Zonal temperature gradient

The Figures 2.10c and 2.10d display the typical bipolar climate change pattern with a warmer troposphere opposing a generally cooler stratosphere (IPCC, 2007;

WMO, 2007). Here, the changes in the latitudinal temperature gradient are of

primary interest because these modify the zonal wind, affecting the life cycle of planetary eddies.

Troposphere

In the warmer climate, enhanced deep convection warms the tropical upper tro- posphere both during summer (JA) and winter (DJ). The Figures 2.10c and 2.10d show that the upper-tropospheric warming patterns during the two seasons roughly resemble, but that mid-tropospheric warmer air penetrates to higher lat- itudes in case of DJ.

In fact, the latent-heat release during DJ (Figure 2.10b) mostly intensifies south of the equator, away from the upper- and mid-tropospheric warming in Figure 2.10d. The additional heat must thus somehow be transported towards the north, which is by the wintertime Hadley-Ferrel circulation, as Figure 2.11b suggests. Also, the DJ mid-tropospheric warming mentioned cannot result from a faster transport since the extratropical Hadley-Ferrel circulation weakens in the warmer climate (Figure 2.11c).

In contrast, the JA latent-heat release mainly strengthens north of the equa- tor (Figure 2.10a). But the additional heat cannot propagate further northwards because the JA Hadley circulation is opposite (Figure 2.11a). Finally, it is im- portant to be aware that the heat is transported and not the latent-heat release, the latter exciting the tropical quasi-stationary eddies.

A variety of global warming experiments show that changes in the latitudi- nal SST distribution merely affect the lower-tropospheric horizontal temperature gradient. For instance, a weaker low-altitude temperature gradient is usually as- sociated with a polar surface warming amplification induced by melting sea ice. At higher levels, on the contrary, the gradient strengthens, mainly due to the enhanced tropical convective heating (Rind et al., 2005a,b, 2002; Timbal et al., 1997).

Here, that finding appears to be valid for the modelled wintertime northern hemisphere (Figure 2.10d) where the Hadley-Ferrel circulation acts to transport the additional tropical heat poleward. The summertime temperature enhance- ment, though, exhibits a minimum at mid-latitudes and parts of the subtropics; the minimum becomes extreme at a latitude of 30-40◦_{N close to the surface, in-}

hence likely to result from the JA zonal belt of negative anomalous SST at 30- 45◦_{N (Figure 2.9e), even though the exact latitudinal position of the minimum}

is not robust. The local maximum latent heating at 400 hPa and 35◦_{N (Figure}

2.10a) appears to be unimportant.

(a) Same as Figure 2.11b, but for JA. The mass transport is clockwise around a stream function minimum, counter-clockwise around a maximum (exception, for this Figure 2.11a only).

(b) Same as Figure 2.2c, but for DJ and two times two orders-of-magnitude wider contours (in integer multiples of±2×1010_kg/s).

(c) Same as Figure 2.11b, but for the anomaly WARM minus COLD during DJ, and one order-of-magnitude tighter contours (in inte- ger multiples of ±2×109 _kg/s).

Figure 2.11: Absolute and anomalous TEM stream functions scaled to display the Hadley-Ferrel circulation.

Stratosphere

In the warmer climate, the stratosphere primarily cools via enhanced long-wave emission by larger concentrations of GHG. Other effects modify that latitude- independent cooling response, here indicated by latitude-dependent temperature changes above the tropopause (Figures 2.10c and 2.10d).

As E39/C lacks an upper stratosphere, altered short-wave irradiance at lower levels is not an issue (Section 2.0.2). Short-wave heating primarily depends on ozone concentrations; on the one hand, weaker tropical ozone-concentrations via

enhanced annual-mean upwelling (Section 2.0.3) may add to the tropical long- wave cooling; on the other hand, stronger ozone concentrations due to enhanced annual-mean downwelling and slower ozone-destructing chemical reactions (Sec- tions 2.0.1, 2.0.2, and 2.0.6) may warm the subtropical lowermost stratosphere (Figures 2.10c and 2.10d).

It is important to be aware that ozone concentrations in the lower strato- sphere at a particular place and point in time do rather depend on the preceding mass transport history than on its instantaneous characteristics (Brasseur and Solomon, 1986). In this context, considering the annual-mean residual circula- tion is thus more meaningful than looking at JA or DJ two-monthly means. Fi- nally, adiabatic temperature changes could intensify the above-mentioned ozone- induced temperature response, but rather depend on the instantaneous anomalous residual circulation.

Polar stratospheric temperature decreases more strongly than that at mid-lat- itudes, intensifying the temperature gradient associated with these two regions, and the gradient strengthens more vigorously in case of DJ than JA. A possi- ble explanation for the gradient changes involves modified differential heating.

Sigmond et al. (2004) compare GCM runs with and without altered SSTs, tro- pospheric and stratospheric GHG concentrations. Their Figure 10a) shows that an exclusive enhancement of stratospheric GHG concentrations strengthens the above-mentioned temperature gradient. Note that stratospheric eddy activity changes are also important, Section 2.7.2 discussing that effect more deeply. Hu and Tung (2002) analyse NCEP/NCAR re-analysis data (Kalnay et al., 1996) and infer that enhanced radiative cooling is producing the recent negative trend in mid-winter polar lower-stratospheric temperature.

The effect of altered stratospheric differential heating is thought to act as fol- lows (Fomichev et al., 2007;Gillett et al., 2003;Hartmann et al., 2000;Forster and Shine, 1999;Kodera and Yamazaki, 1994). For constant uniform ozone concentra- tions, stratospheric short-wave irradiance and hence short-wave heating decrease polewards of the tropics, especially during DJ when the polar region lies in con- stant shade. As long-wave cooling does not directly depend on short-wave solar irradiance, the radiative equilibrium temperature towards higher latitudes both decreases and more sensitively reacts to GHG concentration changes. Therefore, higher GHG and ozone concentrations tend to enhance the DJ meridional tem- perature contrast between middle and polar latitudes. In case of E39/C, part of the effect consequently directly relates to the model boundary condition of GHG concentrations.