scholarly journals Regime Change Behavior during Asian Monsoon Onset

2018 ◽  
Vol 31 (8) ◽  
pp. 3327-3348 ◽  
Author(s):  
Ruth Geen ◽  
F. H. Lambert ◽  
G. K. Vallis
Keyword(s):  
Hadley Circulation ◽  
Stationary Wave ◽  
Realistic Model ◽  
Equation Model ◽  
Monsoon Onset ◽  
Hadley Cell ◽  
Change Behavior ◽  
Summer Hemisphere ◽  
Upper Level ◽  
Asian Monsoons

Abstract As the ITCZ moves off the equator on an aquaplanet, the Hadley circulation transitions from an equinoctial regime with two near-symmetric, significantly eddy-driven cells to a monsoon-like regime with a strong, thermally direct cross-equatorial cell, intense low-latitude precipitation, and a weak summer hemisphere cell. Dynamical feedbacks appear to accelerate the transition. This study investigates the relevance of this behavior to monsoon onset by using primitive equation model simulations ranging from aquaplanets to more realistic configurations with Earth’s continents and topography. A change in the relationship between ITCZ latitude and overturning strength is identified once the ITCZ moves poleward of approximately 7°. Monsoon onset is associated with off-equatorial ascent in regions of nonnegligible planetary vorticity, and this is found to generate a vortex stretching tendency that reduces upper-level absolute vorticity. In an aquaplanet, this causes a transition to the cross-equatorial, thermally direct regime, intensifying the overturning circulation. Analysis of the zonal momentum budget suggests that a stationary wave, driven by topography and land–sea contrast, can trigger a similar transition in the more realistic model configuration, with the wave extending the ascent region of the Southern Hemisphere Hadley cell northward, and enhanced overturning then developing to the south. These two elements of the circulation resemble the East and South Asian monsoons.

Wind–Evaporation Feedback and Abrupt Seasonal Transitions of Weak, Axisymmetric Hadley Circulations

2008 ◽  
Vol 65 (7) ◽  
pp. 2194-2214 ◽  
Author(s):  
William R. Boos ◽  
Kerry A. Emanuel
Keyword(s):  
Angular Momentum ◽  
Axisymmetric Flow ◽  
Hadley Circulation ◽  
Meridional Flow ◽  
Equation Model ◽  
Hadley Cell ◽  
Quasi Equilibrium ◽  
Surface Drag ◽  
Upper Level ◽  
Zonal Momentum

Abstract For an imposed thermal forcing localized off the equator, it is known that conservation of absolute angular momentum in axisymmetric flow produces a nonlinear response once the forcing exceeds a critical amplitude. It is shown here that, for a moist atmosphere in convective quasi-equilibrium, the combination of wind-dependent ocean surface enthalpy fluxes and zonal momentum advection can provide a separate feedback that causes the meridional flow to evolve nonlinearly as a function of a sea surface temperature (SST) forcing, even if an angular momentum–conserving response is not achieved. This wind–evaporation feedback is examined in both an axisymmetric primitive equation model and a simple model that retains only a barotropic and single baroclinic mode. Only SST forcings that do not produce an angular momentum–conserving response are examined here. The wind–evaporation feedback is found to be inhibited in models with linear dynamics because the barotropic component of the Hadley circulation, which is coupled to the baroclinic circulation via surface drag, keeps surface winds small compared to upper-level winds. In models with nonlinear dynamics, the convergence of zonal momentum into the ascending branch of the cross-equatorial Hadley cell can create barotropic westerlies that constructively add to the baroclinic wind at the surface, thereby eliminating the inhibition of the wind–evaporation feedback. The possible relevance of these results to the onset of monsoons is discussed.

The Response of Large-Scale Circulation to Obliquity-Induced Changes in Meridional Heating Gradients

2014 ◽  
Vol 27 (14) ◽  
pp. 5504-5516 ◽  
Author(s):  
Damianos F. Mantsis ◽  
Benjamin R. Lintner ◽  
Anthony J. Broccoli ◽  
Michael P. Erb ◽  
Amy C. Clement ◽  
...  
Keyword(s):  
Heat Transport ◽  
Large Scale ◽  
Climate Model ◽  
Hadley Circulation ◽  
Heat Fluxes ◽  
Meridional Overturning Circulation ◽  
Hadley Cell ◽  
Geophysical Fluid Dynamics Laboratory ◽  
Summer Hemisphere ◽  
Insolation Change

