A possible cause of the AO polarity reversal from winter to summer in 2010 and its relation to hemispheric extreme summer weather
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In 2010, the Northern Hemisphere, in particular Russia and Japan, experienced an abnormally hot summer characterized by record-breaking warm temperatures and associated with a strongly positive Arctic Oscillation (AO), that is, low pressure in the Arctic and high pressure in the midlatitudes. In contrast, the AO index the previous winter and spring (2009/2010) was record-breaking negative. The AO polarity reversal that began in summer 2010 can explain the abnormally hot summer. The winter sea surface temperatures (SST) in the North Atlantic Ocean showed a tripolar anomaly pattern—warm SST anomalies over the tropics and high latitudes and cold SST anomalies over the midlatitudes—under the influence of the negative AO. The warm SST anomalies continued into summer 2010 because of the large oceanic heat capacity. A model simulation strongly suggested that the AO-related summertime North Atlantic oceanic warm temperature anomalies remotely caused blocking highs to form over Europe, which amplified the positive summertime AO. Thus, a possible cause of the AO polarity reversal might be the “memory” of the negative winter AO in the North Atlantic Ocean, suggesting an interseasonal linkage of the AO in which the oceanic memory of a wintertime negative AO induces a positive AO in the following summer. Understanding of this interseasonal linkage may aid in the long-term prediction of such abnormal summer events.
KeywordsAO Hot summer 2010 NAM Atlantic SST Blocking
In Japan, summer 2010 was the warmest in about 100 years of countrywide measurement records. Moreover, summer 2010 was abnormally hot on a planetary scale. For example, Europe, especially Eastern Europe and western Russia, experienced record-breaking hot temperatures, attributed to strong atmospheric blocking over the Euro-Russian region from late June to early August (Matsueda 2011). Additionally, Barriopedro et al. (2011) showed that the spatial extent of the record-breaking temperatures of summer 2010 exceeded the area affected by the previous hottest summer of 2003. Heat anomalies covered almost the entire Eurasian continent in 2010. In contrast, in winter 2009/2010, just a half-year earlier, the continent suffered from anomalously cold weather associated with a record-breaking negative Arctic Oscillation (AO), which is characterized by positive sea level pressure anomalies over the Arctic and negative pressure anomalies over the midlatitudes (Thompson and Wallace 2000). Moreover, in the same winter, a record-breaking negative North Atlantic Oscillation (NAO) caused several severe cold spells over northern and western Europe (Cattiaux et al. 2010). In fact, the strongest negative AO index of the past 30 years was observed in December 2009 (Wang and Chen 2010). This drastic reversal from a record-breaking cold winter to a record-breaking hot summer is preserved in our memory. What if, however, that memory could be preserved not only in our minds but also somewhere on the earth? In particular, might a memory of the strongly negative wintertime 2009/2010 AO have been preserved in the ocean, because of its large thermal heat capacity, which could then be recalled the following summer?
The winter-to-summer evolution of the AO index during 2009/2010 can be summarized as follows: a strongly negative wintertime AO index continued until May, after which it abruptly changed, becoming strongly positive in July and continuing so until the beginning of August. Details of the AO evolution will be described in the following sections. Ogi et al. (2005) pointed out that a strongly positive summertime AO is associated with occurrences of blocking anticyclones, which contributed to the abnormally hot European summer. Trigo et al. (2005) also reported that a blocking anticyclone caused the anomalous hot summer of 2003. The blocking anticyclone over Europe in summer 2003 was shown to be part of a planetary-scale wave train, extending from Europe to eastern Eurasia (Orsolini and Nikulin 2006). The abrupt change of the AO index from strongly negative to strongly positive in 2010 thus corresponded to the change from the abnormally cold winter of 2009/2010 to the abnormally hot summer of 2010, which shows that the AO index is a good indicator of abnormal weather on a planetary-scale, and that extra-seasonal prediction of the AO is a key to long-term forecasting. In this study, we therefore aimed to examine the cause of the 2010 change in the AO from strongly negative to strongly positive.
2 Data and method
Ogi et al. (2004) and Tachibana et al. (2010) demonstrated that in winter, but not in summer, the SV NAM accords well with the AO defined by Thompson and Wallace (2000) and used by the Climate Prediction Center of the U.S. National Oceanic and Atmospheric Administration (NOAA/CPC). Ogi et al. (2005) and Tachibana et al. (2010) also demonstrated that the SV NAM successfully captures anomalous summertime weather conditions associated with blocking anticyclones, such as the hot summer in Europe in 2003, whereas the original AO of Thompson and Wallace (2000), mainly reflects atmospheric variabilities in winter and cannot capture such a hot summer. Therefore, Ogi et al. (2005) redefined the summertime SV NAM as the summer AO. In this study, therefore, we adopted the SV NAM index defined by Ogi et al. (2004) as the AO index, and all references to the AO index in this study mean the SV NAM index.
We used daily data of large-scale atmospheric fields from the National Centers for Environmental Prediction/National Center for Atmospheric Research (NCEP/NCAR) reanalysis data set (Kalnay et al. 1996) to calculate the climatology and anomalies of the meteorological field (i.e., temperature, geopotential height, and wind velocity). Monthly means of sea surface temperature (SST) data are from the NOAA_ERSST_V3 data set, provided by NOAA/OAR/ESRL PSD (http://www.esrl.noaa.gov/psd/) (Smith et al. 2008; Xue et al. 2003). We used monthly mean latent and sensible heat flux data of the Japan 25 year Reanalysis (JRA-25) and the JMA Climate Data Assimilation System to examine the atmosphere–ocean interaction (Onogi et al. 2007). Daily and monthly means of outgoing longwave radiation (OLR) are interpolated OLR data provided by NOAA/OAR/ESRL PSD (Liebmann and Smith 1996). Anomaly fields of individual variables are relative to the multi-year mean climatology from 1979 to 2010 for each month.
