Impact Of Madden-Julian Oscillation Research Articles

The impact of the Madden-Julian oscillation on spring and autumn afternoon diurnal convection in Sri Lanka

This study examines the impact of strong Madden-Julian Oscillation (MJO) phases (P1–P8) on diurnal rainfall patterns focusing on Afternoon Diurnal Convection (ADC) events in Sri Lanka during 2001–2020 spring and autumn. Daily mean rainfall increases (decreases) during the P2-to-P3 (P6-to-P7) MJO phases in both seasons, while the diurnal rainfall amplitude peaks during the P2-to-P3 (P8-to-P1) MJO phases in spring (autumn). ADC events also occur more frequently and intensely during MJO P2-to-P3 (P8-to-P1) in spring (autumn). The MJO’s modulation of diurnal rainfall amplitude and ADC events is more apparent in autumn than in spring. Active MJO phases enhance the westward propagation of diurnal rainfall associated with ADC events, sustained by moisture flux convergence and enhanced upward motion. The prevailing mid-to-upper level easterly wind, combined with deep convection over Sri Lanka, contributes to a more pronounced westward propagation during the P2-to-P3 (P8-to-P1) phases for ADC events in spring (autumn).

Impact of Madden Julian Oscillation on the diurnal cycle of precipitation over the west coast of India

Madden Julian Oscillation (MJO) is an eastward propagating convective system that passes over the Indian and Pacific Oceans with a periodicity of 30–90 days and influences most of the tropical weather systems. This study investigates and quantifies the impact of MJO on the diurnal cycle of precipitation over the west coast of India. The work uses TRMM precipitation, profiles of wind and humidity from ERA5 re-analysis and satellite Infrared window channel measurements from GridSat-B1 for the period 2000–2019. Real-time Multivariate MJO index has been used for the identification of active and suppressed MJO phases. Harmonic analysis is used to derive the diurnal and semi-diurnal amplitudes of precipitation. The active MJO phases enhance the diurnal amplitude of precipitation over the west coast with respect to the climatological diurnal cycle, whereas the suppressed phases reduce it. Active phases help in the formation of mesoscale convective cloud systems with much more spatial extension and temporal duration than that in suppressed phase. This kind of convective organization is observed especially during the active MJO phases of monsoon and post-monsoon seasons. The vertical structure of wind and humidity in the troposphere are modified by MJO where the active (suppressed) phase enhances (decreases) them. The overall modification of tropospheric dynamics, thermodynamics and deep convection by MJO can cause anomalous rainfall activity over the west coast.

The Madden–Julian Oscillation in the Energy Exascale Earth System Model Version 1

AbstractThe present study examines the characteristics of the Madden–Julian Oscillation (MJO) events represented in the Energy Exascale Earth System Model version 1 (E3SMv1), DOE’s new Earth system model. The coupled E3SMv1 realistically simulates the eastward propagation of precipitation and moist static energy (MSE) anomalies associated with the MJO. As in observations, horizontal moisture advection and longwave radiative feedback are found to be the dominant processes in E3SMv1 that lead to the eastward movement and maintenance of the MJO MSE anomalies, respectively. Modulation of the diurnal cycle of precipitation in the Maritime Continent region by the MJO is also well represented in the model despite systematic biases in the magnitude and phase of the precipitation diurnal cycle. On the MJO impact over the midlatitude, E3SMv1 reasonably captures the pattern of the MJO teleconnections across the North Pacific and North America, with improvement in the performance in a high‐resolution version, despite the magnitude being a bit weaker than the observed. Regarding the interannual variability of the MJO, the El Niño‐Southern Oscillation (ENSO) modulation of the zonal extent of MJO’s eastward propagation, as well as associated changes in the mean state moisture gradient in the tropical west Pacific, are well reproduced in the model. However, MJO in E3SMv1 exhibits no sensitivity to the Quasi‐Biennial Oscillation (QBO), with the MJO propagation characteristics being almost identical between easterly QBO and westerly QBO years. Processes that have been suggested as critical to MJO simulation are also examined by utilizing recently developed process‐oriented diagnostics.

