Seasonal Sensitivity of the Cross-Equatorial Hadley Cell Response to Extratropical Thermal Forcings

2020 ◽  
pp. 1-55
Author(s):  
Yen-Ting Hwang ◽  
Po-Chun Chung
Keyword(s):  
Seasonal Variation ◽  
Hadley Cell ◽  
Ocean Heat Uptake ◽  
Tropical Precipitation ◽  
Subtropical Jet ◽  
Community Atmosphere Model ◽  
Key Factor ◽  
Summer Hemisphere ◽  
Sea Surface Temperature Gradient ◽  
Extratropical Forcing

AbstractThis study explores the seasonal sensitivity of tropical circulation responses to an idealized extratropical thermal forcing using Community Atmosphere Model version 5 coupled to a slab ocean. The thermal heating over the Southern Ocean is held constant, and the tropical responses in each month of the year are investigated. An anomalous cross-equatorial cell and a southward tropical rain belt shift occur every month. The anomalous cross-equatorial cell has a strong influence on the strengths of the Hadley cell and the subtropical jet in the winter hemisphere; in contrast, it has nearly no impact on the Hadley cell and the subtropical jet strengths in the summer hemisphere.The seasonal variation of the anomalous cross-equatorial cell is small (<∼ 30% of the annual mean change), and could be understood via the energetic and the sea surface temperature gradient perspectives. Both perspectives point to the seasonality of the anomalous ocean heat uptake within the deep tropics as the key factor explaining the weak seasonality of the anomalous cross-equatorial cell. We propose a hypothesis explaining about 75% of this seasonal variation via the climatological position of the ITCZ relative to the anomalous cross-equatorial cell.The results suggest a modest seasonality in tropical precipitation and circulation responses to extratropical forcing. Also, such seasonality may be partly predicted by the climatological seasonal cycle of the tropical circulations.

scholarly journals Tropospheric Double Jets, Meridional Cells, and Eddies: A Case Study and Idealized Simulations

2007 ◽  
Vol 135 (9) ◽  
pp. 3118-3133 ◽  
Author(s):  
Isabella Bordi ◽  
Klaus Fraedrich ◽  
Frank Lunkeit ◽  
Alfonso Sutera
Keyword(s):  
Radiative Forcing ◽  
General Circulation ◽  
Baroclinic Instability ◽  
Low Frequency ◽  
Circulation Model ◽  
Hadley Cell ◽  
Zonal Wind Anomaly ◽  
Subtropical Jet ◽  
Low Frequency Variability ◽  
Summer Hemisphere

Abstract The observed low-frequency variability of the zonally averaged atmospheric circulation in the winter hemisphere is found to be amenable to an interpretation where the subtropical jet is flanked by a secondary midlatitude one. Observations also suggest that the link between the stratosphere and the troposphere modulates the variability of the tropospheric double-jet structure. Moreover, the summer hemisphere is characterized by a strong midlatitude jet sided by an intermittent subtropical one and easterly winds in the stratosphere. This work addresses the question about the role of eddies in generating and maintaining these key features of the general circulation by means of a simplified general circulation model. Model solutions for different parameter settings and external radiative forcings in the stratosphere are studied with and without eddies active on the system. The following main findings are noted. 1) Eddy dynamics alone, through the baroclinic instability processes in an atmosphere subjected to radiative forcing and dissipation, may account for the observed meridional variance of the tropospheric jets. 2) The Hadley cell can extend to the pole overlying the Ferrel cell, a feature supported by observations in the summer hemisphere. 3) The meridional temperature gradient reversal in the summer stratosphere contributes to the observed low-frequency variability introducing an intermittent formation of a subtropical jet and the occurrence of easterlies in the tropical stratosphere. 4) Poleward propagation of the zonal wind anomaly is, when it occurs, related to the activity of synoptic eddies.

