Clara Orbe

Air‐mass origin in the tropical lower stratosphere: The influence of Asian boundary layer air

A climatology of air-mass origin in the tropical lower stratosphere is presented for the Goddard Earth Observing System Chemistry Climate Model. During late boreal summer and fall, air-mass fractions reveal that as much as 20% of the air in the tropical lower stratosphere last contacted the planetary boundary layer (PBL) over Asia; by comparison, the air-mass fractions corresponding to last PBL contact over North America and over Europe are negligible. Asian air reaches the extratropical tropopause within a few days of leaving the boundary layer and is quasi-horizontally transported into the tropical lower stratosphere, where it persists until January. The rapid injection of Asian air into the lower stratosphere—and its persistence in the deep tropics through late (boreal) winter—is important as industrial emissions over East Asia continue to increase. Hence, the Asian monsoon may play an increasingly important role in shaping stratospheric composition.

Flux distributions as robust diagnostics of stratosphere‐troposphere exchange

[1] We perform the first analysis of stratosphere-troposphere exchange in terms of distributions that partition the one-way flux across the thermal tropopause according to stratospheric residence time t and the regions where air enters and exits the stratosphere. These distributions robustly quantify one-way flux without being rendered ill defined by the short-t eddy-diffusive singularity. Diagnostics are computed with an idealized circulation model with topography only in the Northern Hemisphere (NH) run under perpetual NH winter conditions. Suitable integrations of the flux distribution are used to ∂

The Transit-Time Distribution from the Northern Hemisphere Midlatitude Surface

AbstractThe distribution of transit times from the Northern Hemisphere (NH) midlatitude surface is a fundamental property of tropospheric transport. Here, the authors present an analysis of the transit-time distribution (TTD) since air last contacted the NH midlatitude surface, as simulated by the NASA Global Modeling Initiative Chemistry Transport Model. Throughout the troposphere, the TTD is characterized by young modes and long tails. This results in mean transit times or “mean ages” Γ that are significantly larger than their corresponding modal transit times or “modal ages” τmode, especially in the NH, where Γ ≈ 0.5 yr, while τmode < 20 days. In addition, the shape of the TTD changes throughout the troposphere as the ratio of the spectral width Δ—the second temporal moment of the TTD—to the mean age decreases sharply in the NH from ~2.5 at NH high latitudes to ~0.7 in the Southern Hemisphere (SH). Decreases in Δ/Γ in the SH reflect a narrowing of the TTD relative to its mean and physically correspond ...

Airmass Origin in the Arctic. Part I: Seasonality

AbstractThe first climatology of airmass origin in the Arctic is presented in terms of rigorously defined airmass fractions that partition air according to where it last contacted the planetary boundary layer (PBL). Results from a present-day climate integration of the Goddard Earth Observing System Chemistry–Climate Model (GEOSCCM) reveal that the majority of air in the Arctic below 700 mb last contacted the PBL poleward of 60°N. By comparison, 62% (±0.8%) of the air above 700 mb originates over Northern Hemisphere midlatitudes (i.e., “midlatitude air”). Seasonal variations in the airmass fractions above 700 mb reveal that during boreal winter air from midlatitudes originates primarily over the oceans, with 26% (±1.9%) last contacting the PBL over the eastern Pacific, 21% (±0.87%) over the Atlantic, and 16% (±1.2%) over the western Pacific. During summer, by comparison, midlatitude air originates primarily over land, overwhelmingly so over Asia [41% (±1.0%)] and, to a lesser extent, over North America [2...

Air-mass Origin in the Arctic. Part II: Response to Increases in Greenhouse Gases

AbstractFuture changes in transport from Northern Hemisphere (NH) midlatitudes into the Arctic are examined using rigorously defined air-mass fractions that partition air in the Arctic according to where it last had contact with the planetary boundary layer (PBL). Boreal winter (December–February) and summer (June–August) air-mass fraction climatologies are calculated for the modeled climate of the Goddard Earth Observing System Chemistry–Climate Model (GEOSCCM) forced with the end-of-twenty-first century greenhouse gases and ozone-depleting substances. The modeled projections indicate that the fraction of air in the Arctic that last contacted the PBL over NH midlatitudes (or air of “midlatitude origin”) will increase by about 10% in both winter and summer. The projected increases during winter are largest in the upper and middle Arctic troposphere, where they reflect an upward and poleward shift in the transient eddy meridional wind, a robust dynamical response among comprehensive climate models. The bor...

