Madden‐Julian oscillation (MJO), the dominant mode of intraseasonal variability in the tropical troposphere, has a significant impact on global weather and climate. Here we present that the ...year‐to‐year variation of the MJO activity shows significant changes with the quasi‐biennial oscillation (QBO) in the tropical stratosphere. Specifically, the boreal winter MJO amplitude, evaluated by various metrics, is typically stronger than normal during the QBO easterly phase at 50 hPa and weaker than normal during the QBO westerly phase at 50 hPa. This relationship, which is possibly mediated by the QBO‐related static stability and/or vertical wind shear changes in the tropical upper troposphere and lower stratosphere, is robust whether or not the activeness of the MJO or QBO is taken into account. This result suggests a new potential route from the stratosphere that regulates the organized tropical convection, helping to improve the prediction skill of the boreal winter MJO.
Key Points
MJO strengthens (weakens) during EQBO (WQBO)
The QBO‐MJO connection is significant during boreal wintertime only
The QBO‐MJO connection can be robustly found in various metrics
The Madden–Julian oscillation (MJO), the dominant mode of tropical intraseasonal variability, provides a major source of tropical and extratropical predictability on a subseasonal time scale. This ...study conducts a quantitative evaluation of the MJO prediction skill in state-of-the-art operational models, participating in the subseasonal-to-seasonal (S2S) prediction project. The relationship of MJO prediction skill with model biases in the mean moisture fields and in the longwave cloud–radiation feedbacks is also investigated.
The S2S models exhibit MJO prediction skill out to a range of 12 to 36 days. The MJO prediction skills in the S2S models are affected by both the MJO amplitude and phase errors, with the latter becoming more important at longer forecast lead times. Consistent with previous studies, MJO events with stronger initial MJO amplitude are typically better predicted. It is found that the sensitivity to the initial MJO phase varies notably from model to model.
In most models, a notable dry bias develops within a few days of forecast lead time in the deep tropics, especially across the Maritime Continent. The dry bias weakens the horizontal moisture gradient over the Indian Ocean and western Pacific, likely dampening the organization and propagation of the MJO. Most S2S models also underestimate the longwave cloud–radiation feedbacks in the tropics, which may affect the maintenance of the MJO convective envelope. The models with smaller bias in the mean horizontal moisture gradient and the longwave cloud–radiation feedbacks show higher MJO prediction skills, suggesting that improving those biases would enhance MJO prediction skill of the operational models.
Interannual variation of seasonal-mean tropical convection over the Indo-Pacific region is primarily controlled by El Niño–Southern Oscillation (ENSO). For example, during El Niño winters, ...seasonal-mean convection around the Maritime Continent becomes weaker than normal, while that over the central to eastern Pacific is strengthened. Similarly, subseasonal convective activity, which is associated with the Madden–Julian oscillation (MJO), is influenced by ENSO. The MJO activity tends to extend farther eastward to the date line during El Niño winters and contract toward the western Pacific during La Niña winters. However, the overall level of MJO activity across the Maritime Continent does not change much in response to the ENSO. It is shown that the boreal winter MJO amplitude is closely linked with the stratospheric quasi-biennial oscillation (QBO) rather than with ENSO. The MJO activity around the Maritime Continent becomes stronger and more organized during the easterly QBO winters. The QBO-related MJO change explains up to 40% of interannual variation of the boreal winter MJO amplitude. This result suggests that variability of the MJO and the related tropical–extratropical teleconnections can be better understood and predicted by taking not only the tropospheric circulation but also the stratospheric mean state into account. The seasonality of the QBO–MJO link and the possible mechanism are also discussed.
