A decomposition of ENSO’s impacts on the northern winter stratosphere: competing effect of SST forcing in the tropical Indian Ocean
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- Rao, J. & Ren, R. Clim Dyn (2016) 46: 3689. doi:10.1007/s00382-015-2797-5
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This study applies WACCM, a stratosphere-resolving model to dissect the stratospheric responses in the northern winter extratropics to the imposed ENSO-related SST anomalies in the tropics. It is found that the anomalously warmer and weaker stratospheric polar vortex during warm ENSO is basically a balance of the opposite effects between the SST anomalies in the tropical Pacific (TPO) and that over the tropical Indian Ocean basin (TIO). Specifically, the ENSO-related SST anomalies over the TIO are to induce an anomalously colder and stronger stratospheric polar vortex during warm ENSO, which acts to partially cancel out the much stronger warmer and weaker polar vortex response to the SST anomalies over the TPO. Further analysis indicates that, while the SST forcing from the TPO contributes to the anomalously positive Pacific North America (PNA) pattern in the troposphere and the enhancement of the stationary wavenumber (WN)-1 in the stratosphere during warm ENSO, the TIO SST forcing is to induce an anomalously negative PNA and a reduction of both WN-1 and WN-2 in the stratosphere. Diagnosis of E–P flux confirms that, the anomalously upward propagation of stationary waves in the extratropics mainly lies over the western coast of North America during warm ENSO, which is mainly associated with the TPO-induced positive PNA response and is partially suppressed by the effect of the accompanying TIO SST forcing.
KeywordsEl Niño-Southern Oscillation (ENSO) Indian Ocean basin (IOB) Stratospheric polar vortex WACCM Pacific–North America pattern (PNA)
As one of the major sources of interannual variability in the ocean–atmosphere system, the El Niño-Southern Oscillation (ENSO) events occurring in the tropical eastern Pacific can influence not only the oceanic and atmospheric conditions in the entire tropics but also those across the global extratropics in both hemispheres. Particularly, the significant effects of ENSO on the northern extratropical stratosphere have been identified in observational data (Van Loon et al. 1982; Labitzke and Van Loon 1989; Camp and Tung 2007; Free and Seidel 2009; Xie et al. 2014a, b) and model simulations (Hamilton 1995; Sassi et al. 2004; Manzini et al. 2006; García-Herrera et al. 2006; Cagnazzo and Manzini 2009; Cagnazzo et al. 2009; Ineson and Scaife 2008). It was found that the northern stratospheric polar vortex tends to be anomalously warmer/colder and weaker/stronger during El Niño/La Niña winters (Van Loon et al. 1982; Labitzke and Van Loon 1989; Camp and Tung 2007; Free and Seidel 2009). Numerical experiments with the stratosphere-resolved model WACCM under perpetual January conditions by Taguchi and Hartmann (2006) indicated that the stratospheric sudden warming (SSW) events are twice as likely to occur in El Niño winters than in La Niña winters. It was also reported by Bell et al. (2009) that SSW events occur more frequently in the Reading Intermediate General Circulation Model under El Niño conditions than that under the climatological SST conditions. It was indicated that ENSO regulates the intensity of the stratospheric polar vortex by influencing the upward propagation of the planetary-waves from the troposphere to the stratosphere (Manzini et al. 2006; Camp and Tung 2007; Ineson and Scaife 2008; Ren et al. 2012). Relative to that by La Niña events, the teleconnection pattern in the troposphere induced by El Niño events is to increase the planetary wavenumber (WN)-1 but reduce the planetary WN-2, and the effect of WN-1 overwhelms that of the WN-2 to result in an anomalously warmer and weaker stratospheric polar vortex (Garfinkel and Hartmann 2007, 2008). Nevertheless, there are earlier studies indicating that the relationship between ENSO and the polar stratospheric variability may not be robust or statistically significant (Hamilton 1993, 1995; Baldwin and O’sullivan 1995). A fundamental difficulty in determining ENSO’s effects on the stratosphere is how to separate them from the entangled Quasi Biannual Oscillation (QBO) signals in the extratropical stratosphere (Garfinkel and Hartmann 2007, 2008; Wei et al. 2007; Calvo et al. 2009) due to the phase coincidence of QBO with ENSO in early records (Wallace and Chang 1982; Van Loon and Labitzke 1987; Baldwin and O’sullivan 1995). Later studies by Camp and Tung (2007) showed that the spatial patterns of the ENSO perturbations to the polar stratosphere are nearly orthogonal to that of QBO, and the magnitudes of ENSO perturbations are comparable with QBO perturbations. By analyzing the model results with sufficient data length and without QBO variability, Manzini et al. (2006) and García-Herrera et al. (2006) identified a significant relationship between ENSO and the stratospheric polar vortex variability. They also showed that warm ENSO tends to induce a PNA-like pattern and enhance WN-1 in the stratosphere, thus resulting in the anomalous polar warming during the late ENSO winter and the early subsequent spring.
