The effects of imposed stratospheric cooling on the maximum intensity of tropical cyclones
Using a cloud-resolving model, H. Ramsay presents results that elucidate the effect of stratospheric cooling and sea surface warming on the potential intensity (PI) of tropical cyclones in a recent article in the Journal of Climate. With fixed sea surface temperatures, cooling near and above the tropopause (~90hPa) is shown to increase PI at a rate of 1m/s per degree cooling. With fixed stratospheric temperatures, sea surface warming increases the PI by approximately twice as much, as a rate of about 2m/s per degree warming. These results have significant implications in terms of global tropical cyclone PI trends in response to climate change. Tropical sea surface temperatures have warmed by about 0.15K/decade since the 1970s, while the stratosphere has cooled anywhere from 0.3K/decade to over 1K/decade, depending on the data set used. Therefore, global PI trends in recent decades appear to have been driven more by stratospheric cooling than by surface warming. Find the full abstract here.
A recent JGR article by J. Hsu and co-authors uses the NCAR Community Atmosphere Model to evaluate the sensitivity of stratospheric dynamics to the uncertainty in ozone production related to uncertainty in O2 cross-sections in the Hertzberg Continuum. Reducing the O2 cross-sections by 30% is found to increase ozone abundances in the lower stratosphere, which as a result warms between 60°S -60°N (2K maximum at the equator) and lowers the tropopause height by 100-200m between 30°S -30°N. The study points to the important role of ozone in the lower tropical stratosphere in determining the physical characteristics of the tropical tropopause layer. The full abstract can be found here.
A new ACP article by N.A. Kramarova and co-authors presents a validation of ozone profiles from the Solar Backscatter Ultraviolet (SBUV and SBUV/2) instruments that were recently reprocessed using an updated (version 8.6) algorithm. The SBUV data record covers the period 1970-2011, with a 5 year gap in the 1970s. Validation with MLS (on board the UARS and Aura satellites) and SAGE-II satellite observations, as well as ground-based observations from microwave spectrometers, lidars, Umkehr instruments and balloon-borne ozonesondes. They find that in the stratosphere, between 25 and 1 hPa, the mean biases and standard deviations are mostly within 5% for monthly zonal mean ozone profiles. Above and below this layer the vertical resolution of the SBUV algorithm decreases. In order to account for the lower resolution in the troposphere/lower stratosphere, they combine several layers of data. The drift of the SBUV instruments is also estimated, as well as its potential effect on the long-term stability of the combined data record. The features of individual SBUV(/2) instruments are discussed and recommendations for creating a merged SBUV data set are provided. The full abstract can be found here.
A recent GRL article by S.M. Kang and co-authors explores the impacts of stratospheric ozone depletion on daily precipitation extremes during the austral summer. The two models used both suggest that ozone losses since the late 1970’s have resulted in an increase in frequency and intensity of heavy rainfall events over the southern mid-latitudes. This hemispheric response pattern projects strongly onto a previously identified pattern of seasonal mean precipitation response, both of which are shown to be likely of dynamic rather than thermodynamic origin. The full abstract can be found here.
A recent JGR article by M. Holzer and L. Polvani uses the MATCH transport model to quantify the flux of idealized trace gases across the thermal tropopause as a function of their chemical lifetime. Emissions of the trace species are idealized as time dependent with either a generic anthropogenic pattern or a uniform ocean source. Globally averaged fluxes into the stratosphere normalized by surface emissions are dependent on the tropospheric chemistry, which is idealized as decay with a constant lifetime (τc). For τc=8 days and τc~140 days the fluxes are 1% and 30%, respectively. Interestingly, the flux patterns computed with MATCH are insensitive toτc~ and reveal preferred pathways into the stratosphere – divergent circulation feeding isentropic cross-tropopause transport and isentropic transport to high latitudes. Find the full abstract here.
M. Abalos and co-authors investigate recent contrasting results regarding the seasonality of ozone about the tropical tropopause in a new ACPD article. In the literature, different methods (Lagrangian versus Eulerian, and isentropic versus pressure vertical coordinates) yield different results in terms of ozone transport, and the results must be carefully compared in equivalent terms to avoid misinterpretation. By examining the Lagrangian calculations in the Eulerian formulation, they show that the results are in fact consistent with each other and with a common understanding of the ozone transport processes near and above the tropical tropopause. The analysis of the Transformed Eulerian Mean ozone budget indicates that the annual tropical upwelling cycle is the main forcing of ozone seasonality at altitudes with large vertical gradients in the tropical lower stratosphere. Using a Lagrangian framework, it is found that a large fraction (~50%) of the ozone molecules ascending through the tropical lower stratosphere is of extra-tropical origin and has been in-mixed from mid-latitudes. The full abstract can be found here.
The international Commission on Atmospheric Chemistry and Global Pollution (iCACGP) and the International Global Atmospheric Chemistry (IGAC) project are pleased to announce that the joint 13th Quadrennial iCACGP Symposium/13th IGAC Science Conference will be held in Natal, Brazil, on 22-26 September 2014. Find first announcement.
K. Kodera and co-authors used case studies to investigate the relationship between stratospheric planetary wave reflection and blocking formation in the troposphere in a recent JGR paper. The enhanced upward propagation of a planetary-scale wave packet from the Eurasian sector, involving a Euro-Atlantic blocking, leads to a stratospheric sudden warming (SSW). Following the weakening of the stratospheric westerly jet due to polar warming, the stratospheric planetary wave packet then propagates downward over the American sector, inducing a ridge over the North Pacific as well as a trough over eastern Canada in the upper troposphere. The ridge promotes the formation of a Pacific blocking. This result explains why Pacific blockings tends to form after SSWs, and why they are associated with suppressed upward propagation of planetary waves. The full abstract can be found here.
The SPARC project on Stratospheric Sulfur and Its Role in Climate (SSiRC) is holding its first workshop on 28-30 October 2013 in Atlanta, Georgia, USA. Abstract deadline is 30 August.
