The thermal state of the Bering Sea exhibits interdecadal variations, with distinct changes occurred in 1997–1998. After the unusual thermal condition of the Bering Sea in 1997–1998, we found that the recent climate variability (1999–2010) in the Bering Sea is closely related to Pacific basin-scale atmospheric and oceanic circulation patterns. Specifically, warming in the Bering and Chukchi Seas in this period involves sea ice reduction and stronger oceanic heat flux to the atmosphere in winter. The atmospheric response to the recent warming in the Bering and Chukchi Seas resembles the North Pacific Oscillation (NPO) pattern. Further analysis reveals that the recent climate variability in the Bering and Chukchi Seas has strong covariability with large-scale climate modes in the Pacific, that is, the North Pacific Gyre Oscillation and the central Pacific El Niño. In this study, physical connections among the recent climate variations in the Bering and Chukchi Seas, the NPO pattern and the Pacific large-scale climate patterns are investigated via cyclostationary empirical orthogonal function analysis. An additional model experiment using the National Center for Atmospheric Research Community Atmospheric Model, version 3, is conducted to support the robustness of the results.
KeywordsBering and Chukchi Seas North Pacific Oscillation North Pacific Gyre Oscillation Central Pacific El Niño
The Bering Sea, a northern extension of the North Pacific Ocean, is located between Russia and Alaska and is the third largest semi-enclosed sea in the world. The Bering Sea is connected to the Chukchi Sea, which is a marginal sea of the Arctic Ocean, through the Bering Strait. In a global sense, the Bering Sea acts as the Pacific gateway, through which the North Pacific and the Arctic Ocean exchange heat and water. The Bering Sea is one of the most productive marine resources in the world and provides nearly half of the US fisheries production (National Research Council 1996). For this reason, many oceanographers have tried to understand the variability of the marine ecosystem in the Bering Sea. In particular, its relationship with climate variability has long been the focus of attention (Grebmeier et al. 2006; Overland and Stabeno 2004; Hunt et al. 2002; Kruse 1998; Brodeur et al. 1999). Furthermore, the Bering Sea, as a marginal section of the North Pacific Ocean, is sensitive to Pacific large-scale climate phenomena such as the El Niño/Southern Oscillation (ENSO) (Niebauer 1988) and the Pacific Decadal Oscillation (PDO) (Hare and Mantua 2000; Overland et al. 1999). Understanding the oceanic and atmospheric variability over the Bering Sea, therefore, is essential from both ecological and climatological perspectives.
Atmospheric and oceanic parameters in the Bering Sea vary over a wide range of time scales, from interannual to multi-decadal scales, including trends or climate regime shifts. In particular, recent studies have noted that noteworthy changes in the Bering Sea have occurred recently. For example, the Bering Sea experienced marked warming and a reduction in sea ice during the last decade (Grebmeier et al. 2006; Hunt et al. 2002; Overland and Stabeno 2004; Stabeno et al. 2007). In particular, the striking warming events were concentrated in the years 2000–2005 (Overland et al. 2012). Prominent anomalies in atmospheric and oceanic conditions in the Bering Sea occurred during 1997 and 1998. The sea surface temperature (SST) in the Bering Sea increased up to 5–6 °C above the average for August and September in 1997, and SST continued to be approximately 2 °C higher than average through the summer of 1998. The biological conditions were also anomalous, including major cocolithophorid blooms, salmon returns far below predicted numbers, and the unusual presence of whales over the middle shelf (Hunt et al. 1999; Kruse 1998; Minobe 2002; Napp and Hunt 2001; Schumacher et al. 2003; Stabeno et al. 2001; Stockwell et al. 2001). It is likely that these changes in the Bering Sea are closely related to the state of the North Pacific climate system. Moreover, Minobe (2002) argued that the changes in the Bering Sea in the late-1990s are a part of the Pacific-scale atmospheric and oceanic change, which is regarded as a potential climatic regime shift. The linkage between the Bering Sea and the North Pacific large-scale climate variability was supposed to be established primarily through the atmosphere; in particular, the connection between the two regions seems to vary according to the strength and position of the Aleutian Low (Niebauer 1988; Overland et al. 1999; Stabeno et al. 2001).
