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Trends in trace atmospheric constituents can be driven not also by trends in their (precursor) emissions but also by trends in meteorology. Here we use ground-level ozone as an example to highlight the extent to which unforced, low-frequency climate variability can drive multidecadal trends. Using output from six experiments of the Geophysical Fluid Dynamics Laboratory chemistry-climate model (CM3), we demonstrate that 20 year trends in surface ozone driven by climate variability alone can be as large as those forced by changes in ozone precursor emissions or by anthropogenic climate change. We highlight regions and seasons where surface ozone is strongly influenced by climate variability and thus where a given forced trend may be more difficult to detect. A corollary is that this approach identifies regions and seasons of low variability, where measurement sites may be most effectively deployed to detect a particular trend driven by changing precursor emissions. We find that the representative concentration pathways 4.5 (RCP4.5) and RCP8.5 forced surface ozone trends in most locations emerge over background variability during the first half of the 21st century. Ozone trends are found to respond mostly to changes in emissions of ozone precursors and unforced climate variability, with a comparatively small impact from anthropogenic climate change. Thus, attempts to attribute observed trends to regional emissions changes require consideration of internal climate variability, particularly for short record lengths and small forced trends.

The transport of moisture into the Arctic is tightly connected to midlatitude dynamics. We show that the bulk of the transient poleward moisture transport across 60°N is driven by extreme transport (fluxes greater than the 90th percentile) events. We demonstrate that these events are closely related to the two types of Rossby wave breaking (RWB)—anticyclonic wave breaking (AWB) and cyclonic wave breaking (CWB). Using a RWB tracking algorithm, we determine that RWB can account for approximately 68% of the extreme poleward moisture transport by transients across 60°N in winter and 56% in summer. Additional analysis suggests that the seasonality of such RWB-related moisture transport is determined approximately equally by (1) the magnitude of transport (which is largely a function of the background moisture gradient) and (2) the frequency of RWB occurrence. The seasonality of RWB occurrence is, in turn, tied to the seasonal variation of the latitude of the jet streams—AWB-related (CWB-related) transport occurs more frequently when the jet is shifted poleward (equatorward). The interannual variability of RWB-related transport across 60°N in winter is shown to be strongly influenced by climate variability captured by the El Nino/Southern Oscillation (ENSO) and the North Atlantic Oscillation (NAO). In the positive (negative) phase of ENSO, AWB transports less (more) moisture through the Bering Strait and CWB transports more (less) through western Canada. In the positive (negative) phase of the NAO, AWB transports more (less) moisture through the Norwegian Sea and CWB transports less (more) along the west coast of Greenland. These results highlight that low-frequency climate variability outside of the polar regions can influence the Arctic water vapor by modulating extreme synoptic transport events.

Atmospheric rivers (ARs) can cause wide-ranging impacts upon landfall at high northern latitudes, but comparatively little is known about the dynamics supporting these ARs in contrast to their mid-latitude counterparts. Here ARs near the U.S. West Coast and the Gulf of Alaska during 1979–2015 are compared. ARs are found to occur in both regions with similar frequency, but with different seasonality. Composited atmospheric conditions from the NASA Modern-Era Retrospective Analysis for Research and Applications (MERRA) dataset reveal that a broad height anomaly over the northeast Pacific is influential to AR activity in both regions. When a positive height anomaly exists over the northeast Pacific, AR activity is often deflected poleward toward Alaska, while the U.S. West Coast experiences a decrease in AR activity. The opposing relationship also applies, that is, AR activity is decreased near Alaska and increased along the U.S. West Coast in the presence of a negative height anomaly. Quantitatively, nearly 79% of Gulf of Alaska ARs are associated with a positive northeast Pacific height anomaly and 86% of U.S. West Coast ARs are associated with a negative anomaly. Results suggest that this relationship applies across a range of time scales, to include subseasonal and interannual, not just with respect to individual transient waves. Both ARs and height anomalies are found to be associated with Rossby wave breaking, thereby dynamically linking AR activity with broader North Pacific dynamics.

[1] A Rossby wave breaking identification method is presented which searches for overturning of absolute vorticity contours on pressure surfaces. The results are compared to those from an analysis of isentropic potential vorticity, and it is demonstrated that both yield similar wave breaking distributions. As absolute vorticity is easily obtained from most model output, we present wave breaking frequency distributions from the ERA-Interim data set, thirteen general circulation models (GCMs) and a barotropic model. We demonstrate that a poleward shift of the Southern Hemisphere midlatitude jet is accompanied by a decrease in poleward wave breaking in both the barotropic model and all GCMs across multiple climate forcing scenarios. In addition, it is shown that while anticyclonic wave breaking shifts poleward with the jet, cyclonic wave breaking shifts less than half as much and reaches a poleward limit near 60 degrees S. Comparison of the observed distribution of Southern Hemisphere wave breaking with those from the GCMs suggests that wave breaking on the poleward flank of the jet has already reached its poleward limit and will likely become less frequent if the jet migrates any further poleward with climate change.