Abstract The inter- and intrahemispheric climate responses to a change in obliquity are investigated using the Geophysical Fluid Dynamics Laboratory Climate Model, version 2.1. (GFDL CM2.1). Reduced obliquity causes a weakening of the seasonal insolation contrast between the summer and winter hemispheres and a strengthening of the meridional insolation gradient within the summer hemisphere. The interhemispheric insolation change is associated with weakening of the cross-equatorial Hadley circulation and reduced heat transport from the summer hemisphere to the winter hemisphere, in both the ocean and atmosphere. In contrast, the intrahemispheric insolation change is associated with increased midlatitude summer eddy activity as seen by the increased atmospheric heat transport at those latitudes. Analysis of the zonal mean atmospheric meridional overturning circulation on isentropic surfaces confirms the increase of the midlatitude eddy circulation, which is driven by changes of sensible and latent heat fluxes, as well as changes in the stratification or distribution of entropy. It is suggested that the strengthening of this circulation is associated with an equatorward shift of the ascending branch of the winter Hadley cell.

The large-scale tilt of the eddy divergence in the tropics

2020 ◽  
Author(s):  
Pablo Zurita-Gotor
Keyword(s):  
Momentum Flux ◽  
Large Scale ◽  
Hadley Cell ◽  
Coarse Grained ◽  
Wave Source ◽  
Summer Hemisphere ◽  
Divergence Field ◽  
Upper Level ◽  
The Tropics

<p>This work is concerned with the large-scale structure of the upper-level divergence/precipitation field in the deep tropics. Once the fine ITCZ structure is filtered out, the coarse-grained eddy divergence field is found to tilt eastward moving away from its maximum near the equator in the summer hemisphere. This robust tilt (observed for both hemispheres and seasons) is also present in the classical Gill solution.</p><p>In this presentation we show that the sign of the tilt is intimately linked to the direction of the eddy momentum flux. The observed eastward tilt is such that the momentum flux is directed towards the wave source, suggesting that the observed tilt is determined by wave propagation.</p><p>We also discuss the determination of the tilt in the simple Gill model and its sensitivity to the meridional Hadley flow. We show that the increase in the cross-equatorial momentum flux when the Hadley cell strengthens is associated with an increased tilt of the divergence field in the downstream direction of the flow, supporting the conjecture that the tilt is associated with propagation. </p>

scholarly journals The Hadley Circulation in Reanalyses: Climatology, Variability, and Change

2013 ◽  
Vol 26 (10) ◽  
pp. 3357-3376 ◽  
Author(s):  
H. Nguyen ◽  
A. Evans ◽  
C. Lucas ◽  
I. Smith ◽  
B. Timbal
Keyword(s):  
Large Scale ◽  
Southern Oscillation ◽  
Average Rate ◽  
Hadley Circulation ◽  
Hadley Cell ◽  
Momentum Balance ◽  
Annular Modes ◽  
Modes Of Variability ◽  
Summer Hemisphere ◽  
High Phase

Abstract Analysis of the annual cycle of intensity, extent, and width of the Hadley circulation across a 31-yr period (1979–2009) from all existent reanalyses reveals a good agreement among the datasets. All datasets show that intensity is at a maximum in the winter hemisphere and at a minimum in the summer hemisphere. Maximum and minimum values of meridional extent are reached in the respective autumn and spring hemispheres. While considering the horizontal momentum balance, where a weakening of the Hadley cell (HC) is expected in association with a widening, it is shown here that there is no direct relationship between intensity and extent on a monthly time scale. All reanalyses show an expansion in both hemispheres, most pronounced and statistically significant during summer and autumn at an average rate of expansion of 0.55° decade−1 in each hemisphere. In contrast, intensity trends are inconsistent among the datasets, although there is a tendency toward intensification, particularly in winter and spring. Correlations between the HC and tropical and extratropical large-scale modes of variability suggest interactions where the extent of the HC is influenced by El Niño–Southern Oscillation (ENSO) and the annular modes. The cells tend to shrink (expand) during the warm (cold) phase of ENSO and during the low (high) phase of the annular modes. Intensity appears to be influenced only by ENSO and only during spring for the southern cell and during winter for the northern cell.

scholarly journals Solsticial Hadley Cell ascending edge theory from supercriticality