Meanwhile, the large-scale climate variability in the North Pacific also seems to have undergone a significant change during the last decade. According to recent studies, the amplitude of North Pacific Gyre Oscillation (NPGO), which is characterized by a dipole-like SST pattern in the North Pacific became larger than the PDO amplitude during the recent decade (Bond et al. 2003; Di Lorenzo et al. 2008; Yeh et al. 2011). The NPGO appears to be driven by the atmospheric North Pacific Oscillation (NPO) (Rogers 1981), which is the second dominant mode of sea level pressure variability in the North Pacific (Ceballos et al. 2009; Di Lorenzo et al. 2010; Furtado et al. 2011). The NPO pattern consists of a meridional dipole in sea level pressure, with centers of action on both sides of approximately 50°N.
As noted above, the climate in the Bering Sea and the North Pacific seems to have experienced remarkable changes in the recent decade. It is necessary to contemplate whether the changes in different parts of the Pacific are physically and dynamically linked with each other or not. In this study, therefore, we sought to identify the pattern of the North Pacific variability that is associated with the major physical changes in the Bering Sea, focusing particularly on the recent decade. Our analysis substantiates that the Bering Sea oceanic variability from 1999 to 2010 (i.e., after the unusual conditions in 1997/1998) has a profound impact on atmospheric circulation in the North Pacific, which is characterized by the NPO-like pattern in winter. The accompanying oceanic circulation pattern in the North Pacific features the NPGO pattern, as expected. Furthermore, our analysis found that the recent warming in the Bering Sea had significant covariability with warming in the tropical central Pacific, that is central Pacific (CP) El Niño, which is clearly distinguishable from the canonical eastern Pacific (EP) El Niño (Ashok et al. 2007; Kao and Yu 2009; Kug et al. 2009; Lee and McPhaden 2010; Yeh et al. 2009).
The major analysis tools in the present study are the cyclostationary empirical orthogonal function (CSEOF) method and the regression analysis in CSEOF space. Additional model simulations using NCAR Community Atmospheric Model, version 3 (CAM3), were conducted to test the robustness of the result. The details of the analysis methods are presented in Sect. 2 along with a brief description of the data used in this study. In Sect. 3, recent change in the relationship between the Bering Sea and the North Pacific variability is investigated via CSEOF analysis. Then, the detailed physical conditions over the Bering Sea associated with the warming in 1999–2010 are identified in Sect. 4. The corresponding atmospheric conditions over the North Pacific are described in Sect. 5. Section 6 depicts the large-scale circulation patterns in the Pacific basin associated with the recent Bering Sea warming. Discussion and concluding remarks of this study follow in the last section.
2 Data and methods
Monthly mean oceanic and atmospheric datasets for the 31-year period from 1980 to 2010 were used in the present study. The monthly mean SST data were obtained from the Extended Reconstruction SST, version 3 (ERSST. v3) (Smith et al. 2008). The monthly mean sea ice concentration (SIC) data were acquired from the Hadley Centre sea ice and SST dataset, version 1 (HadISST1) (Rayner et al. 2003), which is archived at the UK Meteorological Office. The atmospheric data including geopotential heights, zonal and meridional winds, air temperatures and net surface energy fluxes (latent heat, sensible heat, and shortwave and longwave radiative fluxes) were taken from the NCEP/DOE AMIP-2 reanalysis (Kanamitsu et al. 2002).
Model experiments are conducted using CAM3, an atmospheric general circulation model developed by the National Center for Atmospheric Research (NCAR), to further assess the atmospheric response over the North Pacific to the changed physical conditions in the Bering and Chukchi Seas in recent decade. The physical and numerical methods used in CAM3 are documented in Collins et al. (2006) and the references therein.