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 Detecting and Tracking Coastal Precipitation in the Tropics: Methods and Insights into Multiscale Variability of Tropical Precipitation

2020 ◽  
Vol 33 (15) ◽  
pp. 6689-6705
Author(s):  
David Coppin ◽  
Gilles Bellon ◽  
Alexander Pletzer ◽  
Chris Scott
Keyword(s):  
Diurnal Cycle ◽  
Large Scale ◽  
Madden Julian Oscillation ◽  
Maritime Continent ◽  
Total Rainfall ◽  
Tropical Precipitation ◽  
Precipitation Estimates ◽  
Summer Hemisphere ◽  
The Tropics ◽  
Climate Prediction Center

AbstractWe propose an algorithm to detect and track coastal precipitation systems and we apply it to 18 years of the high-resolution (8 km and 30 min) Climate Prediction Center CMORPH precipitation estimates in the tropics. Coastal precipitation in the Maritime Continent and Central America contributes to up to 80% of the total rainfall. It also contributes strongly to the diurnal cycle over land with the largest contribution from systems lasting between 6 and 12 h and contributions from longer-lived systems peaking later in the day. While the diurnal cycle of coastal precipitation is more intense over land in the summer hemisphere, its timing is independent of seasons over both land and ocean because the relative contributions from systems of different lifespans are insensitive to the seasonal cycle. We investigate the hypothesis that coastal precipitation is enhanced prior to the arrival of the Madden–Julian oscillation (MJO) envelope over the Maritime Continent. Our results support this hypothesis and show that, when considering only coastal precipitation, the diurnal cycle appears reinforced even earlier over islands than previously reported. We discuss the respective roles of coastal and large-scale precipitation in the propagation of the MJO over the Maritime Continent. We also document a shift in diurnal cycle with the phases of the MJO, which results from changes in the relative contributions of short-lived versus long-lived coastal systems.

scholarly journals Wave and Jet Maintenance in Different Flow Regimes

2016 ◽  
Vol 73 (6) ◽  
pp. 2465-2484 ◽  
Author(s):  
Orli Lachmy ◽  
Nili Harnik
Keyword(s):  
Wave Energy ◽  
Wave Spectrum ◽  
Flow Regimes ◽  
Wave Mode ◽  
Hadley Cell ◽  
Model Parameters ◽  
Mean Flow ◽  
Eddy Heat Flux ◽  
Subtropical Jet ◽  
Zonal Wavenumber

Abstract The wave spectrum and zonal-mean-flow maintenance in different flow regimes of the jet stream are studied using a two-layer modified quasigeostrophic (QG) model. As the wave energy is increased by varying the model parameters, the flow transitions from a subtropical jet regime to a merged jet regime and then to an eddy-driven jet regime. The subtropical jet is maintained at the Hadley cell edge by zonal-mean advection of momentum, while eddy heat flux and eddy momentum flux convergence (EMFC) are weak and concentrated far poleward. The merged jet is narrow and persistent and is maintained by EMFC from a narrow wave spectrum, dominated by zonal wavenumber 5. The eddy-driven jet is wide and fluctuating and is maintained by EMFC from a wide wave spectrum. The wave–mean flow feedback mechanisms that maintain each regime are explained qualitatively. The regime transitions are related to transitions in the wave spectrum. An analysis of the wave energy spectrum budget and a comparison with a quasi-linear version of the model show that the balance maintaining the spectrum in the merged and subtropical jet regimes is mainly a quasi-linear balance, whereas in the eddy-driven jet regime nonlinear inverse energy cascade takes place. The amplitude and wavenumber of the dominant wave mode in the merged and subtropical jet regimes are determined by the constraint that this mode would produce the wave fluxes necessary for maintaining a mean flow that is close to neutrality. In contrast, the equilibrated mean flow in the eddy-driven jet regime is weakly unstable.

Seasonal variation of tropical precipitation chemistry: La Selva, Costa Rica

1997 ◽  
Vol 31 (23) ◽  
pp. 3903-3910 ◽  
Author(s):  
Tamara J. Eklund ◽  
William H. McDowell ◽  
Catherine M. Pringle
Keyword(s):  
Seasonal Variation ◽  
Costa Rica ◽  
Precipitation Chemistry ◽  
La Selva ◽  
Tropical Precipitation

scholarly journals An Explanation for the Sensitivity of the Mean State of the Community Atmosphere Model to Horizontal Resolution on Aquaplanets

2017 ◽  
Vol 30 (13) ◽  
pp. 4781-4797 ◽  
Author(s):  
Adam R. Herrington ◽  
Kevin A. Reed
Keyword(s):  
Moisture Transport ◽  
General Circulation ◽  
Large Scale ◽  
General Circulation Models ◽  
Horizontal Resolution ◽  
Hadley Cell ◽  
Community Atmosphere Model ◽  
The Mean ◽  
Atmosphere Model ◽  
Mean State

