Asymmetric Modulation of the Atlantic Multidecadal Oscillation on ENSO Amplitude

doi:10.21203/rs.3.rs-5007982/v1

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Asymmetric Modulation of the Atlantic Multidecadal Oscillation on ENSO Amplitude

https://doi.org/10.21203/rs.3.rs-5007982/v1

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We investigate the asymmetric modulation of the Atlantic Multidecadal Oscillation (AMO) on El Niño-Southern Oscillation (ENSO) amplitude using long-term observational and reanalysis datasets. Results show that the modulation of the AMO on ENSO amplitude exhibits significant asymmetry in both eastern Pacific (EP) and central Pacific (CP) types and El Niño (El) and La Niña (La) events. Both CP-La and EP-El events are significantly strengthened during the negative AMO phase, while EP-La and CP-El events show no remarkable changes under different AMO phases. Using ocean mixed layer heat budget, we show that the thermocline feedback and net surface heat flux are crucial for the strengthening of CP-La, while zonal advective feedback and thermocline feedback favor the intensification of EP-El during the negative AMO phase. Further analyses reveal that during the negative AMO phase there are relatively colder sea-surface temperatures (SSTs) in the western equatorial Pacific with weakened trade wind, while warmer SSTs occur in the eastern equatorial Pacific with strengthened trade wind, leading to a weakening of zonal SST gradient. As a result, the thermocline deepens (shallows) with the negative (positive) Ekman pumping velocity (EPV) in the eastern (central) equatorial Pacific. These conditions collectively favor the strengthening of CP-La and EP-El events. During the negative AMO phase, the relatively dry background and the zonal wind over the tropical Pacific can lead to significant dry advection toward the central tropical Pacific, which enlarges the local sea-air humidity difference and promotes the release of latent heat flux from the ocean to strengthen CP-La.

Atlantic Multidecadal Oscillation

ENSO amplitude asymmetry

Mixed layer heat budget

Multi-timescale variability

Equatorial Pacific Ocean

As the strongest signal of interannual climate variability and the most important coupled air-sea interaction phenomenon in the global climate system, the El Niño-Southern Oscillation (ENSO) exerts far-reaching influences on global climate (e.g., Bjerknes 1969; Wallace and Gutzler 1981; Wallace et al. 1998; Alexander et al. 2002). Previous studies have shown that the ENSO exhibits asymmetry in its strength, with El Niño events always being stronger than La Niña events (Kang and Kug 2002; An and Jin 2004; Okumura and Deser 2010). Since the beginning of the 21st century, a new type of-sea-surface temperature (SST) warming phenomenon has frequently appeared in the tropical Pacific with its warm SST anomalies (SSTA) centered in the central Pacific rather than the eastern Pacific, differing from the conventional El Niño. These events are defined as Central Pacific (CP) El Niño (CP-El) or Warm Pool El Niño, whereas the classical El Niño events are defined as Eastern Pacific (EP) El Niño (EP-El) or Cold Tongue El Niño (Larkin and Harrison 2005; Ashok et al. 2007; Weng et al. 2007; Kao and Yu 2009; Kug et al. 2009; Ren and Jin 2011). While some argued that different types of La Niña events exhibit little difference in zonal position compared to those of El Niño events (Kug et al. 2009, 2010; Ren and Jin 2011), a growing body of recent studies suggested the necessity of categorizing La Niña events into EP and CP types as well, namely CP-La and EP-La, both in terms of local dynamics and extratropical teleconnections (Cai and Cowan 2009; Shinoda et al. 2011; Tedeschi et al. 2013; Zhang et al. 2015, 2019). Importantly, these two different ENSO types exhibit strikingly different climate impacts, especially in East Asia and Western Europe (Weng et al. 2007; Cai and Cowan 2009; Feng et al. 2010; Zhang et al. 2011, 2012, 2014, 2015, 2019; Yu et al. 2012; Feng and Li 2013; Tedeschi et al. 2013). Thus, it is necessary to further classify the developmental characteristics and climate impacts of these two types of ENSO events.