It is known that the PNA pattern that links the ENSO forcing in the tropics to the atmospheric variability in the extratropics is intimately related to the diabatic heating induced by the ENSO SST anomalies over the tropical eastern Pacific (Wallace and Chang 1982; Simmons et al. 1983; Jin and Hoskins 1995; Newman and Sardeshmukh 1998; Annamalai et al. 2007). However, recent studies have found that, the anomalous diabatic heating over other tropical oceans particularly over the Indian Ocean may also play a role in modulating the PNA pattern (Barsugli and Sardeshmukh 2002; Annamalai et al. 2007). Linear model experiments with idealized tropical forcings showed that the 500-hPa PNA pattern forced by the imposed SST anomalies over the Indian Ocean, tends to destructively interfere with that forced by the idealized SST anomalies over the tropical eastern Pacific (e.g., Simmons et al. 1983; Branstator 1985; Ting and Sardeshmukh 1993; Jin and Hoskins 1995; Newman and Sardeshmukh 1998; Annamalai et al. 2007). Model studies by Kumar and Hoerling (1998), Farrara et al. (2000), and Spencer et al. (2004) also confirmed the possible different effects of the Indian Ocean SST forcing on the PNA pattern from that of Pacific SST forcing. Analysis of the model results from an atmospheric general circulation model (AGCM) in the National Centers for Environmental Prediction (NCEP) by Barsugli and Sardeshmukh (2002) indicated that, there exists a nodal line near 100°E in terms of the sensitivity of the PNA-like responses to the tropical SST forcing, across which the polarity of the PNA response seems to reverse.
Because of the “atmospheric bridge” processes induced by ENSO in the tropics, interannual variations of the SST over the tropical Indian Ocean as well as the SST over the tropical Atlantic, often follow that of the ENSO SST over the tropical eastern Pacific (Klein et al. 1999; Alexander et al. 2002; Kumar and Hoerling 2003). Following the occurrence of warm/cold ENSO (El Niño/La Niña) events in the tropical eastern Pacific, the tropical Indian Ocean and the tropical Atlantic also becomes warmer/colder. Many previous studies have proved the significant lead/lag correlations between the SST anomalies in the tropical eastern Pacific in winter and those in the tropical Indian Ocean (e.g., Cadet 1985; Tourre and White 1995; Lanzante 1996; Nicholson 1997; Klein et al. 1999; Murtugudde and Busalacchi 1999; Yu and Rienecker 1999; Ding and Li 2012) and the tropical Atlantic (e.g., Weare et al. 1976; Covey and Hastenrath 1978; Curtis and Hastenrath 1995; Lanzante 1996; Enfield and Mayer 1997; Nicholson 1997; Klein et al. 1999; Alexander et al. 2002) throughout the winter and subsequent spring. Some studies also related these SST responses in the tropical Indian Ocean and Atlantic Ocean to the zonally homogeneous atmospheric warming/cooling responses in the tropical troposphere to warm/cold ENSO (e.g., Newell and Weare 1976; Angell and Korshover 1978; Angell 1981; Pan and Oort 1983; Reid et al. 1989).
