An official website of the United States government
The Madden–Julian Oscillation (MJO) varies seasonally. Both moist and dry dynamical processes would contribute to this seasonality. Previous results have suggested strong dependence of MJO phase speed on planetary-scale upper tropospheric Kelvin waves interacting with the mean flow. Composites and phase speed spectra assess the association between the Indian Basin MJO circulation and convection with variations in equatorial upper tropospheric background wind patterns, including seasonal variability. Results show that the fastest eastward propagation over the Indian Ocean (>10 ms−1) tends to occur during northern spring when background upper tropospheric easterlies are weakest. Northern winter signals typically advance eastward between 4 and 10 ms−1. Strong easterly background wind conditions during northern summer usually prevent propagation eastward along the equator from the Western Indian Ocean. Results also show relative amplitude variations between the MJO’s upper and lower tropospheric zonal wind signals, with the upper tropospheric circulation signals being disproportionately stronger than the lower tropospheric ones over the Western Hemisphere to East Africa. The upper tropospheric easterly wind anomalies grow over the Western Indian Ocean first, as specific humidity increases in lower tropospheric easterly wind to the east. Then, lower tropospheric westerly wind emerges west of the emerging convection, suggesting that lower tropospheric wind change depends more directly on moist processes than the upper tropospheric wind.
more »
« less
This content will become publicly available on May 1, 2027
Propagation Speed Climatology of Pacific Equatorial Kelvin Waves in Different Background Conditions
Atmospheric equatorial Kelvin waves—convective disturbances that manipulate tropical wind and rainfall patterns—can propagate eastward at speeds ranging from nearly stationary to 30 m/s, with variability determined by moist processes and advection by the background wind. Current studies on Kelvin waves lack a comprehensive climatology that explains how their structure and propagation speeds change in different background states. Thus, this work builds a variable regression model that uses ERA5 reanalysis data to reconstruct Kelvin waves during different background wind shear conditions and phases of the Madden–Julian Oscillation (MJO) and the El Niño–Southern Oscillation (ENSO) over the Pacific. Overall, Kelvin waves tend to speed up during background conditions that generate upper-tropospheric westerlies and slow down during upper-tropospheric easterlies. East Pacific Kelvin waves are faster than West Pacific Kelvin waves because of climatological westerly shear in the former and easterly shear in the latter. However, strong westerly shear over the East Pacific allows extratropical Rossby waves to impede on the Kelvin wave, while strong easterly shear over the West Pacific distorts classical Kelvin wave structure. The results provide references for weather prediction models to accurately resolve the interaction between Kelvin waves and background circulation.
more »
« less
- Award ID(s):
- 2417971
- PAR ID:
- 10690164
- Publisher / Repository:
- MDPI
- Date Published:
- Journal Name:
- Climate
- Volume:
- 14
- Issue:
- 5
- ISSN:
- 2225-1154
- Page Range / eLocation ID:
- 92
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
More Like this
-
-
Abstract Fields of equatorial upper-tropospheric circulation data are lag regressed against wavelet-filtered indices of upper-tropospheric zonal wind anomalies over a range of phase speeds at 50-day periods to study the propagation mechanisms of the upper-tropospheric circulation signal of the Madden–Julian oscillation (MJO) over the Indian Ocean. Results show that the MJO upper-tropospheric zonal wind is accelerated by the geopotential gradient force in quadrature with the existing wind anomaly, yielding its eastward propagation. This effect is offset by Doppler advection of the MJO wind by the easterly background wind. Divergence of mass by the zonal wind propagates the associated geopotential height anomalies in concert with the winds. Results confirm that advection of background wind by the MJO wind amplifies MJO wind anomalies in phase with those anomalies where the background wind is zonally confluent and breaks it down in regions of diffluent background wind. Coincidence of the mass source driven by moist convection and zonally diffluent zonal wind anomalies with falling geopotential height relative to the regions east and west is consistent with planetary-scale Kelvin wave zonal wind signals providing favorable conditions for convection. Circulation data at 100 hPa confirm approximately dry Kelvin wave dynamics driving MJO-associated equatorial circulations eastward.more » « less
-
Abstract The variability of the phase speed of the Madden–Julian oscillation (MJO) is poorly understood. The authors assess how the phase speed of the convective signal of the MJO associates with the background states over eastern Africa and the Indian Ocean. Relaxation of the coupling between tropical modes and their circulation has been previously linked to faster propagation; for example, the MJO speeds up over the eastern Pacific where its convective signal decouples from the circulation. In contrast, our results show that fast MJO events happen to exist during periods of wetter background states (>90 days) from East Africa across the Indian Ocean, whereas slow MJO is associated with dry background states. We found that fast MJO exhibits strong active and inactive phases with a structure suggesting more hierarchical convection. Results indicate that the association of the phase speed of the MJO as seen in the