Low Antarctic continental local weather sensitivity as a consequence of excessive ice sheet orography

2023-01-29 08:02:46

Antarctic response to CO₂-doubling with flattened orography

The important thing discovering of this research is that in response to atmospheric CO₂-doubling, warming over the Antarctic continent is considerably better with flattened orography than with present-day orography in each CESM1.1 and CCSM4.0 (Fig. 2a; examine pink and blue bars). Warming is larger within the annual imply (0.9 Ok better in CCSM4.0 and 0.7 Ok better in CESM1.1) due to considerably better warming over winter (June–July–August (JJA); 1.5 Ok and 1.2 Ok better warming in CCSM4.0 and CESM1.1, respectively) and the shoulder seasons (March–April–Could (MAM) and September–October–November (SON); 1.4 Ok and 1.1 Ok better warming in MAM in CCSM4.0 and CESM1.1, respectively, and 0.8 Ok and 0.4 Ok better warming in SON in CCSM4.0 and CESM1.1, respectively). In each fashions, CO₂-induced warming within the present-day and flattened orography runs is comparable in summer time (DJF).

**Fig. 2: Antarctic (annual and seasonal) floor temperature change and amplification with CO₂-doubling.**

The amplified warming over the Antarctic continent with flattened orography can be evident in Fig. 2b, the place we examine Antarctic amplification, outlined because the ratio between floor warming over the Antarctic continent and warming over the globe, within the present-day and flattened Antarctic orography experiments. Within the annual imply, we discover better amplification of CO₂-forced warming over the Antarctic continent with flattened orography than with present-day orography: the Antarctic amplification ratio will increase from 1.5 to 1.9 in CESM1.1, and from 1.4 to 1.5 in CCSM4.0. Seasonally, better Antarctic amplification with flattened orography is most distinguished in winter (JJA), when the Antarctic amplification ratio will increase from 1.6 to 2.2 in CESM1.1, and from 1.4 to 1.8 in CCSM4.0. Elevated CO₂-forced Antarctic amplification with flattened Antarctic orography can be evident within the shoulder seasons, although it’s stronger in fall (MAM; will increase from 1.7 to 2.3 in CESM1.1, and from 1.5 to 1.8 in CCSM4.0) and weaker in spring (SON; will increase from 1.4 to 1.7 in CESM1.1, and from 1.3 to 1.4 in CCSM4.0). We observe that whereas each fashions are in qualitative settlement, there are appreciable quantitative discrepancies, implying that single mannequin research should be handled with warning.

Since it’s clear that each better Antarctic amplification and better CO₂-forced warming over the Antarctic continent with flattened orography are most distinguished in winter (JJA), for the rest of this research we deal with this season, as we analyze the Antarctic continental warming and produce to mild the mechanisms liable for it.

In Fig. 3, we current the spatial sample of winter season Antarctic floor warming in CESM1.1 and CCSM4.0. In each fashions, elevated CO₂-forced floor warming with flattened Antarctic orography is best over the West Antarctic Ice Sheet and Peninsula, and smallest over areas of the East Antarctic. In CESM1.1, West Antarctic warming extends into the Western Pacific Ocean Sector of the East Antarctic, whereas in CCSM4.0, warming extends in the other way into the Weddell Sea Sector. Over some areas of the West Antarctic, winter season warming is almost 5 Ok better—in each fashions—when Antarctic orography is flattened. The spatial sample of Antarctic floor warming within the winter season resembles that within the annual imply (examine Fig. 3 to Supplementary Fig. 1), suggesting that winter season processes drive the annual imply sign.

**Fig. 3: Antarctic winter season floor temperature change with CO₂-doubling.**

To know why CO₂-forced warming over the Antarctic continent is larger with flattened orography than with present-day orography, we now study how (moist and dry) transport processes into the Antarctic reply in a different way to CO₂-doubling within the two instances.

