Madden-Julian Oscillation (MJO)

Synonyms

30–60 day oscillation; 40–50 day oscillation; Intraseasonal oscillation (ISO); Intraseasonal variability (ISV)

Definition

The MJO is a planetary-scale quasiperiodic oscillation of atmospheric wind and convective cloudiness anomalies that moves slowly eastward along the equator mainly over the tropical Indian and Pacific Oceans with a timescale on the order of 30–60 days.

Introduction

In 1971, Roland Madden and Paul Julian stumbled across a 40–50 day oscillation when analyzing the zonal (east–west) wind data from rawinsondes at Kanton Island (3 °S, 172 °W) over the equatorial western Pacific. Until the early 1980s, little attention was paid to this oscillation, which later became known as the MJO. Since the 1982–1983 El Niño event, low-frequency variations in the tropics, both on intra-annual (less than a year) and interannual (more than a year) timescales, have received much more attention, and the number of MJO-related publications grew rapidly. The MJO turned out to be the dominant form of the intraseasonal (30–90 day) variability in the tropical atmosphere and has many important influences on the global weather and climate system.

The MJO is a naturally occurring mode of variability of the tropical ocean–atmosphere system. It is characterized by an eastward propagation of large regions of both enhanced and suppressed tropical convection, cloudiness and rainfall near the equator mainly over the tropical Indian and Pacific Oceans, and associated large-scale atmospheric circulation (wind) anomalies over the whole globe. The anomalous cloudiness or rainfall usually first emerges over the equatorial western Indian Ocean and intensifies and remains evident as it propagates eastward over the warm ocean waters of the equatorial eastern Indian Ocean and western Pacific, the so-called Indo-Pacific warm pool. This pattern of anomalous cloudiness and rainfall then generally weakens and disappears as it moves over the cooler ocean waters of the equatorial eastern Pacific, the so-called equatorial cold tongue. Along with this eastward-propagating pattern of equatorial cloudiness and rainfall anomalies, there also exist eastward moving distinct baroclinic patterns of lower- and upper-level atmospheric circulation anomalies in the tropics and subtropics. The circulation anomalies extend around the globe and are not confined to the eastern hemisphere as opposed to the cloudiness and rainfall anomalies. When the MJO moves eastward, it modulates the background cloud, rainfall, and circulation in the tropical Indian and Pacific Oceans on timescales shorter than a season but longer than a couple of weeks. The length of a typical MJO cycle is approximately 30–60 days but normally 40–50 days. Thus, the MJO is also known as the 30–60 day oscillation, 40–50 day oscillation, intraseasonal oscillation (ISO), or intraseasonal variability (ISV) after its typical timescale. A complete MJO cycle can be divided into two distinct phases according to the intensity of its convective activity and rainfall: convectively active (enhanced) phase or wet phase and convectively inactive (suppressed) phase or dry phase. The wet phase of the MJO cycle is characterized by enhanced tropical convection and large moist convective storms with higher cloud-top heights, more cloud cover, and heavier rainfall (thus more atmospheric latent heating) than average. In contrast, the dry phase of the MJO cycle is typified by dry and clear conditions with lower cloud-top heights, less cloud cover and rainfall (thus less atmospheric latent heating) than normal. The MJO appears to be predictable with lead times of 2–3 weeks. This can help bridge the gap in environmental forecast skill between that of weather (lead times up to a few days) and that of seasonal-to-interannual climate predictions (lead times from a few months to a few years).

Satellite remote sensing data have played an important role in the MJO studies during the last three decades because of the high spatial (a few kilometers) and temporal (3 h or daily) resolutions and global coverage of the satellite data especially over the tropical oceans where the rawinsondes are sparse and the global reanalyses have had large uncertainties. These satellite-based studies have significantly advanced our knowledge in the MJO description and mechanism as well as its global impacts. They have also led to considerable improvement in our numerical modeling capability and theoretical understanding of the MJO. This entry briefly reviews the central role of satellite remote sensing data in studying the description, mechanisms, and global impacts of the MJO.

Description

The MJO and its eastward-propagating convective feature are most active during the Northern Hemisphere (boreal) winter season (November–April) when the Indo-Pacific warm pool is centered near the equator. During the Northern Hemisphere (boreal) summer season (May–October), the change in the large-scale wind patterns associated with the Asian summer monsoon results in the large-scale convective disturbances propagating northeastward, from the equatorial Indian Ocean into Southeast Asia. The discussions in this entry mainly focus on the boreal winter MJO events although many aspects of these discussions can be equally applied to the boreal summer MJO events.

