MJO Phase Research Articles

The out‐of‐phase variation of surface air temperature anomalies over northern Central Asia between January and February: The role of intraseasonal variations in tropical convection

AbstractThe out‐of‐phase mode of winter surface air temperature anomalies (SATAs) over northern Central Asia (NCA; 45°–65°N, 40°–100°E) between January and February is investigated in this study. This mode corresponds to warm (cold) SATAs in January (February) over NCA and is mainly modulated by the enhanced tropical convection anomalies over the Maritime Continent in previous late December, associated with MJO phase 4. These tropical convection anomalies can induce anomalous tropospheric Rossby‐wave sources over the North Pacific in late December. The eastward‐propagating Rossby‐wave train, triggered by these anomalous Rossby‐wave sources, can cause negative and positive tropospheric geopotential height anomalies over the Greenland–Scandinavia region and NCA in the following early–mid‐January, subsequently leading to warm SATAs over NCA in January. The negative geopotential height anomalies over the Greenland–Scandinavia region in early–mid‐January can trigger upward‐propagating wave activity fluxes (WAFs) into the stratosphere, resulting in negative stratospheric geopotential height anomalies in late January–early February. These stratospheric anomalies, by triggering downward‐propagating WAFs, can in turn lead to positive tropospheric geopotential height anomalies over the Greenland–Scandinavia region in early February. These anomalies over the Greenland–Scandinavia region can maintain themselves in the following mid‐ and late February by feedback of anomalous storm tracks, and cause negative geopotential height anomalies and subsequently cold SATAs over NCA in February by triggered southeastward‐propagating Rossby‐wave train.

Subseasonal Variability of U.S. Coastal Sea Level from MJO and ENSO Teleconnection Interference

Abstract Climate variability affects sea levels as certain climate modes can accelerate or decelerate the rising sea level trend, but subseasonal variability of coastal sea levels is underexplored. This study is the first to investigate how remote tropical forcing from the MJO and ENSO impact subseasonal U.S. coastal sea level variability. Here, composite analyses using tide gauge data from six coastal regions along the U.S. East and West Coasts reveal influences on sea level anomalies from both the MJO and ENSO. Tropical MJO deep convection forces a signal that results in U.S. coastal sea level anomalies that vary based on MJO phase. Further, ENSO is shown to modulate both the MJO sea level response and background state of the teleconnections. The sea level anomalies can be significantly enhanced or weakened by the MJO-associated anomaly along the East Coast due to constructive or destructive interference with the ENSO-associated anomaly, respectively. The West Coast anomaly is found to be dominated by ENSO. We examine physical mechanisms by which MJO and ENSO teleconnections impact coastal sea levels and find consistent relationships between low-level winds and sea level pressure that are spatially varying drivers of the variability. Two case studies reveal how MJO and ENSO teleconnection interference played a role in notable coastal flooding events. Much of the focus on sea level rise concerns the long-term trend associated with anthropogenic warming, but on shorter time scales, we find subseasonal climate variability has the potential to exacerbate the regional coastal flooding impacts. Significance Statement Coastal flooding due to sea level rise is increasingly threatening communities, but natural fluctuations of coastal sea levels can exacerbate the human-caused sea level rise trend. This study assesses the role of tropical influences on coastal subseasonal (2 weeks–3 months) sea level heights. Further, we explore the mechanisms responsible, particularly for constructive interference of signals contributing to coastal flooding events. Subseasonal signals amplify or suppress the lower-frequency signals, resulting in higher or lower sea level heights than those expected from known climate modes (e.g., ENSO). Low-level onshore winds and reduced sea level pressure connected to the tropical phenomena are shown to be indicators of increased U.S. coastal sea levels, and vice versa. Two case studies reveal how MJO and ENSO teleconnection interference played a role in notable coastal flooding events. Much of the focus on sea level rise concerns the long-term trend associated with anthropogenic warming, but on shorter time scales, we find subseasonal climate variability has the potential to exacerbate the regional coastal flooding impacts.

