolved equatorial planetary waves (Kelvin, Rossby, mixed Rossby–gravity (MRG), and inertia–gravity (IG) waves) and small-scale convective gravity waves (CGWs) obtained from an offline CGW parameterization to the 2015/16 QBO disruption are investigated using MERRA-2 global reanalysis data from October 2015 to February 2016. In October and November 2015, anomalously strong negative forcing by MRG and IG waves weakened the QBO jet at 0–5∘ S near 40 hPa, leading to Rossby wave breaking at the QBO jet core in the Southern Hemisphere. From December 2015 to January 2016, exceptionally strong Rossby waves propagating horizontally (vertically) continuously decelerated the southern (northern) flank of the jet. In February 2016, when the westward CGW momentum flux at the source level was much stronger than its climatology, CGWs began to exert considerable negative forcing at 40–50 hPa near the Equator, in addition to the Rossby waves. The enhancement of the negative wave forcing in the tropics stems mostly from strong wave activity in the troposphere associated with increased convective activity and the strong westerlies (or weaker easterlies) in the troposphere, except that the MRG wave forcing is more likely associated with increased barotropic instability in the lower stratosphere.

The Detailed Dynamics of the Hadley Cell. Part II: December–February

AbstractThis paper complements an earlier paper on the June–August Hadley cell by giving a detailed analysis of the December–February Hadley cell as seen in a 30-yr climatology of ERA-Interim data. The focus is on the dynamics of the upper branch of the Hadley cell. There are significant differences between the Hadley cells in the two solsticial seasons. These are particularly associated with the ITCZs staying north of the equator and with mean westerlies in the equatorial regions of the east Pacific and Atlantic in December–February. The latter enables westward-moving mixed Rossby–gravity waves to be slow moving in those regions and therefore respond strongly to upstream off-equatorial active convection. However, the main result is that in both seasons it is the regions and times of active convection that predominantly lead to upper-tropospheric outflows and structures that average to give the mean flow toward the winter pole, and the steady and transient fluxes of momentum and vorticity that balance the Coriolis terms. The response to active convection in preferred regions is shown by means of regressions on the data from the climatology and by synoptic examples from one season. Eddies with tropical origin are seen to be important in their own right and also in their interaction with higher-latitude systems. There is support for the relevance of a new conceptual model of the Hadley cell based on the sporadic nature of active tropical convection in time and space.

Exploring the Factors Influencing the Strength and Variability of Convectively Coupled Mixed Rossby–Gravity Waves

AbstractThe three-dimensional structure, horizontal and vertical propagation characteristics, and convection–circulation coupling of the convectively coupled westward-propagating mixed Rossby–gravity (MRG) waves are examined by classifying the waves based on their amplitude. Convective signals of the MRG waves were identified and isolated using empirical orthogonal function analysis of wavenumber–frequency-filtered outgoing longwave radiation (OLR) data. It was found that about 50% of the MRG waves occur during the August–November months, and this strong seasonality was considered while characterizing the MRG waves. Five strong and five weak MRG wave seasons were identified during 1979–2019, based on seasonal wave amplitude, and through this classification, significant differences in the strength of convection–circulation coupling, zonal scale of circulation, vertical structure, and propagation characteristics of MRG waves were brought out. It was also found that the seasonal mean background state is significantly different during strong and weak MRG wave seasons. While a La Niña–like background state was found to favor enhanced MRG wave activity, the MRG wave activity is mostly suppressed during an El Niño–like background state. The presence of extratropical wave intrusions is another factor that distinguishes the strong MRG wave seasons from the weak ones. Eastward- and northeastward-propagating extratropical wave trains from the South Atlantic to the east Indian Ocean were observed during strong MRG wave seasons.