Impact of Madden Julian Oscillation on the Diurnal Cycle of Precipitation Over the West Coast of India

Impact of Madden Julian Oscillation on the Diurnal Cycle of Precipitation Over the West Coast of India

Global Survey of the MJO and Extreme Precipitation

AbstractThis study examines the modulation of land‐based extreme precipitation around the globe by the Madden‐Julian Oscillation (MJO). The upper‐level convergent phase of the MJO inhibits extreme events over most regions but enhancement in other phases falls in three categories. Over Brazil, Southeast Asia, and Australia, 2‐year rainfall events are most common near the core of the upper‐level divergence as expected. For most other regions in the tropics and subtropics, the extreme events occur along the periphery of the MJO's envelope. Previous regional studies suggest these extremes are driven by the MJO's low‐level circulation either advecting moisture or interacting with orography rather than directly increasing the vertical convection. Finally, extratropical extreme events are more likely associated with the MJO's impact on extratropical wave trains or tropical cyclones. Given the increasing skill of numerical models for predicting the MJO, these results could lead to subseasonal forecasts of extreme events.

The Impact of MJO, Kelvin, and Equatorial Rossby Waves on the Diurnal Cycle over the Maritime Continent

The impacts of the Madden–Julian Oscillation (MJO), Kelvin waves, and Equatorial Rossby (ER) waves on the diurnal cycle of rainfall and types of deep convection over the Maritime Continent are investigated using rainfall from the Tropical Rainfall Measurement Mission Multisatellite Precipitation Analysis and Infrared Weather States (IR–WS) data from the International Satellite Cloud Climatology Project. In an absolute sense, the MJO produced its strongest modulations of rainfall and organized deep convection over the islands, when and where convection is already strongest. The MJO actually has a greater percentage modulation over the coasts and seas, but it does not affect weaker diurnal cycle there. Isolated deep convection was also more prevalent over land during the suppressed phase, while organized deep convection dominated the enhanced phase, consistent with past work. This study uniquely examined the effects of Kelvin and ER waves on rainfall, convection, and their diurnal cycles over the Maritime Continent. The modulation of convection by Kelvin waves closely mirrored that by the MJO, although the Kelvin wave convection continued farther into the decreasing phase. The signals for ER waves were also similar but less distinct. An improved understanding of how these waves interact with convection could lead to improved subseasonal forecast skill.

Madden–Julian Oscillation–Induced Suppression of Northeast Pacific Convection Increases U.S. Tornadogenesis

AbstractThis study investigates the impact of the Madden–Julian oscillation (MJO) on U.S. tornadogenesis using atmospheric reanalysis and model experiments. Our analysis shows that the impact of MJO on U.S. tornadogenesis is most significant in May–July and during MJO phases 3–4 and 5–6 (P3456). These MJO phases are characterized by anomalous ascending motion over the Maritime Continent (MC) and anomalous subsidence over the northeast Pacific (EP), generating anomalous diabatic heating and cooling, respectively. These in turn generate large-scale atmospheric conditions conducive to tornadogenesis in the United States, enhancing the North American low-level jet (NALLJ) and thus increasing the influx of warm and moist air from the Gulf of Mexico to the United States and increasing the low-level wind shear and convective available potential energy along its path. Conversely, during MJO phases 1–2 and 7–8, the opposite patterns of atmospheric anomalies appear over the United States producing unfavorable environments for U.S. tornadogenesis. We further investigate the underlying mechanism for MJO-induced atmospheric circulations conducive to U.S. tornadogenesis using a linear baroclinic model (LBM). The LBM is forced by diabatic heating over the MC and cooling over the EP, which characterizes the P3456 MJO phase. The model experiment reproduces an anomalous ridge over the southern United States and associated anomalous low-level anticyclone that enhances the NALLJ and increases tornadic environmental parameters. Additional sensitivity experiments prescribing the diabatic heating over the MC and diabatic cooling over the EP independently demonstrate that diabatic cooling over the EP is the main driver for producing regional atmospheric conditions favorable for U.S. tornadogenesis.

