An official website of the United States government
Abstract A statistical study of 18 years of diurnal temperature tides observed by the SABER instrument on board the TIMED satellite reveals a substantial response of the tides in the upper atmosphere (>60 km) to the Madden‐Julian Oscillation (MJO) in the tropical troposphere. Nonmigrating tidal amplitudes are modulated at the intraseasonal MJO periods up to ~25% relative to the seasonal mean, twice as much as for the migrating tides (~10%). We fully characterize the tidal response for active MJO days as a function of season and MJO location as prescribed by the MJO index. The MJO modulation of the tides was predicted by models but could not be unequivocally observed before. Our results further point to an important role of background winds that partly cause a different response for equatorial and nonequatorial tidal modes in different seasons, which has implications for the MJO imprint on the ionospheric dynamo region.
more »
« less
Mechanism Studies of Madden‐Julian Oscillation Coupling Into the Mesosphere/Lower Thermosphere Tides Using SABER, MERRA‐2, and SD‐WACCMX
Abstract The Madden‐Julian Oscillation (MJO), an eastward‐moving disturbance near the equator (±30°) that typically recurs every ∼30–90 days in tropical winds and clouds, is the dominant mode of intraseasonal variability in tropical convection and circulation and has been extensively studied due to its importance for medium‐range weather forecasting. A previous statistical diagnostic of SABER/TIMED observations and the MJO index showed that the migrating diurnal (DW1) and the important nonmigrating diurnal (DE3) tide modulates on MJO‐timescale in the mesosphere/lower thermosphere (MLT) by about 20%–30%, depending on the MJO phase. In this study, we address the physics of the underlying coupling mechanisms using SABER, MERRA‐2 reanalysis, and SD‐WACCMX. Our emphasis was on the 2008–2010 time period when several strong MJO events occurred. SD‐WACCMX and SABER tides show characteristically similar MJO‐signal in the MLT region. The tides largely respond to the MJO in the tropospheric tidal forcing and less so to the MJO in tropospheric/stratospheric background winds. We further quantify the MJO response in the MLT region in the SD‐WACCMX zonal and meridional momentum forcing by separating the relative contributions of classical (Coriolis force and pressure gradient) and nonclassical forcing (advection and gravity wave drag [GWD]) which transport the MJO‐signal into the upper atmosphere. Interestingly, the tidal MJO‐response is larger in summer due to larger momentum forcing in the MLT region despite the MJO being most active in winter. We find that tidal advection and GWD forcing in MLT can work together or against each other depending on their phase relationship to the MJO‐phases.
more »
« less
- Award ID(s):
- 1753214
- PAR ID:
- 10374597
- Publisher / Repository:
- DOI PREFIX: 10.1029
- Date Published:
- Journal Name:
- Journal of Geophysical Research: Atmospheres
- Volume:
- 126
- Issue:
- 13
- ISSN:
- 2169-897X
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
More Like this
-
-
Abstract Recent evidence has revealed that strong coupling between the lower atmosphere and the thermosphere (100 km) occurs on intra‐seasonal (IS) timescales ( 30–90 days). The Madden‐Julian Oscillation (MJO), a key source of IS variability in tropical convection and circulation, influences the generation and propagation of atmospheric tides and is believed to be a significant driver of thermospheric IS oscillations (ISOs). However, limited satellite observations in the “thermospheric gap” (100–300 km) and challenges faced by numerical models in characterizing this region have hindered a comprehensive understanding of this connection. This study uses an Ionospheric Connection Explorer (ICON)‐adapted version of the Thermosphere Ionosphere Electrodynamics General Circulation Model, incorporating lower boundary tides from Michelson Interferometer for Global High‐resolution Thermospheric Imaging (MIGHTI) observations, to quantify the impact of the upward‐propagating tidal spectrum on thermospheric ISOs and elucidate connections to the MJO. Thermospheric zonal and diurnal mean zonal winds exhibit prominent ( 20 m/s) tidally driven ISOs throughout 2020–2021, largest at low latitudes near 110–150 km altitude. Correlation analyses confirm a robust connection between thermospheric ISOs, tides, and the MJO. Additionally, Hovmöller diagrams show eastward tidal propagation consistent with the MJO and concurrent Sounding of the Atmosphere using Broadband Emission Radiometry (SABER) observations. This study demonstrates that vertically propagating tides play a crucial role in linking IS variability from the lower atmosphere to the thermosphere, with the MJO identified as a primary driver of this whole‐atmosphere teleconnection. Understanding these connections is vital for advancing our knowledge in space physics, particularly regarding the dynamics of the upper atmosphere and ionosphere.more » « less
-
Abstract Eastward-moving moist deep convection and atmospheric circulation signals associated with the tropical Madden–Julian oscillation (MJO) sometimes break down as they cross the Maritime Continent region, but other times, the signal propagates across the region maintaining amplitude or regaining it over the west Pacific basin. This paper assesses the hypothesis that upper-tropospheric zonal diffluence of the background wind over the Maritime Continent causes much of this Maritime Continent barrier effect and its variation over time, through two mechanisms: 1) by slowing down the MJO as stronger-than-average background upper-tropospheric zonal wind over the Indian Ocean advects the MJO circulation signal westward, slowing its eastward advance, and 2) through the zonal advection of the background wind by subseasonal zonal wind across a region of zonal diffluence of the background wind, which advects the background wind of the opposite sign to the MJO wind. Advection of the opposite-signed background wind counteracts the MJO wind and reduces its associated upper-tropospheric mass divergence, weakening the mechanisms of the upper-tropospheric Kelvin wave component of the MJO circulation. Composites of MJO-associated zonal wind and outgoing longwave radiation signals diminish as they cross the Maritime Continent region when the region’s background zonal winds are diffluent, and composites of data reconstructing the relevant advection terms reveal the direct action of the advection mechanisms. Significance StatementThe Madden–Julian oscillation (MJO) is the leading subseasonal variation of the tropical atmosphere. This project addresses how diffluence of the upper-tropospheric background zonal wind can break down MJO events through advection of and by the background wind.more » « less
-
Abstract A robust linear regression algorithm is applied to estimate 95% confidence intervals on the background wind associated with Madden–Julian oscillation (MJO) upper-tropospheric atmospheric circulation signals characterized by different phase speeds. Data reconstructed from the ERA5 to represent advection by the upper-tropospheric background flow and MJO-associated zonal wind anomalies, together with satellite outgoing longwave radiation anomalies, all in the equatorial plane, are regressed against advection data filtered for zonal wavenumber 2 and phase speeds of 3, 4, 5, and 7 m s −1 . The regressed advection by the background flow is then divided by the negative of the zonal gradient of regressed zonal wind across the central Indian Ocean base longitude at 80°E to estimate the associated background wind that leads to the given advection. The median estimates of background wind associated with these phase speeds are 13.4, 11.2, 10.5, and 10.3 m s −1 easterly. The differences between estimated values at neighboring speeds suggests that advection acts most strongly in slow MJO events, indicating that the slowest events happen to be slow because they experience stronger easterly advection by the upper-tropospheric background wind. Significance Statement The Madden–Julian oscillation (MJO) is the dominant subseasonal rainfall signal of the tropical atmosphere. This project shows that the background wind of the tropical atmosphere most especially slows down the slowest MJO events. Understanding what controls its speed might help scientists better predict events.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
