Response of Monsoon Rainfall to Changes in the Latitude of the Equatorward Coastline of a Zonally Symmetric Continent

AbstractRecent studies have shown that the rapid onset of the monsoon can be interpreted as a switch in the tropical circulation, which can occur even in the absence of land–sea contrast, from a dynamical regime controlled by eddy momentum fluxes to a monsoon regime more directly controlled by energetic constraints. Here we investigate how one aspect of continental geometry, that is, the position of the equatorward coastal boundary, influences such transitions. Experiments are conducted with an aquaplanet model with a slab ocean, in which different zonally symmetric continents are prescribed in the Northern Hemisphere poleward from southern boundaries at various latitudes, with “land” having a mixed layer depth two orders of magnitude smaller than ocean. For continents extending to tropical latitudes, the simulated monsoon features a rapid migration of the convergence zone over the continent, similar to what is seen in observed monsoons. For continents with more poleward southern boundaries, the main precipitation zone remains over the ocean, moving gradually into the summer hemisphere. We show that the absence of land at tropical latitudes prevents the rapid displacement into the subtropics of the maximum in lower-level moist static energy and, with it, the establishment of an overturning circulation with a subtropical convergence zone that can transition rapidly into an angular momentum–conserving monsoon regime.

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Abrupt Seasonal Migration of the ITCZ into the Summer Hemisphere

Journal of the Atmospheric Sciences ◽

10.1175/2007jas2367.1 ◽

2008 ◽

Vol 65 (6) ◽

pp. 1878-1895 ◽

Cited By ~ 14

Author(s):

Peng Xian ◽

Ron L. Miller

Keyword(s):

Angular Momentum ◽

Lower Layer ◽

Mixed Layer Depth ◽

Circulation Model ◽

Atmospheric Dynamics ◽

Solar Heating ◽

Seasonal Migration ◽

Lapse Rate ◽

Seasonal Transition ◽

Summer Hemisphere

Abstract Although the maximum of solar radiation at the top of the atmosphere moves gradually from one hemisphere to the other as part of the seasonal cycle, the intertropical convergence zone (ITCZ) moves abruptly into the summer hemisphere. An axisymmetric circulation model is developed to study this rapid transition. The model consists of an upper and lower layer of the Hadley circulation (HC), with the surface layer attached to a slab ocean and the lower layer connected to the upper layer by a constant lapse rate. The model is forced by solar heating, and the ITCZ is prescribed to coincide with the warmest sea surface temperature (SST). The collocation of tropical rainfall with warm SST allows the model ITCZ migration to be understood in terms of the relative influence of solar heating and atmospheric dynamics upon ocean temperature. Atmospheric dynamics allow the ITCZ to move off the equator by flattening the meridional temperature gradient that would exist in radiative–convective equilibrium. For the present-day tropical oceanic mixed layer depth and ITCZ width, the model exhibits an abrupt seasonal transition of the ITCZ across the equator. It is found that there are two determinative factors on the abrupt transition of the ITCZ: the nonlinear meridional advection of angular momentum by the circulation and ocean thermal inertia. As a result of nonlinear dynamics, angular momentum is well mixed, resulting in minimum atmospheric temperature at the equator and a similar equatorial minimum in SST. This inhibits convection over the equator while favoring a rapid seasonal transition of the ITCZ between the warmer surface water on either side of this latitude.