The MJO can be detectable in several important atmospheric and oceanic parameters, such as atmospheric cloudiness, atmospheric wind speed and direction, atmospheric temperature, atmospheric moisture, surface pressure, surface rainfall, sea surface temperature (SST), and surface heat and freshwater fluxes. However, the fundamental quantities related to the MJO are large-scale organized convection, cloudiness, rainfall, and tropospheric winds. Thus, our discussion of the MJO description mainly centers on how the remote sensing data help us understand the characteristics of large-scale convective cloudiness, rainfall, and tropospheric winds associated with the MJO.

Outgoing longwave radiation (OLR) and infrared window radiance (or brightness temperature) provide broadband and narrowband measures of the total flux of longwave radiation lost to space at the top of the atmosphere. Deep convective clouds in the tropics have cold cloud tops and therefore have low values of OLR and brightness temperature. Typically, an OLR value of less than 200 W m⁻² or a brightness temperature value of less than 220 K indicates the presence of deep convection in the tropics. Because of this simple property of OLR and brightness temperature, they have been widely used as a proxy for deep convection over the tropics, where the background longwave radiation from low clouds or surface is much higher. During the wet phase of the MJO, both OLR and window radiance can be significantly reduced due to higher and colder convective cloud tops. On the other hand, during the dry phase of the MJO, both OLR and window radiance can be significantly enhanced because of clear skies and lower and warmer cloud tops. Thus, OLR and infrared window radiance are good indicators of the convective intensity of the MJO. Over the tropical warm ocean waters where MJO convection is active, satellites are the only way to observe convective cloudiness with large spatial coverage. As a result, the satellite remote sensing data have played a central role in studying the general spatial and temporal structure and eastward-propagating features of the MJO convection and cloudiness during the past three decades.

Based on limited rawinsonde and surface station data, Madden and Julian speculated that the MJO is characterized by slowly eastward-propagating, large-scale oscillations in the tropical convective cloudiness over the equatorial Indian Ocean and western Pacific as the result of an eastward movement of large-scale atmospheric circulation cells oriented in the equatorial zonal plane. Evidence of such eastward-propagating clouds in satellite data was first presented by Arnold Gruber in 1974 and Abraham Zangvil in 1975 who both found large-scale eastward-propagating features near 40–50 days at the equator in the cloud brightness data obtained from the Environmental Science Services Administration (ESSA) satellites (ESSA 3 and 5). However, no further evidence was found until the early 1980s when NOAA OLR data and wind analyses from US National Meteorological Center (NMC) became available. These Advanced Very High Resolution Radiometer (AVHRR) OLR data started from the mid-1970s and were mainly from a series of polar-orbiting satellites, such as the scanning radiometer (SR) series and the Television Infrared Observation Satellite–Next Generation (TIROS-N) series. These OLR data have a twice-daily resolution and a good global coverage each day. By the early 1980s, almost 10 years of daily AVHRR OLR data were archived and available to the research community. In the early to mid-1980s, a series of observational papers on the MJO (at the time still referred to as the 40–50 day or 30–60 day oscillation) using the AVHRR OLR and NMC wind analyses appeared. These studies clearly demonstrated the existence of the slowly eastward propagation of the tropical cloudiness at the intraseasonal timescale and documented many detailed and important convective cloudiness and circulation features of the MJO. These papers also helped to bring the MJO to the attention of the scientific community. The NOAA polar orbiter satellites have been operating almost continuously over the past 30 years. As a result, the NOAA OLR data have a relatively long record (over 30 years) and have been and are still extensively employed for studying the MJO.

Figure 1 shows the infrared brightness for a MJO event from December 7 to 26, 1987, with each panel separated by 5 days. Each map covers the all longitudes (0–360°) between 20 °S and 20 °N. Bright white areas indicate cold high clouds, and dark regions indicate cloud-free or warm low cloud conditions. The slow eastward propagation of cold high clouds associated with the MJO is evident. The cold high clouds first form over the western equatorial Indian Ocean on December 7, 1987 and, over the course of the following ∼20 days, then intensify and propagate eastward across the equatorial Indian Ocean and the Maritime Continent to the equatorial western Pacific Ocean.