The sensitivity of the mean state of the Community Atmosphere Model to horizontal resolutions typical of present-day general circulation models is investigated in an aquaplanet configuration. Nonconvergence of the mean state is characterized by a progressive drying of the atmosphere and large reductions in cloud coverage with increasing resolution. Analyses of energy and moisture budgets indicate that these trends are balanced by variations in moisture transport by the resolved circulation, and a reduction in activity of the convection scheme. In contrast, the large-scale precipitation rate increases with resolution, which is approximately balanced by greater advection of dry static energy associated with more active resolved vertical motion in the ascent region of the Hadley cell. An explanation for the sensitivity of the mean state to horizontal resolution is proposed, based on linear Boussinesq theory. The authors hypothesize that an increase in horizontal resolution in the model leads to a reduction in horizontal scale of the diabatic forcing arising from the column physics, facilitating finescale flow and faster resolved convective updrafts within the dynamical core, and steering the coupled system toward a new mean state. This hypothesis attempts to explain the underlying mechanism driving the variations in moisture transport observed in the simulations.

scholarly journals The Storm-Track Response to Idealized SST Perturbations in an Aquaplanet GCM

2008 ◽  
Vol 65 (9) ◽  
pp. 2842-2860 ◽  
Author(s):  
David James Brayshaw ◽  
Brian Hoskins ◽  
Michael Blackburn
Keyword(s):  
Large Scale ◽  
Storm Track ◽  
Hadley Cell ◽  
Atmospheric Response ◽  
Subtropical Jet ◽  
Poleward Shift ◽  
Sst Anomaly ◽  
Hadley Centre ◽  
Sst Gradient ◽  
Sst Anomalies

Abstract The tropospheric response to midlatitude SST anomalies has been investigated through a series of aquaplanet simulations using a high-resolution version of the Hadley Centre atmosphere model (HadAM3) under perpetual equinox conditions. Model integrations show that increases in the midlatitude SST gradient generally lead to stronger storm tracks that are shifted slightly poleward, consistent with changes in the lower-tropospheric baroclinicity. The large-scale atmospheric response is, however, highly sensitive to the position of the SST gradient anomaly relative to that of the subtropical jet in the unperturbed atmosphere. In particular, when SST gradients are increased very close to the subtropical jet, then the Hadley cell and subtropical jet is strengthened while the storm track and eddy-driven jet are shifted equatorward. Conversely, if the subtropical SST gradients are reduced and the midlatitude gradients increased, then the storm track shows a strong poleward shift and a well-separated eddy-driven jet is produced. The sign of the SST anomaly is shown to play a secondary role in determining the overall tropospheric response. These findings are used to provide a new and consistent interpretation of some previous GCM studies concerning the atmospheric response to midlatitude SST anomalies.

scholarly journals A Lagrangian Analysis of the Northern Hemisphere Subtropical Jet

2014 ◽  
Vol 71 (7) ◽  
pp. 2354-2369 ◽  
Author(s):  
Olivia Martius
Keyword(s):  
Angular Momentum ◽  
Northern Hemisphere ◽  
Potential Vorticity ◽  
South American ◽  
Lagrangian Analysis ◽  
Back Trajectories ◽  
Weather Forecasts ◽  
Subtropical Jet ◽  
Extratropical Forcing ◽  
The Tropics

Abstract This study presents a 5-yr climatology of 7-day back trajectories started from the Northern Hemisphere subtropical jet. These trajectories provide insight into the seasonally and regionally varying angular momentum and potential vorticity characteristics of the air parcels that end up in the subtropical jet. The trajectories reveal preferred pathways of the air parcels that reach the subtropical jet from the tropics and the extratropics and allow estimation of the tropical and extratropical forcing of the subtropical jet. The back trajectories were calculated 7 days back in time and started every 6 h from December 2005 to November 2010 using the Interim European Centre for Medium-Range Weather Forecasts (ECMWF) Re-Analysis (ERA-Interim) dataset as a basis. The trajectories were started from the 345-K isentrope in areas where the wind speed exceeded a seasonally varying threshold and where the wind shear was confined to upper levels. During winter, the South American continent, the Indian Ocean, and the Maritime Continent are preferred areas of ascent into the upper troposphere. From these areas, air parcels follow an anticyclonic pathway into the subtropical jet. During summer, the majority of air parcels ascend over the Himalayas and Southeast Asia. Angular momentum is overall well conserved for trajectories that reach the subtropical jet from the deep tropics. In winter and spring, the hemispheric-mean angular momentum loss amounts to approximately 6%; in summer, it amounts to approximately 18%; and in fall, it amounts to approximately 13%. This seasonal variability is confirmed using an independent potential vorticity–based method to estimate tropical and extratropical forcing of the subtropical jet.