integrated filtered moist static energy with its tendency is stronger than the association of the phase speed as observed in the dry static energy with its tendency which is consistent with the acceleration of the MJO during wet background states. Also, our results indicate that the MJO may be faster during periods of enhanced low-level moisture because these periods have anomalously weak upper-tropospheric easterly background winds, which reduce the westward advection of the MJO by the background easterly wind, resulting in higher eastward phase speed of the MJO. The acceleration of the MJO by the background zonal wind overwhelms the deceleration associated with the moist-wave dynamics. Significance StatementThis study shows that the Madden–Julian oscillation (MJO), which is the dominant subseasonal weather signal in the tropics, moves eastward more quickly across eastern Africa and the Indian Ocean when the region is abnormally moist. The faster propagation does not appear to result from the higher moisture but instead from encountering weaker-than-normal upper-air winds from the east that tend to occur during moist periods.more » « less
-
Abstract Caribbean easterly waves (CEWs) propagate in an environment that is distinct from that of other easterly waves since it exhibits substantial westerly vertical wind shear. In spite of this distinction, their structure, propagation, and growth have not received much attention. A linear regression analysis reveals that these systems exhibit features consistent with moisture modes that are destabilized by moisture–vortex instability. They exhibit large moisture fluctuations and are in weak temperature gradient (WTG) balance, and moist static energy (MSE) growth is partly driven by meridional mean MSE advection by the anomalous winds. However, its circulation tilts vertically against the mean shear, a feature that is often associated with baroclinic instability. To reconcile these differences, a linear stability analysis employing a moist two-layer model is performed using a basic state that resembles the Caribbean Sea during boreal summer. The unstable wave solution from this analysis exhibits a structure that resembles observed CEWs. Excluding the upper troposphere from the stability analysis has little impact on the propagation and growth of the wave, and its circulation still exhibits a westward tilt in height. Thus, baroclinic instability is not the main growth mechanism of CEWs despite their structural similarity to baroclinic waves. Instead, the instability is largely rooted in how the lower-tropospheric circulation interacts with water vapor, as expected from moisture mode theory. These results suggest that tilting against the shear should not be used as the sole diagnostic for baroclinic instability. Baroclinic instability is unlikely to be a primary driver of growth for most oceanic tropical-depression-type waves, in agreement with previous work. Significance StatementThe environment in which Caribbean easterly waves propagate has a vertical wind shear that is like that seen in the midlatitudes, with winds becoming more westerly with height. Furthermore, the center of low pressure of the waves shifts toward the west, as in deepening midlatitude weather systems. This wave structure and shear is different from easterly waves that occur in other regions. However, in spite of the similarity to midlatitude weather systems, we show that Caribbean easterly waves mostly grow from moisture transports in the lower atmosphere. Thus, in spite of the distinct environment and wave structure, Caribbean easterly waves are driven by the same processes as other tropical easterly waves. These results underscore the importance of water vapor in driving tropical circulations. They also indicate that the processes that govern the growth of midlatitude weather may be of less importance in the tropics, even in regions that suggest otherwise.more » « less
-
Duplessy, Jean-Claude (Ed.)Previous research has shown that the equatorial upper tropospheric circulation signal associated with the Madden Julian Oscillation (MJO) over the Indian Ocean behaves like a Kelvin wave, with the eastward-propagation of the associated zonal wind anomaly caused by acceleration by the geopotential gradient force in quadrature with the wind anomaly, with the resultant signal amplified or decayed as the MJO wind advects background zonal wind in regions of background confluent or diffluent flow, with its phase speed adjusted by Doppler shifting by background zonal wind. This paper assesses these previously diagnosed mechanisms in the GEFS V12 forecast model, showing that similar mechanisms occur with the model MJO, but weaker and less organized. Results suggest that, relative to the validation data, the model stalls the MJO upper tropospheric zonal wind anomaly during diffluent background conditions near the Maritime Continent and weakens its amplitude more rapidly than validation data. The stalled propagation leads the model MJO to persist the signals of advection of and by the background wind, but at a reduced rate as the scale of the model MJO wind anomaly diminishes. Results show that beyond the stronger Doppler effect in the model, stalling the model MJO results from filling of the relative geopotential trough collocated with the MJO easterly wind anomaly, leading to breakdown of the geopotential gradient force term that is responsible for propagation of the wind anomaly. Thus, when the Maritime Continent region experiences zonally diffluent flow, the reason for the stronger Maritime Continent barrier effect in the model is that the model does not persist the Kelvin wave propagation mechanism that continues the eastward movement of the easterly wind anomaly in observations.more » « less