Large‐Scale Atmospheric Transport in GEOS Replay Simulations

Offline chemical transport models (CTMs) have traditionally been used to perform studies of atmospheric chemistry in a fixed dynamical environment. An alternative to using CTMs is to constrain the flow in a general circulation model using winds from meteorological analyses. The Goddard Earth Observing System (GEOS) “replay” approach involves reading in analyzed fields every six hours and recomputing the analysis increments, which are applied as a forcing to the meteorology at every model time step. Unlike in CTM, all of the subgrid-scale processes are recalculated on-line so that they are consistent with the large-scale analysis fields, similar in spirit to “nudged” simulations, in which the online meteorology is relaxed to the analysis. Here we compare the transport of idealized tracers in different replay simulations constrained with meteorological fields taken from The Modern-Era Retrospective Analysis for Research and Applications, Version 2 (MERRA-2). We show that there are substantial differences in their large-scale stratospheric transport, depending on whether analysis fields or assimilated fields are used. Replay simulations constrained with the instantaneous analysis fields produce stratospheric mean age values that are up to 30\% too young relative to observations; by comparison, simulations constrained with the time-averaged assimilated fields produce more credible stratospheric transport. Our study indicates that care should be taken to correctly configure the model when the replay technique is used to simulate stratospheric composition.

The role of monsoon-like zonally asymmetric heating in interhemispheric transport

While the importance of the seasonal migration of the zonally averaged Hadley circulation on interhemispheric transport of trace gases has been recognized, few studies have examined the role of the zonally asymmetric monsoonal circulation. This study investigates the role of monsoon-like zonally asymmetric heating on interhemispheric transport using a dry atmospheric model that is forced by idealized Newtonian relaxation to a prescribed radiative equilibrium temperature. When only the seasonal cycle of zonally symmetric heating is considered, the mean age of air in the Southern Hemisphere since last contact with the Northern Hemisphere midlatitude boundary layer, is much larger than the observations. The introduction of monsoon-like zonally asymmetric heating not only reduces the mean age of tropospheric air to more realistic values, but also produces an upper-tropospheric cross-equatorial transport pathway in boreal summer that resembles the transport pathway simulated in the NASA Global Modeling Initiative (GMI) Chemistry Transport Model driven with MERRA meteorological fields. These results highlight the monsoon-induced eddy circulation plays an important role in the interhemispheric transport of long-lived chemical constituents.

Tropospheric transport differences between models using the same large-scale meteorological fields: TRANSPORT IN SPECIFIED DYNAMICS RUNS

The transport of chemicals is a major uncertainty in the modeling of tropospheric composition. A common approach is to transport gases using the winds from meteorological analyses, either using them directly in a chemical transport model or by constraining the flow in a general circulation model. Here we compare the transport of idealized tracers in several different models that use the same meteorological fields taken from Modern-Era Retrospective analysis for Research and Applications (MERRA). We show that, even though the models use the same meteorological fields, there are substantial differences in their global-scale tropospheric transport related to large differences in parameterized convection between the simulations. Furthermore, we find that the transport differences between simulations constrained with the same-large scale flow are larger than differences between free-running simulations, which have differing large-scale flow but much more similar convective mass fluxes. Our results indicate that more attention needs to be paid to convective parameterizations in order to understand large-scale tropospheric transport in models, particularly in simulations constrained with analyzed winds.

Isentropic transport and the seasonal cycle amplitude of CO2: SEASONAL CYCLE AMPLITUDE OF CO2

Department of Atmospheric Science, Colorado State University, Fort Collins, CO, USA. 3 Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA, USA. 4 Joint Institute for Regional Earth System Science and Engineering, University of California, Los Angeles, CA, USA. 5 GESTAR 6 Global Modeling and Assimilation Office, NASA Goddard Space Flight Center 7 Johns Hopkins University, Baltimore, Maryland 8 9