Diurnal cycle of precipitation over coastal sea and small islands in the eastern region of Sumatra including season and Madden Julian Oscillation signatures

The Barisan Mountains influence atmospheric circulation in Sumatra and surrounding areas. Yet, precipitation patterns in eastern Sumatra, especially along the coast, are poorly understood. This study examines diurnal precipitation cycles in coastal seas and small islands of eastern Sumatra, utilizing 2015–2021 IMERG Version 06 rainfall data, data from 21 island rain gauges, and ERA5 reanalysis data to assess precipitation through three parameters: precipitation amount (PA), precipitation frequency (PF), and precipitation intensity (PI). The diurnal cycle of PA and PF is more pronounced than PI. Distinct diurnal precipitation patterns emerge between coastal seas and small islands despite their shared east coast location on Sumatra. Coastal seas exhibit early morning precipitation peaks forming Sumatras squall, influenced by offshore Sumatra and Malay Peninsula propagations, accentuated by high wind speeds. Coastal shape, land topography, and Sumatra's island size also impact coastal sea precipitation. On small islands, rainfall peaks earlier (12:00–15:00 LT) with gentle winds, indicating local convection due to land heating dominance. In addition, the low-amplitude topography of Bangka and Belitung islands also increase rainfall, so PA and PF are higher on these islands than on other small islands. Seasonal analysis reveals rising PA and PF near the equator during the dry southeast monsoon (SEM), linked to the dominant Inter-Tropical Convergence Zone (ITCZ). Conversely, areas like Bangka, Belitung, and the South Coast of Sumatra, farther from the equator, witness higher PA and PF during the wet northwest monsoon (NWM). The South Coast of Sumatra saw a substantial PA increase during NWM, about 60% higher than the southeast dry period. Despite varying cycle amplitudes, peak times for PA and PF remain consistent across seasons. MJO analysis during NWM highlights heightened PA and PF amplitudes across eastern Sumatra, especially over equator-adjacent coastal regions and small islands, reaching 40–70% above non-active MJO phases. In summary, this research examines diurnal precipitation variations in coastal seas and small islands of eastern Sumatra, enhancing understanding of regional climate patterns and factors impacting precipitation cycles in the area.

Dynamical influence of MJO phases on the onset of Indian monsoon

There is a need to understand the onset of monsoon dynamics as the date of onset of monsoon (DOM) is an important parameter in framing all the policy for the imminent season like crop choice, sowing schedule, disaster management, power distribution etc It is observed that the interannual variability of the DOM in India is about 7–8 days, making it more challenge to predict this at long lead. The MJO phases are linked with the different convection centres and hence, influences the global circulation process and the rainfall. In this paper the dynamical influence of the different phases of MJO are being quantified on DOM and its progress in continental India by using the multi-source atmospheric and oceanic parameters like wind structure, outgoing longwave radiation (OLR), sea surface temperature (SST). The linkage of the active and inactive phases of MJO along with the favourable conditions for DOM is obtained by using the pentad analysis of associated parameters in different clusters for both the wet and dry phases of MJO along with the strength for the period 1980–2018. Also the dynamics are studied for the early, normal and late onset years separately to understand the relation better. It is inferred that the wet (dry) phase leads to early (late) monsoon onset over Kerala (MOK) in India. To address the progress of monsoon the DOM in Rajasthan (MOR) is considered and the rainfall anomalies during MOK-MOR period are linked to the MJO phases. It is inferred that the wet MJO phase with negative OLR anomaly triggers the fast progress of monsoon over India. This understanding will surely help operational researchers and the NWP modellers for improving the methodologies for the advanced and accurate prediction of DOM.

Impact of Quasi-Biweekly Oscillation on Southeast Asian Cold Surge Rainfall Monitored by TRMM Satellite Observation

Based on the Tropical Rainfall Measuring Mission (TRMM) satellite observation and ERA5 re-analysis dataset, this paper studies the influence of the northwestward-propagating quasi-biweekly oscillation (QBWO) over the western North Pacific on cold surge rainfall (CSR) over Southeast Asia. Cold surges are the most important driver affecting Southeast Asian rainfall on a synoptic scale. The presence of the QBWO during phases 6–8, in which the associated active convection coupling with a cyclonic circulation reaches Southeast Asia, provides a favorable environment for the increase of CSR. The increase in CSR primarily occurs east of the Philippines, leading to a high likelihood of triggering extreme rainfall. The effects from the QBWO are independent of those from the active MJO phases over Southeast Asia. Additionally, cold surge activity could also be influenced by the QBWO. An examination of the QBWO and MJO indicates that the most preferred phases for the occurrence of cold surges are the time when phase 1 of the QBWO co-exists with phase 7 of the MJO or the time when phase 7 of the QBWO couples with phase 5 of the MJO. Accordingly, about 40% of the total cold surge days would fall in either combination.