Figure 1

Infrared satellite observations for the global tropics (20 °N–20 °S) for (from the top) 03 GMT December 7, 12, 17, 22, and 26, 1987. Bright white areas indicate high clouds and deep convection, and dark regions indicate cloud-free conditions (Based on Global Cloud Imagery (GCI) data courtesy of M. Salby, University of Colorado).

In addition to the large-scale eastward-propagating pattern of MJO convective cloudiness, there exist many fine-scale structures within the convective cloudiness. The high spatial (100 km) and temporal (daily) resolution AVHRR OLR data from NOAA polar orbiters and much higher spatial (a few kilometers) and temporal (3 h) resolution window-channel infrared data from the geostationary satellites, such as Geostationary Meteorological Satellite (GMS) from Japan, are particularly useful in investigating the fine structure of the MJO convective cloudiness due to their high spatial and temporal resolutions. For example, the OLR data indicated many short-period, synoptic-scale convective systems within the planetary-scale 30–60 day fluctuations. Along the equator, these active convective systems move eastward with a phase speed of 10–15 m s⁻¹ and have a horizontal spatial scale of several thousand kilometers and a timescale of less than 10 days. These synoptic-scale, eastward-propagating convective systems within the MJO envelope are referred to as super cloud clusters or more recently convectively coupled Kelvin waves. The GMS infrared window radiance data have revealed that a super cloud cluster consists of many fine-scale cloud clusters. These fine-scale cloud clusters typically propagate westward along the equator with a lifetime of about 1–2 days. Although each cloud cluster moves westward, a super cloud cluster moves eastward due to the successive formation of a new cloud cluster east of the mature-stage cloud cluster. This suggested the existence of a hierarchy of convective activity within the MJO that is still an outstanding avenue of research of today.

In the Tropics, surface rainfall is closely related to convective cloudiness and thus is another key quantity of interest to characterize the MJO. The tropical rainfall can be estimated from the satellite-observed infrared and microwave radiances. The surface rainfall can first be indirectly derived from infrared window radiance and OLR which are very sensitive to cloud-top temperatures that are indirectly tied to surface rainfall. The microwave radiances are very sensitive to the hydrometeors that directly result in surface precipitation and thus can be used more directly to retrieve surface precipitation. The microwave-based rainfall retrievals can be divided into passive and active microwave (radar) retrievals. The passive retrievals can further be divided into the microwave emission-based (sensitive to cloud liquid water) and the microwave scattering-based (sensitive to ice particles and large water drops) rainfall retrievals. Starting from the 1990s, several global rainfall data have been generated from satellite-observed infrared and microwave data and were instrumental in studying the MJO during the last two decades. For example, daily, global oceanic rainfall data retrieved based on microwave emission from the Microwave Sounding Unit (MSU) on the NOAA TIROS-N satellites was first used to study the MJO convective feature in the 1990s. During the late 1990s, the NOAA Climate Prediction Center (CPC) generated a global rainfall data set, referred to CPC Merged Analysis of Precipitation (CMAP), through the merged analysis of precipitation from several sources, such as gauges, satellites, and numerical model outputs. The satellite rainfall estimates for the CMAP includes the window infrared-based rainfall estimate from NOAA geostationary satellites (e.g., Geostationary Operational Environmental Satellites, GOES), the OLR-based rainfall estimate from the NOAA polar-orbiting satellites, the microwave emission-based rainfall estimate from the MSU on the NOAA polar-orbiting satellites and the Special Sensor Microwave/Imager (SSM/I) on the Defense Meteorological Satellite Program (DMSP) satellites, and the microwave scattering-based rainfall estimate from SSM/I. Similar merged global rainfall data were also produced by the Global Precipitation Climatology Project (GPCP) based on similar inputs. However, some subtle differences exist between the CMAP and GPCP precipitation data sets, such as diurnal cycle adjustment and atoll precipitation adjustments. Both CMAP and GPCP rainfall data with pentad (5 day) resolution have been extensively used to study the convective features of the MJO and identify MJO events. Launched in 1997, the Tropical Rainfall Measurement Mission (TRMM) satellite provided the first spaceborne precipitation radar (PR) (active microwave) to monitor global rainfall from space in addition to the passive TRMM Microwave Imager (TMI) instrument and the visible and infrared scanner (VIRS) instrument. The TRMM PR and TMI data have also been used to study the MJO. However, their spatial and temporal sampling is rather coarse. To alleviate these sampling deficiencies of the TRMM PR and TMI, the TRMM Multisatellite Precipitation Analysis (TMPA) project provides a calibration-based sequential scheme for combining precipitation estimates from multiple satellites, as well as gauge analyses where feasible, at fine scales (0.25° × 0.25° and 3 hourly). The input satellite data for the TMPA are mainly from two sources: (1) passive microwave-based precipitation from the TMI on TRMM, the SSM/I on DMSP satellites, the Advanced Microwave Scanning Radiometer–Earth Observing System (AMSR-E) on Aqua, and the Advanced Microwave Sounding Unit-B (AMSU-B) on the NOAA polar-orbiting satellites; (2) window infrared-based precipitation data collected by the international constellation of geostationary satellites. This TMPA data set, also known as the TRMM 3B42, has relatively better retrieval accuracy and sampling at fine spatial and temporal scales. It has been used extensively for the recent MJO studies.

