13.2 CAN THE CCM3 GLOBAL CLIMATE MODEL ACCURATELY SIMULATE THE ARCTIC OSCILLATION? Raymondwhich D. Mooring may be recovered using tropospheric Georgia Institute of Technology, Atlanta, Georgia data or a combination of tropospheric and ABSTRACT The Arctic Oscillation (AO) is the stratospheric data (Baldwin and Dunkerton, leading mode of mean sea level pressure from 1999). The AO is robust (meaning it is 20N poleward. This robust phenomenon is insensitive to the details of the calculations annular in nature and manifests itself on many performed on the data used to identify it different time scales ranging from multi- (Baldwin and Dunkerton, 2001)) and is best decadal to interannual. After filtering summarized as having one center of action simulated sea-level pressure and surface over the Arctic region, displaced toward temperature datasets, and an observed AO Greenland, and an opposing ring at index, spectral analysis indicates that the first midlatitudes with prominent features over both PCA of the pressure time series correlates the Atlantic and Pacific oceans (Ambaum et with the AO index. The second PCA of al., 2001; Baldwin and Dunkerton, 1999; surface temperature also correlates with the Baldwin and Dunkerton, 2001; Deser, 2000). AO index at several frequencies. This study This annular AO is a result of internal determines that the low frequency (defined as dynamical feedbacks with the climate system, signals with 8-month periods or longer) AO and as such can show a large response to signal can be accurately simulated in the rather modest external forcings (Hartman et CCM3 fields of sea level pressure and surface al., 2000). Hartmann et al. (2000) and temperature. Thompson et al. (2000) suggest that since atmospheric climate models can simulate the INTRODUCTION Thompson and Wallace observed structure, amplitude, and time series (1998) defined the first EOF of the mean sea- of the tropospheric AO by specifying the level pressure (SLP) from 20°N poleward to be atmospheric composition and boundary the Arctic Oscillation (AO). The AO is a well- conditions, the AO itself is free (meaning that it defined naturally occurring mode of variability, will occur in the absence of any external which may be recovered using tropospheric forcing). This also suggests that the AO is a data or a combination of tropospheric and Corresponding author address: Raymond D. Mooring, Georgia Institute of Technology, 1 School of Earth and Atmospheric Sciences, Atlanta, GA 30332-0340; email: [email protected] 13.2 real physical structure and not just a of action than any coordinated behavior of the consequence of the EOF analysis used to Atlantic and Pacific centers of action (Deser, define it. 2000). The NAO, usually represented by sea The AO is hemispheric in nature level pressure anomalies of opposite sign (Thompson and Wallace, 1998) and between the Icelandic low and the tropical transcends many different time scales North Atlantic (Barnett, 1985), is a response to (Hartmann et al., 2000; Ambaum et al. 2001). regional forcing over the North Atlantic. Wallace (2000) has shown that the AO is Because of its emphasis on atmosphere- observable on intraseasonal and interannual ocean interaction, it is suggested that the AO time scales while Hurrell (1995) explains that is best studied on interannual time scales and quasistationary planetary waves in the longer. But because it transcends geographic atmosphere produce spatially coherent large and time scale classifications, it is useful in patterns of anomalies in local surface predicting climate and in studying polar, variables on interannual and longer time stratospheric, and atmospheric dynamics scales. Thompson et al. (2000) suggest that (Wallace, 2000). Some researchers, like the AO is prominent over a wide range of Deser (2000) and Higgins et al. (2000), frequencies in all seasons because its zonal suggest that the AO encompasses the NAO. symmetry doesn’t require local sources and Deser (2000) asserts that the leading EOF sinks to counteract local tendencies induced includes the leading EOF of each of its by advection. The AO is evident throughout subdomains. Because the AO is a the year in the troposphere but its amplitude hemispheric phenomenon and the NAO is and meridional scales are somewhat larger primarily a North Atlantic phenomenon, this during the cold season (Thompson and suggests that AO does indeed encompass the Wallace, 2000). NAO. In fact, the AO is nearly Not to be confused with the well- indistinguishable from the NAO in the Atlantic studied North Atlantic Oscillation (NAO), the sector (0.95 temporal correlation) in monthly annular character of the AO is more of a data (Deser, 2000). Hurrell (1995) found a reflection of the dominance of its Arctic center correlation between the first mode of SLP and 2 13.2 a NAO index of 0.91. Ambaum et al. (2001) and Wallace (1998) even found the AO signal explains that the NAO and AO are highly in springtime SAT. correlated because