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Response of the Hadley Circulation to Climate Change in an Aquaplanet GCM Coupled to a Simple Representation of Ocean Heat Transport

Journal of the Atmospheric Sciences ◽

10.1175/2010jas3553.1 ◽

2011 ◽

Vol 68 (4) ◽

pp. 769-783 ◽

Cited By ~ 65

Author(s):

Xavier J. Levine ◽

Tapio Schneider

Keyword(s):

Heat Transport ◽

General Circulation ◽

Climate Models ◽

Circulation Model ◽

Hadley Circulation ◽

Dynamical Regime ◽

Ocean Heat Transport ◽

Simple Representation ◽

Wide Range ◽

Momentum Fluxes

Abstract It is unclear how the width and strength of the Hadley circulation are controlled and how they respond to climate changes. Simulations of global warming scenarios with comprehensive climate models suggest the Hadley circulation may widen and weaken as the climate warms. But these changes are not quantitatively consistent among models, and how they come about is not understood. Here, a wide range of climates is simulated with an idealized moist general circulation model (GCM) coupled to a simple representation of ocean heat transport, in order to place past and possible future changes in the Hadley circulation into a broader context and to investigate the mechanisms responsible for them. By comparison of simulations with and without ocean heat transport, it is shown that it is essential to take low-latitude ocean heat transport and its coupling to wind stress into account to obtain Hadley circulations in a dynamical regime resembling Earth’s, particularly in climates resembling present-day Earth’s and colder. As the optical thickness of an idealized longwave absorber in the simulations is increased and the climate warms, the Hadley circulation strengthens in colder climates and weakens in warmer climates; it has maximum strength in a climate close to present-day Earth’s. In climates resembling present-day Earth’s and colder, the Hadley circulation strength is largely controlled by the divergence of angular momentum fluxes associated with eddies of midlatitude origin; the latter scale with the mean available potential energy in midlatitudes. The importance of these eddy momentum fluxes for the Hadley circulation strength gradually diminishes as the climate warms. The Hadley circulation generally widens as the climate warms, but at a modest rate that depends sensitively on how it is determined.

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Energetic Constraints on the Width of the Intertropical Convergence Zone

Journal of Climate ◽

10.1175/jcli-d-15-0767.1 ◽

2016 ◽

Vol 29 (13) ◽

pp. 4709-4721 ◽

Cited By ~ 37

Author(s):

Michael P. Byrne ◽

Tapio Schneider

Keyword(s):

Radiative Forcing ◽

General Circulation ◽

Hadley Circulation ◽

Intertropical Convergence Zone ◽

Future Climate Change ◽

Convergence Zone ◽

Moist Static Energy ◽

Wide Range ◽

Gross Moist Stability ◽

Static Energy

Abstract The intertropical convergence zone (ITCZ) has been the focus of considerable research in recent years, with much of this work concerned with how the latitude of maximum tropical precipitation responds to natural climate variability and to radiative forcing. The width of the ITCZ, however, has received little attention despite its importance for regional climate and for understanding the general circulation of the atmosphere. This paper investigates the ITCZ width in simulations with an idealized general circulation model over a wide range of climates. The ITCZ, defined as the tropical region where there is time-mean ascent, displays rich behavior as the climate varies, widening with warming in cool climates, narrowing in temperate climates, and maintaining a relatively constant width in hot climates. The mass and energy budgets of the Hadley circulation are used to derive expressions for the area of the ITCZ relative to the area of the neighboring descent region, and for the sensitivity of the ITCZ area to changes in climate. The ITCZ width depends primarily on four quantities: the net energy input to the tropical atmosphere, the advection of moist static energy by the Hadley circulation, the transport of moist static energy by transient eddies, and the gross moist stability. Different processes are important for the ITCZ width in different climates, with changes in gross moist stability generally having a weak influence relative to the other processes. The results are likely to be useful for analyzing the ITCZ width in complex climate models and for understanding past and future climate change in the tropics.