Effect of tropical sub‐seasonal variability on heatwaves over India

AbstractOne of the important consequences of changing climate is the increase in temperature extremes and heat stress over many regions of the world. Understanding the various factors that modulate these events on different time scales is of utmost importance due to their great societal relevance. For example, understanding and prediction on sub‐seasonal time scales is crucial for reducing losses due to extremes and diligent management of limited resources. In this study, the effect of tropical intraseasonal oscillations like convectively coupled equatorial waves (CCEWs) such as equatorial Rossby wave (ER), Kelvin wave, mixed Rossby gravity waves and the Madden and Julian Oscillations (MJO) in modulating heatwaves occurrence over India is explored. We find that these oscillations significantly modulate extreme heat events. The extent of the modulation is also found to be highly dependent on the location, type and phase of these waves. The maximum effect is observed in case of ER with positive phase (defined as high outgoing longwave radiation phase) enhancing the frequency of the events by almost twice the normal values, and supporting long duration and more intense heatwaves while the opposite phase totally suppresses the heatwave events. The effect of MJO is region dependent with positive (negative) phase increasing (decreasing) the heatwave frequency over South and Eastern (North‐western) parts of the country. Kelvin and MT waves are also found to have lesser but significant effect on heatwaves, and mostly reduces the occurrence of heatwaves. Overall, the study shoes that the ER wave has the most significant effect on the frequency and duration of heatwaves over the study region.

Dynamical Roles of Mixed Rossby–Gravity Waves in Driving Convective Initiation and Propagation of the Madden–Julian Oscillation: General Views

AbstractMotivated by the previous case study, this work shows that dynamical variations of mixed Rossby–gravity waves with tropical depression–type circulations (MRGTDs) are possible drivers of convective initiation and propagation of the Madden–Julian oscillation (MJO) by performing statistical analysis. MJO events initiated in the Indian Ocean (IO) in boreal winter are objectively identified solely using outgoing longwave radiation data. The lagged-composite analysis of detected MJO events demonstrates that MJO convection is initiated in the southwestern IO (SWIO), where strong MRGTD–convection coupling is statistically found. Further classification of MJO cases in terms of intraseasonal convection and MRGTD activities in the SWIO suggests that 26 of 47 cases are related to more enhanced MRGTDs, although they can also be secondarily affected by Kelvin waves. For those MRGTD-enhanced MJO events, MJO convective initiation is primarily triggered by low-level MRGTD circulations that develop via the enhancement of downward energy dispersion in accordance with upper-tropospheric baroclinic conversion. This is supported by the modulation of MRGTD structure associated with zonal wave contraction due to upper-tropospheric zonal convergence, and plentiful moisture advected into the western IO. Following this MRGTD-induced MJO triggering and midtropospheric premoistening in the IO contributed by MRGTD shallow circulations as well as intraseasonal winds during the MJO-suppressed phase, low-level MRGTD winds with eastward group velocity successively trigger convection to the east, which helps MJO convective propagation over the IO. The interannual atmospheric variability may affect whether the presented MRGTD-related processes are effective or not.

Atmospheric Subseasonal Variability and Circulation Regimes: Spectra, Trends, and Uncertainties

AbstractThe globally integrated subseasonal variability associated with the two main atmospheric circulation regimes, the balanced (or Rossby) and unbalanced (or inertia–gravity) regimes, is evaluated for the four reanalysis datasets: ERA-Interim, JRA-55, MERRA, and ERA5. The results quantify amplitudes and trends in midlatitude traveling and quasi-stationary Rossby wave patterns as well as in the equatorial wave activity across scales. A statistically significant reduction of subseasonal variability is found in Rossby waves with zonal wavenumber k = 6 along with an increase in variability in wavenumbers k = 3–5 in the summer seasons of both hemispheres. The four reanalyses also agree regarding increased variability in the large-scale Kelvin waves, mixed Rossby–gravity waves, and westward-propagating inertio-gravity waves with the lowest meridional mode. The amplitude and sign of trends in inertia–gravity modes with smaller zonal scales and greater meridional modes differ between the ERA-Interim and JRA-55 datasets on the one hand and the ERA5 and MERRA data on the other. An increased variability in the ERA-Interim and JRA-55 accounts for positive trends in their total subseasonal variability.

