When the atmosphere is more opaque to longwaves than to shortwaves (panel a), the ground temperature is higher than the nearby atmosphere in radiative equilibrium. This jump in temperature causes strong meridional circulation near the surface, especially when the rotation effect is weak (e.g. the tropical regions). This circulation fills the entire troposphere with airs that are almost at rest with the surface, and only above the tropopause, when the stratification is strong enough, the zonal wind U can deviate from 0 following the thermal wind balance.
When the atmosphere is more opaque in shortwaves (panel b), the ground temperature becomes lower than the nearby atmosphere in radiative equilibrium, and therefore meridional circulation becomes suppressed. Zonal wind is only close to zero in the boundary layer, and in the free atmosphere, the zonal wind can vary following the thermal wind balance.
Assuming a semi-gray atmosphere, we analytically solve for the radiative equilibrium temperature profile, based on which we attempt to predict the meridional circulation strength following the “Held-2000 scaling” , and the equatorial superrotation.
If you are interested, please see our paper for more details.
The meridional overturning circulation has two regimes. Under weak insolation and fast rotation, the circulation is dominantly driven by momentum forcing (including friction, eddy momentum transport etc.), and is thermally-indirect in most of the low latitudes (mechanism). Under strong insolation and slow rotation, the circulation is dominantly driven by thermal forcing (including diabatic heating, eddy heat transport etc.), and is thermally-direct.
As circulation transforms from a thermally indirect circulation to a thermally direct one, the upper air starts to be filled with low angular momentum air from the extratropics (panel c versus panel b), meanwhile, the low latitudes start to be dominated by downward motion, largely reducing the cloud fraction (panel f versus panel e).
Kang. W. 2019, Regime transition between eddy-driven and moist-driven circulation on High Obliquity Planets, Astrophysical Journal (accepted)
Given that high obliquity aquaplanets tend to be warmer than low obliquity equivalents, we investigate whether the ocean on the warmer high obliquity planets are more vulnerable to strong insolation. The stratospheric water vapor concentration (solid curves) is much higher under high obliquity especially when climate is warm. The stratospheric humidity seems to be better correlated with the maximum surface temperature over the globe throughout the year, rather than the global annual mean surface temperature.
In addition to the overall warmer climate, the extremely warm period during the polar days, and the misalignment between the cold trap and the spots where water vapor is sent to stratosphere, each is responsible for 1 order of magnitude increase of the stratospheric humidity.
On one hand, a wetter stratosphere makes the ocean more vulnerable to strong insolation, on the other hand, it significantly increases the chance to directly detect surface originated water vapor.
We first find the high obliquity planets to be warmer than their low obliquity equivalents, even when the climate is very warm (1800 W/m2 will blow up the low obliquity experiment), indicating the previously proposed ice-albedo feedback may not be the full story.
Through a series of mechanism-denying experiments, we conclude that the relative warmness stems from the low cloud albedo under high obliquity, which in turn is because the high surface heat capacity makes cloud formation largely lag behind the substellar point migration. As shown below, the high obliquity is warmer than the low obliquity even without ice-albedo feedback, but the temperature difference vanishes or even reverses without cloud radiative effects or without the seasonal variation of insolation.
Will the general circulation be just reversed when reversing the meridional temperature gradient?
The simple answer is NO. With a reversed meridional temperature gradient, circulation becomes thermally indirect, shallow, and weak. Explanation is as follows:
Mid-latitude baroclinic eddies become bottom amplified, and this can be explained by the changes of the most unstable mode in the generalized Eady model.
Driven by surface friction (which is required to balance u’v’), the Hadley cell is thermally indirect while the Ferrel cell is thermally direct, in the reverse temperature gradient case, as opposed to the normal gradient case.
The eddy-driven Hadley circulation aids to the surface friction driven circulation in the normal case, but cancels it in the reverse case. This partially explained why the reverse case has a weak and shallower circulation compared to the normal case.
The meridional circulation in the reverse case is shallow also because the eddies are bottom amplified in the reverse case and thus there is no strong eddy drag in the upper atmosphere to drive circulation.
The mid-lat Ferrel cell is thermally indirect (direct) in the normal (reverse) case, acting to restore (reduce) the meridional temperature gradient there. This may partially explain why the eddy activity gets much weaker in the reverse case, which then leads to a much weaker meridional circulation.
With seasonal variability (Held Suarez experiment with equilibrium temperature varying with season)…
Annual mean meridional overturning circulation is almost identical to the circulation in the perpetual annual mean simulation.
Solstice Hadley cell is much stronger than the annual mean, and it is driven by eddy heat transport rather than the eddy momentum transport as in the perpetual annual mean simulation.
The zonal asymmetry of the MJO forcing itself also plays a role. In response to a circumglobal MJO-like forcing, or a longitudinal restricted one located at different locations, the SSWs can be either enhanced or suppressed . Forced by circumglobal MJO-like forcing with increasing amplitude, the SSW first gets enhanced then suppressed as shown in the black solid curve below; this is a result of the competition between the enhancement mechanism and the suppression mechanism.
Direct transmission of MJO-excited waves, modification of the stationary pattern in the mid-latitude, and the amplitude of the mid-latitude generated transient waves, all three of them play a role in determining the SSW’s fate. The key process controlling the MJO impact on SSW was found to be the strengthening and weakening of the stationary pattern through the interaction with the MJO forcing. Shown below are the mid-latitude jet response to MJO forcing restricted in B window (where the current MJO forcing located at) and C window (where the future MJO is predicted to expand to). The former (latter) strengthens (weakens) the stationary pattern in the jet, and ends up enhancing (suppressing) the SSWs. Notice that the MJO-excited waves contribute in both cases to enhance SSWs.
What happens in the transition stage from the unforced background flow to the forced one? We ran 800-ensemble simulations starting from different initial conditions, with the MJO-like forcing turned on at t=0 (initial MJO phase is different from one and another), and closed the momentum budget. The results indicate that the eddy momentum transport, u’v’, drives the climatology response.
Seeing that the MJO-related signal can only propagate to the Arctic in some specific phase (See Fig.1), we anticipated that the zonal asymmetry of either the background state or the forcing itself plays a role and pursued it in Kang and Tziperman 2018 .
Using idealized circumglobal MJO forcing with k=1 structure, we studied the role played by the background zonal asymmetry. MJO-forced waves can only penetrate the jet in appearance of the background stationary wave pattern. In perfect zonally symmetric experiment (0% below), the transmission rate of the MJO-forced waves decreases to almost zero, consistent with the lackness of MJO-related signal in the Arctic stratosphere (Fig. 5). The transmission rate in the MJO forced experiments (MJO[N]) seem to follow a universal rule as the background changing experiments.
As the transmission of MJO-forced waves being prohibited by the zonally symmetric background flow, the Arctic stratosphere responds in an opposite manner compared to the zonally asymmetric background case: the temperature goes down, the PNJ gets stronger, and the SSW becomes less frequent.
The reason for the suppression of SSWs and the cooling of the Arctic stratosphere is revealed in the U wind response above. The MJO forcing at the Equator works as a wave source, exciting Rossby waves propagating outside the Equator, and in the mean while, transporting momentum to the Equator. As a result, the mid-latitude jet decelerates, the temperature gradient (baroclinicity) falls due to the thermal wind constraint, weakening both the stationary waves and the transient waves which feed on the baroclinicity there.