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1 Intensification of Northern Hemisphere Subtropical Highs in a Warming Climate Wenhong Li, Laifang Li, Mingfang Ting, and Yimin Liu 1. Data and Methods The data used in this study consists of the atmospheric circulation field from the 40-year European Centre for Medium-Range Weather Forecasts (ECMWF) Re-Analysis (ERA-40) data set from The ERA-40 reanalysis is being used in the study for the following reasons: first, the relatively longer period of data availability of the ERA-40 reanalysis ( ) allows us to better study the changes of the subtropical highs; second, ERA-40 datasets reasonably represent the surface location of the centers of the subtropical highs when compared with real surface observations (ICOADS); third, the ERA-40 reanalysis does not show substantial discrepancies from NCEP/NCAR and other higher-resolution reanalysis including JRA ( ) and MERRA ( ) in terms of the climatological intensity of the subtropical highs over overlapping periods. The 925hPa stream function field was chosen to characterize the summer subtropical highs in this study. The intensities of the anticyclones are defined as the zonally asymmetric component of stream function. Summer seasonal means are obtained by averaging over the months of June, July, and August (JJA) in the Northern Hemisphere. Historically, there are two methods to define the intensity of subtropical highs over oceans: the maximum stream function 2, and the domain averaged stream function over the climatological subtropical high regions 3. The domains in the study are (20-40 N, E) for the NPSH, and (20-35 N W) for the NASH, respectively. Similar trends in the NATURE GEOSCIENCE 1

2 intensities of the subtropical highs have been found using the above two methods; also see Figures 1 and 2 in the main text. We have examined the 925hPa stream function field from the coupled ocean-atmospheric general circulation models for the IPCC AR4 that is available at the Program for Climate Model Diagnosis and Intercomparison (PCMDI). Three sets of simulations are considered in the study: pre-industrial runs (PICNTRL), 20 th century forced runs (20C3M), and the 21 st century simulations under the emission scenario A1B (A1B). There are 23 models in the pre-industrial controlled run, the 20 th century forced runs, and the 21 st century simulations. The three-dimensional atmospheric diabatic heating rate is calculated as a residual from the thermodynamic energy equation 4-6 using monthly data, then averaged over the months of JJA in the Northern Hemisphere. Total column diabatic heating consists of four heating components, sensible heating (SH), longwave radiation (LO), shortwave radiation (SW), and condensational heating (CO). SH in Fig. 3 of the main text is obtained from surface sensible heat flux from the models; CO is converted from precipitation rate, H L Pr, where 6 1 L J kg and Pr is the precipitation rate from the models. LO and SW are calculated as follows: LO= upwelling longwave radiation surface - downwelling longwave radiation surface -outgoing longwave radiation SW= TOA incident shortwave radiation TOA -TOA reflected shortwave radiation TOA - downwelling shortwave radiation surface + upwelling shortwave radiation surface 2 NATURE GEOSCIENCE

3 SUPPLEMENTARY INFORMATION In the summer subtropics (15 N-45 N), sensible heating exceeding 50 Wm -2 is mainly observed in western North America between 90 W-120 W, Europe, western Asia and Africa, thus the above regions are defined as western continents 2,7. Similarly, condensation heating ( 50 Wm -2 ) is located in the eastern North America (east of 90 W), Indian and East Asian summer monsoon regions; those regions are mentioned as eastern continents in the paper. Longwave radiative cooling ( -210 Wm -2 ) are dominant over the eastern Atlantic and Pacific between 20 W-60 W, 120 W-150 W, respectively; these regions are referred to as eastern oceans following our previous studies 2,7. Thus, vertical distribution of the total diabatic heating in Fig. 4 of the main text represents mainly the profile of SH, LO, and CO over the western continents, eastern oceans, and eastern continents, respectively. 2. Comparison of the heating field obtained from the CMIP3 multi-model ensemble (MME) mean to that from the NCEP/DOE reanalysis (NCEP 2) Summertime total column diabatic heating patterns obtained from the CMIP3 MME mean in the 20 th century are evaluated by comparing to patterns from the NCEP2 reanalysis (Fig. S1). Figure S1 shows a reasonable agreement of the total heating pattern in the subtropics between the CMIP3 MME mean and that obtained from the NCEP2 reanalysis in boreal summer (JJA). However, total column heating over the eastern North Pacific and North Atlantic obtained NATURE GEOSCIENCE 3

4 from the CMIP3 models (Fig. S1b) is about 30% weaker than that from the NCEP2 (Fig. S1a), indicating a weaker modeled longwave radiative cooling over the regions. The atmospheric positive heating areas from the CMIP3 models are relatively smaller over the warm pool region, as well as the Asia/India continents, than those from the NCEP2 reanalysis, although the magnitude and area of the heating is comparable over western North America. The relatively weaker longwave radiative cooling over the eastern North Atlantic and heating over western Euro-Asia in the CMIP3 MME mean simulate a weaker NASH in the 20 th century (Fig. 1 in the main text) according to the arguments of Hoskins (1991) 8, Wu and Liu (2003) 7 and Miyasaka and Nakamura (2005) 9. Figure S1 also shows that the CMIP3 models reasonably capture the dominant heating component in the North Hemisphere (Fig. S1c). The MME mean heating field indicates a prominent longwave radiative cooling over the eastern North Pacific and eastern North Atlantic; sensible heating over western North America, Africa and western Asia; and condensation heating over eastern North America, as well as the Indian and East Asian summer monsoon regions, and the eastern Pacific, similar to those in Wu and Liu (2003) Idealized General Circulation Model 4 NATURE GEOSCIENCE