Subseasonal midlatitude prediction skill following Quasi-Biennial Oscillation and Madden–Julian Oscillation activity

Abstract. The Madden–Julian Oscillation (MJO) is known to force extratropical weather days to weeks following an MJO event through excitation of stationary Rossby waves, also referred to as tropical–extratropical teleconnections. Prior research has demonstrated that this tropically forced midlatitude response leads to increased prediction skill on subseasonal to seasonal (S2S) timescales. Furthermore, the Quasi-Biennial Oscillation (QBO) has been shown to possibly alter these teleconnections through modulation of the MJO itself and the atmospheric basic state upon which the Rossby waves propagate. This implies that the MJO–QBO relationship may affect midlatitude circulation prediction skill on S2S timescales. In this study, we quantify midlatitude circulation sensitivity and prediction skill following active MJOs and QBOs across the Northern Hemisphere on S2S timescales through an examination of the 500 hPa geopotential height field. First, a comparison of the spatial distribution of Northern Hemisphere sensitivity to the MJO during different QBO phases is performed for European Centre for Medium-Range Weather Forecasts (ECMWF) ERA-Interim reanalysis and ECMWF and the National Centers for Environmental Prediction (NCEP) hindcasts. Secondly, differences in prediction skill in ECMWF and NCEP hindcasts are quantified following MJO–QBO activity. In both hindcast systems, we find that regions across the Pacific, North America, and the Atlantic demonstrate an enhanced MJO impact on prediction skill during strong QBO periods with lead times of 1–4 weeks compared to MJO events during neutral QBO periods.

Seasonality of the MJO Impact on Upper Troposphere–Lower Stratosphere Temperature, Circulation, and Composition

Abstract Seasonal differences in the impact of the Madden–Julian oscillation (MJO) on tropical and extratropical upper troposphere–lower stratosphere (UTLS) temperature, circulation, and trace gases are examined using trace gases (ozone, carbon monoxide, and water vapor) and temperature from measurements from the Microwave Limb Sounder (MLS) and meteorological fields from the Modern-Era Retrospective Analysis for Research and Applications, version 2 (MERRA-2). During boreal winter months (November–February), atmospheric fields exhibit a well-known planetary-scale perturbation consistent with the upper-level flow modeled by Gill, with twin high and low pressure extratropical systems associated with a Rossby wave response. However, the circulation anomalies in the UTLS differ during boreal summer months (June–September), when background UTLS circulation north of the equator is dominated by the Asian summer monsoon anticyclone. The twin high and low pressure extratropical systems are much weaker but with a stronger equatorial Kelvin wave front that encircles the globe as the MJO propagates eastward. These differences are explained in terms of seasonal variations in vertically propagating Kelvin waves that strongly depend on the zonal structure of the climatological background winds. The trace gas response to the MJO is strongly coherent with circulation anomalies showing strong seasonal differences. The stronger equatorial Kelvin wave front during the summer produces enhanced upwelling in the tropical tropopause layer, resulting in significant cooling of this region, reduced ozone and water vapor, and enhanced carbon monoxide.

Assessing the skill of all-season diverse Madden–Julian oscillation indices for the intraseasonal Amazon precipitation

