Howard K. Roscoe

Frost flowers on sea ice as a source of sea salt and their influence on tropospheric halogen chemistry

[1] Frost flowers grow on newly-formed sea ice from a saturated water vapour layer. They provide a large effective surface area and a reservoir of sea salt ions in the liquid phase with triple the ion concentration of sea water. Recently, frost flowers have been recognised as the dominant source of sea salt aerosol in the Antarctic, and it has been speculated that they could be involved in processes causing severe tropospheric ozone depletion events during the polar sunrise. These events can be explained by heterogeneous autocatalytic reactions taking place on salt-laden ice surfaces which exponentially increase the reactive gas phase bromine (‘‘bromine explosion’’). We analyzed tropospheric bromine monoxide (BrO) and the sea ice coverage both measured from satellite sensors. Our model based interpretation shows that young ice regions potentially covered with frost flowers seem to be the source of bromine found in bromine explosion events. INDEX TERMS: 0322 Atmospheric Composition and Structure: Constituent sources and sinks; 1640 Global Change: Remote sensing; 3309 Meteorology and Atmospheric Dynamics: Climatology (1620); 3339 Meteorology and Atmospheric Dynamics: Ocean/atmosphere interactions (0312, 4504); 3360 Meteorology and Atmospheric Dynamics: Remote sensing. Citation: Kaleschke, L., et al. (2004), Frost flowers on sea ice as a source of sea salt and their influence on tropospheric halogen chemistry, Geophys. Res. Lett., 31, L16114, doi:10.1029/ 2004GL020655.

Antarctic climate change and the environment: an update

We present an update of the ‘key points’ from the Antarctic Climate Change and the Environment (ACCE) report that was published by the Scientific Committee on Antarctic Research (SCAR) in 2009. We summarise subsequent advances in knowledge concerning how the climates of the Antarctic and Southern Ocean have changed in the past, how they might change in the future, and examine the associated impacts on the marine and terrestrial biota. We also incorporate relevant material presented by SCAR to the Antarctic Treaty Consultative Meetings, and make use of emerging results that will form part of the Intergovernmental Panel on Climate Change (IPCC) Fifth Assessment Report

The role of eddies in the southern ocean temperature response to the southern annular mode.

The role of eddies in modulating the Southern Ocean response to the southern annular mode (SAM) is examined,usinganoceanmodelrunatmultipleresolutionsfromcoarsetoeddyresolving.Thehigh-resolution versionsof the model showan increase in eddy kinetic energy that peaks 2‐3 yr after a positive anomalyin the SAM index. Previous work has shown that the instantaneous temperature response to the SAM is characterized by predominant cooling south of 458S and warming to the north. At all resolutions the model captures this temperatureresponse. This responseis also evidentin the coarse-resolution implementation of the model with no eddy mixing parameterization,showing that eddiesdo not play an important role in the instantaneous response. On the longer time scales, an intensification of the mesoscale eddy field occurs, which causes enhanced poleward heat flux and drives warming south of the oceanic Polar Front. This warming is of greater magnitude and occurs for a longer period than the initial cooling response. The results demonstrate that this warming is surface intensified and strongest in the mixed layer. Non-eddy-resolving models are unable to capturethedelayededdy-driventemperatureresponsetotheSAM.Theauthorsthereforequestiontheability

The impact of the mixing properties within the Antarctic stratospheric vortex on ozone loss in spring