of their overlap in the The AO is not confined to a zonal or Atlantic sector. However, the NAO is worthy meridional propagation, but can propagate to be studied in its own right. It is the leading vertically. Thompson and Wallace (1998) regional EOF of mean sea level pressure suggest that the AO is important because of (Ambaum et al., 2001; Qian et al., 2000) in the the structural resemblance to the dominant Atlantic and the second mode of sea level mode of circulation variability in the lower pressure in a complex EOF (Barnett, 1985). stratosphere. Nikulin and Repinskaya (2001) The AO signature is found not only in found the second EOF of monthly winter total SLP, but in other tropospheric variables as ozone anomalies in midlatitude Northern well. Thompson et al. (2000) found an AO Hemisphere to be associated with the AO. contribution in Northern Hemisphere Baldwin and Dunkerton (1999) found that the wintertime trends in surface air temperature correlation between an AO index and zonal- (SAT), precipitation, total column ozone, and mean temperature exceeds 0.9 at 150 hPa tropopause height, all of which are well and 80°N, but stratospheric AO defined. For instance, Thompson and Wallace teleconnections only seem to exist in the (2000) found that the leading mode of month- active season (Thompson and Wallace, 2000). to-month variability in geopotential height is Hartmann et al. (2000) indicated that much fundamentally zonally symmetric. Enfield and theoretical and observational evidence exists Mestas-Nunez (1999) suggested that the third to support the notion that the troposphere can mode of the detrended non-ENSO complex drive stratospheric variability, but suggest that EOF in sea surface temperature (SST) wave propagation, potential vorticity induction, anomaly is the AO. Likewise, Yasunaka and and mass redistribution on the stratosphere Hanawa (2002) found the second EOF of SST can all drive the troposphere dynamically. to have an elongated zonal signal in both the Baldwin and Dunkerton (1999, 2001) also North Atlantic and North Pacific. This found a downward propagation through the signature is very similar to the AO. Thompson tropopause of the AO signal in low-pass 3 13.2 filtered (but not unfiltered) data. Baldwin and in the analysis. The data was converted to Dunkerton (1999) suggest that the large anomalies by subtracting the grid-averaged stratospheric anomalies are precursors to 20-year climatology from 1970-1989. changes in tropospheric weather patterns. In An EOF analysis was performed on fact, Baldwin and Dunkerton (2001) found AO the anomaly matrices and the leading three surface pressure to lag middle stratospheric spatial eigenmodes were recorded. The circulation by about sixty days. leading mode of the sea-level pressure (1SLP) The purpose of this study is to and the second leading mode of the surface determine if the CCM3 global climate model temperature (2TS) was plotted to determine can simulate the AO in the monthly mean SLP the stationary pattern of the AO. These and surface temperature (TS) fields. An patterns were described visually and annual cycle version of the original CCM with compared to the description of the AO found in no anomalous boundary conditions isolated the literature to see if any preliminary the NAO signal (Barnett, 1985). The model conclusions could be found to attest to the used in this study was forced by a global SST model’s successful simulation of the AO. dataset. In 1999, Robertson et al. reported The principal components of the that prescribing the SST variability over the temporal eigenmodes were filtered twice using North Atlantic in accordance with observations the low-pass Hanning Window. The monthly in a global climate model simulation AO Index (created by projecting the daily 1000 substantially increases the interannual hPa height anomalies poleward of 20°N onto variability of the AO. the loading pattern of the AO) of the Climate Prediction Center was used and filtered using DATA AND METHODS The simulated data the same filtering process. used in this analysis is from the CCM3 global Squared-coherence values were climate model. The twenty-year period, calculated between the AO index and 1SLP. 1970-1989, is considered in the study. The The 1SLP and AO are significantly correlated values of SLP and TS are used. The monthly at a given frequency when the squared- mean values were reported on a 2.8° x 2.8° coherence value at that frequency is greater grid. Only data from 20° N poleward was used 4 13.2 than the 90% confidence level (0.25). The decadal time scale because my record length analysis was limited to phases ranging from contains only one cycle of this signal. This approximately 8 months to 10 years. No method was repeated with the 2TS time conclusions were made about the squared- series. Conclusions were drawn pertaining to coherence between the AO and 1SLP (or the ability of the CCM3 to simulate the AO between the AO and 2TS) on the multi- signal.