High ground-level ozone (O3) concentrations adversely impact human health by inducing respiratory diseases and threaten food security by lowering crop yields (Brown and Bowman, 2013; WHO, 2013; Chuwah et al., 2015). Considering the ecotoxicity of O3, understanding the physical and chemical mechanisms that control atmospheric O3 concentrations is of great importance. Surface O3 is produced in the atmosphere via photochemical processing of multiple precursors including volatile organic compounds (VOCs), carbon monoxide (CO) and nitrogen oxides (NO, NO2). These precursors originate from both natural and anthropogenic sources (Vingarzan, 2004; Simon et al., 2014; Jiang et al., 2016). In addition to local production, transport of O3 and its precursors from upwind regions and the upper atmosphere can also influence surface O3 abundance. Stratospheric intrusion events, which lead to vertical downmixing of ozone-rich air, can significantly elevate surface O3 during spring (Grewe, 2006; Lin et al., 2012b; Zhang et al., 2014). The long-range transport of O3 and its precursors has been extensively studied, and their intercontinental impacts have been evaluated using measurements and model simulations (Parrish et al., 1993; Fehsenfeld et al., 1996; Wild and Akimoto, 2001; Creilson et al., 2003; Simmonds et al., 2004; Fiore et al., 2009; Brown-Steiner and Hess, 2011; Lin et al., 2012a, 2014).

Both photochemistry and dynamic transport collectively affect surface O3 levels. Important meteorological factors that can impact both photochemistry and transport include atmospheric circulations, solar radiation, air temperature, and relative humidity. Atmospheric circulation considerably determines the timescale and pathway of O3 transport (Bronnimann et al., 2000; Auvray and Bey, 2005; Hess and Mahowald, 2009). The efficiency of O3 transport varies concurrently with atmospheric circulations on different scales. Knowland et al. (2015) demonstrated the important role of midlatitude storms in redistributing O3 concentrations during springtime. The North Atlantic Oscillation (NAO) significantly affects surface and tropospheric O3 concentrations over most of Europe by influencing the intercontinental transport of air masses (Creilson et al., 2003; Christoudias et al., 2012; Pausata et al., 2012). Lamarque and Hess (2004) indicated that the Arctic Oscillation (AO) can modulate springtime tropospheric O3 burdens over North America. The shift in the position of the jet stream associated with climate change was found to strongly affect summertime surface O3 variability over eastern North America (Barnes and Fiore, 2013). Increases in solar radiation and air temperature can increase the rate of the chemical production of O3 and modulate the biogenic emissions of O3 precursors (Guenther, 1993; Sillman and Samson, 1995; Peñuelas and Llusià, 2001), especially over highly polluted regions (Ordónez et al., 2005; Rasmussen et al., 2012; Pusede et al., 2015). Increases in humidity can enhance the chemical destruction of O3 and shorten its atmospheric lifetime (Johnson et al., 1999; Camalier et al., 2007). Therefore, changes in meteorological conditions on various spatial and temporal scales play key roles in determining the surface O3 distribution. Understanding the mechanisms and feedbacks of the interactions between O3 and climate has received increasing attention and will be essential for future surface O3 mitigation (Jacob and Winner, 2009; Doherty et al., 2013).

Sea surface temperature (SST) is an important indicator that characterizes the state of the climate system. Its variations strongly perturb the mass and energy exchange between the ocean and atmosphere (Small et al., 2008; Gulev et al., 2013), which influence atmospheric circulation, atmospheric temperature, and specific humidity (Sutton and Hodson, 2005; Frankignoul and Sennéchael, 2007; Li et al., 2008) from regional to global scales (Glantz et al., 1991; Wang et al., 2000; Goswami et al., 2006). It also affects cloud formation and consequently influences incoming solar radiation (Deser et al., 1993; Fallmann et al., 2017). Numerous studies have shown that SST changes over different oceans and at different latitudes lead to significantly different meteorological and climate responses (Webster, 1981; Lau and Nath, 1994; Lau, 1997; Sutton and Hodson, 2007; Sabeerali et al., 2012; Ueda et al., 2015). Details on the SST–climate relationships over individual oceanic regions are summarized in Kushnir et al. (2002).