Calculations of equivalent length from an artificial advected tracer provide new insight into the isentropic transport processes occurring within the Antarctic stratospheric vortex. These calculations show two distinct regions of approximately equal area: a strongly mixed vortex core and a broad ring of weakly mixed air extending out to the vortex boundary. This broad ring of vortex air remains isolated from the core between late winter and midspring. Satellite measurements of stratospheric H2O confirm that the isolation lasts until at least mid-October. A three-dimensional chemical transport model simulation of the Antarctic ozone hole quantifies the ozone loss within this ring and demonstrates its isolation. In contrast to the vortex core, ozone loss in the weakly mixed broad ring is not complete. The reasons are twofold. First, warmer temperatures in the broad ring prevent continuous polar stratospheric cloud (PSC) formation and the associated chemical processing (i.e., the conversion of unreactive chlorine into reactive forms). Second, the isolation prevents ozone-rich air from the broad ring mixing with chemically processed air from the vortex core. If the stratosphere continues to cool, this will lead to increased PSC formation and more complete chemical processing in the broad ring. Despite the expected decline in halocarbons, sensitivity studies suggest that this mechanism will lead to enhanced ozone loss in the weakly mixed region, delaying the future recovery of the ozone hole.

Slant column measurements of O3 and NO2 during the NDSC intercomparison of zenith-sky UV-visible spectrometers in June 1996

In June 1996, 16 UV-visible sensors from 11 institutes measured spectra of the zenith sky for more than 10 days. Spectra were analysed in real-time to determine slant column amounts of O3 and NO2. Spectra of Hg lamps and lasers were measured, and the amount of NO2 in a cell was determined by each spectrometer. Some spectra were re-analysed after obvious errors were found. Slant columns were compared in two ways: by examining regression analyses against comparison instruments over the whole range of solar zenith angles; and by taking fractional differences from a comparison instrument at solar zenith angles between 85° and 91°. Regression identified which pairs of instruments were most consistent, and so which could be used as universal comparison instruments. For O3, regression slopes for the whole campaign agreed within 5% for most instruments despite the use of different cross-sections and wavelength intervals, whereas similar agreement was only achieved for NO2 when the same cross-sections and wavelength intervals were used and only one half-days data was analysed. Mean fractional differences in NO2 from a comparison instrument fall within ±7% (1-sigma) for most instruments, with standard deviations of the mean differences averaging 4.5%. Mean differences in O3 fall within ±2.5% (1- sigma) for most instruments, with standard deviations of the mean differences averaging 2%. Measurements of NO2 in the cell had similar agreement to measurements of NO2 in the atmosphere, but for some instruments measurements with cell and atmosphere relative to a comparison instrument disagreed by more than the error bars.

Validation of Ground-Based Visible Measurements of Total Ozone by Comparison with Dobson and Brewer Spectrophotometers

Comparisons of total column ozone measurements from Dobson, Brewer and SAOZ instruments are presented for the period 1990 to 1995 at seven stations covering the mid- and the high northern latitudes, as well as the Antarctic region. The main purpose of these comparisons is to assess, by reference to the well established Dobson network, the accuracy of the zenith-sky visible spectroscopy for the measurement of total ozone. The strengths and present limitations of this latter technique are investigated. As a general result, the different instruments are found to agree within a few percent at all stations, the best agreement being obtained at mid-latitudes. On average, for the mid-latitudes, SAOZ O3 measurements are approximately 2% higher than Dobson ones, with a scatter of about 5%. At higher latitudes, both scatter and systematic deviation tend to increase. In all cases, the relative differences between SAOZ and Dobson or Brewer column ozone are characterised by a significant seasonal signal, the amplitude of which increases from about 2.5% at mid-latitude to a maximum of 7.5% at Faraday, Antarctica. Although it introduces a significant contribution to the seasonality at high latitude, the temperature sensitivity of the O3 absorption coefficients of the Dobson and Brewer instruments is shown to be too small to account for the observed SAOZ/Dobson differences. Except for Faraday, these differences can however be largely reduced if SAOZ AMFs are calculated with realistic climatological profiles of ozone, pressure and temperature. Other sources of uncertainties that might affect the comparison are investigated. Evidence is found that the differences in the air masses sampled by the SAOZ and the other instruments contribute significantly to the scatter, and the impact of the tropospheric clouds on SAOZ measurements is displayed.