3 Changes in the relationship between the Bering Sea and the North Pacific
The PC time series of the first CSEOF of the SSTA in the Bering and Chukchi Seas (black bar) and that of the North Pacific (blue line) are presented in Fig. 2. One notable feature in Fig. 2 is the remarkable difference in the relationship between the two PC time series before and after 1999. The two PC time series are significantly correlated (r = 0.74) with each other in 1999–2010, but the correlation is obscure (r = 0.04) in 1980–1998. This result implies that the link between the Bering and Chukchi Seas and the North Pacific SST variability during the recent decade (1999–2010) is clearly distinguishable from that in the previous period (1980–1998). The Bering Sea appears to be more closely connected to the North Pacific in terms of SST variations after it experienced the unusual physical conditions during 1997–1998.
4 Atmospheric and oceanic variability in the Bering Sea during 1999–2010
The superimposed shading in Fig. 5 represents the regressed net surface energy flux anomaly. A positive value defines an upward flux from the ocean to the atmosphere. As illustrated in Fig. 5, there is a strong spatial coherence between SIC and net surface flux anomalies, although the sign of correlation is reversed between warm (April–August) and cold (November–March) seasons. Negative SIC anomaly generally coincides with negative flux anomaly in the warm season, but with positive flux anomaly in the cold season. The magnitude of the upward flux in the cold season is much larger than the downward flux in the warm season.
In the Arctic and subarctic region including the Bering and Chukchi Seas, two of the most important sea ice related processes that affect air-sea interaction are the ice-albedo feedback and the oceanic heat transport (Screen and Simmonds 2010a, b). Melting sea ice exposes the open water that effectively absorbs incoming sunlight than the sea ice, which leads to an increase in surface air temperature; this so-called ice-albedo feedback can only be active in summer with sufficient insolation (Deser et al. 2000). Thus, the downward heat flux anomaly in the warm season in Fig. 5 may be attributed to the increased absorption of solar radiation due to ice-albedo feedback. The flux patterns in the warm season are also consistent with the Arctic Ocean being more efficient in absorbing atmospheric heat during summer (Screen and Simmonds 2010b).
In winter, on the other hand, the primary air–sea interaction mechanism is the oceanic heat transport, since the albedo effect is suppressed due to the low insolation (Screen and Simmonds 2010a, b). Screen and Simmonds (2010b) proposed two explanations for the relationship between SIC and net surface flux in winter season. As a direct response to reductions in winter sea ice cover, heat is released from the relatively warm ocean surface to the colder atmosphere above. As an indirect response, sea ice reduction in summer also facilitates increased heat transfer from the ocean to the atmosphere in winter; sea ice cover reduction in summer leads to heat storage in the ocean, which is released in winter. Indeed, Fig. 5 exhibits striking spatial coherence between the negative SIC and the upward flux anomalies in the cold season, which supports that melting sea ice facilitates increased oceanic heat loss. An implication is that sea ice plays a significant role in forcing the atmosphere above by regulating energy flux transfer.
5 North Pacific atmospheric circulation
5.1 Regression analysis
The accompanying air temperature anomalies, shown as shading in Fig. 7, display warm air temperature anomalies to the north of 50°N with centers in the Bering Sea region. The vertical sections of temperature (shading) and geopotential height anomalies (contour) averaged over the Bering Sea region (180°–140°W) are shown in Fig. 7. Warming over the Bering Sea is not confined to the lower troposphere but is also evident in the upper troposphere. In December, positive temperature anomalies over the Bering Sea region reach approximately 400 hPa and a baroclinic geopotential height structure with a nodal point near 600 hPa is observed. The linear model result of Hoskins and Karoly (1981) indicates that this baroclinic response is forced by diabatic heating in the lower troposphere associated with surface heat flux deriving from the imposed boundary forcing. In January, on the other hand, an equivalent barotropic structure is observed. The warm air temperature anomaly over the Bering Sea reaches a mature state in January, exhibiting amplification and expansion to near the tropopause. As a result, the thickness of the atmospheric layer increases further, and the atmospheric circulation adjusts to the equivalent barotropic structure. The adjustment process from an initial baroclinic structure to an equivalent barotropic one was substantiated in atmospheric general circulation models (Peng et al. 2003; Deser et al. 2007). The barotropic dipole structure in January is maintained throughout winter, although the anomaly center varies in location and amplitude. The meridional dipole structure is roughly characterized as an NPO-like pattern. Although the NPO pattern is conventionally known as an intrinsic mode of variability (e.g., Rogers 1981), an NPO-like pattern seems to appear also as a result of an atmospheric response to the recent thermal forcing in the Bering Sea.