In association with the eastward-propagating equatorial convective cloud and rainfall system are strong variations in lower- and upper-level large-scale atmospheric wind fields along the equator and in the subtropics. Unlike the convective cloudiness that is mostly confined over the equatorial Indian and western Pacific Oceans, the large-scale wind anomalies of the MJO extend globally along the equator and into the subtropics. For example, along the equator, low-level zonal winds converge into the convective center, while upper-level zonal winds diverge away from the convective center. These lower- and upper-level zonal winds are interconnected through ascending (upward vertical movement) moist air within the convective center and descending dry air outside the convective center. These large-scale zonal winds propagate eastward together with the convective cloudiness along the equator and can reach into the western hemisphere (eastern Pacific, Atlantic, and Africa). In addition to these zonal winds along the equator are large-scale gyre circulations extending into the subtropics in both the lower and upper troposphere that are tied to the eastward-propagating convective cloudiness and zonal winds along the equator. For example, in the lower troposphere, a subtropical cyclonic couplet (counterclockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere) flanks or lies to the west of the MJO convective region, while a subtropical anticyclonic couplet (clockwise in the Northern Hemisphere and counterclockwise in the Southern Hemisphere) lies to the east of the MJO convective region. On the other hand, in the upper troposphere, a subtropical anticyclonic couplet flanks or lies to the west of the MJO convective region, while a subtropical cyclonic couplet lies to the east of the MJO convective region due to the baroclinic nature of the tropical large-scale wind fields. The near equatorial large-scale zonal wind anomalies are a Kelvin wave response to the MJO convective heating, while the off equatorial meridional wind anomalies are a Rossby wave response to the MJO convective heating. The Kelvin wave is named after Lord Kelvin, who studied water waves along a vertical side boundary under rotation conditions. In this case, the equator, where the vertical component of the Earth’s rotation vector changes sign, serves the vertical side boundary. The Rossby wave is due to the latitudinal variation of the vertical component of the Earth’s rotation and is named after C. G. Rossby, who was the first to clearly isolate the so-called Rossby wave dynamics (a balance between inertia and rotation). These large-scale patterns of convective cloudiness and wind fields are components of what are collectively referred to as equatorial waves or convectively coupled equatorial waves. It is the upper-level circulation features of these waves that allow the convective signatures of the MJO over the Indo-Pacific warm pool to influence weather “downstream” over the eastern Pacific and Atlantic (e.g., hurricanes and tropical cyclones) as well as the midlatitudes (e.g., precipitation extremes along the US west coasts).

In terms of satellite observations, tropospheric winds are difficult to observe directly except via cloud tracking, such as the cloud-drift winds derived from the Multi-angle Imaging SpectroRadiometer (MISR) and NOAA geostationary satellites. However, the surface winds can be measured directly using the spaceborne radar scatterometers, such as the SeaWinds instrument on NASA’s Quick Scatterometer (QuikSCAT) satellite. The importance of the cloud-drift winds and QuikSCAT surface winds for the MJO study has been recognized but still in the early stages of exploration. The overall large-scale dynamic structure of the MJO, especially in the upper levels, is still mainly derived from the global reanalyses or radiosondes at the moment.