Has the Antarctic Vortex Split before 2002

In late September 2002, the Antarctic ozone hole was seen to split into two parts, resulting in large increases in ozone at some stations and the potential for significant modification of chlorofluorocarbon (CFC)-induced ozone loss. The phenomenon was dynamical (a split vortex), causing large increases in stratospheric temperature above stations normally within the vortex. Temperatures at Halley, Antarctica, at 30 hPa increased by over 60 K, and temperatures at South Pole at 100 hPa increased by over 25 K. It is important to know if this has happened before, since if it happens in the future, it would significantly alter the total hemispheric ozone loss due to chlorine from CFCs, particularly if it happens in August or September. Temperatures in winter and spring measured at Halley or the South Pole since 1957 and 1961, respectively, show no other comparable increases until the final warming in late spring, except for two dates in the 1980s at Halley when meteorological analyses show no vortex split. There are very few periods of measurements missing at both Halley and the South Pole, and analyses in those few periods show no vortex split. Measurements in August and September at sites normally near the edge of the vortex show very few suspicious dates, and analyses of those few suspicious dates again show no vortex split. It is concluded that the vortex has probably not split before the final warming since Antarctic records began in the late 1950s, and almost certainly not in August or September.

Ozone and NO2 air-mass factors for zenith-sky spectrometers: intercomparison of calculations with different radiative transfer models

Calculations of air-mass factors (AMFs) for ground-based zenith-sky UV-visible spectrometers are now well developed in laboratories where stratospheric constituents are measured with this technique. An intercomparison between results from the different radiative transfer models used to calculate AMFs at twilight is presented here. The comparison was made for ozone AMFs at 510 nm and for NO2 AMFs at 440 nm. Vertical profiles were specified. Results are presented firstly for calculations in a pure Rayleigh atmosphere, then including background aerosols. Relative differences between calculated AMFs from different models cause relative errors in vertical columns of ozone and NO2 measured by zenith-sky spectrometers. For commonly used averages over solar zenith angles, these relative errors are ±2.3% in the vertical column of ozone and ±1.1% in the vertical column of NO2. Refinements to the calculations, suggested by the intercomparison, should reduce these errors to ±1.0% for ozone and ±0.5% for NO2.

Retrieval of NO2 vertical profiles from ground‐based UV‐visible measurements: Method and validation

Vertical profiles of NO2 are retrieved from ground-based UV-visible slant columns by sequential estimation, using a forward model that consists of a stacked box photochemical model and a radiative transfer model. The retrieval method is characterized, and a rigorous error analysis is presented. The vertical resolution of the retrieved profiles is shown to vary from 5 to 10 km, depending on the retrieval altitude. The retrieved profiles are found to be moderately sensitive to the assumed vertical profiles of ozone and aerosol, to the range of solar zenith angles of the observations, and to the error in the slant column. However, in the altitude range 10 to 35 km they are relatively insensitive to a priori information and agreed well with profiles from simultaneous balloon-borne measurements.

The Final Warming Date of the Antarctic Polar Vortex and Influences on its Interannual Variability

Abstract More than 40 years of radiosonde data from two Antarctic stations are examined for changes in the date of the final stratospheric warming that occurs each year as the vortex breaks up in spring/summer. A new measure of this date is derived that does not rely on specification of a threshold, as has been common previously. The date of final warming takes between 10 and 40 days to progress from 30 to 100 hPa and occurs 20–30 days later in the 1990s than in the 1960s. Multiple linear regression analyses of these final warming dates, and also of the vertical profile of the southern annular mode (SAM), are presented. Only a weak signal is found for a linear trend, but a significant response is found throughout the atmosphere to ozone mass deficit (OMD), representing stratospheric ozone loss. In the SAM a significant response to the combined influence of solar variability and the quasi-biennial oscillation (QBO) is also found. The seasonal evolution of these signals in the NCEP Reanalysis zonal mean tem...