5.2 Comparison with the CAM3 model simulation
6 Covariability of Bering Sea SST and Pacific large-scale circulation
Based on CSEOF analysis and the CAM3 model simulation, it is identified that the warming and sea ice reduction in the Bering and Chukchi Seas during the last decade are closely related to the atmospheric NPO-like pattern. The NPO pattern, a key atmospheric mode in the Pacific basin, is well known for its strong influence on weather patterns over Eurasia and North America, and in particular storm tracks, temperatures, and precipitation (Seager et al. 2005; Linkin and Nigam 2008). The NPO pattern, of course, exerts a strong influence over the Pacific basin; Di Lorenzo et al. (2008) found that the NPO serves as the atmospheric forcing pattern for the oceanic NPGO pattern. Previous studies also emphasized that the regional Bering Sea variability is closely related to the state of the entire Pacific climate system (Niebauer 1988; Minobe 2002).
As noted above, the North Pacific atmospheric response to the recent warming in the Bering and Chukchi Seas during winter is the NPO-like pattern (Fig. 10a). The corresponding SSTA over the North Pacific bears a strong resemblance to that of the NPGO pattern; the positive SSTA in the eastern North Pacific extends across the Pacific to the western Bering Sea and the Sea of Okhotsk resulting in meridional gradient of SSTA along approximately 40°N. The physical relationship between the NPO and the NPGO SSTA pattern presented in Fig. 10a can be explained by the air-sea interaction. As illustrated in Fig. 7b, the southerly wind anomaly over the eastern North Pacific advects warm and moist air, leading to warm air temperature anomaly in this region. Note that this warm air temperature anomaly extended southward in the eastern North Pacific does not yet appear in December, since the NPO pattern is not fully established (Fig. 7a). The associated net surface flux anomaly in the eastern North Pacific exhibits downward flux (Fig. 11a), which subsequently produces a positive SSTA (Fig. 10a). The northerly wind, in contrast, advects cold and dry air into the central North Pacific, resulting in an upward heat flux anomaly and cold SSTA. Although the air-sea interaction appears to be inactive in spring and summer (Fig. 11b, c), positive SSTA is observed over the northeastern Pacific for several seasons (Fig. 10b, c).
Another important feature in Figs. 10 and 11 is that the oceanic and atmospheric variations in the North Pacific extend to the subtropical and tropical Pacific. Figure 10 displays the positive SSTA extending southwestward from Baja California to the tropical central Pacific. Also, the negative geopotential height extends across much of the Pacific basin from the mid-latitudes to the tropics. This North Pacific-tropical Pacific connection is evident not only in winter but also in spring and summer. The accompanying tropical Pacific SSTA features warming over the tropical central Pacific, that is, CP El Niño (Fig. 10). The atmospheric flow over the tropical Pacific displays westerly anomaly extending from the equatorial western to central Pacific and easterly anomaly over the eastern Pacific (Fig. 11). Di Lorenzo et al. (2010) suggested that the CP El Niño is tightly linked to the NPGO pattern via atmospheric teleconnection of the NPO. More recent studies have noted that the mid-latitude NPO forcing is particularly influential in the initial establishment of SSTA in the tropical central Pacific for CP El Niño events (Furtado et al. 2011; Kim et al. 2012; Yu and Kim 2011). This covariability of the NPO, NPGO and CP El Niño is clearly demonstrated in the regression patterns over the Pacific basin in connection with the warming of the Bering and Chukchi Seas during 1999–2010 (Figs. 10, 11). Indeed, the CP El Niño events in the recent decade, such as 2002/2003 and 2004/2005, concur with the positive SSTA in the Bering and Chukchi Seas region (see the PC time series in Fig. 2). Although data analysis in this study is inconclusive in regard to the cause-effect relationship among the Bering and Chukchi Seas warming, the NPO and the CP El Niño, their physical connectivity has become clearer and stronger during the recent decade.