Madden–Julian Oscillation (MJO) impact on the Amazon intraseasonal precipitation assessed by different MJO indices is investigated through an analysis of composite events and observed case studies. The MJO indices diagnosing skill is described in detail using reanalysis, satellite, and gauge-based gridded rainfall data. Three types of existing MJO indices are considered: (1) OLR-based MJO (OMI index); (2) dynamically- or circulation-based MJO (VPM index); and (3) combined convectively- and dynamically-based MJO (Wheeler–Hendon RMM index). Our results suggest that, in the large-scale (i.e., around the global tropics), even in the regional domain, the MJO OLR-only index well-represent the dynamical and convective features associated with the intraseasonal variability. On the other hand, each index gives diverging results on rainfall characterization over the Amazon Basin (AB). For instance, the cumulative distribution of precipitation for each MJO phase and index depicts considerable differences in the main climatic regions of the AB, indicating a diverging intraseasonal representation for extreme rainfall values. In addition, when event-by-event is assessed, details as well as the identification of events themselves can differ among indices. This characteristic is particularly observed during extreme rainfall events in the AB. A significant percentage of MJO activity is detected only by the MJO OLR-based index. Because the large-scale zonal circulation dominates the dynamically-based indices (RMM and VPM), the MJO impact in these indices is not an appropriate measure of convective MJO activity. Since the convective component of the MJO is our primary objective, the results presented in this study show that the OLR-based MJO index is able to better account for the MJO impacts over the AB. The indices considered in this study are often used for monitoring and forecasting the MJO activity over South America. However, given the dissimilarity of the representation of precipitation in the AB, our findings also support the consideration of a regional index for monitoring and forecasting the MJO impacts.

Influence of Subseasonal Variability on the Diurnal Cycle of Precipitation on a Mountainous Island: The Case of New Caledonia

Abstract The relationship between the large-scale intraseasonal variability, synoptic wind regimes, and the local daily variability of precipitation over the main island of New Caledonia (southwest tropical Pacific) is investigated with a focus on the austral summer wet season (November–April). The average diurnal cycle of precipitation over the island is characterized by a sharp afternoon maximum around 1600 local time, with significant differences between the windward east coast, the leeward west coast, and the mountain range. The afternoon peak is related to the afternoon sea-breeze circulation and to the diurnal cycle of convection over land. In general, its magnitude follows the same evolution as the daily mean. In agreement with past studies, a clear modulation of the Madden–Julian oscillation (MJO) on both the diurnal cycle of precipitation and the probability of occurrence of four robust wind regimes can be identified in the New Caledonia region during the wet season. From the evidence that there is a qualitative correspondence between the effects of both the MJO phases and the wind regimes on features in the diurnal cycle of precipitation, a simple model is proposed to inspect the MJO forcing mediated by wind regimes on the diurnal variability of rain. The complete decomposition of the MJO impact shows that the modulation of diurnal cycle by the MJO relies on complex interactions between the MJO and synoptic winds that involve both large-scale MJO convective anomalies and MJO-induced modification of wind patterns.

Subseasonal to Seasonal Prediction of Wintertime Northern Hemisphere Extratropical Cyclone Activity by S2S and NMME Models

AbstractIn this study, the prediction of wintertime extratropical cyclone activity from subseasonal to seasonal timescale in current dynamical models' reforecasts is investigated. On seasonal time scales, the North American Multi‐Model Ensemble (NMME) models show skillful predictions over the eastern North Pacific, North America and the western North Atlantic with at least 5 months lead. The prediction skill is highly related to El Nino‐Southern Oscillation (ENSO), as using the ENSO‐related SST pattern gives rise to prediction skill with very similar spatial pattern and amplitude. On subseasonal time scales, models in the Seasonal to Sub‐seasonal Prediction (S2S) dataset have skillful predictions up to 4 weeks lead over regions from the eastern North Pacific to the western Atlantic, as well as northern Europe, the eastern Atlantic and East Asia. Generally, forecast skill improves with a larger ensemble size. The subseasonal prediction skill from the Pacific to the western Atlantic is related to ENSO, and that over eastern Atlantic, Europe and East Asia are associated with stratospheric polar vortex anomalies. Current models do not show much skill from the Madden‐Julian Oscillation (MJO), as the MJO impact on extratropical cyclone activity is not well captured by the models. European Centre for Medium‐Range Weather Forecasts (ECMWF) model has the highest single model subseasonal prediction skill. The prediction skill in the ECMWF model is higher than its estimated potential predictability, likely because the signal‐to‐noise ratio is too low in the model hindcasts.