The different role played by the SST forcing in the Indian Ocean from that in the tropical eastern Pacific, in modulating the PNA pattern, seems to suggest that the SST forcing in the tropical Indian Ocean, though it is ENSO-induced, may act to counter with the original ENSO SST forcing in the tropical eastern Pacific and to weaken the otherwise stronger effects of ENSO on the extratropical stratosphere, regarding the intimate associations of the PNA anomalies in the troposphere with the planetary wave activity in the extratropical stratosphere in northern winter. However, because of the difficulties in distinguishing the effects of the SST forcings from different tropical oceans in observations, specific evidence on the roles of different tropical oceans in modulating the ENSO’s effects on the extratropical stratosphere is still lack. Most modeling studies usually consider the canonical ENSO SST anomalies over the eastern Pacific as the tropical ENSO forcing when they examine the responses of the extratropical atmosphere to ENSO, though the extratropical responses to ENSO in their simulations are usually much stronger than that in the observation [e.g., Fig. 5 in Taguchi and Hartmann (2006); Figs. 2, 3 in Bell et al. (2009); Fig. 4 in Ineson and Scaife (2008); Fig. 9 in Lan et al. (2012)].
The objective of this study is to use the WACCM and conduct a series of numerical experiments with the ENSO-related tropical SST forcing confined to different ocean basins for a dissection of the ENSO’s effects on the extratropical stratosphere. Particularly, we will provide modeling evidence to demonstrate, how the anomalous tropical SST forcings from different oceans, coordinate with one another to modulate the PNA pattern in the troposphere and contribute to the consequent ENSO’s effects on the extratropical stratosphere. The results will further clarify the dynamical linkage between the ENSO-related tropical forcings and the atmospheric responses in the extratropical stratosphere, and advance our knowledge of the dynamical roles of different tropical oceans in contributing to the ENSO’s effect on the extratropical atmosphere.
The remainder of this paper is organized as follows. Section 2 describes the data, model, experiment design, and analysis procedure. Section 3 demonstrates the different responses of the extratropical stratosphere to the ENSO-related tropical SST forcings over different ocean basins. Section 4 analyzes the dynamical connections between the tropical forcings over different ocean basins and the circulation responses from the troposphere to the stratosphere. Summary and conclusions are presented in Sect. 5.
2 Model, data and analysis procedure
We employed the version 4 of the stratosphere-resolved AGCM model WACCM (WACCM4, Garcia et al. 2007) which is developed by the National Center for Atmospheric Research (NCAR). The WACCM4 is one of the atmosphere components of the NCAR Community Earth System Model CESM (version 1.0.4). It is also a superset of the Community Atmospheric Model version 4 (CAM4), and includes all of the physical parameterizations in CAM4 (Neale et al. 2013). WACCM is a “high top” chemistry–climate model with 66 vertical levels extending from the surface to 5.1 × 10−6 hPa (approximately 150 km), and with a horizontal resolution of 1.9° (latitude) × 2.5° (longitude). The main improvements in version 4 include the updated parameterization schemes for non-orographic gravity waves generated by frontal systems and convection, and for surface stress due to unresolved topography (Garcia et al. 2007; Richter et al. 2010). It was found that the improved parameterization for surface stress or the turbulent mountain stress has led to a dramatic improvement on frequency of SSW events in northern winter (Richter et al. 2010; Marsh et al. 2013). The relatively high reproducibility of WACCM for the stratospheric variability as well as the ENSO signals in stratosphere has been confirmed in many previous studies (e.g., Sassi et al. 2004; Taguchi and Hartmann 2006; Garcia et al. 2007; Garfinkel and Hartmann 2008; Taguchi 2010; Calvo and Marsh 2011; Xie et al. 2012; Hegyi et al. 2014).