7 Discussion and conclusion
The climate in the Bering Sea exhibits interdecadal variation, with distinct anomalous conditions occurred in 1997 and 1998. After unusual physical conditions during 1997–1998, recent (1999–2010) climate variability in the Bering Sea exhibits a closer connectivity with the North Pacific. CSEOF analysis reveals that the recent warming in the Bering Sea and the resultant reduction in SIC involves stronger oceanic heat loss in winter. In accordance with the increased heat transfer from the Bering Sea to the atmosphere, atmospheric circulation over the North Pacific exhibits an NPO-like pattern in winter (January–March). An experiment with the CAM3 model confirms and supports this connection between the Bering Sea and the North Pacific.
In the previous period (1980–1998), on the other hand, SST, sea ice, and energy flux in the Bering and Chukchi Seas seem to exhibit a weaker physical connectivity in comparison with the recent decade. Consequently, a strong physical connection between the climate variability in the Bering and Chukchi Seas and that of the North Pacific cannot be established in the previous period. Note that the R 2 value of the 1,000-hPa geopotential height regressed onto the first CSEOF of the Bering Sea SSTA is merely 0.3 in the earlier period, indicating that there is no significant physical relationship (or covariability) between the two regions. The atmospheric response in the North Pacific to the increased energy flux in the Bering and Chukchi Seas, that is, the NPO-like pattern, is a peculiar aspect of the recent decade (1999–2010).
The NPO pattern exerts strong influence over the North Pacific, serving as the atmospheric forcing for the oceanic NPGO pattern (Di Lorenzo et al. 2010). Furthermore, several recent studies argued that the initial establishment of SST anomalies in the tropical central Pacific is intimately tied to mid-latitude atmospheric variability, that is, the NPO pattern (Furtado et al. 2011; Kim et al. 2012; Yu and Kim 2011). The important links between the CP El Niño, the NPO and the NPGO constitute a prominent low-frequency variability in the Pacific basin in the recent decade. Indeed, several recent studies suggested that primary decadal-scale changes in the state of the North Pacific and the tropical Pacific occurred in the late 1990s (Bond et al. 2003; Yeo et al. 2012; Xiang et al. 2012). The CP El Niño has occurred more frequently (Kug et al. 2009; Lee and McPhaden 2010; Yeh et al. 2009) and the NPGO amplitude has been generally larger than the PDO amplitude (Bond et al. 2003; Di Lorenzo et al. 2008) since the late 1990s.
It is interesting to note that the prominent patterns of Pacific variability (i.e., NPGO and CP El Niño) in recent years appear as the regression patterns of the first CSEOF mode of the Bering Sea SSTA for 1999–2010. This reflects stronger connection of the climate variability in the Bering Sea and the prominent climate modes over the Pacific basin in the recent decade. Even though large-scale climate variability in the Pacific has been known as a dominant source of Bering Sea climate variability, the EP El Niño and the PDO do not actually account for much of the interannual variability of the Bering Sea climate; EP El Niño accounts for ~7 % of the sea ice variance in the Bering Sea (Niebauer 1988) and the variance of Bering Sea SSTA explained by the PDO is ~10 % (Minobe 2002). In contrast, Bering Sea variability in the recent decade is significantly (R 2 > 0.9) connected to the NPGO-like pattern and the CP El Niño. This implies that the Bering Sea in the recent decade serves as an integral component of a prominent low-frequency variability in the Pacific along with the NPGO and the CP El Niño.
Considering the short analysis period, it is hard to identify the cause-effect relationship between variability in the Bering Sea, and that of the Pacific basin. Therefore, investigations based on model experiments are required to further explore the detailed mechanisms of this physical connection. Additional analysis of the global warming scenario datasets under the auspice of the Intergovernmental Panel on Climate Change (IPCC) Fifth Assessment Report (AR5) will provide better insight into the interaction between the Bering Sea and the Pacific under the global warming state.
This research was supported by the project entitled “Ocean Climate Change: Analysis, Projections, Adaptation (OCCAPA)” funded by the Ministry of Land, Transport, and Maritime Affairs, Korea.
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