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Science Update: Seasonality above the tropical tropopause: reconciling the Eulerian and Lagrangian perspectives of transport processes

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.

Science Update: Flux of idealized surface trace gases into the lowermost stratosphere dependent on lifetimes

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.

Science Update: Modelling evidence of ozone depletion’s impact on extreme precipitation during the austral summer

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.

Science update: Thermal characteristics of the cold-point tropopause region in CMIP5 models

J. Kim and co-authors focus on the cold-point tropopause region as simulated by CMIP-5 models in a new JGR article. They look at the climatology, seasonality, and intraseasonal to interannual variability of the temperature field near the cold-point tropopause (CPT) in all CMIP-5 models for both historical simulations and future projections (using RCP8.5). The CPT temperature is estimated using both the 100-hPa and zero-lapse-rate (ZLR) temperatures. The historical simulations reproduce the spatio-temporal structure of the CPT temperature as well as the interannual variability associated with ENSO and the intraseasonal variability associated with equatorial waves successfully. However, the models show non-negligible biases in several aspects: 1) most models have a warm bias around the CPT; 2) large inter-model differences occur in the amplitude of the seasonal cycle in 100-hPa temperature; 3) several models overestimate lower stratospheric warming in response to volcanic aerosols; 4) temperature variability associated with the quasi-biennial oscillation and Madden-Julian oscillation is absent in most models; 5) equatorial waves near the CPT exhibit a wide range of variations among the models with unrealistically persistent Kelvin waves in several models. In terms of future projections, the models predict a robust warming at both the 100hPa and ZLR levels, but cooling at the 70hPa level. Most models also project a weakened seasonal cycle of temperature at both 100hPa and 70hPa levels. These findings may have important implications for cross-tropopause water vapour transport and related global climate change and variability. The full abstract can be found here.

Science update: GeoMIP (Geoengineering Model Intercomparison Project) climate model response

A recent JGR article by B. Kravitz and co-authors presents results from one of the GeoMIP model experiments (experiment G1). This experiment looks at the climate response of 12 models to an abrupt quadrupling of CO2 from preindustrial concentrations brought into radiative balance via a globally uniform reduction in insolation. The model results indicate that this reduction offsets global mean surface temperature to a large extent and also prevents 97% of Arctic sea-ice loss. However, compared to the preindustrial climate, the tropics are cooler (-0.3K) while the poles are warmer (+0.8K). Annual mean precipitation minus evaporation anomalies remain largely unchanged, except over some tropical regions where precipitation is slightly reduced. Global average net primary production increases by 120% above simulated preindustrial levels, mainly because of CO2 fertilisation, but also because of reduced plant heat stress compared to a world without geoengineering. Importantly, however, all models show that uniform solar geoengineering in the G1 experiment cannot simultaneously return regional and global temperature and hydrologic cycle intensity to preindustrial levels. The full abstract can be found here.

Science update: Influence of the propagation of stratospheric planetary waves on tropospheric blockings

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.