The Intergovernmental Panel on Climate Change Fifth Assessment Report (IPCC, 2013) provides strong evidences in Chapter 2 that global SSTs are generally increasing due to the impacts of anthropogenic forcings on global climate change. In addition, regional SST exhibits natural periodic or irregular oscillations with timescales ranging from months to decades. The El Niño–Southern Oscillation (ENSO) is the most influential natural SST variability that originates in the tropical Pacific and has worldwide climate impacts (Philander, 1983; Wang et al., 2012). The Pacific Decadal Oscillation (PDO), defined by ocean temperature anomalies in the northeast and tropical Pacific Ocean, is another long-lived, El Niño-like pattern that persists for several decades (Mantua and Hare, 2002). Over the Indian Ocean, SST anomalies feature a seesaw structure between the western and eastern equatorial regions, known as the Indian Ocean Dipole (IOD) mode (Saji et al., 1999). The North Atlantic Ocean exhibits various modes of low-frequency SST variability (Kushnir, 1994; Wu and Liu, 2005; Fan and Schneider, 2012; Taboada and Anadon, 2012). The mechanisms responsible for SST variability include ocean circulation variability, wind stress, and ocean–atmosphere feedbacks (Frankignoul, 1985; Deser et al., 2010). Aerosols and greenhouse gases (GHGs) emitted from anthropogenic and natural sources also contribute to regional SST variability through modulation of the solar radiation received by the ocean surface (Rotstayn and Lohmann, 2002; Wu and Kinter, 2011; Hsieh et al., 2013; Ding et al., 2014; Meehl et al., 2015).

Considering the distinct roles of regional SST variability in modulating regional climate systems, the impact of regional SST changes on the surface O3 distribution needs to be explored. Lin et al. (2015) found that more frequent deep stratospheric intrusions appear over the western US during strong La Niña springs because of the meandering of the polar jet towards this region. This process can remarkably increase surface O3 levels in the western US. The La Niña-like decadal cooling of the eastern equatorial Pacific Ocean in the 2000s weakened the long-range transport of O3-rich air from Eurasia towards Hawaii during spring (Lin et al., 2014). Liu et al. (2005) revealed that El Niño winters are associated with stronger transpacific pollutant transport, which also has implications for the long-range transport of O3. Except for the ENSO impacts, very few studies to date have directly addressed the linkage between SST and O3. Therefore, a comprehensive understanding of the response of surface O3 to SST changes in individual ocean basins is lacking and necessary.

To fill this gap, this study focuses on examining the sensitivity of O3 evolution over four polluted continental regions in the Northern Hemisphere (i.e., North America (NA, 15–55∘ N, 60–125∘ W), Europe (EU, 25–65∘ N, 10∘ W–50∘ E), East Asia (EA, 15–50∘ N, 95–160∘ E) and South Asia (SA, 5–35∘ N, 50–95∘ E), defined in Fiore et al., 2009) with respect to nearby basin-scale SST changes. We describe the design of numerical experiments and model configuration in Sect. 2. Surface O3 responses to regional SST changes are given in Sect. 3. Relevant mechanisms governing the SST–O3 relationships are discussed in Sect. 4. The impact of basin-scale SST changes on intercontinental transport of O3 is described in Sect. 5. Conclusions are drawn in Sect. 6.

