Drivers of multidecadal variability in JJA ozone concentrations in the eastern United States Lu Shen, Loretta J. Mickley School of Engineering and Applied Sciences, Harvard University Acknowledgments: Jacob group and AQAST and NIH funding

Positive Atlantic Multi-decadal Oscillation (AMO) and its spatial pattern in the sea surface temperature AMO is defined as the SST averaged over the extra-tropical North Atlantic minus the regression on global mean temperature K 2

The JJA anomalous surface air temperature in the eastern US and AMO from 1895 to 2013 Averaged ΔT between warm and cold phases of AMO is 0.79 K over the eastern US. Temperature AMO 3 Data: NCDC, NOAA, linear trend is removed

r 4 A warm AMO means a drier and warmer summer over US ( ) r Data: NCDC, NOAA AMO, 11-year moving average is used Our results confirm earlier studies, e.g. Sutton et al. [2005], that a warm AMO will lead to decreased precipitation and increased surface air temperature in US. Negative PDSI means drought Correlation of AMO and JJA temperatures Correlation of AMO and PDSI drought index Significant at 0.05 level

Warm AMO reduces 500 hPa zonal wind speed and shifts polar jet polarward. From the cold to warm phase of AMO, polar jet in the northeast US shifts north by ~2.3°. CMIP5 models predict only ~1° poleward shift from 2000 to 2100 due to climate change. m s -1 Δ 500 hPa zonal wind speed : warm – cold AMO Mean JJA Jet latitude in Warm AMO Cold AMO Data: 20 Century Reanalysis 5

Stagnation definition [Wang, 1999; Horton,2012 and 2014]: (1)daily mean surface wind speed is < 3.2 ms -1 (2)daily mean 500 mb wind speed is < 13 ms -1 (3)daily mean precipitation is <1 mm. Δ JJA Stagnation days: warm – cold AMO Days Warm AMO leads to increased JJA stagnation in the eastern United States. Correlation of AMO and stagnation days (JJA, 11-year moving average) r 2.6 Days in East Data: 20 Century Reanalysis 6

Present MDA8 ozone (AQS, ) Present climate (NCEP, ) (1) Local meteorology: T, RH, NS and EW wind. (2) Synoptic circulations: SVD predictors calculated with T, RH, and 1000 hPa, 850 hPa and 500 hPa winds. Step 1: Build statistical models using observations. Step 2: Apply model to 20th Century Reanalysis. Calculate the ozone concentrations using the meteorology in 20 Century Reanalysis Assumption: The anthropogenic emissions and global background ozone remain constant. 20th Century Reanalysis: (1)Only observations of SSTs and SLPs are used. (2)Spatial resolution: 200km×200km More details about this method are in my poster this afternoon Step 3: Calculate MDA8 ozone assuming constant emissions for

Cross-validated skills expressed in the correlation of determinant (R 2 ) between observed and predicted monthly PM 2.5 using Consideration of synoptic circulation improves the predicative capability of statistical ozone model Cross-validated R 2 : obs vs. prediction R2R2 R 2 =50.1% Data: NCEP, AQS, (1) local meteorology (2) Synoptic circulations (3) Global background Influence of meteorology at different spatial scales To characterize the influence of synoptic circulations, we have developed a new method based on the SVD of spatial correlations between ozone and the meteorological variables in the surrounding regions. Consideration of synoptic meteorology improves the R 2 from 37% to 50%. 8

Influence of AMO on surface ozone in the eastern US Estimated MDA8 ozone in the eastern US from 1871 to 2012 (11 year moving average) 20th Century Reanalysis T NCDC Temperature MDA8 ozone Note that we assume constant emission of ozone precursors. The AMO drives a multidecadal variability in JJA ozone concentrations in the eastern United States. The amplitude of the ozone oscillation is ~2-3 ppbv. 9

A cold phase of AMO is expected in the coming decades. Constructed AMO indices using different definitions from 1861 to 2013 The coming cold phase of AMO should partly compensate the effect of global warming in the US, which relieves the pressure in ozone air quality management. We are now testing the AMO effect using the GISS/GEOS-Chem model with specified SSTs. AMO index 11-year moving average 10

Conclusions / Implications The AMO appears to drive multidecadal variability in mean JJA MDA8 ozone in the eastern US, with anomalies as great as 3 ppbv. A complete picture of ozone air quality management in coming decades requires the consideration of AMO influence. Present GCMs cannot capture AMO well. 11