Abstract Observational evidence of two extratropical pathways to forcing tropical convective disturbances is documented through a statistical analysis of satellite-derived OLR and ERA5 reanalysis. The forcing mechanism and the resulting disturbances are found to strongly depend on the structure of the background zonal wind. Although Rossby wave propagation is prohibited in easterlies, modeling studies have shown that extratropical forcing can still excite equatorial waves through resonance between the tropics and extratropics. Here this “remote” forcing pathway is investigated for the first time in the context of convectively coupled Kelvin waves over the tropical Pacific during northern summer. The extratropical forcing is manifested by eddy momentum flux convergence that arises when extratropical eddies propagate into the subtropics and encounter their critical line. This nonlinear forcing has similar wavenumbers and frequencies with Kelvin waves and excites them by projecting onto their meridional eigenstructure in zonal wind, as a form of resonance. This resonance is also evidenced by a momentum budget analysis, which reveals the nonlinear forcing term is essential for maintenance of the waves, while the remaining linear terms are essential for propagation. In contrast, the “local” pathway of extratropical forcing entails the presence of a westerly duct during northern winter that permits Rossby waves to propagate into the equatorial east Pacific, while precluding any sort of resonance with Kelvin waves due to Doppler shifting effects. The intruding disturbances primarily excite tropical “cloud plumes” through quasigeostrophic forcing, while maintaining their extratropical nature. This study demonstrates the multiple roles of the extratropics in forcing in tropical circulations and illuminates how tropical–extratropical interactions and extratropical basic states can provide be a source of predictability at the S2S time scale. Significance Statement This study seeks to understand how circulations in the midlatitudes excite the weather systems in the tropics. Results show that the mechanisms, as well as the types of tropical weather systems excited, are strongly dependent on the mean large-scale wind structure. In particular, when the large-scale wind blows from east to west, a special type of eastward-moving tropical weather system, the Kelvin wave, is excited owing to its resonance with remote eastward-moving weather systems in the extratropics. On the contrary, when the average wind blows from west to east, midlatitude systems are observed to intrude into the lower latitudes and directly force tropical convection, the cloud plumes, while maintaining their extratropical nature. These results speak to how the midlatitudes can excite distinct types of tropical weather systems under different climatological wind regimes. Understanding these tropical weather systems and their interactions with the midlatitudes may ultimately help to improve predictions of weather beyond 2 weeks.
more »
« less
Interaction between the MJO and High-Frequency Waves over the Maritime Continent in Boreal Winter
The two-way interaction between Madden–Julian oscillation (MJO) and higher-frequency waves (HFW) over the Maritime Continent (MC) during boreal winter of 1984–2005 is investigated. It is noted from observational analysis that strengthened (weakened) HFW activity appears to the west (east) of and under MJO convection during the MJO active phase and the opposite is seen during the MJO suppressed phase. Sensitivity model experiments indicate that the control of HFW activity by MJO is through change of the background vertical wind shear and specific humidity. The upscale feedbacks from HFW to MJO through nonlinear rectification of condensational heating and eddy momentum transport are also investigated with observational data. A significantly large amount (25%–40%) of positive heating anomaly ([Formula: see text]) at low levels to the east of MJO convection is contributed by nonlinear rectification of HFW. This nonlinear rectification is primarily attributed to eddy meridional moisture advection. A momentum budget diagnosis reveals that 60% of MJO zonal wind tendency at 850 hPa is attributed to the nonlinear interaction of HFW with other scale flows. Among them, the largest contribution arises from eddy zonal momentum flux divergence [Formula: see text]. Easterly (westerly) vertical shear to the west (east) of MJO convection during the MJO active phase causes the strengthening (weakening) of the HFW zonal wind anomaly. This leads to the increase (decrease) of eddy momentum flux activity to the east (west) of the MJO convection, which causes a positive (negative) eddy zonal momentum flux divergence in the zonal wind transitional region during the MJO active (suppressed) phase, favoring the eastward propagation of the MJO.