Multiscale Nature of Atmospheric Rivers Park, Chanil; Son, Seok‐Woo; Guan, Bin
Geophysical research letters,
28 May 2023, Letnik:
50, Številka:
10
Journal Article
Recenzirano
Odprti dostop
This study provides evidence for the multiscale nature of atmospheric rivers (ARs) by differentiating them based on high‐ (HF) and low‐frequency (LF) moisture transports. The HF‐dominant ARs exhibit ...migratory behavior as they are typically accompanied by extratropical cyclones. Their spatial distribution is seasonally synchronized with midlatitude storm activity. On the other hand, the LF‐dominant ARs stay in place as they are associated with quasi‐stationary circulation. They prevail in the subtropical monsoon regions in the summer hemisphere. The ARs are often jointly affected by HF and LF processes. Such intermediate ARs are frequently observed along the poleward boundary of subtropical highs. The latter two AR types are locally more persistent than the HF‐dominant ARs, implying the importance of LF dynamics in long‐lasting AR impacts. We suggest that multiscale analysis of ARs will offer valuable insights into their diversity and hydrological impacts.
Plain Language Summary
Atmospheric rivers (ARs) are narrow corridors of intense water vapor transport often concurrent with extratropical cyclones. Through case studies and climatological analyses, however, this study shows that ARs are not only associated with high‐frequency (HF) moisture transport such as those driven by extratropical cyclones but also related to low‐frequency (LF) moisture transport driven by quasi‐stationary circulation. The HF‐dominant ARs are observed along the midlatitude storm tracks, while the LF‐dominant ARs appear in summer monsoon regions in low latitudes. There are also a non‐negligible number of ARs jointly influenced by HF and LF processes, particularly along the poleward flank of subtropical highs. Noticeably, ARs strongly influenced by LF processes have a longer local duration than others. It implies that LF dynamics, which have received relatively less attention, need to be incorporated for a better understanding and prediction of the long‐lasting AR impacts. This study highlights that ARs have diverse characteristics by HF and LF processes. Taking their multiscale nature into account thus will be helpful when delineating the dynamics and hydrological impacts of ARs.
Key Points
High‐frequency moisture transport related to atmospheric rivers (ARs) is synchronized with transient storm activity in midlatitudes
The ARs are also fueled by low‐frequency (LF) moisture transport along the boundary of subtropical highs and in the summer monsoon regions
The LF moisture transport plays a critical role in locally long‐duration AR events, increasing the risk of high‐rank AR events
Stratospheric sudden warming (SSW) events exhibit pronounced interannual variability. Based on zonal wind reversals at 60°N and 10 hPa, it has been suggested that SSW events occur more preferentially ...during El Niño–Southern Oscillation (ENSO) winters (both El Niño and La Niña winters) than during ENSO-neutral winters. This relationship is reevaluated here by considering seven different SSW definitions. For all definitions, SSW events are detected more frequently during El Niño winters than during ENSO-neutral winters, in agreement with a strengthened planetary-scale wave activity. However, such a systematic relationship is not found during La Niña winters. While three SSW definitions, including the wind-reversal definition, show a higher SSW frequency during La Niña winters than during ENSO-neutral winters, other definitions show no difference or even lower SSW frequency during La Niña winters. This result, which is qualitatively insensitive to the choice of reanalysis datasets, ENSO indices, and SST datasets, indicates that the reported ENSO–SSW relationship is dependent on the details of the SSW definition. This result is interpreted in terms of different background wind, latitudinal extent of wind reversal, and planetary-scale wave activity during El Niño and La Niña winter SSW events.
The surface warming in recent decades has been most rapid in the Arctic, especially during the winter. Here, by utilizing global reanalysis and satellite datasets, it is shown that the northward flux ...of moisture into the Arctic during the winter strengthens the downward infrared radiation (IR) by 30–40 W m−2over 1–2 weeks. This is followed by a decline of up to 10% in sea ice concentration over the Greenland, Barents, and Kara Seas. A climate model simulation indicates that the wind-induced sea ice drift leads the decline of sea ice thickness during the early stage of the strong downward IR events, but that within one week the cumulative downward IR effect appears to be dominant. Further analysis indicates that strong downward IR events are preceded several days earlier by enhanced convection over the tropical Indian and western Pacific Oceans. This finding suggests that sea ice predictions can benefit from an improved understanding of tropical convection and ensuing planetary wave dynamics.