Elevated moisture advection with flattened orography

It’s well-known that when compelled with CO₂, almost all local weather fashions reply with a rise in poleward latent warmth transport²⁴: that is clearly seen in our runs with CESM1.1 and CCSM4.0 (Fig. 4, stable blue strains). Such elevated poleward latent warmth transport is predicted on theoretical grounds, since atmospheric particular humidity relies upon exponentially on temperature via the Clausius–Clapeyron relation, which reinforces the meridional gradient of precipitable water in a hotter world²⁵. Conversely, poleward dry static vitality transport declines in lots of fashions with CO₂ forcing²⁴, together with CESM1.1 and CCSM4.0 (Fig. 4, pink strains), since polar-amplified warming weakens the meridional temperature gradient.

**Fig. 4: Atmospheric vitality transport response to CO₂-doubling.**

When Antarctic orography is flattened, we discover a bigger enhance in CO₂-forced latent warmth transport towards the Antarctic continent than when orography is at its present-day peak. That is evident in each CESM1.1 and CCSM4.0 (Fig. 4, panels a and b, respectively; examine stable and dashed blue strains), equivalent to a 0.05 PW enhance in latent warmth transport throughout 70S in each fashions; this represents a 100% enhance within the poleward latent warmth transport response to CO₂-doubling in CCSM4.0, and a 50% enhance in CESM1.1.

To know why latent warmth transport at excessive Southern latitudes will increase extra with CO₂ forcing when Antarctic orography is flattened, we take into account the spatial construction of the moist isentropes (isolines of each the equal potential temperature, θ_E, and the MSE) in each flattened and present-day Antarctic orography experiments (Fig. 5). Within the latitude-height airplane, flattened Antarctic orography is related to flattened moist isentropes over the pole, in comparison with present-day Antarctic orography the place isentropes are extra sloped (examine, for instance, the 270 Ok moist isentrope in Fig. 5, panels a, c with these in panels b, d); that is true whether or not atmospheric CO₂ is at preindustrial concentrations (Fig. 5, black contours) or doubled (Fig. 5, pink contours).

**Fig. 5: Moist isentropic surfaces and atmospheric particular humidity change with CO₂-doubling.**

Within the extratropics, moist isentropes successfully act as isolines of the poleward moisture transport streamfunction^26,27, significantly for moisture transport to the Antarctic continent²⁸. The moist static vitality (MSE), the sum of atmospheric latent warmth, wise warmth, and geopotential, is conserved alongside a moist isentrope, and may be written as

$${rm{MSE}} = L_vq + C_pT + g{Phi},$$

(1)

the place L_v is the latent warmth of fusion, q is the particular humidity, C_p is the particular warmth of dry air at fixed stress, T is the temperature, g is the acceleration as a consequence of gravity, and Φ is the peak relative to the geoid. When moist isentropes are flatter (as when Antarctic orography is flattened), air parcels acquire much less geopotential as they circulate poleward, and subsequently retain extra moisture. That is additionally evident within the change in particular humidity with CO₂-doubling (Fig. 5, colours): the (poleward) gradient of this particular humidity change alongside a moist isentropic floor, ∇(∆q)|_θE, is larger with present-day Antarctic orography (panels a, c) than with flattened Antarctic orography (panels b, d), indicating that extra moisture is misplaced alongside this trajectory with present-day orography than with flattened orography (examine, for instance, the gradient of the change in particular humidity, denoted by the blue colours, alongside the 270 Ok moist isentrope in Fig. 5a, c with these in 5b, d).

As poleward latent warmth transport in the direction of the Antarctic continent will increase extra in response to CO₂-doubling when Antarctic orography is flattened (see Fig. 4), condensational heating within the boundary layer over Antarctica additionally will increase extra: that is seen by evaluating the left and proper columns in Fig. 6, the place condensational heating has been calculated within the lowest 200 hPa of the atmospheric column. Flattening of moisture transport trajectories (by flattening moist isentropes, see Fig. 6) ends in much less condensational depletion of poleward-moving moisture on the fringe of the continent, thereby allowing extra moisture to condense over the continental inside. Equally, when Antarctic orography is flattened, CO₂-forced precipitation will increase are smaller over the sting of the Antarctic continent, and considerably better over the continental inside (see Supplementary Fig. 2). In each CCSM4.0 and CESM1.1 flattened orography experiments, the rise in condensational heating within the boundary layer with CO₂– doubling roughly happens over the identical areas the place CO₂-forced temperature will increase are best (significantly over the West Antarctic in each fashions; examine Fig. 3a, b to Fig. 6a, b), indicating that condensational heating performs a key function in warming these areas.