2.2 Data and experiment design
The monthly SST fields covering the period 1950–2010 are extracted from the SST and sea–ice datasets provided by the Meterological Office, Hadley Centre for Climate Prediction and Research (Rayner et al. 2006). The area mean SST anomalies in the Niño3 region (5°S–5°N, 150°–90°W) are used to define the monthly Niño3 index as a representation of ENSO signal. The climatological mean air temperature, geopotential height, and wind fields, which are used for validation of the WACCM simulations, are from the European Center for Medium Range Weather Forecasting interim reanalysis data (ERA-Interim). The ERA-Interim data has a horizontal resolution of 1.5° (latitude) × 1.5° (longitude) and 37 vertical levels from 1000 hPa to 1 hPa (Dee et al. 2011).
Experimental SST forcing descriptions
Forcing field descriptions
Climatological SST fields of 1950–2010
Same as ClmSST, but the ENSO-related SST anomalies in Fig. 1a added in the tropical Oceans (30°S–30°N, 0°–360°)
Same as ClmSST, but the ENSO-related SST anomalies in Fig. 1a added in the tropical Pacific Ocean (30°S–30°N, 135°E–70°W)
Same as ClmSST, but the ENSO-related SST anomalies in Fig. 1a added in the tropical Indian Ocean (30°S–30°N, 30°E–135°E)
Same as ClmSST, but the ENSO-related SST anomalies in Fig. 1a added in the tropical Atlantic Ocean (30°S–30°N, 70°W–10°E)
2.3 Analysis methods
3 Stratospheric responses to the tropical ENSO-related SST forcings from different ocean basins
3.1 Winter stratospheric climatology in WACCM
3.2 Stratospheric responses in boreal winter
When the ENSO SST forcing is confined to the tropical eastern Pacific in the TPO experiment, or the coordination of the SST forcings in the tropical Indian Ocean and the tropical Atlantic Ocean are excluded, it is seen that the responses in the tropical troposphere do become slightly weaker than that in the “ENSO” experiment. In spite of this, the stratospheric polar vortex responses to warm ENSO do not become relatively weaker, but instead become much stronger than that in the “ENSO” experiment (Fig. 3c, d vs. Fig. 3a, b). This seems to suggest that, the ENSO-associated SST forcings in the tropical Indian Ocean or in the tropical Atlantic do contribute to the ENSO effects in the tropical troposphere, but may act to partially cancel out that in the polar stratosphere. To further confirm this, we show in Fig. 3e, f and 3g, h the atmospheric responses to the ENSO-associated SST forcing imposed in the tropical Indian Ocean only (TIO experiment) and in the tropical Atlantic only (TAO experiment), respectively. It is seen that, though the TIO SST forcing also induces a tropical tropospheric warming response as the TPO forcing, it does not yield a weakening stratospheric polar jet response or a stratospheric polar warming response, but oppositely, a significant response of stratospheric polar cooling and strengthening of the stratospheric polar jet. Comparing with the responses to TIO, the responses to TAO in Fig. 3g, h are much weaker and insignificant. The trivial atmospheric responses to TAO suggest that the cancelling effect may mainly lie in the tropical Indian Ocean. By comparing Fig. 3e, f with 3i, j, it is further clear that the difference in responses to tropical SST forcings between “ENSO” and TPO can be largely accounted for by the effects of TIO.
3.3 Seasonal evolutions of the stratospheric responses
When the ENSO SST forcing is only considered in the tropical eastern Pacific and that in the tropical Indian Ocean and Atlantic is turned off (the TPO experiment), it is seen that the seasonal evolutions of the stratospheric responses are quite similar to that in the “ENSO” experiment, but the stratospheric polar vortex responses to warm ENSO are much stronger than that in the “ENSO” experiment (Fig. 4c, d vs. Fig. 4a, b). This proves once again that the ENSO-associated SST forcings in the tropical Indian Ocean partially cancel out the polar stratospheric responses to the tropical eastern Pacific ENSO SST forcing. The TIO experiment in Fig. 4e, f indeed induces a significant response of stratospheric polar cooling and polar jet strengthening throughout the warm ENSO winter. By comparing Fig. 4e, f with Fig. 4g, h, it is seen that the difference in responses to the tropical SST forcings between “ENSO” and TPO can be mainly attributed to the effect of the TIO SST forcing.