El Niño phase-dependent high-frequency variability in Western Equatorial Pacific

The intensities of high-frequency (HF) variability with period less than 90 days at different phases of El Nino events were investigated through observational data analysis. A large asymmetry in the HF variability intensity between the developing phase and decaying phase (i.e., pre-peak stage versus post-peak stage) of eastern Pacific (EP) El Nino is revealed, while the amplitude and spatial pattern of the sea surface temperature anomaly during these two stages are almost same. The diagnosis shows that the asymmetry is significant not only on intraseasonal time scale (20–90 days) but also on synoptic time scale (less than 20 days). The anatomy analysis further unveils that the asymmetric synoptic variability between the two episodes arises from the asymmetric intensities of the equatorial Rossby and mixed Rossby gravity (MRG) waves. We suggest that the stronger vertical easterly wind shear in the pre-peak stage than that in the post-peak stage plays a vital role in causing the stronger synoptic equatorial Rossby and MRG waves in the pre-peak stage. Meanwhile, the drier atmosphere and more descending motion in the post-peak stage contribute to the weakened intraseasonal and synoptic variabilities in that stage. The aforementioned weakened easterly wind shear, drier atmosphere and more descending motion in the post-peak stage can be traced back to the occurrence of the anomalous anticyclone circulation over the western North Pacific since the decaying phase of El Nino. The essential role of large-scale environmental conditions in modulating the HF variability during the two episodes is further confirmed by modeling experiments.

A Simulation Study of Kelvin Waves Interacting with Synoptic Events during December 2016 in the South China Sea and Maritime Continent

Abstract This study evaluates the model simulation of interaction between convectively coupled tropical disturbances in the South China Sea (SCS) and Maritime Continent (MC). The Model for Prediction Across Scales (MPAS) is used to simulate the major interaction events in December 2016 with a fixed 60-km horizontal resolution and a variable 60–15-km resolution. Compared with an observational analysis, the overall spatial and temporal evolution of simulated rainfall and circulation reveals the capability of MPAS for reproducing equatorial Kelvin waves (KWs), and the interactions with equatorial Rossby waves and off-equatorial mixed Rossby–gravity (MRG)/TD-type waves up to a 5–7-day lead in both fixed 60-km and variable 60–15-km resolutions. Two interaction events are further examined. One involves an MRG/TD wave, prevailing northeasterlies, and a Borneo vortex developed in SCS during 6–11 December. The other involves a KW converging with the easterly trade wind that led to an MRG/TD-type wave and the formation of Typhoon Nock-ten during 16–20 December. The MPAS 60–15-km resolution tends to produce stronger precipitation and more coherent vorticity structures in both interaction events. Increasing the resolution to 15 km contributes to better representation of finer spatial vorticity and rainfall structures.

Vertical Velocity Profiles in Convectively Coupled Equatorial Waves and MJO: New Diagnoses of Vertical Velocity Profiles in the Wavenumber–Frequency Domain

Abstract A new diagnostic framework is developed and applied to ERA-Interim to quantitatively assess vertical velocity (omega) profiles in the wavenumber–frequency domain. Two quantities are defined with the first two EOF–PC pairs of omega profiles in the tropical ocean: a top-heaviness ratio and a tilt ratio. The top-heaviness and tilt ratios are defined, respectively, as the cospectrum and quadrature spectrum of PC1 and PC2 divided by the power spectrum of PC1. They represent how top-heavy an omega profile is at the convective maximum, and how much tilt omega profiles contain in the spatiotemporal evolution of a wave. The top-heaviness ratio reveals that omega profiles become more top-heavy as the time scale (spatial scale) becomes longer (larger). The MJO has the most top-heavy profile while the eastward inertio-gravity (EIG) and westward inertio-gravity (WIG) waves have the most bottom-heavy profiles. The tilt ratio reveals that the Kelvin, WIG, EIG, and mixed Rossby–gravity (MRG) waves, categorized as convectively coupled gravity waves, have significant tilt in the omega profiles, while the equatorial Rossby (ER) wave and MJO, categorized as slow-moving moisture modes, have less tilt. The gross moist stability (GMS), cloud–radiation feedback, and effective GMS were also computed for each wave. The MJO with the most top-heavy omega profile exhibits high GMS, but has negative effective GMS due to strong cloud–radiation feedbacks. Similarly, the ER wave also exhibits negative effective GMS with a top-heavy omega profile. These results may indicate that top-heavy omega profiles build up more moist static energy via strong cloud–radiation feedbacks, and as a result, are more preferable for the moisture mode instability.