Large-scale transport into the Arctic: the roles of the midlatitude jet and the Hadley Cell

Abstract

Abstract. Transport from the Northern Hemisphere (NH) midlatitudes to the Arctic plays a crucial role in determining the abundance of trace gases and aerosols that are important to Arctic climate via impacts on radiation and chemistry. Here we examine this transport using an idealized tracer with a fixed lifetime and predominantly midlatitude land-based sources in models participating in the Chemistry Climate Model Initiative (CCMI). We show that there is a 25 %–45 % difference in the Arctic concentrations of this tracer among the models. This spread is correlated with the spread in the location of the Pacific jet, as well as the spread in the location of the Hadley Cell (HC) edge, which varies consistently with jet latitude. Our results suggest that it is likely that the HC-related zonal-mean meridional transport rather than the jet-related eddy mixing is the major contributor to the inter-model spread in the transport of land-based tracers into the Arctic. Specifically, in models with a more northern jet, the HC generally extends further north and the tracer source region is mostly covered by surface southward flow associated with the lower branch of the HC, resulting in less efficient transport poleward to the Arctic. During boreal summer, there are poleward biases in jet location in free-running models, and these models likely underestimate the rate of transport into the Arctic. Models using specified dynamics do not have biases in the jet location, but do have biases in the surface meridional flow, which may result in differences in transport into the Arctic. In addition to the land-based tracer, the midlatitude-to-Arctic transport is further examined by another idealized tracer with zonally uniform sources. With equal sources from both land and ocean, the inter-model spread of this zonally uniform tracer is more related to variations in parameterized convection over oceans rather than variations in HC extent, particularly during boreal winter. This suggests that transport of land-based and oceanic tracers or aerosols towards the Arctic differs in pathways and therefore their corresponding inter-model variabilities result from different physical processes.

Evaluating Simulations of Interhemispheric Transport: Interhemispheric Exchange Time Versus SF6 Age

“…We consider all models in each project and do not separate free‐running or specified dynamics models. (Note, Orbe et al, , ; and Yang, Waugh, Orbe, et al, , have shown that the spread in transport among specified dynamics models is comparable to that for free‐running models in CCMI.) There are small differences in the SF 6 emissions used between the TransCom and CCMI models (TransCom models used EDGAR v4.0, whereas CCMI models used EDGAR v4.2), but the τ ex and SF 6 age calculations are not sensitive to these differences as both calculations are more dependent on meridional gradient of SF 6 concentrations and not on emissions.…”

Jet Stream‐Surface Tracer Relationships: Mechanism and Sensitivity to Source Region

“…All tracers decay uniformly at a loss rate of τ = 50 days −1 . Tracers with the same loss have been used in prior studies (e.g., Orbe et al, 2017Orbe et al, , 2018Shindell et al, 2008;Yang et al, 2019). Although not the primary focus of our analysis, we also explore how the lifetime of tracers impacts their relationship with the jet by simulating χ 40−50 with loss rates of τ = 5, 25, 100 and 150 days −1 .…”

Dependence of Northern Hemisphere Tropospheric Transport on the Midlatitude Jet Under Abrupt CO2 Increase

“…It is therefore important to know the processes controlling this transport and how the transport will change with climate. Previous studies comparing simulations of real and idealized tracers show a large spread among models (Monks et al., 2015; Orbe et al., 2017, 2018; Shindell et al., 2008; Yang et al., 2019). Furthermore, there remain large uncertainties in the role of different processes in causing changes, such as the relative role of changes in the large‐scale circulation, isentropic mixing, and convection (e.g., Orbe et al., 2018; Yang et al., 2019).…”