Seasonally and regionally averaged surface O3 changes in each SST perturbation simulation for the four highly populated continental regions and three ocean basins defined in our study are given in Tables 1 and S1 in the Supplement, respectively. The responses of the surface O3 concentrations to basin-scale SST changes (i.e., ±1 ∘C) are mainly below 3 ppbv in the Northern Hemisphere (Tables 1 and S1), though larger anomalies (i.e., up to 5 ppbv) are also observed over the eastern coast of China, the Indian subcontinent, and certain oceanic areas (Figs. 1 and S2). This SST–O3 sensitivity is comparable to previous findings. For instance, Bloomer et al. (2009) reported a positive O3–temperature relationship of 2.2–3.2 ppbv ∘C-1 across the rural eastern US. Wu et al. (2008) found that summertime surface O3 may increase by 2–5 ppbv over the northeastern US in the 2050s. Additionally, Fiore et al. (2009) demonstrated an intercontinental decrease in surface O3 of no more than 1 ppbv in response to 20 % reductions in anthropogenic emissions within a continental region. Our study indicates that basin-scale SST changes alone may exert significant effects on the surface O3 above a specific ocean basin and its surrounding continents.

As shown in Fig. 1, seasonal changes of up to 5 ppbv in the mean surface O3 concentration are observed during boreal summers, mainly in coastal regions and remote oceans. Surface O3 changes in response to positive and negative SST anomalies generally exhibit a consistent spatial pattern but are opposite in sign, suggesting robust relationships between surface O3 levels and SST anomalies (Fig. 1). An increase in summertime SST over a specific ocean basin tends to increase the surface O3 concentration over the upwind regions but reduce this concentration over downwind continents. For instance, a 1 ∘C warming over the North Pacific leads to a widespread decrease of approximately 1 ppbv in surface O3 over the North Pacific, North America, and the North Atlantic (Table S1) but may enhance the surface O3 by nearly 3 ppbv over southern China. Similarly, the SST warming over the North Atlantic decreases the surface O3 levels by 1–2 ppbv over the North Atlantic and Europe but increases (∼ 1 ppbv) those over North America and the North Pacific. For the North Indian Ocean, positive SST anomalies tend to increase the surface O3 over the Indian Ocean and Africa but decrease the surface O3 over South and East Asia (Fig. 1). During the boreal winter, a widespread decrease in surface O3 associated with the warming of different oceans is observed. Significant changes (e.g., up to 5 ppbv) mainly occur over remote ocean areas. Over populated continents, the response of the surface O3 to basin-scale SST changes is typically insignificant. Details are shown in Fig. S2.

Our simulations reveal that different oceans can exert distinct region-specific effects on the O3 distribution. The effects of three individual warming–cooling cases (i.e., Pacific-W, Atlantic-W, and Indian-W and Pacific-C, Atlantic-C, and Indian-C) on surface O3 distributions are further summed up to compare with the combined warming–cooling cases (i.e., ALL-W and ALL-C). The responses of surface O3 to a hemispheric SST anomaly generally resemble the sum of responses to individual regional SST changes (see Figs. S3 and S4). This indicates that the effect of a generalized SST warming on surface O3 can be decomposed into individual regional SST forcings. We now analyze the processes that impact the dependence of SST on the O3 distribution using simulations that increase the SST.

The findings above indicate that, in general, a basin-scale SST increase in the Northern Hemisphere is more likely to enhance atmospheric stability at midlatitudes, which may suppress air pollutants from lofting to the free troposphere. This process potentially has large effects on O3 intercontinental transport. Following previous work (e.g., Doherty et al., 2013; Fang et al., 2011), we use passive CO-like tracers to demonstrate the potential effect of regional SST changes on long-range O3 transport. A warming of North Pacific SSTs by 1 ∘C tends to increase the East Asian CO tracer concentrations by nearly 6 % at the surface (Fig. 8b), which is accompanied by a significant reduction (∼ 4 %) in eastward transport to North America. Similarly, for the North American tracer, a warming of North Atlantic SSTs by 1 ∘C increases (∼ 1 %) the concentrations in North America but decreases (3–4  %) the concentrations over downwind Europe (Fig. 8d). The response of the South Asian CO tracer to North Indian Ocean warming also shows a decreasing tendency over downwind regions, but the patterns are more complicated over the source region in this case (Fig. 8e). Because the CO-like tracers added in the simulation have a fixed decay lifetime, their concentration changes are completely caused by the SST-induced transport anomalies. The decrease in CO tracer concentrations over downwind regions suggests that the warming of basin-scale SST tends to suppress the long-range transport of air pollutants. Additionally, in the Pacific-W case, changes in the East Asian CO tracer (Fig. 8a) generally resemble the changes in surface O3 over East Asia (Fig. 7a), indicating the dominant effect of physical transport on the O3 distribution over East Asia. Regarding the North American CO tracer in response to the North Atlantic warming or the South Asian CO tracer in response to the North Indian Ocean warming, their concentration changes are spatially inconsistent with those of O3 (see Figs. 7 and 8). This further indicates the distinct roles that different basin-scale SSTs play in nearby air quality.