Mechanisms

To understand the mechanisms responsible for the initiation and maintenance of the MJO, it is important to quantify the evolution of the thermodynamic environment and surface conditions associated with the MJO. In particular, documenting the three-dimensional temperature and moisture structure of the MJO is crucial in advancing our theoretical understanding of the MJO. The availability of satellite-based temperature and moisture soundings makes these studies possible. For example, tropical mean tropospheric temperature, lower-troposphere (surface–300 mb) temperature, and upper-troposphere (500–100 mb) temperature were derived from the MSU channels in the early 1990s. These data were used to study the relationship between MJO convection and temperature anomalies in the 1990s. It was found that when the MJO is amplifying (e.g., over eastern Indian Ocean), convective heating anomalies are positively correlated to temperature anomalies. This implies production of eddy available potential energy (EAPE), which can in turn be used to drive atmospheric motion and sustain the MJO. When the MJO is decaying (e.g., over the eastern Pacific or east of the Date Line), temperature anomalies are nearly in quadrature with convective heating anomalies. As a result, their correlation and production of EAPE are small which is no longer an energy source for the MJO. For water vapor, the TIROS Operational Vertical Sounder (TOVS) provided the water vapor fields at five different levels in the troposphere in the 1990s. These data were used to study the three-dimensional structure and evolution of water vapor over the life cycle of the MJO in the early 2000s. The composite evolution of moisture shows markedly different vertical structures as a function of longitude. There is a clear westward tilt with the height of the moisture maximum associated with the MJO propagating eastward across the Indian Ocean. These disturbances evolve into nearly vertically uniform moist anomalies as they reach the western Pacific. Near-surface (below 850 mb) positive water vapor anomalies were observed to lead the convection anomaly by 5 days over the Indian Ocean and western Pacific. Upper-level positive water vapor anomalies were observed to lag the peak in the convection anomaly by 5–10 days, as the upper troposphere is moistened following intense convection. In the eastern Pacific, the moisture variations then become confined to the lower levels (below 700 mb), with upper-level water vapor nearly out of phase.

While the MSU and TOVS provided useful initial insights into the three-dimensional temperature and moisture structure of the MJO, their vertical resolution was too low to describe the detailed vertical structure, especially near the tropopause and boundary layer. Recently, global atmospheric moisture and temperature profiles with a much higher vertical resolution were produced by the Atmospheric Infrared Sounder (AIRS)/Advanced Microwave Sounding Unit on the NASA Aqua satellite. The AIRS data provide an unprecedented opportunity to document the vertical moist thermodynamic structure and spatial–temporal evolution of the MJO. Figure 2 presents the pressure-longitude cross sections of the temperature anomaly and its relationship to the convective rainfall anomaly for a composite MJO cycle (from−20 day to +20 day, separated by 10 days each). In the Indo-Pacific warm pool, the temperature anomaly exhibits a trimodal vertical structure: a warm anomaly in the free troposphere (800–250 hPa) and a cold anomaly near the tropopause (above 250 hPa) and in the lower troposphere (below 800 hPa) for the wet phase and vice versa for the dry phase. The moisture anomaly also shows markedly different vertical structures as a function of longitude and the strength of the convection anomaly. Most significantly, the AIRS data demonstrate that, over the Indian Ocean and western Pacific, enhanced convection and precipitation is generally preceded in both time and space by a low-level warm and moist anomaly and followed by a low-level cold and dry anomaly. This zonal asymmetry in the low-level moisture and temperature anomaly provides a favorable moist thermodynamic condition for the eastward propagation of the MJO. Furthermore, the comparison between the AIRS observations and the National Center for Environmental Prediction (NCEP)/National Center for Atmospheric Research (NCAR) and NCEP/Department of Energy (DOE) reanalyses revealed the poor representation of the low-level moisture and temperature structure associated with the MJO in these reanalyses particularly over the equatorial Indian and Pacific Oceans, where there are very few conventional data to constrain the reanalyses. Despite their reliance on (imperfect) numerical models, these reanalyses have been widely used as “observations” to validate MJO theories and model simulations.

Figure 2

Pressure-longitude cross sections of equatorial mean (averaged from 8 °S to 8 °N) temperature anomaly for a composite MJO cycle based on AIRS data from 2002 to 2005. The color red denotes warm anomalies, while the color blue indicates cold anomalies. The superimposed solid black line denotes rainfall anomaly from TRMM (Reproduced from Tian et al., 2006, their Figure 3, by permission of American Meteorological Society).

