The Detailed Dynamics of the Hadley Cell. Part II: December–February

Hoskins, Brian J.; Yang, Gui‐Ying

doi:10.1175/jcli-d-20-0504.1

Cited by 22 publications

(22 citation statements)

References 30 publications

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“…One strong reason for the usefulness of the long‐wave approximation is that it retains the Kelvin wave, which is of central importance for large‐scale tropical motions that are affected by rotation. The long‐wave approximation thus captures variability associated with the Walker circulation, but it is not informative for the Hadley cell dynamics and cross‐equatorial winds projecting onto MRG waves (Yang and Hoskins, 2017; Hoskins et al ., 2020; Hoskins and Yang, 2021).…”

Section: Dynamical Regimes From Planetary To Mesoscalesmentioning

confidence: 99%

Waves and coherent flows in the tropical atmosphere: New opportunities, old challenges

Stephan

Žagar

Shepherd

2021

Quart J Royal Meteoro Soc

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Atmospheric science relies on numerical models to simulate the complex multiscale nature of atmospheric variability, but our confidence in weather and climate predictions relies on theory and simplified models that describe scale interactions at a mechanistic level and can provide causal accounts of atmospheric behaviour. Global simulations at kilometre‐scale resolution are now feasible and offer new opportunities to the atmospheric science community for testing and expanding our understanding of climate variability and change. Taking full advantage of this new tool requires smart strategies for evaluating and analysing the output, especially as kilometre‐scale climate modelling will be limited to relatively short simulations with a rather small number of realizations. We here review some of the available tools for diagnosing and studying the dynamics of waves, coherent flows, and the interactions between them in terms of their ability to provide causal accounts of the behaviour seen in observations and in comprehensive simulation models. We describe their successes but also some of their limitations. The limitations are seen to be especially pronounced in the Tropics, where clouds, convection and atmospheric circulation are inextricably linked. The lack of a natural spatial truncation scale in the Tropics has given rise to many theoretical challenges, but it is for precisely this reason that the Tropics are where we might expect the largest gain from global kilometre‐scale models.

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Section: Dynamical Regimes From Planetary To Mesoscalesmentioning

confidence: 99%

Waves and coherent flows in the tropical atmosphere: New opportunities, old challenges

Stephan

Žagar

Shepherd

2021

Quart J Royal Meteoro Soc

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“…The present study supports the conceptual perspective on the Hadley circulation introduced and discussed in Hoskins et al . (2020) and Hoskins and Yang (2021), and sheds more light on the structure and variability of the Hadley circulation and its close connection to tropical moist convection. The analysis reveals how tropical moist convection drives Atlantic Hadley circulation variability, both temporally and spatially, over large geographic distances on time‐scales of days to months mediated by equatorial waves.…”

Section: Discussionmentioning

confidence: 90%

“…From the vorticity perspective as put forward by Hoskins et al (2020) and Hoskins and Yang (2021), since the meridional wind anomaly is strong in the Northern Hemisphere during the period, the absolute vorticity is expected to be close to zero. In other words, the positive planetary vorticity in the Northern Hemisphere has to be balanced approximately by upper-level negative relative vorticity.…”

Section: A Hadley Circulation Surge Event and The Impact On Trade Win...mentioning

confidence: 99%

“…Hoskins et al . (2020) and Hoskins and Yang (2021) recently presented a comprehensive and detailed investigation of the Hadley circulation, arguably the most fundamental feature of tropical weather and climate. They suggested complementing and refining the traditional zonal‐mean and time‐mean theory in two ways: the two studies acknowledge the central role of moist convection as a driving factor of Hadley circulation variability, and suggest that the asymmetries in longitude and fluctuations on time‐scales of days to months are of order one importance in the upper branch of the Hadley Cell.…”

Section: Introductionmentioning

confidence: 99%

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Tropical moist convection as an important driver of Atlantic Hadley circulation variability

Tomassini

Yang

2022

Quart J Royal Meteoro Soc

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The exact role of moist deep convection and associated latent heating in the tropical Hadley circulation has been debated for many years. This study investigates the connection between moist convection and the strength of the upper‐level meridional circulation over the tropical Atlantic, focusing mainly on one particular boreal winter season. There is a close relationship between events of strong organised deep convection and enhanced meridional upper‐level wind on many occasions. A process‐based analysis of specific events suggests that moist convection impacts Hadley circulation variability on time‐scales of days to months through equatorial wave dynamics. Equatorial waves play an important role, both directly by contributing to the Hadley circulation via their meridional wind component and also indirectly by triggering moist convection through low‐level convergence. Specific Hadley circulation surge events, short‐term, regionally confined intensifications of the upper‐level meridional circulation, can be attributed to enhanced organised moist convection and equatorial wave activity in many cases, with implications for trade wind cloudiness. The findings thus elucidate how the mean Hadley circulation is shaped by and composed of temporally and spatially varying convection–circulation interactions.

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“…1c and Fig. 2f), although some discrepancies can be observed in the tropics and subtropics where impact of tropical convection cannot be ignored 9 . This analysis indicates that the rather narrow climatological dip, ranging between 25°N and 40°N, arises from the combined effect of the two eddy fluxes.…”

Section: Introductionmentioning

confidence: 95%

The precipitation distribution set by eddy fluxes: the case of boreal winter

Yoo

Lee

2023

npj Clim Atmos Sci

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The latitudinal precipitation distribution shows a secondary peak in midlatitudes and a minimum in the subtropics. This minimum is widely attributed to the descending branch of the Eulerian Hadley cell. This study however shows that the precipitation distribution aligns more closely with the transformed Eulerian mean (TEM) vertical motion. In Northern Hemisphere winter, maximum TEM descent (ascent) and precipitation minimum (maximum) are collocated at ~20°N (~40°N). The subtropical descent is mostly driven by the meridional flux of zonal momentum by large-scale eddies, while the midlatitude ascent is driven by the meridional flux of heat by the eddies. When the poleward eddy momentum flux is sufficiently strong, however, the secondary precipitation peak shifts to 60°N corresponding to the location of the TEM ascent driven by the eddy momentum flux. Moisture supply for the precipitation is aided by evaporation which is enhanced where the TEM descending branch brings down dry air from the upper troposphere/lower stratosphere. This picture is reminiscent of dry air intrusions in synoptic meteorology, suggesting that the descending branch may embody a zonal mean expression of dry air intrusions. Moist air rises following the TEM ascending branch, suggesting that the ascending branch may be interpreted as a zonal mean expression of warm conveyor belts. This study thus offers a large-scale dynamics perspective of the synoptic description of precipitation systems. The findings here also suggest that future changes in the eddy momentum flux, which is poorly understood, could play a pivotal role in determining the future precipitation distribution.

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The Detailed Dynamics of the Hadley Cell. Part II: December–February

Cited by 22 publications

References 30 publications

Waves and coherent flows in the tropical atmosphere: New opportunities, old challenges

Waves and coherent flows in the tropical atmosphere: New opportunities, old challenges

Tropical moist convection as an important driver of Atlantic Hadley circulation variability

The precipitation distribution set by eddy fluxes: the case of boreal winter

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