Further investigations of zonal wind suggest that an increase in SST over different oceans consistently decreases the westerly winds at lower midlatitudes (25–45∘ N) in the Northern Hemisphere but increases these winds at higher latitudes (Fig. 9). In general, increases in the geopotential height induced by basin-scale SST warming are more significant at midlatitudes than at other latitudes, which is consistent with the air temperature changes. Consequently, the meridional geopotential height gradient decreases at lower latitudes but increases at higher latitudes, leading to corresponding changes in the westerly winds. The latitude band at 25–45∘ N covers many polluted regions (i.e., North America and East Asia). A weakened westerly wind may reduce long-range O3 transport. As demonstrated in Sect. 4.3, the basin-scale SST increases also exert negative effects on the upward transport of air masses at midlatitudes. Therefore, the decreases in CO tracer concentrations over downwind regions (Fig. 8a and c) can be explained by both suppressed vertical transport and weakened westerly winds. In the Indian-W case, the SST increase over North India leads to a low-pressure anomaly above the Arabian Sea due to the enhanced deep convection (as discussed in Sect. 4.3). The corresponding anomalous cyclonic circulation may be responsible for the dipole of the South Asian CO tracer changes over the source region depicted in Fig. 8e.

In addition, we also find a hemispheric-scale decrease in peroxyacetyl nitrate (PAN), a reservoir of O3 precursors (NOx and HOx) that facilitate the long-range transport of O3, during the warming of different oceans (Fig. S14). This decrease is likely caused by the increase in the thermal decomposition of PAN in response to the air temperature increase (Jacob and Winner, 2009; Doherty et al., 2013).

Thus, it is reasonable to infer that, in general, the increased thermal decomposition of PAN, the weakened midlatitude westerlies, and the reduced vertical air transport may exert a joint reduction effect on the intercontinental transport of O3 during basin-scale SST increases.

In this paper, we investigate the responses of surface O3 to basin-scale SST anomalies in the Northern Hemisphere. The latest version of CESM (version 1.2.2) is used in our simulation, forced with climatological and stationary SST anomalies (±1 ∘C) in the North Pacific, North Atlantic, and North Indian oceans, respectively. The responses of surface O3 associated with these SST changes are evaluated. Results of similar magnitude but opposite sign are observed for the SST warming versus cooling simulations for each ocean basin, suggesting robust connections between the SST anomalies and surface O3 changes. The regionally and seasonally averaged surface O3 changes over four continental regions (i.e., NA, EU, EA, and SA) produce wide seasonal and regional variability (varying from 1 to 3 ppbv). The warming of the North Pacific leads to nearly 3 ppbv increases in the surface O3 over southern China in summer, with corresponding decreases over North America (∼ 1 ppbv). Similarly, the North Atlantic SST warming elevates the surface O3 pollution over North America while reducing the surface O3 (nearly 1–2 ppbv) over Europe. Changes in the North Indian SST exert significant impacts (1–3 ppbv) over South and East Asia during the entire year.