Figure 3

Geographical regions of a number of impacts of the boreal winter MJO on the global climate system. (1) Influencing tropical weather, alternative periods of wetter/drier conditions in the tropical Indian Ocean and western Pacific. (2) Modulating the diurnal cycle of tropical deep convection and rainfall in the tropical Indian Ocean and western Pacific. (3) Modulating the onsets and breaks of the Australian and South American monsoon systems. (4) Influencing ENSO cycle over the equatorial central and eastern Pacific. (5) Impacting tropical cyclone genesis over the tropical Indian Ocean and western Pacific. (6) Influencing the development of heavy rainfall events over US west coast. (7) Changing the subtropical total-column ozone over the eastern Hemisphere and Pacific Ocean. (8) Affecting the aerosol and air pollution over the equatorial Indian and western Pacific Oceans as well as the tropical Africa and Atlantic Ocean. (9) Influencing the ocean surface Chl across the tropical ocean coasts.

The ocean surface fluxes of heat (solar and infrared radiation, latent and sensible heat), mass (rainfall and water vapor flux), and momentum (wind) are considered important for the initiation and eastward propagation of the MJO by some researchers. Understanding the manner and degree the atmospheric components of the MJO are coupled to ocean surface fluxes is vital to understand the MJO dynamics and establish MJO theories. This coupling includes how the atmospheric components of the MJO influence the ocean surface heat, mass, and momentum fluxes and how the ocean surface provides the needed heat and moisture sources for the MJO convection. Surface heat fluxes have typically been very difficult to measure remotely from space although satellites offer the only viable way to estimate these quantities with regular temporal and spatial samplings over the tropical oceans. Apart from precipitation discussed above, satellite measurements of clouds and water vapor have been used with atmospheric radiative transfer models to provide estimates of solar and infrared radiation fluxes at the ocean surface. Moreover, satellite estimates of ocean surface winds (e.g., QuikSCAT and SSM/I discussed above) have been used in conjunction with water vapor measurements from satellites to construct estimates of latent heat (or evaporative) flux from the ocean. These types of observations, in conjunction with satellite SST retrievals (e.g., Advanced Very High Resolution Radiometer (AVHRR) or TMI), have been used to study how the MJO convection interacts with the ocean surface and explore the degree the ocean and atmosphere are coupled at intraseasonal and other timescales. Satellite-based ocean surface wind speed observations (e.g., QuikSCAT and SSM/I discussed above) have been used to derive momentum fluxes (e.g., wind stress) at the ocean surface. These observations have been critical in documenting and understanding the role of the MJO in influencing the development and evolution of El Niño and Southern Oscillation (ENSO) events. ENSO is the most important interannual variability in the coupled tropical atmosphere ocean system with a dominant timescale of 2–7 years and has significant impacts on the global weather and climate.

During the dry phase of the MJO, suppressed convection is associated with decreased cloud cover and increased surface insolation and anomalous surface easterlies. These anomalous surface easterlies act to decrease the surface wind speed because the background surface winds are weak westerlies in the equatorial Indian and western Pacific Oceans, hence decreasing the surface latent heat flux (or evaporation). Increased surface shortwave radiation and reduced surface evaporation contribute to the warming of SST for the dry phase. During the subsequent wet phase of the MJO, enhanced convection is associated with increased cloud cover and decreased surface insolation. As a result, the SST warming trend is arrested and a cooling trend is initiated. Subsequently, the continued cooling of the upper ocean is accelerated by increased westerly surface winds leading to enhanced surface evaporation and increased entrainment of cold water from below the thermocline. Then the wet phase is followed by another dry phase when SST warming occurs. Therefore, over the Indian Ocean and western Pacific, the enhanced convection is usually led by a warm SST anomaly to the east due to enhanced insolation and decreased evaporation and followed by a cold SST anomaly to the west due to decreased insolation and enhanced evaporation. When the convective anomaly approaches the Date Line, the surface evaporation anomaly and surface solar radiation anomaly tend to cancel each other. Thus, the SST anomaly is rather small over the eastern Pacific, so does the convective anomaly. This convection–SST phase relationship leads many scientists to believe that the MJO is a coupled mode of the tropical ocean–atmosphere system.