Response of tropical convection over the western Pacific to stratospheric polar vortex during boreal winter

AbstractIt has been extensively discussed that stratospheric polar vortex significantly contribute to the variability and predictability of tropospheric weather and climate over midlatitudes and polar regions. However, we currently have little understanding of the exact influence of stratospheric polar vortex on tropical troposphere. Here, the exact responses of tropical convection to Northern Hemisphere (NH) extreme stratospheric polar vortex events (i.e., strong vortex [SV] events and weak vortex [WV] events) are investigated in this study. We find that the western Pacific region is particularly noteworthy when focusing on the spatial distribution of tropical convective activity. Significant enhancement of convective activity over the western Pacific is clearly observed in about 2 weeks after WV events, while opposite responses to SV events can also be expected. The change in convective activity over the western Pacific is considerably contributed by the intraseasonal variability, and thus MJO phase 7 (3) is dominant after WV (SV) events. The seesaw pattern of zonal‐mean convective activity near equator may be a property of MJO itself, not definitely caused by the stratospheric polar vortex as previously thought. A significantly linear correlation between anomalous convective activity and static stability near tropopause over the western Pacific is found, suggesting that circumstances in the tropical upper troposphere play a crucial role in the evolution of tropical convection, especially for the MJO.

VARIABILITY ANALYSIS OF SIGNIFICANT WAVE HEIGHTS AND WIND WAVES IN RIAU ARCHIPELAGO SEA PART ALKI 1

The Riau Archipelago Sea is part of the Indonesian Archipelago Sea Lane (ALKI) 1, with a very high intensity of crossing ships. Analysis of surface wind speed and significant wave height is the most important for the safety and performance of offshore shipping. This research aims to study wave characteristics and wind speed by identifying the main factors that affect significant wave height and surface wind speed. Dominant factors that affect significant wave height and wind speed are needed to decide on the safest path and the best time before crossing in ALKI-1. Temporal and spatial analysis of the seasonal variability of significant wave height and wind speed using ECMWF data for 18 years. The data used are significant wave height data and wind speed every 6 hours during the period 2000 – 2018. Three observation points are used for temporal analysis, it is found that significant wave height and wind speed are influenced by two main factors, namely MJO and Monsoon. Maximum significant wave height and wind speed that occurs in the SON period for points 3 and the DJF period at points 1 and 2. MJO affects directly from phases one to eight for observation points 1, 2, and 3. The 4, 7, and 8 MJO phases affect the value of wave height and wave speed significantly, and the 1, 2, and 5 MJO phases affect wave height and wind speed weakly.
 Keywords: ALKI 1, MJO, MJO phase, Monsoon, Significant wave height, Wind speed.

Effect of vertical overturning circulation scale and moist static energy tendency on MJO phase speed

The combined effects of vertical overturning circulation (VOC) zonal length scale and moist static energy (MSE) tendency zonal asymmetry on MJO phase speed were investigated based on diagnosis of ERA-Interim data over a 40-year period (1979–2019). In a key region (80°–100°E), 128 MJO events were selected. It was found that the larger the VOC zonal length scale, the faster the MJO eastward propagation. The correlation coefficient between them was 0.52, exceeding the 99% confidence level. A significant positive correlation (0.59) was also identified between the phase speed and MSE tendency zonal asymmetry. A linear regression model based on the aforementioned two parameters was constructed, and the phase speed could be estimated based on the model. The correlation coefficient between the reconstructed phase speed and its observed counterpart was 0.73, exceeding the 99% confidence level with an F-test. A composite analysis of the fast and slow groups indicated that the VOC zonal length scale was modulated by the background state. An El Niño (La Niña)-like SST pattern and associated precipitation anomalies promoted a larger (smaller) VOC zonal length scale and thus a faster (slower) propagation speed. A sensitivity test with a reference point in a different longitudinal zone (120°–140°E) was conducted. Again, there were significant relationships between the MJO phase speed and the two parameters. The correlation between the reconstructed and observed phase speed was 0.67, exceeding the 99% confidence level.摘要本文通过对1979–2019年ERA-I再分析资料进行诊断分析, 研究了MJO垂直环流 (VOC) 纬向尺度和湿静力能 (MSE) 趋势纬向不对称性对MJO传播速度的综合影响. 研究结果表明, MJO传播速度与VOC的纬向尺度和MSE趋势纬向梯度之间存在显著的正相关关系. 基于上述两个参数, 本文建立了线性回归模型, 该模型可以较好的估计MJO的传播速度. 本文对上述结果在不同的地理区域 (120°–140°E) 进行了敏感性测试. MJO传播速度仍然与这两个参数之间有显著的正相关关系.

