Abstract. We have developed objective criteria for choosing the location of the northern hemisphere polar vortex boundary region and the onset and breakup dates of the vortex. By determining the distribution of Ertel's potential vorticity (Epv) on equivalent latitudes, we define the vortex edge as the location of maximum gradient of Epv constrained by the location of the maximum wind jet calculated along Epv isolines. We define the vortex boundary region to be at the local maximum convex and concave curvature in the Epv distribution surrounding the edge. We have determined that the onset and breakup dates of the vortex on the 450 K isentropic surface occur when the maximum wind speed calculated along Epv isolines rises above and falls below approximately 15.2 m s-•. We use 1992-1993 as a test case to study the onset and breakup periods, and we find that the increase of polar vortex Epv values is associated with the dominance of the term in the potential vorticity equation involving the movement of air through the surface due to the diabatic circulation. We also find that the decrease is associated with the dominance of the term involving radiatively induced changes in the stability of the atmosphere.

Chemical ozone destruction occurs over both polar regions in local winter-spring. In the Antarctic, essentially complete removal of lower-stratospheric ozone currently results in an ozone hole every year, whereas in the Arctic, ozone loss is highly variable and has until now been much more limited. Here we demonstrate that chemical ozone destruction over the Arctic in early 2011 was--for the first time in the observational record--comparable to that in the Antarctic ozone hole. Unusually long-lasting cold conditions in the Arctic lower stratosphere led to persistent enhancement in ozone-destroying forms of chlorine and to unprecedented ozone loss, which exceeded 80 per cent over 18-20 kilometres altitude. Our results show that Arctic ozone holes are possible even with temperatures much milder than those in the Antarctic. We cannot at present predict when such severe Arctic ozone depletion may be matched or exceeded.

We show here that stratospheric water vapor variations play an important role in the evolution of our climate. This comes from analysis of observations showing that stratospheric water vapor increases with tropospheric temperature, implying the existence of a stratospheric water vapor feedback. We estimate the strength of this feedback in a chemistry-climate model to be +0.3 W/(m 2 ·K), which would be a significant contributor to the overall climate sensitivity. One-third of this feedback comes from increases in water vapor entering the stratosphere through the tropical tropopause layer, with the rest coming from increases in water vapor entering through the extratropical tropopause.climate change | lowermost stratosphere | overworld D oubling carbon dioxide in our atmosphere by itself leads to a global average warming of ∼1.2°C. However, this direct warming from carbon dioxide drives other changes, known as feedbacks, that increase the eventual warming to 2.0-4.5°C. Thus, much of the warming predicted for the next century comes not from direct warming by carbon dioxide but from feedbacks.The strongest climate feedback is the tropospheric water vapor feedback (1, 2). The troposphere is the bottom 10-15 km of the atmosphere, and there are physical reasons to expect it to become moister as the surface warms (3)-and, indeed, both observations (4-6) and climate models (7, 8) verify this. Because water vapor is itself a greenhouse gas, tropospheric moistening more than doubles the direct warming from carbon dioxide.Stratospheric water vapor is also a greenhouse gas (9) whose interannual variations may have had important climatic consequences (10). This opens the possibility of a stratospheric water vapor feedback (11, 12) whereby a warming climate increases stratospheric water vapor, leading to additional warming. In this paper, we investigate this possibility. AnalysisMicrowave Limb Sounder Observations of the Overworld. Stratospheric water vapor can best be understood by subdividing the stratosphere into two regions: the overworld, that part of the stratosphere above the altitude of the tropical tropopause (∼16 km), and the lowermost stratosphere, that part of the extratropical stratosphere below that altitude (13) (see also figure 1 of ref. 14). Air enters the overworld exclusively through the tropical tropopause layer (TTL), where cold temperatures regulate the humidity of the air (14, 15) (we hereafter refer to the water content of air entering the overworld as H 2 O ov-entry ). Variations in H 2 O ov-entry can therefore be traced to variations in TTL temperatures. Fig. 1 shows monthly average tropical 82-hPa (∼18-km altitude) water-vapor volume-mixing-ratio anomalies observed by the Aura Microwave Limb Sounder (MLS) (16) (all tropical averages in this paper are over 30°N-30°S; anomalies are the remainder after the average annual cycle has been subtracted). These data are a good approximation of H 2 O ov-entry because this air has just entered the overworld and production of water from methane oxidation is negli...

Abstract-Aura, the last of the large Earth Observing System observatories, was launched on July 15, 2004. Aura is designed to make comprehensive stratospheric and tropospheric composition measurements from its four instruments, the High Resolution Dynamics Limb Sounder (HIRDLS), the Microwave Limb Sounder (MLS), the Ozone Monitoring Instrument (OMI), and the Tropospheric Emission Spectrometer (TES). With the exception of HIRDLS, all of the instruments are performing as expected, and HIRDLS will likely be able to deliver most of their planned data products. We summarize the mission, instruments, and synergies in this paper.

We have used a detailed cirrus cloud model to evaluate the physical processes responsible for the formation and persistence of subvisible cirrus near the tropical tropopause and the apparent absence of these clouds at midlatitudes. We find that two distinct formation mechanisms are viable. Energetic tropical cumulonimbus clouds transport large amounts of ice water to the upper troposphere and generate extensive cirrus outflow anvils. Ice crystals with radii larger than 10 -20 •um should precipitate out of these anvils within a few hours, leaving behind an optically thin layer of small ice crystals (•vis -0.01 -0.2, depending upon the initial ice crystal size distribution). Given the long lifetimes of the clouds, wind shear is probably responsible for the observed cloud thickness <_ I km. Ice crystals can also be generated in situ by slow, synoptic scale uplift of a humid layer. Given the very low temperatures at the tropical tropopause (_-85øC), synoptic-scale uplift can generate the moderate ice supersaturations (less than 10%) required for homogeneous freezing of sulfuric acid aerosols. In addition, simulations suggest that relatively large ice crystal number densities should be generated (more than 0.5 cm-a). The numerous crystals cannot grow larger than about 10-20/zm given the available vapor, and their low fall velocities will allow them to remain in the narrow supersaturated region for at least a day. The absorption of infrared radiation in the thin cirrus results in heating rates on the order of a few K per day. If this energy drives local parcel temperature change, the cirrus will dissipate within several hours. However, if the absorbed radiative energy drives lifting of the cloud layer, the vertical wind speed will be about 0.2 cm-s -•, and the cloud may persist for days with very little change in optical or microphysical properties. The fact that these clouds form most frequently over the tropical western Pacific is probably related (through the nucleation physics) to the very low tropopause temperatures in this region. Simulations using midlatitude tropopause temperatures near -65øC suggest that at the higher temperatures, fewer ice crystals nucleate, resulting in more rapid crystal growth and cloud dissipation by precipitation. Hence, the lifetime of thin cirrus formed near the midlatitude tropopause should be limited to a few hours after the synoptic-scale system that initiated cloud formation has passed. r Introduction Th•n, persistent ice clouds have been detected near the tropical tropopause by satellite measurements [e.g., (n•s) measurements, Prabhakara et al. [1993] found that the seasonal average cloud cover produced by thin cirrus is as large as 50% over the tropical western Pacific 21,361 21,362 JENSEN ET AL.-THIN CIRRUS DEHYDRATION in all seasons. Wang et al. [1994] also observed optically thin cirrus near the tropopause in excess of 50% of the time in this region using the Stratospheric Aerosol and Gas Experiment (SAGE) II solar occultation particle extinction measurements. Optical lidar m...