The Atlantic Multidecadal Oscillation (AMO), a leading pattern of multidecadal variability, which is characterized by a basin-wide SST warming or cooling in the North Atlantic with a long periodicity of 50–70 year (Kerr 2000). A positive (negative) AMO phase means a SST warming (cooling) in most part of the North Atlantic. It is believed that the AMO is related to oceanic dynamics, and is manifested as the surface expression of the Atlantic Meridional Overturning Circulation (AMOC) (Wang et al. 2014; McCarthy et al. 2015; Sun et al. 2019). Due to observational limitations, the physical mechanisms underlying the AMO, however, remain undetermined. Nevertheless, the AMO plays a crucial role in changing global climate, particularly in modulating the ENSO, which has been well documented (Dong et al. 2006; Kang et al. 2014; Levine et al. 2017; Gong et al. 2020). Specifically, the AMO has a negative correlation with ENSO amplitude; that is, ENSO amplitude is strengthened (suppressed) during the negative (positive) AMO. Dong et al. (2006) pointed out that the AMO can affect ENSO amplitude via an atmospheric bridge. During the positive AMO, the reduced Pacific trade wind would deepen the thermocline in the equatorial Pacific through the Sverdrup balance, thus weakening ENSO amplitude. Kang et al. (2014) proposed that the weakening of ENSO amplitude during the positive AMO is due to the easterly wind stress anomalies related to the colder mean SST in the central equatorial Pacific. Levine et al. (2017) found that the AMO can modulate the multidecadal variability of the ENSO through strongly influencing the Walker circulation. Based on analysis of ocean mixed layer heat budget, Gong et al. (2020) demonstrated that the thermocline feedback is the primary process in regulating ENSO growth during the negative phase of the AMO.

The aforementioned studies indicate that it is necessary to consider the asymmetry of the ENSO in its cold and warm phases, as well as the zonal positions, for examining the modulation of ENSO amplitude by the AMO. So far, only a few studies have explored the modulation of the AMO on ENSO amplitude asymmetry. Kang et al. (2014) found that the westward shift and weakening of ENSO zonal wind stress anomalies in the central equatorial Pacific during the positive AMO may lead to more frequent CP-El events; and Yu et al. (2015) suggested that a positive AMO may strengthen the Pacific meridional mode (PMM) and the coupling between subtropical and tropical Pacific, causing the frequent occurrence of CP-El. However, these studies mainly focused on the frequency of CP-El, with limited discussion on the intensity and changes associated with La Niña.

Here we aim to investigate the impact of the AMO on ENSO amplitude asymmetry by comparing ENSO events in terms of their different phases and zonal positions using observational and reanalysis datasets. The rest of this paper is organized as follows. In Section 2, we introduce the datasets and methods used in this study. In Section 3, we show differences of ENSO amplitude and their related air-sea responses during different AMO phases, including the underlying physical processes. Summary and discussion are given in Section 4.

2.1 Datasets

The datasets used in this study include: (1) monthly mean SST from the Hadley Center Sea Ice and SST dataset version 1.1 (HadISST v1.1; Rayner et al. 2003) with a resolution of 1°×1° for the period of 1900–2015; (2) monthly mean ocean temperature, zonal velocity (u), meridional velocity (v), vertical velocity (w), zonal wind stress ($\:{{\tau\:}}_{x}$), and meridional wind stress ($\:{{\tau\:}}_{y}$) from the Simple Ocean Data Assimilation version 2.2.4 (SODA 2.2.4; Carton and Giese 2008) with a resolution of 0.5°×0.5° for the period of 1900–2010; (3) monthly mean sea level pressure, 850-hPa wind, 2-m specific humidity, and surface heat flux from the NOAA-Twentieth Century Reanalysis Version 3 (NOAA-20CRv3; Slivinski et al. 2019) with a resolution of 1°×1° for the period of 1900–2015.