scholarly journals On the Discrepancies in Tropical Belt Expansion between Reanalyses and Climate Models and among Tropical Belt Width Metrics

2017 ◽  
Vol 30 (4) ◽  
pp. 1211-1231 ◽  
Author(s):  
Nicholas Davis ◽  
Thomas Birner
Keyword(s):  
Climate Models ◽  
Hadley Cell ◽  
Sufficient Evidence ◽  
Climate Indices ◽  
Radiative Forcings ◽  
Subtropical Jet ◽  
Surface Climate ◽  
Belt Width ◽  
Hadley Cells ◽  
Robust Response

Abstract The arid subtropics are situated at the edges of the tropical belt, where subsidence in the Hadley cells suppresses precipitation. Any meridional shift in these edge latitudes could have significant impacts on surface climate. Recent studies have investigated past and future changes in the tropical belt width and have found discrepancies in the rates of expansion estimated with different metrics and between climate models and reanalyses. Here, CMIP5 simulations and four modern reanalyses are analyzed using an ensemble of objective tropical belt width metrics to reexamine if such inconsistencies exist. The authors do not find sufficient evidence to demonstrate this discrepancy between models and reanalyses, as reanalysis trends in the tropical belt width fall within the range of model trends for any given metric. Furthermore, only metrics based on the Hadley cells are found to exhibit robust historical and future expansion. Metrics based on the subtropical jet and the tropopause show no robust response. This differentiation may be due to the strong correlation, on all time scales, between the Hadley cell edge latitudes and the latitudes of the eddy-driven jets, which consistently shift poleward in response to radiative forcings. In contrast, the subtropical jet and tropopause metrics appear to be decoupled from the Hadley cells and the eddy-driven jets and essentially measure a different tropical belt. The tropical belt width metrics are inconsistently correlated with surface climate indices based on precipitation and surface evaporation. This may make assessing the surface impacts of observed and future tropical expansion challenging.

The Transition to a Subtropical Jet Regime and Its Maintenance

2014 ◽  
Vol 71 (4) ◽  
pp. 1389-1409 ◽  
Author(s):  
Orli Lachmy ◽  
Nili Harnik
Keyword(s):  
Baroclinic Instability ◽  
Spherical Model ◽  
Hadley Cell ◽  
Vertical Shear ◽  
Momentum Balance ◽  
Mean Flow ◽  
Intermediate Scale ◽  
Subtropical Jet ◽  
Thermally Driven ◽  
Baroclinic Modes

Abstract An abrupt transition from a merged jet regime to a subtropical jet regime is analyzed using a two-layer modified quasigeostrophic (QG) spherical model. Unlike the common version of QG models, this model includes advection of the zonal mean momentum by the ageostrophic mean meridional circulation, allowing for a relatively realistic momentum balance in the tropics and subtropics. The merged jet is a single jet inside the Ferrel cell created by a merging of the subtropical and eddy-driven jets, and the subtropical jet is a mainly thermally driven jet at the Hadley cell edge. The maintenance of each type of jet depends on the dominant baroclinic modes. In the merged jet regime, the spectrum is dominated by intermediate-scale (wavenumbers 4–6) fast waves at the midlatitudes that grow close to the jet maximum. In the subtropical jet regime, the spectrum is dominated by long (wavenumbers 1–3) slow westward-propagating waves at high latitudes and somewhat weaker intermediate-scale slow waves at the midlatitudes. In the subtropical jet regime, waves equilibrate at weaker amplitudes than in the merged jet regime. A mechanism is found that explains why baroclinic instability is weaker in the subtropical jet regime, although the vertical shear of the mean flow is stronger, which has to do with the lower-level potential vorticity (PV) structure. The relevance of these results to the real atmosphere seams to hold in local zonal sections but not for the zonal mean.

Sign in / Sign up

Export Citation Format

Share Document