2021 ◽  
Author(s):  
Spencer A. Hill ◽  
Simona Bordoni ◽  
Jonathan L. Mitchell
Keyword(s):  
Large Scale ◽  
Analytical Approximation ◽  
Hadley Cell ◽  
Rossby Number ◽  
Absolute Vorticity ◽  
Rotation Rates ◽  
Planetary Rotation ◽  
Wide Range ◽  
Summer Hemisphere ◽  
Earth’S Climate

AbstractHow far the Hadley circulation’s ascending branch extends into the summer hemisphere is a fundamental but incompletely understood characteristic of Earth’s climate. Here, we present a predictive, analytical theory for this ascending edge latitude based on the extent of supercritical forcing. Supercriticality sets the minimum extent of a large-scale circulation based on the angular momentum and absolute vorticity distributions of the hypothetical state were the circulation absent. We explicitly simulate this latitude-by-latitude radiative-convective equilibrium (RCE) state. Its depth-averaged temperature profile is suitably captured by a simple analytical approximation that increases linearly with sinφ, where φ is latitude, from the winter to the summer pole. This, in turn, yields a one-third power-law scaling of the supercritical forcing extent with the thermal Rossby number. In moist and dry idealized GCM simulations under solsticial forcing performed with a wide range of planetary rotation rates, the ascending edge latitudes largely behave according to this scaling.

scholarly journals The Role of Hadley Circulation and Lapse-Rate Changes for the Future European Summer Climate

2018 ◽  
Vol 32 (2) ◽  
pp. 385-404 ◽  
Author(s):  
Roman Brogli ◽  
Nico Kröner ◽  
Silje Lund Sørland ◽  
Daniel Lüthi ◽  
Christoph Schär
Keyword(s):  
Hadley Circulation ◽  
Global Climate Models ◽  
Hadley Cell ◽  
Lapse Rate ◽  
Climate Change Signal ◽  
Climate Projections ◽  
Minor Importance ◽  
Near Surface ◽  
Summer Warming ◽  
The Mediterranean

Abstract By the end of the century, climate projections for southern Europe exhibit an enhanced near-surface summer warming in response to greenhouse gas emissions, which is known as the Mediterranean amplification. Possible causes for this amplified warming signal include a poleward Hadley cell expansion as well as tropospheric lapse-rate changes. In this work, regional climate model (RCM) simulations driven by three different global climate models (GCMs) are performed, representing the RCP8.5 emission scenario. For every downscaled GCM, the climate change signal over Europe is separated into five contributions by modifying the lateral boundary conditions of the RCM. This simulation strategy is related to the pseudo–global warming method. The results show that a poleward expansion of the Hadley cell is of minor importance for the Mediterranean amplification. During summer, the simulated Hadley circulation is weak, and projections show no distinct expansion in the European sector. The north–south contrast in lapse-rate changes is suggested as the most important factor causing the Mediterranean amplification. Lapse-rate changes are projected throughout Europe, but are weaker over the Mediterranean than over northern Europe (around 0.15 vs 0.3 K km−1 by the end of the century). The weaker lapse-rate changes result in a strong near-surface summer warming over the Mediterranean, since the upper-tropospheric warming is of similar magnitude throughout Europe. The differing lapse-rate changes can be understood as a thermodynamic response to lower-tropospheric humidity contrasts.

scholarly journals Axisymmetric Hadley Cell Theory with a Fixed Tropopause Temperature Rather than Height

2020 ◽  
Vol 77 (4) ◽  
pp. 1279-1294
Author(s):  
Spencer A. Hill ◽  
Simona Bordoni ◽  
Jonathan L. Mitchell
Keyword(s):  
Thermal Field ◽  
Hadley Circulation ◽  
Hadley Cell ◽  
Lapse Rate ◽  
Cell Theory ◽  
Single Column ◽  
Single Column Model ◽  
Local Surface Temperature ◽  
Hadley Cells ◽  
Tropopause Temperature