3.4 Downward propagations of the extratropical stratospheric signals
Once the ENSO SST forcing is only confined to the tropical eastern Pacific (the TPO experiment), the seasonal evolutions of the extratropical responses are quite similar to that in the “ENSO” experiment, but the downward propagating signals are much stronger and more significant than that in the “ENSO” experiment (Fig. 5c, d vs. Fig. 5a, b). This confirms that the ENSO-associated SST forcings in the tropical Indian Ocean partially cancel out the polar stratospheric responses to the tropical eastern Pacific ENSO SST forcing. The TIO experiment in Fig. 5e, f indeed induces a downward propagating response, out-of-phase with that in the TPO experiment, throughout the ENSO winter. By comparing Fig. 5e, f with 5g, h, it once again assures that the difference between the “ENSO” and the TPO in response to the tropical SST forcings can be largely explained by the opposite effect of the TIO SST forcing.
4 Understanding the cancelling effect of the Indian Ocean on the stratospheric responses to the ENSO SST forcing from the tropical eastern Pacific
4.1 Tropical latent heating perturbations and tropical geopotential height anomalies
Once the tropical SST forcings are merely confined to the tropical Pacific, the positive height response over the tropical Indian Ocean in the TPO experiment becomes relatively weaker than that in the “ENSO” experiment. The relatively weaker height response in the TPO experiment may be caused by the absence of the TIO SST forcings. When the SST forcings are only considered in the tropical Indian Ocean, as shown in Fig. 6g, the maximum latent heating perturbations lie over the tropical Indian Ocean. However, the latent heating perturbations over the tropical Pacific and Atlantic oceans are quite weak and insignificant and therefore ignorable, suggesting that the ENSO-related TIO SST forcing has a relatively local effect on the tropical convections. The contributions of the TIO SST forcing to the local positive height response can been clearly seen from Fig. 6g and further verified by the response difference between the “ENSO” and TPO experiment, with the positive height responses at 200 hPa over the tropical Indian Ocean (Fig. 6i). Therefore, the positive TIO SST anomalies following the maturing warm ENSO favor the zonally symmetric air warming and thus the positive height response at 200 hPa in the tropics.
From the above analysis, it can also be inferred that the positive height responses always straddle the nearby equatorial latent heating regions. A pair of positive height centers are located off the equator and distributed in both hemispheres, which is a typical forced Rossby wave response. There also exists a belt of positive height responses eastward of the heating sources along the equator, as a typical forced equator-trapped Kelvin wave response (Gill 1980), leading to the zonally symmetric warming in the tropical troposphere.
4.2 Extratropical circulation responses to tropical heating anomalies
Accompanied with the positive tropical height responses in the upper troposphere, anomalous cyclones are excited poleward. The extratropical circulation responses to the tropical latent heating over the TIO and TPO are quite different, especially in the Pacific–North America pattern. Specifically, a canonical PNA-like spatial pattern was reproduced in the “ENSO” and the TPO experiments with an anomalous low located over the Aleutian Islands (a deepened Aleutian low) and an anomalous high centered over Canada–Greenland (an intensified Canadian Arctic high), as indicated in Fig. 6c, e. The most obvious difference between the “ENSO” and the TPO experiments is that the positive PNA-like response in Fig. 6e seems to be stronger than that in Fig. 6c. Nevertheless, when the ENSO-related forcing is simply confined to the tropical Indian Ocean, an opposite spatial pattern is reproduced in the TIO experiment, with an anomalous high over the North Pacific (a weakened Aleutian low) and an anomalous low over Canada–Greenland (a weakened Canadian Arctic high), which together project onto the negative phase of PNA, as shown in Fig. 6g. This agrees with the results of Hoerling et al. (2004) and Annamalai et al. (2007), and can be further verified by the difference between the “ENSO” and the TPO experiments in Fig. 6i.