MJO Anomalies relationship with Volcanic Eruption in Indonesian Archipelago

Madden-Julian Oscillation (MJO) is a large-scale atmospheric phenomenon that crosses the equator with a propagation area ranging from 15° North to 15° South, and moves from west to east, precisely from the Indian Ocean to the Pacific. MJO propagation is usually characterized by the rising of Sea Surface Temperature (SST) in Indian Ocean respectively. MJO anomaly is closely related to the variability of weather in the area it passes, including the Indonesian Archipelago. The impact of MJO anomaly in Indonesian Archipelago itself is suspected to cause not only excessive rainfall, but also a very long consecutive No Rain Days (NRD), with in the end can also trigger massive crop failures. In addition to SST, there are also external events that can affect the duration of the MJO, i.e. volcanic eruptions. Therefore, the study aims to determine the effect of volcanic eruption events on MJO anomalies, through the SST variable as an MJO anomaly parameter and also the occurrence of volcanic eruptions in the same time span. Data from both variables were taken over a period of 35 years, from 1982 to 2016. The SST data is a daily average data, obtained from ESRL NOAA, which is then filtered by the Bandpass Filter method to reinforce the oscillation of data. The MJO anomaly that seen from the SST fluctuations is then matched with time data and the duration of volcanic eruption events over the same time span. It found out that there are three MJO anomalies occurred during the time span of 35 years, and it is hypothesized that they were clearly in correspondence between the duration of volcanic eruption events with the incidence of MJO anomalies in Indonesian Archipellago.

Combined Effect of the Madden-Julian Oscillation and Arctic Oscillation on Cold Temperature Over Asia

The combined effect of the Madden−Julian oscillation (MJO) and Arctic oscillation (AO) on the temperature variation over Asia is investigated using the thermodynamic budget equation. Because of the distinct geographical origin of the two atmospheric modes, the influence of AO is more dominant in the higher latitude, whereas the MJO impact is more predominant in the lower latitude. Hence, the physical process responsible for the surface cold anomaly is different for northern and southern Asia. Cold anomaly appears in most of Asia 20–25 days after the MJO phase 6 (corresponding to the phase 2–3). However, more strengthened cold anomaly occurs over northern Asia under the negative AO state and it is caused by advection of temperature anomaly by climatological northerly wind associated with the East Asia winter monsoon flow. On the other hand, much stronger cold anomaly is seen over southern Asia under the positive AO state for the same lag day of the initial MJO phase 6. Aside from the upward overturning circulation forced by the tropical MJO over the subtropics and lower midlatitudes, the weakening of the East Asia subtropical jet by the positive AO induces additional upward motion over southern Asia to adjust the thermal wind balance. The combined effect of the MJO and AO also influences on the occurrence of extreme cold event. Under the negative (positive) AO phase, the extreme cold event occurrence probability over northern (southern) Asia increases by 90% (60%) compared to that for all winter days. The relative increase rate for the MJO phases 2–3 is ~30% over southern Asia. The cold event occurrence probability for the combined modes is about the twice that for only MJO impact, suggesting that an incorporation of both modes enhance the predictability of extreme cold event.

Modelling the role of Atlantic air–sea interaction in the impact of Madden–Julian Oscillation on South American climate

This study addresses the role of Atlantic air–sea interaction in the remote influence of the Madden–Julian Oscillation (MJO) on eastern South American climate during austral summertime. To disentangle the different processes involved, reanalysis data as well as a regional climate model run in coupled mode and as a stand‐alone atmosphere are used. The simulations are able to represent the observed influences of the MJO in precipitation and surface air temperature. In particular, in both setups the model is able to represent adequately the atmospheric teleconnections associated with the MJO, which involves the development of a barotropic cyclonic anomaly over South America between 30°S and 60°S, which favours a southwards shift of the South Atlantic Convergence Zone (SACZ) and a warming in eastern Brazil. Moreover, model simulations support the hypothesis that air–sea interaction is important to set up the strength of the rainfall response in the SACZ. That is, the development of a local warm SST anomaly forced by heat flux anomalies associated with the direct MJO impact in turn feeds back into the atmosphere generating a stronger surface convergence that shifts the SACZ southwards. In the absence of this SST‐forced response the SACZ still shifts southwards, but anomalies are much weaker and less extensive. We also found that the coupled model represents more adequately the remotely forced MJO temperature signal over eastern Brazil, probably due to a too strong response of the stand‐alone model to prescribed sea surface temperature.