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Energetic Constraints on the Intertropical Convergence Zone Position in the Observed Seasonal Cycle From Modern‐Era Retrospective Analysis for Research and Applications, Version 2 (MERRA‐2)

Geophysical Research Letters ◽

10.1029/2020gl088506 ◽

2020 ◽

Vol 47 (16) ◽

Cited By ~ 1

Author(s):

Ho‐Hsuan Wei ◽

Simona Bordoni

Keyword(s):

Retrospective Analysis ◽

Seasonal Cycle ◽

Intertropical Convergence Zone ◽

Convergence Zone ◽

Energetic Constraints ◽

Modern Era

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Multiple Zonal Jets in a Differentially Heated Rotating Annulus*,+

Journal of Physical Oceanography ◽

10.1175/jpo-d-13-0255.1 ◽

2014 ◽

Vol 44 (9) ◽

pp. 2273-2291 ◽

Cited By ~ 11

Author(s):

Carlowen A. Smith ◽

Kevin G. Speer ◽

Ross W. Griffiths

Keyword(s):

Active Role ◽

Taylor Number ◽

Dynamical Regime ◽

Zonal Jets ◽

Multiple Jets ◽

Momentum Fluxes ◽

Baroclinic Flow ◽

Circumpolar Current ◽

The Antarctic ◽

Gap Width

Abstract A laboratory experiment of multiple baroclinic zonal jets is described, thought to be dynamically similar to flow observed in the Antarctic Circumpolar Current. Differential heating sets the overall temperature difference and drives unstable baroclinic flow, but the circulation is free to determine its own structure and local stratification; experiments were run to a stationary state and extend the dynamical regime of previous experiments. A topographic analog to the planetary β effect is imposed by the gradient of fluid depth with radius supplied by a sloping bottom and a parabolic free surface. New regimes of a low thermal Rossby number (RoT ~ 10−3) and high Taylor number (Ta ~ 1011) are explored such that the deformation radius Lρ is much smaller than the annulus gap width L and similar to the Rhines length. Multiple jets emerge in rough proportion to the smallness of the Rhines scale, relatively insensitive to the Taylor number; a regime diagram taking the β effect into account better reflects the emergence of the jets. Eddy momentum fluxes are consistent with an active role in maintaining the jets, and jet development appears to follow the Vallis and Maltrud phenomenology of anisotropic wave–turbulence interaction on a β plane. Intermittency and episodes of coherent meridional jet migration occur, especially during spinup.

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Relative roles of energy and momentum fluxes in the tropical response to extratropical thermal forcing

Journal of Climate ◽

10.1175/jcli-d-20-0151.1 ◽

2020 ◽

pp. 1-74

Author(s):

Yen-Ting Hwang ◽

Hung-Yi Tseng ◽

Kuan-Chen Li ◽

Sarah M. Kang ◽

Yung-Jen Chen ◽

...

Keyword(s):

Mixed Layer ◽

Time Scale ◽

Momentum Flux ◽

Mixed Layer Depth ◽

Hadley Cell ◽

Slight Reduction ◽

Layer Depth ◽

Tropical Ocean ◽

Momentum Fluxes ◽

Energy And Momentum

AbstractThis study investigates the transient responses of atmospheric energy and momentum fluxes to a time-invariant extratropical thermal heating in an atmospheric model coupled to an aquaplanet mixed layer ocean with the goal of understanding the mechanisms and time-scales governing the extratropical-to-tropical connection. Two distinct stages are observed in the teleconnection: (1) A decrease in the meridional temperature gradient in midlatitudes leads to a rapid weakening of the eddy momentum flux and a slight reduction of the Hadley cell strength in the forced hemisphere. (2) The subtropical trades in the forced hemisphere decrease and reduce evaporation. The resulting change to sea surface temperature leads to the development of a cross-equatorial Hadley cell, and the Intertropical Convergence Zone shifts to the warmer hemisphere. The Hadley cell weakening in the first stage is related to decreased eddy momentum flux divergence, and the response time-scale is independent of the mixed layer depth. In contrast, the time taken for the development of the cross-equatorial cell in the latter stage increase as the mixed layer depth increases. Once developed, the deep tropical cross-equatorial cell response is an order of magnitude stronger than the initial subtropical response and dominates the anomalous circulation. The analysis 31 combines the momentum and energetic perspectives on this extratropical-to-tropical teleconnection and moreover shows that the subtropical circulation changes associated with the momentum budget occur with a time-scale that is distinct from the deep tropical response determined by the thermal inertia of the tropical ocean.

