An official website of the United States government
Abstract Interactions between large-scale waves and the Hadley Cell are examined using a linear two-layer model on anf-plane. A linear meridional moisture gradient determines the strength of the idealized Hadley Cell. The trade winds are in thermal wind balance with a weak temperature gradient (WTG). The mean meridional moisture gradient is unstable to synoptic-scale (horizontal scale of ∼1000 km) moisture modes that are advected westward by the trade winds, reminiscent of oceanic tropical depression-like waves. Meridional moisture advection causes the moisture modes to grow from “moisture-vortex instability” (MVI), resulting in a poleward eddy moisture flux that flattens the zonal-mean meridional moisture gradient, thereby weakening the Hadley Cell. The amplification of waves at the expense of the zonal-mean meridional moisture gradient implies a downscale latent energy cascade. The eddy moisture flux is opposed by a regeneration of the meridional moisture gradient by the Hadley Cell. These Hadley Cell-moisture mode interactions are reminiscent of quasi-geostrophic interactions, except that wave activity is due to column moisture variance rather than potential vorticity variance. The interactions can result in predator-prey cycles in moisture mode activity and Hadley Cell strength that are akin to ITCZ breakdown. It is proposed that moisture modes are the tropical analog to midlatitude baroclinic waves. MVI is analogous to baroclinic instability, stirring latent energy in the same way that dry baroclinic eddies stir sensible heat. These results indicate that moisture modes stabilize the Hadley Cell, and may be as important as the latter in global energy transport.
more »
« less
Eddy Influences on the Hadley Circulation
Abstract Variations in the width and strength of the Hadley cells are associated with many radiative, thermodynamic, and dynamical forcings. The physical mechanisms driving these responses remain unclear, in part because of the interactive nature of eddy‐mean flow adjustment. Here, a modeling framework is developed which separates the mean flow and time‐mean eddy flow in a gray radiation general circulation model with simple representations of ocean heat transport and ozone. In the absence of eddies, with moist convection and weak numerical damping, the Hadley cell is confined to the upper troposphere and has a vanishingly small poleward momentum flux. Eddies allow the cell to extend down to the surface, double its heat transport, and flux momentum poleward, the latter two being basic consequences of a deepening of the circulation. Because of convection and damping—which mimics, in part, the effect of eddy stresses—previous work may have underestimated the impact of eddies on earth's circulation. Quasigeostrophic eddy fluxes are sufficient to produce Hadley and Ferrel cells, but with a substantially greater Hadley cell strength than when all eddy impacts are considered, including eddy fluxes of moisture, mass, and momentum and eddy impacts on surface fluxes and clouds.
more »
« less
- Award ID(s):
- 1643167
- PAR ID:
- 10460446
- Publisher / Repository:
- DOI PREFIX: 10.1029
- Date Published:
- Journal Name:
- Journal of Advances in Modeling Earth Systems
- Volume:
- 11
- Issue:
- 6
- ISSN:
- 1942-2466
- Page Range / eLocation ID:
- p. 1563-1581
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
More Like this
-
-
Abstract 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.more » « less
-
null (Ed.)Abstract While there is substantial evidence for tropospheric jet shift and Hadley cell expansion in response to greenhouse gas increases, quantitative assessments of individual mechanisms and feedback for atmospheric circulation changes remain lacking. We present a new forcing-feedback analysis on circulation response to increasing CO 2 concentration in an aquaplanet atmospheric model. This forcing-feedback framework explicitly identifies a direct zonal wind response by holding the zonal mean zonal wind exerting on the zonal advection of eddies unchanged, in comparison with the additional feedback induced by the direct response in zonal mean zonal wind. It is shown that the zonal advection feedback accounts for nearly half of the changes to the eddy-driven jet shift and Hadley cell expansion, largely contributing to the subtropical precipitation decline, when the CO 2 concentration varies over a range of climates. The direct response in temperature displays the well-known tropospheric warming pattern to CO2 increases, but the feedback exhibits negative signals. The direct response in eddies is characterized by a reduction in upward wave propagation and a poleward shift of midlatitude eddy momentum flux (EMF) convergence, likely due to an increase in static stability from moist thermodynamic adjustment. In contrast, the feedback features a dipole pattern in EMF that further shifts and strengthens midlatitude EMF convergence, resulting from the upper-level zonal wind increase seen in the direct response. Interestingly, the direct response produces an increase in eddy kinetic energy (EKE), but the feedback weakens EKE. Thus, the forcing-feedback framework highlights the distinct effect of zonal mean advecting wind from direct thermodynamic effects in atmospheric response to greenhouse gas increases.more » « less
-
Abstract Weak-temperature-gradient influences from the tropics and quasigeostrophic influences from the extratropics plausibly constrain the subtropical-mean static stability in terrestrial atmospheres. Because mean descent acting on this static stability is a leading-order term in the thermodynamic balance, a state-invariant static stability would impose constraints on the Hadley cells, which this paper explores in simulations of varying planetary rotation rate. If downdraft-averaged effective heating (the sum of diabatic heating and eddy heat flux convergence) too is invariant, so must be vertical velocity—an “omega governor.” In that case, the Hadley circulation overturning strength and downdraft width must scale identically—the cell can strengthen only by widening or weaken only by narrowing. Semiempirical scalings demonstrate that subtropical eddy heat flux convergence weakens with rotation rate (scales positively) while diabatic heating strengthens (scales negatively), compensating one another if they are of similar magnitude. Simulations in two idealized, dry GCMs with a wide range of planetary rotation rates exhibit nearly unchanging downdraft-averaged static stability, effective heating, and vertical velocity, as well as nearly identical scalings of the Hadley cell downdraft width and strength. In one, eddy stresses set this scaling directly (the Rossby number remains small); in the other, eddy stress and bulk Rossby number changes compensate to yield the same, ~Ω−1/3scaling. The consistency of this power law for cell width and strength variations may indicate a common driver, and we speculate that Ekman pumping could be the mechanism responsible for this behavior. Diabatic heating in an idealized aquaplanet GCM is an order of magnitude larger than in dry GCMs and reanalyses, and while the subtropical static stability is insensitive to rotation rate, the effective heating and vertical velocity are not.more » « less
-
null (Ed.)Abstract Eddy heat flux plays a fundamental role in the Southern Ocean meridional overturning circulation, providing the only mechanism for poleward heat transport above the topography and below the Ekman layer at the latitudes of Drake Passage. Models and observations identify Drake Passage as one of a handful of hot spots in the Southern Ocean where eddy heat transport across the Antarctic Circumpolar Current (ACC) is enhanced. Quantifying this transport, however, together with its spatial distribution and temporal variability, remains an open question. This study quantifies eddy heat flux as a function of ACC streamlines using a unique 20-yr time series of upper-ocean temperature and velocity transects with unprecedented horizontal resolution. Eddy heat flux is calculated using both time-mean and time-varying streamlines to isolate the dynamically important across-ACC heat flux component. The time-varying streamlines provide the best estimate of the across-ACC component because they track the shifting and meandering of the ACC fronts. The depth-integrated (0–900 m) across-stream eddy heat flux is maximum poleward in the south flank of the Subantarctic Front (−0.10 ± 0.05 GW m −1 ) and decreases toward the south, becoming statistically insignificant in the Polar Front, indicating heat convergence south of the Subantarctic Front. The time series provides an uncommon opportunity to explore the seasonal cycle of eddy heat flux. Poleward eddy heat flux in the Polar Front Zone is enhanced during austral autumn–winter, suggesting a seasonal variation in eddy-driven upwelling and thus the meridional overturning circulation.more » « less