Impacts

During the past three decades, the MJO has been shown to have important influences on various weather and climate phenomena over the globe at many timescales, such as the diurnal cycle, tropical weather, monsoon onsets and breaks, ENSO, tropical hurricanes and cyclones, extreme precipitation events, extratropical and high-latitude circulation, and weather patterns. Given evidence that the MJO is predictable with lead times of 2–3 weeks, the strong modulation of the global climate system by the MJO implies that many other components of the global climate system may be predictable with similar lead times. Some examples of the MJO impacts are listed and described below, and the regions of impacts are illustrated in Figure 3.

• First, the MJO significantly impacts the tropical synoptic weather, such as alternative periods of wet and dry conditions, especially over the tropical Indian Ocean and west Pacific, through its influences of tropical rainfall and cloudiness (Figure 3). During the wet phase of the MJO, the tropical atmosphere is very moist and cloudy with heavy rainfall. In contrast, during the dry phase of the MJO, the tropical atmosphere experiences dry and clear conditions with plenty of sunshine.

• Second, the MJO can influence the diurnal cycle of tropical deep convection and rainfall through its effect on the background state over the equatorial Indian and western Pacific oceans (Figure 3). The diurnal cycle is enhanced over both land and water during the convectively active phase of the MJO, while it is reduced during the convectively suppressed phase of the MJO. However, the diurnal phase is not significantly affected by the MJO.

• Third, the MJO can substantially modulate the intensity of monsoon systems around the globe, such as the Australian and South American monsoons for boreal winter and the Asian and North American monsoons for boreal summer (Figure 3). The wet phase of the MJO can affect both the onset timing and intensity of the monsoon, while the dry phase of the MJO can prematurely end a monsoon and also initiate breaks during already existing monsoons.

• Fourth, there is evidence that the MJO influences the ENSO cycle (Figure 3). It was argued that the westerly wind bursts associated with the MJO over the equatorial western Pacific are an important trigger for an El Niño event. The MJO may not cause an El Niño event, but can contribute to the speed of development, and perhaps the overall intensity of an ENSO cycle.

• Fifth, the MJO is known to modulate tropical cyclone activity in the Indian Ocean, Pacific Ocean, Gulf of Mexico, and Atlantic Ocean by providing a large-scale environment that is favorable (unfavorable) for storm development (Figure 3). For example, westerly wind anomalies at the surface in and just behind the area of enhanced convection of the MJO may generate cyclonic (anticyclonic) rotation north (south) of the equator, respectively. At the same time, in the upper levels, anticyclonic (cyclonic) rotation develops along and just behind the area of enhanced convection resulting in a means to reduce vertical wind shear and increase upper-level divergence – both of which are favorable for tropical cyclone development and intensification. The strongest tropical cyclones tend to develop during the wet phase of the MJO. As the MJO progresses eastward, the favored region for tropical cyclone activity also shifts eastward from the Indian Ocean to the Pacific Ocean and eventually to the Atlantic Ocean.

• Sixth, boreal winter extreme precipitation events along the US west coast are often connected with the pattern of tropical rainfall and circulation anomalies associated with the MJO (Figure 3). When the heavy tropical rainfall associated with the MJO is concentrated at the Maritime Continent, a strong blocking anticyclone is located in the Gulf of Alaska with a strong polar jet stream around its northern flank. During this time, the US west coast typically experiences a dry spell. When the enhanced tropical rainfall associated with the MJO shifts to the central Pacific and weakens, a deep low pressure system typically forms near the Pacific Northwest coast and can bring up to several days of heavy rain and possible flooding to the Pacific Northwest coast. These events are often referred to as “Pineapple Express” events, so named because a significant amount of deep tropical moisture traverses the Hawaiian Islands on its way toward western North America.

Although most quantities/processes/phenomena of interest, such as hurricanes, monsoons, and extratropical circulation and weather patterns, are not wholly described by satellite data, the satellite-based OLR or rainfall data were usually used to identify the MJO events and often to characterize aspects of the impacts (e.g., rainfall). Some studies totally depend on the available satellite data. For example, in the study of the MJO impact on the diurnal cycle of tropical deep convection, the International Satellite Cloud Climatology Project cloud product was employed to characterize the diurnal cycle, and the TRMM 3B42 precipitation product was used to identify the MJO events.