Apart from the waveform responses, annular mode responses can also be found in the sensitivity experiments. In the “ENSO” and especially the TPO experiments, from the southern United States and Mexico to the North Atlantic and well into Western Europe, a belt of highly significant low-pressure anomalies is a very prominent feature of the upper tropospheric geopotential height anomaly pattern. The positive upper tropospheric geopotential height anomaly center appears over South Greenland and the Davis Strait, also covers large parts of Canada and the North Pole, and it extends to the east towards Iceland, Spitsbergen and the Russian Arctic (Fig. 6c, e). The high-latitude positive and mid-latitude negative geopotential height anomalies clearly project onto a strong negative phase of the northern annular mode (NAM) in Fig. 6d, f, with the latter’s amplitude greater than the former’s. This is consistent with the results of Fletcher and Kushner (2011) and Graf et al. (2014). By contrast, the TIO experiment yields a negative geopotential height response from the Canadian Arctic and the Davis Strait to Greenland and Iceland and well into Svalbard and the Russian Arctic, and a positive geopotential response from the southern United States and Mexico to the North Atlantic and well into Southwestern Europe as indicated in Fig. 6g, which together corresponds to a positive NAM response in Fig. 6h. This is also proved by the difference between the “ENSO” and TPO experiments in Fig. 6j. The response to the TIO and that to the TPO forcing exhibit similar patterns but are opposite signed. This is mainly attributed to the fact that the thermodynamic forcing is shifted by 180° longitude. In other words, since the diabatic heating is shifted by 180° longitude, the extratropical wave is also shifted by 180° in the zonal direction (The SST anomaly center in the TIO is just ~180 degrees longitude from that in the TPO). A PNA is mainly projected to the planetary wavenumber-1 in the upper troposphere, which can propagate into the stratosphere. If the zonal wavenumber-1 is shifted by 180 degrees in the zonal direction, the phase is thus changed to the opposite polarity. Since positive/negative NAM corresponds to the westerly/easterly anomalies in the circumpolar regions, the NAM response can be connected with the downward propagating signals from upper levels (see Fig. 5), while the anomalous PNA-like response is obviously a manifestation of the stationary Rossby wave responses.
4.3 Extratropical planetary wave activities
4.3.1 Different behaviors of WN-1 and WN-2
In the “ENSO” and the TPO experiments, an anomalous low response over North Pacific and anomalous high response over Canada were reproduced, as indicated in Fig. 7a, d, which is consistent with Fig. 6c, e. The extratropical anomalous response centers amplify the climatological stationary waves (i.e., an anomalous low response superposed on the Aleutian low, and an anomalous high response superposed on the Canadian continental high). From Fig. 7b, e, it can be seen that the strengthening climatological waves in the upper troposphere in the “ENSO” and TPO experiments are dominanted by the contributions of WN-1. On the contrary, the WN-2 response centers are nearly out of phase with the climatological WN-2 in Fig. 7c, f.
Conversely, in the TIO experiment, an anomalous high response over North Pacific and an anomalous low response over Canada were produced (see Fig. 7g), which is consistent with Fig. 6g. The extratropical anomalous response centers weaken the climatological stationary wave (i.e., an anomalous high response superposed on the Aleutian low, and an anomalous low response superposed on the Canadian continental high). The response in TIO is obviously opposite to the TPO and therefore weakens the ENSO signal in the extratropics. The depressed climatological waves in the extratropics in TIO are partially from the contributions of WN-1 in Fig. 7h associated with the negative PNA pattern in Fig. 6g, and partially from WN-2 in Fig. 7i, which will be further proved below by analyzing the E–P flux. Different from the competitive relationships between the zonal WN-1 and WN-2 responses in the “ENSO” and “TPO” experiments, the WN-1 and WN-2 responses to the TIO forcing coordinate with each other to weaken the climatological stationary waves. Such characteristic in the TIO experiment can also be found in the difference between the “ENSO” and the TPO experiment shown in Fig. 7j, l.
