Surface Ozone‐Meteorology Relationships: Spatial Variations and the Role of the Jet Stream

Abstract

We investigate the relationships among summertime ozone (O 3), temperature, and humidity on daily timescales across the Northern Hemisphere using observations and model simulations. Temperature and humidity are significantly positively correlated with O 3 across continental regions in the midlatitudes (∼35-60 • N). Over the oceans, the relationships are consistently negative. For continental regions outside the midlatitudes, the O 3-meteorology correlations are mixed in strength and sign but generally weak. Over some high latitude, low latitude, and marine regions, temperature and humidity are significantly anticorrelated with O 3. Daily variations in transport patterns linked to the position and meridional movement of the jet stream drive the relationships among O 3 , temperature, and humidity. Within the latitudinal range of the jet, there is an increase (decrease) in O 3 , temperature, and humidity over land with poleward (equatorward) movement of the jet, while over the oceans poleward movement of the jet results in decreases of these fields. Beyond the latitudes where the jet traverses, the meridional movement of the jet stream has variable or negligible effects on surface-level O 3 , temperature, and humidity. The O 3-meteorology relationships are largely the product of the jet-induced changes in the surface-level meridional flow acting on the background meridional O 3 gradient. Our results underscore the importance of considering the role of the jet stream and surface-level flow for the O 3-meteorology relationships, especially in light of expected changes to these features under climate change. Plain Language Summary The relationship of ozone (O 3) with meteorological variables such as temperature and humidity at the Earth's surface varies in strength and sign. Some regions, such as continental parts of the midlatitudes, experience increases in O 3 as the temperature or humidity rises. However, this is not the case over the entire Northern Hemisphere. We use detailed computer simulations of atmospheric chemistry to show that these relationships are primarily the result of changes in meteorology, not changes in emissions or chemistry. The relationship between O 3 and meteorological variables is related to the north-south movement of the jet stream, powerful eastward-flowing air currents located near the tropopause that can encircle the hemisphere. Specifically, we find that the jet stream influences the O 3-meteorology relationships due to its effect on the northward and southward advection of O 3 , temperature, and humidity and not due to cyclones and the associated frontal activity, as has been previously suggested. Our results are relevant for understanding the present-day O 3-meteorology relationships and how climate change may impact O 3 pollution.

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

“…In this case, the latitude where r (V, ϕ jet ) = 0 lies north of r (χ, ϕ jet ) = 0 by ∼5°, and other processes such as changes in zonal winds or convection could be important for the tracer-jet relationships in this region and season. These results support Kerr et al (2020) and provide strong evidence linking the tracer-jet relationships to (1) the source region of the tracers and (2) the V-jet relationship (Figure 3).…”

“…This mechanism explains (1) the variation in sign of the jet-tracer relationship with tracer meridional gradients, (2) the land-ocean differences in the jet-tracer relationship for tracers with mid-latitude sources, and (3) seasonal differences in the jet-tracer relationship. Furthermore, this mechanism explains both the latitudinal and land-ocean differences in the JJA jet-O 3 relationship reported in Kerr et al (2020) and also helps explain seasonality in the jet-O 3 relationship. Although not shown in Kerr et al (2020), the sign of the jet-O 3 relationship over North America and Eurasia changes from positive during JJA to negative in DJF, which is broadly consistent with χ 40−50 ( Figure 2c and 2d and 4c and 4d).…”

“…A recent study by Kerr et al (2020) provided further support for a link between variability in the upper-level jet and surface-level O 3 but also showed substantial spatial variations in the relationship. They showed that the daily variability in surface-level O 3 during boreal summer (JJA) is significantly correlated with the meridional position of the jet across the Northern Hemisphere mid-latitudes, but the sign of the relationship differed between land and ocean (with O 3 increasing over land but decreasing over the oceans when the jet is in a poleward position).…”

“…A simple convolution-based smoothing is applied in longitudinal space to address potential longitudinal discontinuities in the jet's position (i.e., "jumps" in ϕ jet ) using a box-shaped function with a width of ∼10° longitude. Identifying ϕ jet using 500 hPa winds follows previous work by Barnes and Fiore (2013) and Kerr et al (2020).…”

“…The analysis presented in Section 3.1 has shown that a large fraction of daily tracer variability is related to meridional movement of the jet but does not show the mechanism(s) involved or why the signs of the tracer-jet relationships varies. Kerr et al (2020) suggested that the jet stream affects surface-level O 3 by altering the near-surface meridional flow (V). We test this hypothesis using our suite of tracers.…”