The Impact of the Interaction between Madden-Julian Oscillation and Cold Surge, on Rainfall over Western Indonesia

The Madden-Julian Oscillation and Cold Surge phenomena have been known to cause increased rainfall, with the capacity to trigger hydrometeorological disasters, in western Indonesia. However, further investigations are required regarding the interaction between these phenomena on rainfall pattern. Therefore, this study aims to analyze the interaction between MJO and CS over western Indonesia, particularly by using land-based rainfall observation data from multiple stations, as previous studies were dominated by the use of gridded data from remote observations. This study utilized in-situ observation data obtained from 4329 weather observations and rain stations between 1989 and 2018. Subsequently, quality control performed based on data availability exceeding 70% over a 30-year period resulted in 303 selected stations to be used for further analysis. Meanwhile, the RMM index, as well as reanalysis data of mean sea level pressure and 925 hPa meridional wind, were also applied for MJO and CS identification. According to the composite analysis, the effect of CS on MJO phases tends to increase precipitation by about 50%, over western Indonesia, with maximum increase ranging from 200 to 400% over the northeastern coast of Sumatra, around Karimata Strait (Riau Islands and West Kalimantan), as well as the northern coast of Java. These areas are exposed to the sea and have direct access to the wind-terrain interaction. In addition, the highest rainfall anomaly due to the MJO-CS interaction occurs around Karimata Strait, followed by northern Sumatra and Java, with spatially averaged rainfall anomaly reaching 5 mm/day over the area.

Differences in Mechanisms of Orographic Rainfall over West Sumatra (Case Study: 10 April and 23 April 2004)

Two different mechanisms of orographic rainfall enhancement  in West Sumatra were investigated utilizing observed data during the Coupling Processes in the Equatorial Atmosphere (CPEA)-I campaign. The variation of the atmospheric conditions during the campaign was shown by rainfall, surface wind, humidity, and stability index. An X-band Doppler radar captured the atmospheric conditions related to the enhancement of orographic rainfall mechanisms. The dry and less stable atmospheric conditions resulted in the convective type of rainfall. In contrast, the humid and stable atmospheric conditions brought the large-scale rainfall in the mountainous region where the events took place coincided with the inactive and active MJO phases..

Impact of rainfall intensity, monsoon and MJO to rainfall merging in the Indonesian maritime continent

Merging is an estimation technique used to improve the accuracy of rainfall predictions by combining two rainfall predictions. In fact, the performance of remote sensing estimates varies in each place and time where the influence of the time has a relationship with the global weather phenomenon. The aim of this study is to investigate the influence of the two most influential phenomena on rainfall in the Indonesian maritime continent: the monsoon and Madden–Julian Oscillation. This assessment also analyzed the impact of rainfall intensity. While the change in correlation, root-mean-square-error (RMSE) and mean-absolute-error (MAE) will be used to assess the effectiveness of merging of monsoons and MJO. The result shows that the intensity of rainfall apparently affect the accuracy of merging, where the moderate-intensity has low RMSE and MAE and high correlation compared to heavy or very heavy rainfall. While comparing other phases and season found, the 5th phase of MJO and rainy seasons have the best performance. Moreover, among modification methods, the modification of conditional merging (CM) is the best merging technique for all seasons and MJO’s phases.