The study of MJO impact on wave height and wind speed in Indonesian Seas

Indonesia is passed by an atmospheric phenomenon, called the Madden-Julian Oscillation (MJO), which has an impact on the wave height in the Indonesian Seas. The significant wave height is simulated using WAVEWATCH-III (WW3) numerical model in Indonesian region (90 °E-150 °E, 20 °N-20 °S) forced by surface winds from Cross-Calibrated Multi-Platform (CCMP), Navy Global Environmental Model (NAVGEM), and Navy Operational Global Atmospheric Prediction System (NOGAPS). This simulation is concentrated on MJO phase 3, 4, and 5 which passed through Indonesia and its adjacent waters that occurred in particular time between 1990-2015. In this study, the impact of MJO was analyzed during every monsoon season. In addition, wind speed analysis was carried out to further enrich the analysis of the MJO impact. The simulation result shows that MJO exerts the highest impact during phase 5 and DJF, which contributes to the increase of wind speed (WS) and significant wave height (SWH) in Indonesian inner seas by 6 m/s and 30 cm, respectively, and in southern Lesser Sunda Island by 8 m/s and 1.2 m, respectively. MJO can also contribute to decreasing of the WS and SWH, when it occurred during DJF and MAM phase 3, and JJA phase 4. There is no noticabe change of WS and SWH during SON.

Role of Madden–Julian Oscillation in Modulating Monsoon Retreat

The Madden–Julian oscillation (MJO) is the major fluctuation in tropical weather on a seasonal scale. The impact of MJO on different epochs, viz., onset, advance and active break is well known. There can be several MJO events in a season and it may enhance/suppress the retreat process. The present study aims to find the MJO-modulated retreat of monsoon. The results suggest that the fastest retreat of monsoon occurred in the years 2007 and 2008, while slowest retreat of monsoon occurred in the year 1979. The retreat features of the Indian summer monsoon (ISM) are investigated with the MJO phase and amplitude variations. The daily MJO indices for the retreat period 1979–2016 are used. The results reveal that the MJO strength decreases during the transition phase (i.e., summer monsoon to winter monsoon transition). The monsoon retreat is most favored by strong MJO phase 4 and phase 5. The fastest retreat of monsoon occurred in the years 2007 and 2008, while the slowest retreat of monsoon occurred in the year 1979. There exists a weak positive correlation between the MJO amplitude and the retreat period of monsoon. The monsoon retreat is most favored by strong MJO phase 4 and phase 5. The MJO amplitude variations during MJO phases 1–8 suggest that the MJO amplitude decreases with increase in retreat period. The MJO-modulated retreat results in slow retreat of monsoon, whereas fast and normal retreat of monsoon is seen on rare occasions. Weak MJO events lead to normal retreat of monsoon.

Impact of Madden Julian oscillation (MJO) and other meteorological phenomena on the heavy rainfall event from 19th − 28th December, 2014 over Sri Lanka