Abstract The mechanism of North Pacific (NP) blocking formation is investigated by conducting a reanalysis-based budget analysis of the quasigeostrophic geopotential tendency equation. It is ...confirmed that the amplification of NP blocking anomalies primarily results from vorticity fluxes with a minor contribution of heat fluxes. In winter, the cross-frequency vorticity fluxes, resulting from interactions between high-frequency eddies and the slowly varying background flow, dominate the blocking formation. The cross-frequency vorticity fluxes, however, become substantially weaker and comparable to the low-frequency vorticity fluxes in summer. This seasonality indicates that the mechanism of NP blocking formation varies with seasons due to the different background flow. It is further found that NP blocking formation is not sensitive to the region of formation (i.e., western vs eastern NP) nor to the type of wave breaking (i.e., cyclonic vs anticyclonic wave breaking).
Abstract
This study highlights the importance of the diabatic process in the heavy rainfall events (HREs) that are initiated on the eastern slope of the Tibetan Plateau and move to the lower reaches ...of the Yangtze River basin. These HREs, which cause significant socioeconomic losses in the Yangtze River basin, are typically maintained for 3 days. They develop when a large amount of moisture converges on the eastern slope of the Tibetan Plateau. By solving the quasigeostrophic (QG) omega equation, it is revealed that the vertical motion of HREs is organized by both dynamic and diabatic forcings, with the latter being dominant. The stationary boundary forcing on the eastern slope of the Tibetan Plateau also contributes to the initial organization of the HREs. While the dynamic vertical motion does not change much and the boundary forcing becomes negligible after the initial organization, diabatic vertical motion becomes more dominant in QG vertical motion (∼80%) as HREs develop and move downstream. The potential vorticity (PV) tendency budget analysis reveals that the development and eastward movement of the HRE-related surface cyclone is primarily associated with diabatic PV production to the east of the cyclone where a large amount of moisture converges. This result implies that the long-traveling HREs along the Yangtze River basin are highly self-maintaining in nature.
Abstract
The detailed dynamical mechanisms of the upper-tropospheric circulation response to the Madden–Julian oscillation (MJO) convection are examined by integrating a primitive equation model. A ...series of initial-value calculations with the climatological boreal winter background flow forced by the MJO-like thermal forcing successfully capture the key aspects of the observed circulation response to the MJO convection. This suggests that a large fraction of MJO-related circulation anomalies are direct responses to tropical heating in both the tropics and extratropics and can be largely explained by linear dynamics.
It is found that MJO-like dipole heatings not only intensify tropical upper-tropospheric anomalies but also weaken them at certain regions because of the interaction between equatorial Kelvin and Rossby waves. The Rossby wave train primarily excited by horizontal divergence of upper-level perturbation flow propagates northeastward and then heads back to the equator. In this way, Rossby wave activity once generated over the subtropical Indian Ocean tends to enhance the equatorial upper-tropospheric anomalies over the tropical Atlantic and West Africa that have already been created by the zonally propagating equatorial Rossby and Kelvin waves. A ray path tracing reveals that a successive downstream development of Rossby wave train mostly results from the large-scale waves with zonal wavenumbers 2–3 in the Northern Hemisphere and 3–5 in the Southern Hemisphere.
The sensitivity tests show that the overall results are quite robust. It is found, however, that the detailed circulation response to the MJO-like forcing is somewhat sensitive to the background flow. This suggests that MJO-related circulation anomalies may have nonnegligible long-term variability and change as background flow varies.
Abstract
The dynamical mechanism by which the quasi-biennial oscillation (QBO) might influence the temperature anomaly, associated with the Madden–Julian oscillation (MJO), in the equatorial upper ...troposphere and lower stratosphere (UTLS) is examined by conducting a series of initial-value experiments using a dry primitive equation model. The observed temperature response to the MJO convection becomes colder and more in phase with the convection during easterly QBO (EQBO) than westerly QBO (WQBO) phases. This QBO-dependent MJO temperature anomaly in the UTLS is qualitatively reproduced by model experiments in which EQBO or WQBO background state is artificially imposed above 250 hPa while leaving the troposphere unaltered. As in the observations, the localized cold anomaly in the UTLS becomes strengthened and steepened with EQBO-like background state than WQBO-like one. It turns out that the QBO zonal wind, instead of temperature, plays a major role in determining the localized UTLS temperature anomaly by modulating wave energy dispersion.
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