Sources of Tropical Subseasonal Skill in the CFSv2

Abstract This study applies Fourier filtering to a combination of rainfall estimates from TRMM and forecasts from the CFSv2. The combined data are filtered for low-frequency (LF, ≥120 days) variability, the MJO, and convectively coupled equatorial waves. The filtering provides insight into the sources of skill for the CFSv2. The LF filter, which encapsulates persistent anomalies generally corresponding with SSTs, has the largest contribution to forecast skill beyond week 2. Variability within the equatorial Pacific is dominated by its response to ENSO, such that both the unfiltered and the LF-filtered forecasts are skillful over the Pacific through the entire 45-day CFSv2 forecast. In fact, the LF forecasts in that region are more skillful than the unfiltered forecasts or any combination of the filters. Verifying filtered against unfiltered observations shows that subseasonal variability has very little opportunity to contribute to skill over the equatorial Pacific. Any subseasonal variability produced by the model is actually detracting from the skill there. The MJO primarily contributes to CFSv2 skill over the Indian Ocean, particularly during March–May and MJO phases 2–5. However, the model misses opportunities for the MJO to contribute to skill in other regions. Convectively coupled equatorial Rossby waves contribute to skill over the Indian Ocean during December–February and the Atlantic Ocean during September–November. Convectively coupled Kelvin waves show limited potential skill for predicting weekly averaged rainfall anomalies since they explain a relatively small percent of the observed variability.

Subseasonal Forecast Skill over the Northern Polar Region in Boreal Winter

AbstractPentad (5-day averaged) forecast skill over the Arctic region in boreal winter is evaluated for the subseasonal to seasonal prediction (S2S) systems from three operational centers: the European Centre for Medium-Range Weather Forecasts (ECMWF), the U.S. National Centers for Environmental Prediction (NCEP), and Environment and Climate Change Canada (ECCC). The results indicate that for a lead time longer than about 10 days the forecast skill of 2-m air temperature and 500-hPa geopotential height in the Arctic area is low compared to the tropical and midlatitude regions. The three S2S systems have comparable forecast skill in the northern polar region. Relatively high skill is observed in the Arctic sector north of the Bering Strait in pentads 4–6. Possible sources of S2S predictability in the polar region are explored. The polar forecast skill is found to be dependent on the phase of the Arctic Oscillation (AO) in the initial condition; that is, forecasts initialized with the negative AO are more skillful than those starting from the positive AO. This is likely due to the influence of the stratospheric polar vortex. The tropical MJO is found to also influence the prediction skill in the polar region. Forecasts starting from MJO phases 6–7, which correspond to suppressed convection in the equatorial eastern Indian Ocean and enhanced convection in the tropical western Pacific, tend to be more skillful than those initialized from other MJO phases. To improve the polar prediction on the subseasonal time scale, it is important to have a well-represented stratosphere and tropical MJO and their associated teleconnections in the model.

Spatial Correction of Multimodel Ensemble Subseasonal Precipitation Forecasts over North America Using Local Laplacian Eigenfunctions

Abstract The extent to which submonthly forecast skill can be increased by spatial pattern correction is examined in probabilistic rainfall forecasts of weekly and week-3–4 averages, constructed with extended logistic regression (ELR) applied to three ensemble prediction systems from the Subseasonal-to-Seasonal (S2S) project database. The new spatial correction method projects the ensemble-mean rainfall neighboring each grid point onto Laplacian eigenfunctions and then uses those amplitudes as predictors in the ELR. Over North America, individual and multimodel ensemble (MME) forecasts that are based on spatially averaged rainfall (e.g., first Laplacian eigenfunction) are characterized by good reliability, better sharpness, and higher skill than those using the gridpoint ensemble mean. The skill gain is greater for week-3–4 averages than week-3 leads and is largest for MME week-3–4 outlooks that are almost 2 times as skillful as MME week-3 forecasts over land. Skill decreases when using more Laplacian eigenfunctions as predictors, likely because of the difficulty in fitting additional parameters from the relatively short common reforecast period. Higher skill when increasing reforecast length indicates potential for further improvements. However, the current design of most subseasonal forecast experiments may prove to be a limit on the complexity of correction methods. Relatively high skill for week-3–4 outlooks with winter starts during El Niño and MJO phases 2–3 and 6–7 reflects particular opportunities for skillful predictions.