Process analysis indicates that dry deposition and vertical diffusion are two major processes governing the surface O3 balance. The increase in SST in different ocean basins tends to increase the contributions of vertical diffusion to surface O3 over upwind regions while greatly restraining those over downwind continents. These processes generally lead to widespread decreases in surface O3, which are partially offset by increases in air-temperature-dependent chemical production rates. Specifically, the chemical production changes are mainly responsible for the surface O3 increases over North America in response to the North Atlantic SST warming but exert a negative effect on South Asia in response to the North Indian SST warming. Decreases in the convective transport of O3 to the surface associated with North Indian warming are significant over South Asia and exert a negative impact on surface O3 concentrations. Advective transport has a positive effect on surface O3 in southern China in the Pacific-W case.

We further show that air temperature is an important factor controlling the surface O3 responses to SST anomalies. Reductions in the surface O3 chemical production in South Asia associated with North Indian SST warming can be explained by the corresponding SST-induced decreases in ground-level air temperature and solar radiation. Meanwhile, the widespread increase in air temperature associated with basin-scale SST warming is more likely to promote O3 production over other highly polluted regions.

Conversely, SST increases at low latitudes over different oceans enhance deep convection in summer, which promotes convergence at the surface, as well as upward motions at low latitudes. The corresponding surface pressure anomalies centered over the east coast of East Asia associated with the North Pacific warming and over the Arabian Sea associated with the North Indian warming tend to increase the surface O3 above through exchanges with the surrounding highly polluted air. The basin-scale SST increases in the Northern Hemisphere reduce the tropospheric temperature gradient at midlatitudes that restrains vertical transport of O3 over continents and weakens the westerlies at lower midlatitudes. The response of the CO tracer also suggests that these factors may jointly exert a negative effect on the intercontinental transport of O3.

This study highlights the sensitivity of O3 evolution to basin-wide SST changes in the Northern Hemisphere and identifies the key chemical or dynamical factors that control this evolution. Idealized and spatially uniform SST anomalies are used to explore the general mechanisms governing SST–O3 relationships. We find that the SST changes over tropical regions exert considerable impacts on surface O3 levels. The increase in tropical SST over different ocean basins enhances deep convection, which further triggers large-scale subsidence over nearby and remote regions. These enhanced convective activities also tend to release more latent heat over the upper troposphere and significantly increase the air temperature there. These processes influence large-scale circulation patterns and lead to opposite surface O3 responses over upwind and downwind regions related to a specific ocean basin. This finding provides valuable implications for the potential surface O3 change in response to future warming or cooling of individual oceans.

Additionally, the sensitivity tests with 1 ∘C SST warming and cooling superimposed onto all three ocean basins further show in general that the SST forcing on O3 distribution is geographically additive. A number of studies have used the decomposed SST anomalies for different regions to identify their relevant roles in a particular climate response (e.g., Sutton and Hodson, 2005; Camargo et al., 2013; Ueda et al., 2015). A linear assumption that the influence of large-scale SST anomaly pattern on the atmosphere can be generally constructed by the linear combination of the influences of individual SST patches has been verified by previous studies, especially for the tropical regions where the signal-to-noise ratio is higher (e.g., Fan et al., 2016; Seager and Henderson, 2016). Therefore, our study also helps to understand the roles different ocean basins in the Northern Hemisphere play in modulating surface O3 distributions in a global-wide SST warming condition associated with climate change.

Overall, this study may guide the management of regional O3 pollution by considering the influence of specific SST variability. However, cautions should be taken in interpreting our results in the real world since observed surface O3 variabilities are induced by various factors including O3 precursor emissions and atmospheric conditions. Realistic SST anomalies over different oceans are more complicated (usually not uniformly distributed) and often intercorrelated with each other (Fan et al., 2016). They may exert joint effects on modulating surface O3 distributions. To provide more precise understanding about the SST–O3 relationship over a specific region, additional sensitivity tests regarding smaller patches of SST variability are necessary.