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

Abstract

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.

Mechanisms of Future Predicted Changes in the Zonal Mean Mid-Latitude Circulation

“…Recent tests of the transient response to abrupt 4 × CO 2 in the SH in CMIP5 models shows the eddy momentum flux and eddy-driven jet evolve on a faster timescale than the maximum absolute value of the upper tropospheric meridional temperature gradient from 0 to 45 • S and the subtropical jet strength (compare the orange and purple lines to the blue and pink lines in Fig. 3), which are connected to the tropical starting point [42]. Using similar CMIP5 experiments, Chemke and Polvani [9] showed the eddy momentum flux also responds faster than the surface meridional temperature gradient in the subtropics.…”

“…Future progress in understanding the zonal mean midlatitude circulation response to increased CO 2 relies on (1) carefully designed climate model experiments that attempt to falsify the thermodynamic starting points and assumptions underlying the proposed mechanisms and (2) subsequently using the remaining mechanisms to create emergent constraints. Several approaches can be used to test mechanisms, for example, (1) imposing diabatic perturbations based on the response to increased CO 2 in radiative-convective equilibrium either in the absence [60] or presence of the climatological circulation, (2) quantifying the transient evolution of the circulation response to an abrupt CO 2 increase [9,11,17,36,42,67,68], (3) imposing latitudinally dependent CO 2 concentration [54], (4) quantifying the seasonality of the mechanisms, and (5) nudging parameters to their climatological value, e.g., turning off wind induced surface heat exchange [43]. Emergent constraints (see [19]) are needed in order to quantify and compare the modeled mechanisms to those in the real atmosphere.…”

Is the subtropical jet shifting poleward?

“…The Hadley cell edge is correlated with the eddy-driven jet but only very weakly correlated with either the subtropical jet position or tropopause break (Davis and Rosenlof 2012;Solomon et al 2016;Davis and Birner 2017;Waugh et al 2018). Interestingly, the subtropical jet strength is correlated with Hadley cell edge in CMIP5 models (Menzel et al 2019). The weak correlations between the upper-atmospheric metrics and the Hadley cell edge suggest that the upper-and lower-atmospheric metrics measure different aspects of the tropical edge (Davis and Birner 2017) and that the suitability of upper-atmospheric methods needs to be considered (Waugh et al 2018).…”

Seasonal and Annual Changes of the Regional Tropical Belt in GPS-RO Measurements and Reanalysis Datasets

“…Changes in the zonal wind associated with HC (Fig. 7a) include a weakening of the STJ over the Pacific-a pattern that projects onto the zonal mean (Waugh et al 2018;Menzel et al 2019). The Pacific is likely where some of the strongest coupled changes in the regional overturning circulation occur when the zonal-mean HC is wider (Staten et al 2019).…”

“…This casts some doubts on the utility of the TTB as a metric of tropical width that is relevant to surface climate. On the other hand, the TTB seems to vary with the subtropical jet (STJ) latitude (Davis and Birner 2017;Waugh et al 2018), which likewise is expected to shift poleward (Kushner et al 2001;Yin 2005;Miller et al 2006;Fu and Lin 2011;Swart and Fyfe 2012;Staten et al 2012Staten et al , 2014Barnes and Polvani 2013;Ceppi and Hartmann 2013;Ceppi et al 2014Ceppi et al , 2018Simpson et al 2014;Staten and Reichler 2014), even if not at the same rate as lower tropospheric metrics (Davis and Birner 2017;Waugh et al 2018;Menzel et al 2019). However, previous studies primarily discuss the above advantages and limitations of the zonal-mean TTB with the exception of the agreement between metrics shown in MK18.…”