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The Role of the Cloud Radiative Effect in the Sensitivity of the Intertropical Convergence Zone to Convective Mixing

Journal of Climate ◽

10.1175/jcli-d-17-0794.1 ◽

2018 ◽

Vol 31 (17) ◽

pp. 6821-6838 ◽

Cited By ~ 9

Author(s):

Joshua Talib ◽

Steven J. Woolnough ◽

Nicholas P. Klingaman ◽

Christopher E. Holloway

Keyword(s):

Energy Transport ◽

Hadley Circulation ◽

Intertropical Convergence Zone ◽

Convergence Zone ◽

Convective Mixing ◽

Surface Latent Heat Flux ◽

Mixing Rates ◽

Cloud Radiative Effects ◽

Static Energy

Studies have shown that the location and structure of the simulated intertropical convergence zone (ITCZ) is sensitive to the treatment of sub-gridscale convection and cloud–radiation interactions. This sensitivity remains in idealized aquaplanet experiments with fixed surface temperatures. However, studies have not considered the role of cloud-radiative effects (CRE; atmospheric heating due to cloud–radiation interactions) in the sensitivity of the ITCZ to the treatment of convection. We use an atmospheric energy input (AEI) framework to explore how the CRE modulates the sensitivity of the ITCZ to convective mixing in aquaplanet simulations. Simulations show a sensitivity of the ITCZ to convective mixing, with stronger convective mixing favoring a single ITCZ. For simulations with a single ITCZ, the CRE maintains the positive equatorial AEI. To explore the role of the CRE further, we prescribe the CRE as either zero or a meridionally and diurnally varying climatology. Removing the CRE is associated with a reduced equatorial AEI and an increase in the range of convective mixing rates that produce a double ITCZ. Prescribing the CRE reduces the sensitivity of the ITCZ to convective mixing by 50%. In prescribed-CRE simulations, other AEI components, in particular the surface latent heat flux, modulate the sensitivity of the AEI to convective mixing. Analysis of the meridional moist static energy transport shows that a shallower Hadley circulation can produce an equatorward energy transport at low latitudes even with equatorial ascent.

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Monsoon Dynamics with Interactive Forcing. Part I: Axisymmetric Studies

Journal of the Atmospheric Sciences ◽

10.1175/jas3916.1 ◽

2007 ◽

Vol 64 (5) ◽

pp. 1417-1430 ◽

Cited By ~ 95

Author(s):

Nikki C. Privé ◽

R. Alan Plumb

Keyword(s):

Boundary Layer ◽

Angular Momentum ◽

General Circulation ◽

Large Scale ◽

Circulation Model ◽

Meridional Flow ◽

Monsoon Circulation ◽

Moist Static Energy ◽

Summer Hemisphere ◽

Static Energy

Abstract The applicability of axisymmetric theory of angular momentum conserving circulations to the large-scale steady monsoon is studied in a general circulation model with idealized representations of continental geometry and simple physics. Results from an aquaplanet setup with localized subtropical forcing are compared with a continental case. It is found that the meridional circulation that develops is close to angular momentum conserving for cross-equatorial circulation cells, both in the aquaplanet and in the continental cases. The equator proves to be a substantial barrier to boundary layer meridional flow; flow into the summer hemisphere from the winter hemisphere tends to occur in the free troposphere rather than in the boundary layer. A theory is proposed to explain the location of the monsoon; assuming quasiequilibrium, the poleward boundary of the monsoon circulation is collocated with the maximum in subcloud moist static energy, with the monsoon rains occurring near and slightly equatorward of this maximum. The model results support this theory of monsoon location, and it is found that the subcloud moist static energy distribution is determined by a balance between surface forcing and advection by the large-scale flow.

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