This study revealed the influence of Madden Julian oscillation (MJO) and cold surge (CS) on the extreme rainfall events that occurred in Sri Lanka from 19th to 28th December 2014. During this period, the Northeast Monsoon winds over the Bay of Bengal were strengthened by a CS, which intensified convective activity over the Bay of Bengal. Simultaneously, convective phase 3 of the MJO, which was located over the eastern Indian Ocean is associated with a positive rainfall anomaly over Sri Lanka. Westerly wind anomalies associated with MJO phase 3 shifted southward due to the strengthening of northeasterly winds by the CS, forming an incipient vortex to the southeast of Sri Lanka. The low level moisture transport was enhanced by this vortex circulation together with strong cross-equatorial moisture flux transportation. The convectively active phase of MJO provided favourable conditions such as low level convergence, cyclonic relative vorticity, and deep convection for the intensification of vortex into a tropical depression over the southwest Bay of Bengal. Strong upward motion and combination of strong low level convergence with upper-level divergence allowed evacuation of the ascending air mass, aiding the intensification of deep convection. This persistent vertical coupling structure allowed deep convection of moist warm air, which led to heavy rainfall during the period. Combination of a number of factors, such as local terrain effects, CS at low levels, MJO wet phase and a vortex to southeast of Sri Lanka led to an intensification of the tropical depression that caused an extreme rainfall event over the Eastern and Northeastern parts of Sri Lanka.

Impact of Madden-Julian Oscillation (MJO) on global distribution of total water vapor and column ozone

The Madden-Julian Oscillation (MJO) is the leading mode of intra-seasonal variability in the tropical troposphere, characterized by an eastward moving ‘pulse’ of cloud and rainfall near the equator. In this study, total precipitable water (TPW) and total column ozone (TCO) datasets from ECMWF ERA-Interim reanalysis were used to analyse the impact of the MJO on the distribution of water vapor and column ozone in the tropics from 1979 to 2013. The results show that seasonal variations of TPW modulated by the MJO are maximized in the tropics of about 10°S-10°N during boreal winter, while the variation in TCO is maximized in the mid-latitudes of about 30°S - 40°N in the same season. The composite analysis shows that MJO modulates TPW and TCO anomalies eastward across the globe. The underlying mechanism of the MJO’s impact on TPW is mainly associated with variation of tropical convection modulated by the MJO, while the underlying mechanism of the MJO’s impact on TCO is mainly associated with an intra-seasonal variability of tropopause height modulated by the MJO activity. This knowledge helps to improve the prediction skill of the intra-seasonal variation of water vapor and column ozone in the tropics during boreal winter.

Impacts of the Madden‐Julian oscillation on intraseasonal precipitation over Northeast Brazil

ABSTRACTThe impacts of the Madden‐Julian oscillation (MJO) on precipitation over Northeast Brazil (NEB, also known as Nordeste) are evaluated based on daily raingauge data from 492 stations over 30‐year period (1981–2010). Composites of precipitation, outgoing longwave radiation and moisture‐flux anomalies are performed for each phase of the MJO, and over all four seasons, based on the Jones–Carvalho MJO index. To distinguish the MJO signal from other patterns of climate variability, daily data are filtered using a 20–90 day band‐pass filter; only days classified as MJO events are considered in the composites. The results show strong seasonality of the MJO's impact on precipitation. The most spatially coherent signals of precipitation anomalies occur in the austral summer, when about 80% of the raingauge stations showed increased precipitation in phases 1–2 and suppressed precipitation in phases 5–6 of the oscillation. Although the MJO impacts precipitation on intraseasonal timescales in all seasons in most locations, these impacts vary in magnitude and depend on the phase of the oscillation. Precipitation anomalies over NEB are explained by the interaction of convectively coupled Kelvin‐Rossby waves with the dominant climatic features in each season. During the austral summer and spring, westerly regimes increase precipitation over most NEB. In the austral winter and fall, precipitation anomalies exhibit more complex spatial variability. In these seasons, precipitation anomalies in coastal areas depend on the strength of the South Atlantic anticyclone, which is largely modulated by Rossby waves. The strengthening of the anticyclone intensifies the convergence of the trade winds in coastal areas and precipitation windward of the coastal range. Conversely, the intensification of the subsidence is responsible for precipitation deficits in the lee side of the range. These conditions are typically observed when easterly regimes dominate over tropical South America and NEB decreasing moisture flow from the Amazon.