**Fig. 6: Condensational heating change with CO₂-doubling.**

Elevated wise warmth advection with flattened orography

Whereas better moisture transport amplifies CO₂-forced warming over the Antarctic continent when Antarctic orography is flattened, dry processes additionally contribute to better warming. Earlier research on how flattened Antarctic orography impacts the imply state local weather recommend a variety of native adjustments within the atmospheric circulation, together with weakening of the polar vortex^14,17, slowing of the eddy-driven jet^14,16,22, elevated baroclinicity nearer to the pole^14,21 and a cessation of katabatic circulate over the continent^15,16,18. In each CCSM4.0 and CESM1.1, the latter is related to the disappearance of the polar cell within the mass overturning streamfunction when orography is flattened (Fig. 7, examine contours in panels a, c with b, d; additionally see refs. ^15,22).

**Fig. 7: Mass overturning streamfunction and temperature advection change with CO₂-doubling.**

The presence (or absence) of the polar cell determines how meridional temperature advection responds to CO₂-doubling over the excessive Southern latitudes. When Antarctic orography is at present-day peak, the temperature advection response is northward within the decrease troposphere (Fig. 7a, c, colours), as heat temperature anomalies are advected away from the Antarctic continent by the decrease department of the polar cell. When Antarctic orography is flattened, however, the meridional temperature advection response is in the direction of the continent (i.e., southward; Fig. 7b, d, colours) within the decrease troposphere, as hotter boundary layer air from areas the place sea ice has retreated is extra readily advected poleward when the polar cell is absent.

Earlier research have proven that CO₂-forced GCMs simulate comparable advection of decrease tropospheric heat anomalies from newly ice-free ocean areas within the Arctic to adjoining land areas: whereas summer time season sea ice decline and ocean blended layer warming within the Arctic straight trigger (delayed) surface-amplified warming over the Arctic Ocean in fall and winter⁷, warming over boreal land areas is primarily via heat air advection from the polar oceans²⁹. Subsequently, when Antarctic orography is flattened, there may be better similarity between the poles in how advection of boundary layer air from newly ice-free ocean waters warms land areas in winter.

Higher surface-amplified warming with flattened orography

On account of each the dry and moist transport processes described above, Antarctic continent floor temperatures heat extra with CO₂-doubling when Antarctic orography is flattened (recall Figs. 2 and 3).

The overlying ambiance additionally warms with CO₂-doubling, and that warming can be extra surface-amplified with flattened Antarctic orography than with present-day orography (Fig. 8, examine pink with black strains). As we have now proven, such surface-amplified warming is a results of better atmospheric vitality transport convergence (each dry and moist) into the decrease troposphere when Antarctic orography is flattened, mixed with robust atmospheric static stability in winter which prevents convective switch of those anomalies aloft (recall the secure stratification implied by moist isentropes in Fig. 4; additionally see ref. ³⁰).

**Fig. 8: Atmospheric temperature change with CO₂-doubling.**

Higher surface-amplified warming with CO₂-doubling when Antarctic orography is flattened additionally results in a stronger (optimistic) lapse charge suggestions over the Antarctic continent itself: 1.2 W/m²/Ok (0.8 W/m²/Ok) with flattened Antarctic orography in comparison with 0.5 W/m²/Ok (0.3 W/m²/Ok) with present-day Antarctic orography in CESM1.1 (CCSM4.0), which we have now computed utilizing the radiative kernel methodology³¹ with a CAM3-derived temperature kernel³². The optimistic lapse charge suggestions within the excessive latitudes is likely one of the most essential radiative elements liable for polar-amplified warming in response to CO₂ forcing, significantly that of the Arctic^{9,10,30,33,34}. Its enhance when Antarctic orography is flattened is in keeping with the rise in Antarctic (continental) amplification reported right here (recall Fig. 2).