5 SUPPLEMENTARY INFORMATION The idealized general circulation model (GCM) used in this study is a time-dependent, global, hydrostatic, primitive equation model derived from Hoskins and Simmons (1975) 10. It is nonlinear, spectral in the horizontal, and uses finite differences in σ coordinates in the vertical. The horizontal resolution is triangular truncation 42, and there are 15 levels in the vertical. The model is discussed in detail by Hoskins and Rodwell (1995) 11 and has been updated recently by Dr. Mike Blackburn in the Department of Meteorology, the University of Reading ( Both the upper and lower-level subtropical anticyclone flows were well simulated in summer. The model uses a linear damping to the anomaly of vorticity and divergence in the lowest two levels, sigma=0.967, 0.887, on timescales of 4 and 1.5 days, respectively. Newtonian cooling is applied to the anomaly of temperature at all levels, generally with a timescale of 25 days but decreasing gradually to 8 days at top level and 5.0 days at bottom level for historical reasons. These values are the same as those of Liu et al. (2007) 12, who studied the formation and oscillation of the subtropical anticyclone at the upper troposphere. In addition to a bi-harmonic horizontal diffusion with damping, a vertical diffusion is also included to guarantee long integrations. It is similar to that used by Ambrizzi and Hoskins (1997) 13, with a coefficient of 1 m 2 s -1, but is applied to the anomaly of the vorticity, divergence and temperature fields in spectral NATURE GEOSCIENCE 5

6 space. Without vertical diffusion, the flow at upper troposphere near equator tends to show gridscale structures. The summer (JJA) basic state climatology of the European Centre for Medium-Range Weather Forecasts (ECMWF) of is maintained in the idealized GCM through 3-D restoration terms. The forcing is derived as the MME heating changes from the 20 th century ( ) to the 21 st century ( ) from the CMIP3 models and is constantly applied in the model during the integration. The resulting stream function field (contour) is then plotted in Figure 5 in the main text compared to the climatology (shaded). The intensity change of the Northern Hemisphere subtropical highs is thus demonstrated to be predominately caused by diabatic heating change in boreal summer from the 20 th to the 21 st century. 6 NATURE GEOSCIENCE

7 SUPPLEMENTARY INFORMATION Figure S1. Total column diabatic heating obtained from the NCEP2 reanalysis (upper panel) and the CMIP3 MME mean (middle panel), as well as main local heating (bottom panel) from the CMIP3 models in boreal summer (JJA). (Unit: W/m 2 ) NATURE GEOSCIENCE 7

8 References 1 Uppala, S. M. et al. The ERA-40 re-analysis. Quart. J. Roy. Meteor. Soc. 131, (2005). 2 Liu, Y. & Wu, G. Progress in the study on the formation of the summertime subtropical anticyclone. Adv. Atmos. Sci. 21, (2004). 3 Davis, R. E., Hayden, B. P., Gay, D. A., Phillips, W. L. & Jones, G. V. The North Atlantic Subtropical anticyclone. J. Climate 10, (1997). 4 Chan, S. C. & Nigam, S. Residual Diagnosis of Diabatic Heating from ERA-40 and NCEP Reanalyses: Intercomparisons with TRMM. J. Climate 22, (2009). 5 Hoskins, B. J. et al. Diagnostics of the global atmospheric circulation. based on ECMWF analysis Department of Meteorology, University of Reading, Compiled as part of the U. K. Universities Global Atmospheric Modelling Project, WMO/TD-NO. 326, 217 pp. (1989). 6 Nigam, S. On the dynamical basis for the Asian summer monsoon rainfall - El Nino relationship. J. Climate 7, (1994). 7 Wu, G. & Liu, Y. Summertime quadruplet heating pattern in the subtropics and the associated atmospheric circulation. Geophys. Res. Lett. 30, 1201, (2003). 8 Hoskins, B. J. Towards a PV-theta view of the general circulation. Tellus. Ser. AB 43, (1991). 9 Miyasaka, T. & Nakamura, H. Structure and formation mechanisms of the Northern hemisphere summertime subtropical highs. J. Climate 18, (2005). 10 Hoskins, B. J. & Simmons, A. J. A multi-layer model and the semi-implicit method. Quart. J. Roy. Meteor. Soc. 101, (1975). 11 Hoskins, B. J. & Rodwell, M. J. A model of the Asian summer monsoon, Part I: The Global Scale. J. Atmos. Sci. 52, (1995). 12 Liu, Y., Hoskins, B. J. & Blackburn, M. Impacts of the Tibetan topography and heating on the summer flow over Asia. J. Meteor. Soc. Japan 85B, 1-19 (2007). 13 Ambrizzi, T. & Hoskins, B. J. Stationary Rossby wave propagation in a baroclinic atmosphere. Quart. J. Roy. Meteor. Soc. 123, (1997). 8 NATURE GEOSCIENCE