more »
« less
- Award ID(s):
- 1643297
- NSF-PAR ID:
- 10142973
- Date Published:
- Journal Name:
- Journal of Climate
- Volume:
- 32
- Issue:
- 13
- ISSN:
- 0894-8755
- Page Range / eLocation ID:
- 3819 to 3835
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
More Like this
-
-
Abstract Recent study indicates that the non-instantaneous interaction of convection and circulation is essential for evolution of large-scale convective systems. It is incorporated into cumulus parameterization (CP) by relating cloud-base mass flux of shallow convection to a composite of subcloud moisture convergence in the past 6 h. Three pairs of 19-yr simulations with original and modified CP schemes are conducted in a tropical channel model to verify their ability to reproduce the Madden–Julian oscillation (MJO). More coherent tropical precipitation and improved eastward propagation signal are observed in the simulations with the modified CP schemes based on the non-instantaneous interaction. It is found that enhanced feedback between shallow convection and low-level moisture convergence results in amplified shallow convective heating, and then generates reinforced moisture convergence, which transports more moisture upward. The improved simulations of eastward propagation of the MJO are largely attributed to higher specific humidity below 600 hPa in the free troposphere to the east of maximum rainfall center, which is related to stronger boundary layer moisture convergence forced by shallow convection. Large-scale horizontal advection causes asymmetric moisture tendencies relative to rainfall center (positive to the east and negative to the west) and also gives rise to eastward propagation. The zonal advection, especially the advection of anomalous specific humidity by mean zonal wind, is found to dominate the difference of horizontal advection between each pair of simulations. The results indicate the vital importance of non-instantaneous feedback between shallow convection and moisture convergence for convection organization and the eastward MJO propagation.more » « less
-
null (Ed.)Abstract The diversity of the Madden-Julian Oscillation (MJO) in terms of its maximum intensity, zonal extent and phase speed was explored using a cluster analysis method. The zonal extent is found to be significantly correlated to the phase speed. A longer zonal extent is often associated with a faster phase speed. The diversities of zonal extent and speed are connected with distinctive interannual sea surface temperature anomaly (SSTA) distributions and associated moisture and circulation patterns over the equatorial Pacific. An El Niño–like background SSTA leads to enhanced precipitation over the central Pacific, allowing a stronger vertically overturning circulation to the east of the MJO. This promotes both a larger east-west asymmetry of column-integrated moist static energy (MSE) tendency and a greater boundary-layer moisture leading, serving as potential causes of the faster phase speed. The El Niño–like SSTA also favors the MJOs intruding further into the Pacific, causing a larger zonal extent. The intensity diversity is associated with the interannual SSTA over the Maritime Continent and background moisture condition over the tropical Indian Ocean. An observed warm SSTA over the Maritime Continent excites a local Walker cell with a subsidence over the Indian Ocean, which could decrease the background moisture, weakening the MJO intensity. The intensity difference between strong and weak events would be amplified due to distinct intensity growth speed. The faster intensity growth of a strong MJO is attributed to a greater longwave radiative heating and a greater surface latent heat flux, as both of which contribute to a greater total time change rate of the column-integrated MSE.more » « less
-
The modulation of the Madden–Julian Oscillation (MJO) intensity by eastern Pacific (EP) type and central Pacific (CP) type of El Niño was investigated using observed data during the period of 1979–2013. MJO intensity is weakened (strengthened) over the equatorial western Pacific from November to April during EP (CP) El Niño. The difference arises from distinctive tendencies of column-integrated moist static energy (MSE) anomaly in the region. A larger positive MSE tendency was found during the convection developing period in the CP MJO than the EP MJO. The tendency difference is mainly caused by three meridional moisture advection processes: the advection of the background moisture by the intraseasonal wind anomaly, the advection of intraseasonal moisture anomaly by the mean wind and the nonlinear eddy advection. The advections’ differences are primarily caused by different intraseasonal perturbations and high-frequency activity whereas the background flow and moisture gradient are similar. The amplitudes in the intraseasonal suppressed convection anomaly over the central Pacific is critical in modulating the three meridional moisture advection processes. The influences on the central Pacific convection anomaly from seasonal mean moisture in two types of El Niños are discussed.more » « less
-
more » « less
A 200-hPa zonal momentum budget is performed to examine the role that western North Pacific tropical cyclones (TCs) play in helping to organize intraseasonal extratropical circulation anomalies that occur with the Madden–Julian oscillation (MJO). Zonal wind is linearly decomposed into components that occur on MJO time scales (i.e., 20–100-day periods), as well as those that occur with lower and higher frequency. Dates during Northern Hemisphere fall that feature nonrecurving TCs within a search radius centered on a South China Sea grid point when the MJO is convectively active over the Maritime Continent and west Pacific warm pool are used to generate composites of relevant budget terms. These composites are then compared to others that are based on the full list of dates that feature a convectively active MJO in the same location during NH fall without regard for TC presence. Composite results highlight the primary momentum sources that guide the evolution of the NH extratropical zonal wind and associated mass field in each event set. TCs help to accelerate the East Asian subtropical jet that evolves with the MJO by modulating the high-frequency subtropical circulation over Southeast Asia. The phasing of this circulation with its underlying MJO time-scale component enables it to transfer momentum to the emerging subtropical jet. This momentum is integrated into the more slowly evolving flow and carried forward by other processes, which leads to the development of a westerly momentum surge along the subtropical jet that spans the length of the North Pacific Ocean.
- Free Publicly Accessible Full Text
-
Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1175/JCLI-D-18-0511.1