Favoured cycles and other features of anomalous forecast uncertainty as revealed by time series of TIGGE data

AbstractThe time evolution of forecast uncertainty for a given lead time is examined using 9 years of daily ensemble data from the TIGGE archive and a classical time‐series approach, with a focus on anomalies in the ensemble variance of T+120h 500 hPa geopotential height averaged over select regions. Spectral analysis suggests the presence of favoured cycles in the anomaly time series, manifested by strong peaks in the spectra at certain periods between 10 and 40 days. Such favoured cycles imply that the time evolution of the forecast uncertainty has some element of predictability. Coincident with these cycles are episodes of two‐week to monthly time‐scale local trends (“ramp ups” and “ramp downs”) in the variance anomaly series. A correlation analysis with typical flow indices is carried out to gauge what physical processes might influence the behaviour of the variance anomaly time series. From the broadest perspective, the Eady index is clearly the most influential of the indices and it is inferred that low‐frequency peaks in anomalously positive Eady index presage low‐frequency peaks in anomalously positive variance. Considering particular regions and conditioning on season and MJO phase helps reveal the influences of the other indices. There are also indications that anomalously high forecast uncertainty preferentially occurs during specific changes in sign of the indices. Nonetheless, the correlation analysis suggests that much of the behaviour of the variance anomaly time series remains to be explained. Principal component analysis and k‐means cluster analysis identify a leading mode of variability in which the variance is jointly elevated or suppressed across the hemisphere (with concentration in the Pacific and Atlantic Oceans storm tracks).

A multiscale analysis of the tornadoes of 30–31 May 2019 in south-central Chile

Abstract On 30 and 31 May 2019, tornadoes occurred in the cities of Los Angeles and Talcahuano/Concepcion in south-central Chile, in a region where tornado activity is not common. The main goal of this study was to analyze these tornadoes across multiple scales: synoptic, mesoscale, and subseasonal. On the synoptic scale, the tornadoes were associated with an anomalous 500-hPa trough and associated surface cyclone to the west of Chile. A strong (20+ m s−1) low-level jet accompanied this trough, potentially enhanced by flow blocking by the Andes. A relatively warm and saturated surface layer combined with cold upper-level temperatures in the trough to yield 200–500 J kg−1 of CAPE on both days. This CAPE was accompanied by high levels of both deep-layer and low-level shear. Storm motions inferred by lightning swaths and GOES-IR imagery, along with estimates of storm motion and updraft helicity from a high-resolution WRF simulation, suggested this CAPE-shear combination was sufficient for the tornadic thunderstorms to be supercells. Finally, anomalies of sea level pressure, 500-hPa height, and surface dew point temperature from 27 to 31 May 2019 resembled long-term composite anomalies for MJO phases 1 and 2, suggesting a subseasonal link between the extreme event in Chile and convection in the tropics.

Probabilistic Skill of Subseasonal Surface Temperature Forecasts over North America

Abstract The skill of surface temperature forecasts up to 4 weeks ahead is examined for weekly tercile category probabilities constructed using extended logistic regression (ELR) applied to three ensemble prediction systems (EPSs) from the Subseasonal-to-Seasonal (S2S) project (ECMWF, NCEP, and CMA), which are verified over the common period 1999–2010 and averaged with equal weighting to form a multimodel ensemble (MME). Over North America, the resulting forecasts are characterized by good reliability and varying degrees of sharpness. Skill decreases after two weeks and from winter to summer. Multimodel ensembling damps negative skill that is present in individual forecast systems, but overall, does not lead to substantial skill improvement compared to the best (ECMWF) model. Spatial pattern correction is implemented by projecting the ensemble mean temperatures neighboring each grid point onto Laplacian eigenfunctions, and then using those amplitudes as new predictors in the ELR. Forecasts and skill improve beyond week 2, when the ELR model is trained on spatially averaged temperature (i.e., the amplitude of the first Laplacian eigenfunction) rather than the gridpoint ensemble mean, but not at shorter leads. Forecasts are degraded when adding more Laplacian eigenfunctions that encode additional spatial details as predictors, likely due to the short reforecast sample size. Forecast skill variations with ENSO are limited, but MJO relationships are more pronounced, with the highest skill during MJO phase 3 up to week 3, coinciding with enhanced forecast probabilities of above-normal temperatures in winter.

