Recovery of the ozone hole and increasing greenhouse-gas concentrations have opposite effects on the jet stream. New model experiments indicate that they will cancel each other out over coming decades, leaving storm tracks at a stand still.
Over about the past 30 years, depletion of the ozone layer over Antarctica has had a greater effect on climate in the Southern Hemisphere than rising greenhouse-gas concentrations, causing a polewards shift of wind and precipitation patterns. Since about 2000, the so-called ozone hole has stopped enlarging and it is expected to close completely sometime after the middle of the century, in response to a ban on the production and use of ozone-depleting substances under the Montreal Protocol. This raises the question of whether wind and precipitation patterns will revert to their pre-ozone-hole conditions as the ozone layer recovers. Two new studies — one by Polvani and colleagues1 in Geophysical Research Letters and the other by McLandress and colleagues2 in Journal of Climate — show that as the ozone hole closes, increasing greenhouse-gas concentrations will counter the effects of ozone recovery, preventing atmospheric circulation from returning to 'normal'.
The ozone layer — located approximately 13 to 40 km above the surface of the Earth — protects life on Earth by absorbing ultraviolet radiation from the Sun. Depletion of the layer not only increases harmful ultraviolet radiation at the Earth's surface, but also affects Southern Hemisphere climate from the South Pole to the subtropics3, 4. The depletion of Antarctic ozone occurs primarily during late winter/early spring, causing a cooling of the polar stratosphere owing to reduced absorption of ultraviolet radiation. This cooling leads to a delayed summertime response in the lower atmosphere, characterized by a polewards shift of the jet stream that is associated with westerly winds at mid latitudes. This shift has direct consequences for weather at the surface because the jet stream — which is a band of strong winds about 7–12 km above the surface — determines the tracks of storms at mid and high latitudes.
Recent studies show that ozone depletion also causes a polewards expansion of the southern boundary of the so-called Hadley cell — a circulation loop that dominates the tropical atmosphere. In the Hadley cell, which is symmetric about the Equator, air rises near the Equator, flows towards the pole at a height of about 10 to 15 km, descends in the subtropics (at about 30° S and 30° N) and flows back towards the Equator near the surface. The descending branch contributes to the subtropical dry zones, so polewards expansion of the Hadley cell would be accompanied by a polewards expansion of these dry zones, with consequences for water resources and food production in these areas5, 6.
The human-induced increase in greenhouse-gas concentrations is expected to have a similar effect as Antarctic ozone depletion on the location of the Southern Hemisphere jet stream and the Hadley-cell boundary7. During the period from 1960 to 2000 these two human-induced climate drivers joined forces, causing a polewards shift of the summertime mid-latitude jet stream in the Southern Hemisphere on the order of about 230 to 300 km, with ozone depletion being the dominant force2, 4. While greenhouse-gas concentrations are anticipated to continue rising over coming decades, the Antarctic ozone hole is expected to close, so these two factors will begin to exert competing influences on the location of the westerly jet and dry zones (Fig. 1). The 'winner' of this competition has not yet been determined.
Climate models of various complexities have been compared to try to determine how climate will respond to these counteracting forces over the coming decades, but these studies have yielded conflicting results8, 9. This is believed in part to be due to oversimplification of chemical and dynamical interactions in the atmosphere in the models. It is also partly due to the lack of systematic investigation of the impact of the two anthropogenic climate forcings when acting alone and in combination. The new studies by Polvani et al.1 and McLandress et al.2 largely overcome these shortcomings.
McLandress and co-workers2 used a model that simulates the chemistry and physics of the atmosphere coupled to a model that simulates the ocean and sea ice to address the question, running it several times for the period from 1960 to 2100. Their model not only calculates the evolution of the ozone layer, but also calculates the sea surface temperatures that result from changes in ozone-depleting substances and greenhouse-gas concentrations. Taking a slightly different approach, Polvani and co-authors1 forced an atmospheric model with pre-defined reduction of the ozone hole and sea surface temperatures taken from simulations made with a coupled ocean–atmosphere model.
Both studies come to the same conclusion. In model experiments in which only the ozone-hole recovery is simulated (greenhouse-gas concentrations do not change and are kept at current levels), the jet stream and southern Hadley-cell border move back to their pre-ozone latitude. In model experiments in which only greenhouse-gas concentrations increase, the jet stream keeps moving polewards. However, when both ozone recovery and greenhouse-gas forcings are included in the simulations, the jet stream and Hadley-cell border stay more or less at their current locations.
Thus, the recovery of the ozone hole is expected to cancel out changes in climate conditions due to increasing greenhouse-gas concentrations over the coming decades. Importantly, however, both studies also indicate that these cancellations are a summertime event. During other seasons, when greenhouse-gas forcing has no competition from ozone forcing, the jet stream and Hadley-cell boundary are expected to shift towards the South Pole.
Is the outcome of this competition fully understood, and are the surmised cancellations certain? Even though the studies agree, several uncertainties remain about the degree to which recovery of the ozone hole will cancel out future circulation changes driven by increasing greenhouse gases. It will depend on how quickly the ozone hole recovers; a faster recovery would displace the jet stream back towards the Equator in the near future. It will also depend on the rate at which greenhouse gases are emitted. A higher rate than the moderate increase assumed in the model simulations will tend to keep the jet stream moving towards the South Pole. So, although the studies by Polvani and colleagues1 and McLandress and colleagues2 certainly advance our understanding of likely changes in Southern Hemisphere circulation, it remains to be seen whether the shift will be polewards or Equatorwards; let the tug of war begin.
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