Figure 8 shows that the zonal deviation and wave responses generally exhibit a westward tilt and their amplitudes are generally amplified with height increasing. Such a structure corresponds to an upward propagation of Rossby waves, and will contribute to a positive eddy heat flux response in the “ENSO” and the TPO experiments in Fig. 8a, f. It is the opposite in the TIO experiment in Fig. 8g, i, and therefore in the difference between the “ENSO” and the TPO experiments in Fig. 8j, l.
It can be inferred that the contrasting wave responses between the TIO and TPO forcings are caused not only by WN-1, but also by WN-2. While the anomalous WN-1 effects oppositely to the projected positive (for the “ENSO” and the TPO experiments) and negative (for the TIO experiment) PNA response in the upper troposphere, the WN-2 response is always out of phase with the climatological WN-2.
4.3.2 E–P flux and its divergence
Vertical component of E–P fluxes (Fz; units: 103 kg s−2) at 50 hPa averaged in 45°–75°N for total zonal waves and WN 1–2 for each experiment
5 Summary and discussion
Since a developing tropical IOB warming event initiating from winter generally follows a maturing El Niño event, many previous studies based on reanalysis and model outputs have also included the TIO signal when they checked the stratospheric polar vortex response to ENSO events. Different from them, this study applies the WACCM, a stratosphere-resolved GCM model to dissect the stratospheric responses in the northern winter extratropics to the imposed ENSO-related SSTA in different tropical ocean basins. The teleconnections between the TPO SST and the stratospheric polar vortex intensity and between the TIO SST and the stratospheric polar vortex intensity are examined in different sensitivity experiments.
WACCM not only reproduced quite well the climatology of the stratospheric circulations, including the stratospheric polar vortex intensity, but also the interannual variability of the northern winter stratospheric polar vortex. When forced with the ENSO-related SST in the TPO, the stratospheric polar vortex is anomalously warmer and weaker than normal, consistent with the results in many previous studies cited in this article; conversely, when forced with the ENSO-related SST merely in the TIO, the stratospheric polar vortex is anomalously colder and stronger than climatology. Specifically, warm forcing in TPO can enhance the climatological WN-1 pattern and decline the climatological stationary WN-2 pattern in the extratropics, with the increase of the former overwhelming the decrease of the latter. The increase in WN-1 is mainly the result of the projected positive PNA response in the extratropics with the Aleutian low deepened and the Canadian continental high intensified. By contrast, the warm forcing in the TIO weakens both the climatological WN-1 and WN-2 patterns and induces a negative PNA response with the Aleutian low weakened and the Canadian continental high undermined. Diagnosis on E–P flux confirms that, the anomalously upward propagation of stationary wave in the extratropics mainly lies over the western coast of North America in the TPO experiment, while the TIO warming suppresses the upward propagation of stationary waves there.
This paper proves that SSTA in the TIO modulate the influence of ENSO on the extratropical circulation. Previous studies have also noted the opposite-signed zonal-mean responses to SSTA over the TPO and the TIO (e.g., Branstator and Haupt 1998; Barsugli and Sardeshmukh 2002; Annamalai et al. 2007; Fletcher and Kushner 2011). Specifically, a positive-PNA and negative-NAM response corresponds to an imposed warming in the tropical Pacific, whereas a negative-PNA and positive-NAM signal is forced by an imposed warming in the tropical Indian Ocean. Our study further indicated a strong in-phase/out-of-phase relationship between WN-1/WN-2 response and the background stationary WN-1/WN-2 counterpart in the TPO, which is also consistent with the result of Fletcher and Kushner (2011). We also found an out-of-phase relationship between WN-1 (or WN-2) response and their background stationary counterpart for imposed warming in the TIO.