Intra-annual variability of the North West Shelf of Australia and its impact on the Holloway Current: Excitement and propagation of coastally trapped waves

Shelf circulation on the North West Shelf (NWS) of Australia is dominated by seasonal variations of the alongshore Holloway Current forced by the monsoonal winds. However, the intra-annual variability of the current remains poorly understood. In this study, a combination of model outputs and two years of mooring time series data were used to analyse the intra-annual variability of the Holloway Current. The dominant intra-annual variability of the alongshore current on the NWS were at intraseasonal and semiannual frequencies, as revealed by sea level data. Whereas the interannual variability in the region is mostly forced remotely by tropical Pacific processes, the intraseasonal and semiannual signals are mostly driven by variations of regional winds off the northern coast of Australia. Phase analysis of sea level anomalies indicated that the excitation and propagation of Coastally Trapped Waves (CTWs) are forced by Madden-Julian Oscillations (MJO; intraseasonal) and semiannual wind anomalies. Intraseasonal CTWs are excited over the eastern parts of the Kimberley shelf, whereas semiannual waves are generated further to the east, over the northern Australian coast. The phase speed of intraseasonal CTWs is ~5.5 ms−1, characteristic of the first CTW mode, and 2.5 ms−1for semiannual CTWs, characteristic of the second CTW mode. The fluctuations in the velocity field over the NWS translates into significant alongshore current transport anomalies, ranging on average from ~0.4 Sv to ~0.6 Sv, for intraseasonal and semiannual frequency, respectively. Southwestward (northeastward) intraseasonal transport anomalies peak during Phase 1 (Phase 5) of the MJO cycle (out of the 8 phases of MJO) on the Kimberley shelf, whereas the corresponding transport anomalies on the Pilbara shelf peak during Phase 3 (Phase 7). Semiannual southwestward (northeastward) transport anomalies on the Kimberley shelf peak in April and October (January and July), preceding those on the Pilbara shelf by one month.

Interaction of MJO and diurnal cycle observed by radiosonde data during the first YMC campaign in Bengkulu (November 2017 - January 2018)

The interaction of MJO and diurnal cycle in the Maritime Continent has been identified from previous studies but more observations are needed to understand it clearly. The first campaign of Years of the Maritime Continent (YMC) was conducted in Bengkulu from 16 November 2017 to 15 January 2018. Among various kind of observations, the three-hourly radiosonde sounding was conducted in Bengkulu station during the campaign. This study analyses the radiosonde data to investigate the thermodynamic profile of the mean diurnal cycle (MDC) at different MJO phases. The MDC of potential temperature anomaly (PTA), relative humidity, winds, convective available potential energy (CAPE) and total precipitable water (TPW) in different MJO phases were compared to the MDC values during the campaign (average condition). The obtained MDC PTA has a similar pattern with the results from the Pre-YMC campaign (November - December 2015). During the active MJO phase (3-4), RH is much higher and the zonal westerly wind is up to 6 m/s faster than the average condition at most of the time and elevation below 15 km, and TPW is amplified about 1-4 mm. In contrast, the condition is reversed during inactive MJO phase (1-2). In addition, the CAPE peaks occurred at 06 - 09 UTC and followed by TPW peaks 6 hours later.

Application of Fuzzy Clustering Technique for Analysis of North Indian Ocean Tropical Cyclone Tracks

ABSTRACT A fuzzy, c-means (FCM) clustering technique is explored to investigate the track of tropical cyclones over the North Indian Ocean (NIO) for the period (1976-2014). A total of five clusters is objectively identified based on partition index, partition coefficient, Dunn Index and separation index. The results obtained during analysis emphasized that each cluster has the unique features in terms of their genesis location, landfall, travel duration, trajectory, seasonality, accumulated cyclone energy and Intensity. Analysis of large scale environmental parameters, constructed preceding day of genesis show some of these parameters to be potential precursors to TC formation for almost all the clusters, most prominently, mid-tropospheric humidity, zonal wind, vorticity and outgoing long wave radiation of the main developing regions. The individual clusters have the several distinct features in their seasonal cycles. The cluster C5 shows distinct bimodal distributions where as other clusters are formed throughout the year. ENSO influenced the cyclone frequency in two of the five clusters. The MJO is found to play an important role in the genesis of the cyclone. The post monsoon season cyclone frequency is more in MJO phase 2, 3 and 4. The technique (FCM) can be used as a guideline in terms of the probable affected zone of TC Tracks by the operational forecasters.