Abstract Axisymmetric Hadley cell theory has traditionally assumed that the tropopause height (Ht) is uniform and unchanged from its radiative–convective equilibrium (RCE) value by the cells’ emergence. Recent studies suggest that the tropopause temperature (Tt), not height, is nearly invariant in RCE, which would require appreciable meridional variations in Ht. Here, we derive modified expressions of axisymmetric theory by assuming a fixed Tt and compare the results to their fixed-Ht counterparts. If Tt and the depth-averaged lapse rate are meridionally uniform, then at each latitude Ht varies linearly with the local surface temperature, altering the diagnosed gradient-balanced zonal wind at the tropopause appreciably (up to tens of meters per second) but the minimal Hadley cell extent predicted by Hide’s theorem only weakly (≲1°) under standard annual-mean and solsticial forcings. A uniform Tt alters the thermal field required to generate an angular-momentum-conserving Hadley circulation, but these changes and the resulting changes to the equal-area model solutions for the cell edges again are modest (<10%). In numerical simulations of latitude-by-latitude RCE under annual-mean forcing using a single-column model, assuming a uniform Tt is reasonably accurate up to the midlatitudes, and the Hide’s theorem metrics are again qualitatively insensitive to the tropopause definition. However imperfectly axisymmetric theory portrays the Hadley cells in Earth’s macroturbulent atmosphere, evidently its treatment of the tropopause is not an important error source.

scholarly journals Pacific Mean-State Control of Atlantic Multidecadal Oscillation–El Niño Relationship

2020 ◽  
Vol 33 (10) ◽  
pp. 4273-4291 ◽  
Author(s):  
Dongmin Kim ◽  
Sang-Ki Lee ◽  
Hosmay Lopez ◽  
Marlos Goes
Keyword(s):  
El Niño ◽  
El Nino ◽  
Hadley Circulation ◽  
Atlantic Multidecadal Oscillation ◽  
Ocean Dynamics ◽  
Easterly Wind ◽  
Atmospheric Teleconnections ◽  
Upper Level ◽  
The Individual ◽  
Mean State

AbstractWe investigate the potential impacts of the interdecadal Pacific oscillation (IPO) and Atlantic multidecadal oscillation (AMO) on El Niño and the associated atmosphere and ocean dynamics by using the Community Earth System Model–Large Ensemble Simulation (CESM-LENS). The individual effects of IPO and AMO on El Niño frequency and the underlying atmosphere–ocean processes are well reproduced in CESM-LENS and agree with previous studies. However, the sensitivity of El Niño frequency to the AMO is robust mainly during the negative IPO phase and very weak during the positive IPO phase. Further analysis suggests that the atmospheric mean state in the Pacific is much amplified during the negative IPO phase, facilitating the AMO-induced interocean atmospheric teleconnections. More specifically, during the negative IPO phase of the amplified mean state, the positive AMO enhances ascending motion from the northeastern Pacific, which in turn increases subsidence into the southeast Pacific through local anomalous Hadley circulation. The associated low-level easterly wind anomalies in the central equatorial Pacific are also reinforced by amplified upper-level divergence over the Maritime Continent to enhance the negative IPO, which is unfavorable for El Niño occurrence. Conversely, the negative AMO nearly cancels out the suppressing effect of the negative IPO on El Niño occurrence. During the positive IPO phase of the weakened atmospheric mean state, however, the AMO-induced interocean atmospheric teleconnections are much weaker; thus, neither the positive nor the negative AMO has any significant impact on El Niño occurrence.

scholarly journals The Role of the Divergent Circulation for Large-Scale Eddy Momentum Transport in the Tropics. Part I: Observations

2019 ◽  
Vol 76 (4) ◽  
pp. 1125-1144 ◽  
Author(s):  
Pablo Zurita-Gotor
Keyword(s):  
Momentum Flux ◽  
Large Scale ◽  
Zonal Flow ◽  
Stationary Wave ◽  
Hadley Cell ◽  
Momentum Transport ◽  
Mean Flow ◽  
Meridional Transport ◽  
The Tropics

Abstract This work investigates the role played by the divergent circulation for meridional eddy momentum transport in the tropical atmosphere. It is shown that the eddy momentum flux in the deep tropics arises primarily from correlations between the divergent eddy meridional velocity and the rotational eddy zonal velocity. Consistent with previous studies, this transport is dominated by the stationary wave component, associated with correlations between the zonal structure of the Hadley cell (zonal anomalies in the meridional overturning) and the climatological-mean Rossby gyres. This eddy momentum flux decomposition implies a different mechanism of eddy momentum convergence from the extratropics, associated with upper-level mass convergence (divergence) over sectors with anomalous westerlies (easterlies). By itself, this meridional transport would only increase (decrease) isentropic thickness over regions with anomalous westerly (easterly) zonal flow. The actual momentum mixing is due to vertical (cross isentropic) advection, pointing to the key role of diabatic processes for eddy–mean flow interaction in the tropics.

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