Recently, a number of studies have documented the MJO impacts on atmospheric composition, air quality, and biogeochemical cycle. This discovery has critically depended on the availability of satellite data. For example, the MJO impact on tropical total-column ozone has been recently characterized using the satellite-observed tropical total-column ozone from the AIRS and Total Ozone Mapping Spectrometer (TOMS). It was found that tropical total ozone intraseasonal variations are large (∼ ±10 Dobson unit) and comparable to those in the annual and interannual timescales. These intraseasonal total ozone anomalies are mainly evident in the subtropics over the Pacific and eastern hemisphere, with a systematic relationship to the MJO convection and wave dynamics discussed earlier (Figures 3 and 4a). The subtropical negative ozone anomalies typically flank or lie to the west of the equatorial anomalous convection and are collocated with the subtropical upper-troposphere anticyclones generated by the equatorial anomalous convective heating. On the other hand, the subtropical positive ozone anomalies generally lie to the east of the equatorial anomalous convection and are collocated with the subtropical upper-troposphere cyclones generated by the equatorial anomalous convective heating. The subtropical ozone anomalies are anticorrelated with geopotential height anomalies near the tropopause and thus mainly associated with the ozone variability in the stratosphere rather the troposphere.

Figure 4

(a) Map of total-column ozone anomaly for a composite MJO cycle based on TOMS/SBUV data from 1980 to 2006. The color red denotes high ozone anomalies, while the color blue indicates low ozone anomalies. The superimposed solid black line denotes rainfall anomaly from CMAP (Reproduced from Tian et al., 2007, by permission of American Geophysical Union). (b) As (a) but for TOMS aerosol index anomaly (Reproduced from Tian et al., 2008, by permission of American Geophysical Union). (c) As (a) but for CMAP rainfall anomalies. (d) As (a) but for SeaWiFS ocean surface Chl anomaly (Reproduced from Waliser et al., 2005, by permission of American Geophysical Union).

Another example, the recent availability of multiple, global satellite aerosol products from TOMS, Moderate Resolution Imaging Spectroradiometer (MODIS), and Advanced Very High Resolution Radiometer (AVHRR) has made the investigation of the MJO modulation of aerosols possible. Large aerosol variations are found over the equatorial Indian and western Pacific Oceans where MJO convection is active, as well as the tropical Africa and Atlantic Ocean where MJO convection is weak, but the background aerosol level is high (Figures 3 and 4b). Although significant uncertainties still exist in the satellite aerosol retrievals, the satellite data indicate that the MJO and its associated cloudiness, rainfall, and circulation variability may systematically influence the aerosol variability.

The impacts of the MJO on the carbon monoxide (CO) abundances in the tropical tropopause layer (TTL) were also recently reported based on the Aura Microwave Limb Sounder (MLS) CO data. The effects of the eastward propagation of MJO on CO abundances in the TTL are evident. This indicates that the anomalous deep convection associated with the MJO can inject CO from the lower troposphere up to the TTL.

The availability of satellite-derived ocean surface chlorophyll (Chl) from Sea-viewing Wide Field-of-view Sensor (SeaWiFS) provided a means for the discovery of the MJO impacts on oceanic biology. It was found that the MJO produces systematic and significant variations in ocean surface Chl in a number of regions across the tropical Oceans, including the northern Indian Ocean, a broad expanse of the northwestern tropical Pacific Ocean, and a number of near-coastal areas in the far eastern Pacific Ocean (Figures 3 and 4c, d). This indicates a need to further investigate the MJO modulation of the biogeochemical cycle properties and higher levels of food chains.

Summary

The MJO is a large-scale quasiperiodic oscillation of tropical atmospheric circulation and convection anomalies that moves slowly eastward along the equator mainly over the tropical Indian and Pacific Oceans with a timescale on the order of 30–60 days. The MJO is the dominant form of the intraseasonal variability in the tropical atmosphere and has many important influences on the global weather and climate system. Since the 1970s, the satellite remote sensing data have played a fundamental role in advancing our knowledge in the MJO, particularly in terms of its description, theoretical mechanisms, and global impacts. First, the satellite data provided us the fundamental knowledge of the convective and dynamic features of the MJO. Second, the satellite data presented us the three-dimensional thermodynamic structure and the surface condition (e.g., SST and surface heat flux) evolution associated with the MJO that helped us to better understand the MJO and propose theoretical description. Third, the satellite data offered us the opportunity to discover the global impacts of the MJO that have relevance to societal concerns such as extreme precipitation events, atmospheric composition, air quality, and biological markers in the ocean.

Cross-references

Aerosols

Radars