3 Strongly positive AO days
4 Oceanic footprint left by the previous winter’s negative AO
5 Steady responses to the oceanic forcing in the Atlantic region
Taking together the results presented in Sects. 3, 4, and 5, we suggest that an oceanic memory of the strongly negative wintertime AO may have influenced the strongly positive summertime AO. A negative wintertime NAO would cause warm SST anomalies in high- and low-latitude regions of the Atlantic, as suggested by Xie and Tanimoto (1998) and Tanimoto and Xie (2002). Because the horizontal structures of the NAO and the AO in the Atlantic sector in winter 2009/2010 are similar (See Fig. 4), the strongly negative wintertime AO would maintain the warm SST anomaly in this region. The downward latent and sensible heat flux anomaly over the high latitudes and the tropical Atlantic (Fig. 3) in winter and spring indicates that anomalous heating of the ocean by the atmosphere occurred from winter to spring during the strongly negative phase of the AO in winter 2009/2010. Because the thermal heat capacity of the ocean is large, the sea surface stored this warmth (i.e., the SST anomaly remained positive) into the following summer.
In May and June, the heat flux anomaly changed from downward to upward in the tropics (see Fig. 3), and in July and August, the center of the upward anomaly moved westward. The area of the upward heat flux anomaly coincided with the area of the warm SST anomaly from May to August. The warm SST during the summer following the strongly negative wintertime AO therefore heated the atmosphere, activating atmospheric convection. The OLR anomalies also indicate high convective activity in the tropical Atlantic region (Fig. 5), suggesting a remote influence of the Atlantic SST upon the occurrence of an anticyclone over Europe. This Atlantic SST influence has been pointed out by many studies (e.g., Cassou et al. 2005; García-Serrano et al. 2008). García-Serrano et al. (2008) showed that a midlatitude anticyclonic anomaly related to tropical convection can excite a Rossby wave. Our numerical experiment using the linear model showed that the atmospheric response to the tripolar SST pattern clearly resulted in an anomalous height and wind pattern that caused a blocking high over Europe (Figs. 6, 7), however, the modeled geopotential amplitude is weaker than the observations. This discrepancy is because a linear model cannot represent the dynamical instability due to, for example, wave–wave interaction. Therefore, the model indicates that although the oceanic memory in the Atlantic is a trigger, by itself it is insufficient to cause a blocking high to develop. Weak, positive OLR anomalies along the Gulf Stream were associated with anticyclonic surface winds on strongly positive AO days (Fig. 5). The observed wave activity flux (Fig. 2a) also seems to emanate from that region. This midlatitude signature implies that strengthening of the positive geopotential anomalies over Europe was associated with the Atlantic tripolar SST anomaly.
The positive geopotential anomaly in the area of the polar jet stream caused eastward propagation of Rossby waves, and the unusual amplification of Rossby waves might have led to the formation of blocking anticyclones. These findings are in agreement with previous studies. For example, Tachibana et al. (2010) reported that a blocking anticyclone over the Atlantic sector that induces blocking over the Russian Far East is associated with a long-lasting, strongly positive AO caused by wave–mean flow interactions. As a result of these interactions, the positive AO pressure pattern can continue for a long time. In addition, Orsolini and Nikulin (2006) pointed out that the blocking anticyclone over Europe in summer 2003 was part of a wave train extending from Europe to eastern Eurasia. The linear model did not simulate an anticyclonic anomaly in the Russian Far East. To simulate the influence of an anomalous wintertime negative AO on an anomalous positive AO in the following summer due to a long-lasting oceanic memory, an atmosphere–ocean coupled high-resolution model simulation is needed. We reserve this experiment for future studies.
Of course, the set of processes introduced here is just one possible explanation for the formation of the strongly positive summer AO in 2010. For example, summertime SST anomalies in the Mediterranean Sea (Feudale and Shukla 2010) might simultaneously induce a strongly positive summer AO. Although the effect of the oceanic memory of a negative AO during the previous winter might be smaller than the effects of simultaneous events, the previous winter’s footprint may at least play a role in the reversal of the AO polarity from a strongly negative wintertime AO to a strongly positive summertime AO. If this reversal pattern recurs, it might be possible to predict the summer AO from the wintertime AO. The more negative the winter AO anomaly is, the deeper the footprint left in the ocean would be, suggesting that a winter-to-summer reversal of the AO might occur only in years when the negative wintertime AO anomaly is large. In addition to an oceanic memory effect, other memory effects such as anomalous snow accumulation on the Eurasian continent or elsewhere in the Northern Hemisphere, as suggested by Ogi et al. (2003) and Barriopedro et al. (2006), may also contribute to the reversal of AO polarity. To test these possibilities, statistical analyses of multi-year data and simulation by a full coupled atmosphere–ocean–land global climate model are the next step.
We extend special thanks to V. A. Alexeef, M. Honda, H. E. Hori, and J. Inoue for their very helpful comments on this study. We also thank two anonymous reviewers for their valuable comments and suggestions to improve the quality of the paper. This study was supported by Grant-in-Aid for challenging Exploratory Research 22654055, and a part of this study was supported by “Green Network of Excellence” Program (GRENE Program) Arctic Climate Change Research Project.
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