A weaker stratospheric polar vortex is expected in the El Niño winter. Due to the atmospheric chaos, a stronger polar vortex and an El Niño event may also coexist in observations. It needs to be further studied whether the decrease in WN-2 for each case (e.g., an imposed warming in the TPO, in the TIO, or in both) and the decrease in WN-1 for imposed warming in the TIO can partially explain the concurrence of an El Niño event and a stronger stratospheric polar vortex event in the observations. Few previous studies have given reasonable explanations for this. Much attention has been paid to the general influence of the tropical Pacific ENSO SST on the northern winter stratospheric polar vortex (i.e., El Niño/La Niña and weaker/stronger stratospheric polar vortex), with less attention on the warming in other ocean basins. It would be worth exploring the relative importance of the warming over the two ocean basins to the relationship between ENSO and the stratospheric polar vortex, as also suggested by Li and Lau (2013).
Toniazzo and Scaife (2006) confirmed the statistically significant influence of ENSO on winter North Atlantic climate by the stratosphere pathway, which can also be seen in our “ENSO” and TPO experiments (negative NAM/NAO response in Fig. 6). However, the northern stratospheric response to the ENSO-related SST forcing in the TAO is rather weak in the stratosphere. The weak modulation of the ENSO-related SST forcing in the TAO on the northern winter stratospheric polar vortex may be attributed to the weak SST anomalies in our TAO experiment. The role of the TAO SST forcing on the northern winter circulation has been reported in literature. For example, results from linear models (Li et al. 2007) and AGCMs (Mechoso and Lyons 1988; Watanabe and Kimoto 1999; Robertson et al. 2000; Peng et al. 2005; Sutton and Hodson 2007; Wu et al. 2007; Wang et al. 2008) suggested that a positive TAO SST anomaly tend to produce a south–north dipole in geopotential heights, also much like the NAO. Since the tropical Atlantic warming develops even later than the tropical IOB warming, and SSTA in the tropical Atlantic are still very weak in northern winter when El Niño has matured (Fig. 1), signals are seemingly insignificant and feeble for the imposed weak warming in the tropical Atlantic (Fig. 3g, h). It can also be verified by the similar features between the “ENSO” minus TPO and the TIO experiment (i.e., ENSO–TPO ≈ TIO), though the SSTA in the tropical Atlantic were included in the “ENSO” experiment. Due to the limited scope of the article, the influence of the tropical Atlantic conditions needs to be further studied.
Since the TIO experiment uses specified SST anomalies, there is possibility that ocean forcing in the atmosphere may be misleaded to some degrees. The TIO experiment may deviate from the realistic world where the TIO SST anomalies are forced remotely by ENSO and are passive responses. For example, Copsey et al. (2006) used the HadAM3 model and found that the negative sea level pressure (or positive rainfall) trend overlies the warming SST trend in the Indian Ocean, which is inconsistent with observations. It necessitates further investigations with fully coupled models in the future. As is known to the scientific community, on the longer timescale, the tropical Indian Ocean is getting warmer with a growth rate different from that in other ocean basins in recent decades, whereas the eastern equatorial Pacific is cooling. At the same time, the northern winter stratospheric polar vortex shows a cooling trend and the stratospheric NAM is also strengthened toward its positive phase (Ren and Yang 2012; Rao et al. 2015). It is worthwhile to study whether the mechanism found in this article can provide an explanation for the concurrence of the two phenomena. Also, coupled models tend to reproduce a colder eastern tropical Pacific SST pattern (Wei et al. 2007) and therefore a stronger northern winter stratospheric polar vortex than that in observations. Given the fact that the TIO SST seems to be more efficient (1-K warming in the TIO induced stronger response in the stratosphere than that in the TPO) in modulation of the northern winter stratospheric polar vortex intensity, our findings may be used to understand model biases in reproducing the stratospheric polar vortex by attributing them to the TPO or the TIO SST biases, which obviously can help model developers to improve model’s reproducibility of SST and the stratospheric polar vortex.
This work are jointly supported by research grant from the National Natural Science Foundation of China (41575041, 41430533 and 91437105), a Chinese Academy of Sciences project (Grant No. XDA11010402) and a China Meteorological Administration Special Public Welfare Research Fund (Grant No. GYHY201406001). The authors thank the reviewers and editors for their helpful comments and kind suggestions. We acknowledge the UK Met Office providing HadISST dataset. We also thank the NCAR providing the WACCM model.