Exploiting the Abrupt 4 × CO 2 Scenario to Elucidate Tropical Expansion Mechanisms

The subtropical jet (STJ) is thought to coexist with the edge of the Hadley cell (HC). However, recent studies reveal that the location of the STJ is poorly correlated with the latitude of the poleward edge of the HC. Here we use output from the Coupled Model Intercomparison Project Phase 5 to show that a weaker STJ is associated with a more poleward HC edge interannually, but there is a strengthening of the STJ and expansion of the HC in response to increased CO2. The HC expansion caused by increased CO2 is much more rapid than the strengthening of the STJ. It is suggested that the differing response times and relationships between interannual variations and increased CO2 are due to differing sensitivities of the HC and STJ to shifts in the eddy momentum fluxes.

Section: Introductionsupporting

confidence: 87%

Section: Forced Responsementioning

confidence: 99%

Section: Forced Responsementioning

confidence: 99%

See 1 more Smart Citation

Disconnect Between Hadley Cell and Subtropical Jet Variability and Response to Increased CO₂

Menzel

Waugh

Grise

2019

“…In order to understand why the Northern Hemisphere might be less responsive to the same SST forcing, we compute the changes in the vertical static stability ( C st ) for each hemisphere and season. In the Southern Hemisphere, it is well established that changes in subtropical static stability are closely connected with the expansion of the Hadley cell (Chemke & Polvani, ; Lu et al, ). The relationship between static stability and Hadley cell extent in the Northern Hemisphere in comprehensive models has been less explored, but it has generally been found that the statistical connections are weaker (e.g., cf.…”

Section: Resultsmentioning

confidence: 99%

Hemispheric Asymmetry of Tropical Expansion Under CO₂ Forcing

Watt‐Meyer

Frierson

2019

The degree of Hadley cell expansion under global warming will have a substantial impact on changing rainfall patterns. Most previous studies have quantified changes in total tropical width, focused on the Southern Hemisphere Hadley cell or considered each hemisphere's response to a multitude of anthropogenic forcings. It is shown here that under exclusive CO2 forcing, climate models predict twice as much Hadley cell expansion in the Southern Hemisphere relative to the Northern Hemisphere. This asymmetry is present in the annual mean expansion and all seasons except boreal autumn. It is robust across models and Hadley cell edge definitions. It is surprising since asymmetries in simulated Hadley cell expansion are typically attributed to stratospheric ozone depletion or aerosol emission. Its primary cause is smaller sensitivity of the Northern Hemisphere Hadley cell to static stability changes. The pattern of sea surface warming and the CO2 direct radiative effect also contribute to the asymmetry.

“…Given the significant effect of Arctic sea ice loss on φ Ψ500 , we next seek to elucidate the mechanisms. Held () developed a theory for the HC width, which is able to explain the projected widening of the HC (e.g., Chemke & Polvani, ). In that theory, the edge of the HC (φ H00 ) is determined by the latitude where the vertical shear of the mean angular momentum conserving flow at the upper branch of the HC becomes baroclinically unstable and scales as follows:

ϕ_{normalH 00} \propto {((), \frac{N H_{e}}{normalΩ a})}^{1 false/ 2},

where H e is subtropical tropopause height,

N^{2} = \frac{g}{θ} \frac{\partial θ}{\partial z}

is subtropical static stability (averaged between 400 and 800 mb), and Ω is Earth's rotation rate.…”

Section: Resultsmentioning

confidence: 99%

The Effect of Arctic Sea Ice Loss on the Hadley Circulation

Chemke

Polvani

Deser

2019

One of the most robust responses of the climate system to future greenhouse gas emissions is the melting of Arctic sea ice. It is thus essential to elucidate its impacts on other components of the climate system. Here we focus on the response of the annual mean Hadley cell (HC) to Arctic sea ice loss using a hierarchy of model configurations: atmosphere only, atmosphere coupled to a slab ocean, and atmosphere coupled to a full‐physics ocean. In response to Arctic sea ice loss, as projected by the end of the 21st century, the HC shows negligible changes in the absence of ocean‐atmosphere coupling. In contrast, by warming the Northern Hemisphere thermodynamic coupling weakens the HC and expands it northward. However, dynamic coupling acts to cool the Northern Hemisphere which cancels most of this weakening and narrows the HC, thus opposing its projected expansion in response to increasing greenhouse gases.