Madden–Julian oscillation
The Madden–Julian oscillation is characterized by an eastward progression of large regions of both enhanced and suppressed tropical rainfall, observed mainly over the Indian and Pacific Ocean.
A strong relationship between the leading mode of intraseasonal variability of the North American Monsoon System, the MJO and the points of origin of tropical cyclones is also present.
A period of warming sea surface temperatures is found five to ten days prior to a strengthening of MJO-related precipitation across southern Asia.
A break in the Asian monsoon, normally during the month of July, has been attributed to the Madden–Julian oscillation after its enhanced phase moves off to the east of the region into the open tropical Pacific Ocean.
[28] An increase in frequency of MJO phases with convective activity over the eastern Pacific might have contributed to the drying trend seen in the Congo Basin in the last few decades.
[31] However, observations suggest that the 1982-1983 El Niño developed rapidly during July 1982 in direct response to a Kelvin wave triggered by an MJO event during late May.
In the tropical Pacific, winters with weak-to-moderate cold, or La Niña, episodes or ENSO-neutral conditions are often characterized by enhanced 30- to 60-day Madden–Julian oscillation activity.
A recent example is the winter of 1996–1997, which featured heavy flooding in California and in the Pacific Northwest (estimated damage costs of $2.0–3.0 billion at the time of the event) and a very active MJO.
Such winters are also characterized by relatively small sea surface temperature anomalies in the tropical Pacific compared to stronger warm and cold episodes.
Typical wintertime weather anomalies preceding heavy precipitation events in the Pacific Northwest are as follows:[34] Throughout this evolution, retrogression of the large-scale atmospheric circulation features is observed in the eastern Pacific–North American sector.
Many of these events are characterized by the progression of the heaviest precipitation from south to north along the Pacific Northwest coast over a period of several days to more than one week.
[34] A coherent simultaneous relationship exists between the longitudinal position of maximum MJO-related rainfall and the location of extreme west coast precipitation events.
[34] In 2019, Rostami and Zeitlin[35] reported a discovery of steady, long-living, slowly eastward-moving large-scale coherent twin cyclones, so-called equatorial modons, by means of a moist-convective rotating shallow water model.
Crudest barotropic features of MJO such as eastward propagation along the equator, slow phase speed, hydro-dynamical coherent structure, the convergent zone of moist-convection, are captured by Rostami and Zeitlin's modon.
It is shown that such eastward-moving coherent dipolar structures can be produced during geostrophic adjustment of localized large-scale pressure anomalies in the diabatic moist-convective environment on the equator.
If moist convection is strong enough, a dipolar cyclonic structure, which appears in the process of adjustment as a Rossby-wave response to the perturbation, transforms into a coherent modon-like structure in the lower layer, which couples with a baroclinic Kelvin wave through a zone of enhanced convection and produces, at initial stages of the process, a self-sustained slowly eastward-propagating zonally- dissymmetrical quadrupolar vorticity pattern.
By means of a new multi-layer pseudo-spectral moist-convective Thermal Rotating Shallow Water (mcTRSW) model in a full sphere, they presented a possible equatorial adjustment beyond Gill's mechanism for the genesis and dynamics of the MJO.
Interaction of the BKW, after circumnavigating all around the equator, with a new large-scale buoyancy anomaly may contribute to excitation of a recurrent generation of the next cycle of MJO-like structure.