2.2 Methods

a. Definition of AMO index

Following Enfield et al. (2001) and Wang et al. (2009), the AMO index is defined as SST anomalies averaged in the North Atlantic region (75°-7.5°W, 0°-60°N). Since the AMO has a periodicity of 50–70 years, a 21-year low-pass filter is applied before calculating the AMO index. Thus, a positive AMO index indicates the positive AMO phase, and vice versa.

b. Definition of ENSO events and their types

The Niño3.4 index is defined as the monthly mean SST anomaly in the Niño3.4 region (170°-120°W, 5°S-5°N). El Niño (La Niña) events are defined as five consecutive overlapping 3-month periods with running mean Niño3.4 index at or above 0.5°C (at or below -0.5°C) according to the National Climate Prediction Center (CPC).

Following Ren and Jin (2011), EP and CP indices (EPI and CPI) are calculated using a mathematical rotation of the Niño3 and Niño4 indices based on the averaged SST anomalies in the regions of (150°-90°W, 5°S-5°N) and (160°E-150°W, 5°S-5°N), respectively. El Niño (La Niña) events with EPI greater (smaller) than CPI are classified as EP events, while those with CPI greater (smaller) than EPI are defined as CP events. Following these criteria, various types of ENSO events under different AMO phases are classified (Table 1).

Table 1

Selected CP/EP-ENSO events during +/-AMO phase from 1900 to 2015 in this study
	+AMO	-AMO
CP-El Niño	2004/2005, 2009/2010, 2014/2015	1923/1924, 1968/1969, 1977/1978, 1987/1988,1994/1995
CP-La Niña	1933/1934, 1950/1951, 1998/1999, 2000/2001, 2010/2011, 2011/2012	1924/1925, 1973/1974, 1975/1976, 1983/1984, 1988/1989
EP-El Niño	1902/1903, 1904/1905, 1905/1906, 1925/1926, 1930/1931, 1939/1940, 1940/1941, 1951/1952, 1957/1958, 2002/2003, 2006/2007	1911/1912, 1913/1914, 1914/1915, 1918/1919, 1963/1964, 1965/1966, 1969/1970, 1972/1973, 1976/1977, 1982/1983, 1986/1987, 1991/1992,1997/1998
EP-La Niña	1903/1904, 1938/1939, 1942/1943, 1945/1946, 1949/1950, 1954/1955, 1955/1956, 1999/2000, 2007/2008	1908/1909, 1909/1910, 1910/1911, 1916/1917, 1964/1965, 1970/1971, 1971/1972, 1984/1985, 1995/1996

c. Mixed layer heat budget

Following Jin et al. (2003) and Hong et al. (2008), the anomalous mixed layer heat budget equation can be written as follows:

$$\:\frac{\partial\:{T}^{{\prime\:}}}{\partial\:t}=\:-({u}^{{\prime\:}}\frac{\partial\:\stackrel{-}{T}}{\partial\:x}+\stackrel{-}{u}\frac{\partial\:{T}^{{\prime\:}}}{\partial\:x}+{u}^{{\prime\:}}\frac{\partial\:{T}^{{\prime\:}}}{\partial\:x})-({v}^{{\prime\:}}\frac{\partial\:\stackrel{-}{T}}{\partial\:y}+\stackrel{-}{v}\frac{\partial\:{T}^{{\prime\:}}}{\partial\:y}+{v}^{{\prime\:}}\frac{\partial\:{T}^{{\prime\:}}}{\partial\:y})-({w}^{{\prime\:}}\frac{\partial\:\stackrel{-}{T}}{\partial\:z}+\stackrel{-}{w}\frac{\partial\:{T}^{{\prime\:}}}{\partial\:z}+{w}^{{\prime\:}}\frac{\partial\:{T}^{{\prime\:}}}{\partial\:z})+\frac{{Q}_{net}^{{\prime\:}}}{\rho\:{C}_{p}H}+R$$

where T denotes vertical-mean mixed layer temperature; u, v and w represent the three-dimensional ocean current components; $\:\partial\:/\partial\:x$, $\:\partial\:/\partial\:y$ and $\:\partial\:/\partial\:z$ indicate the three-dimensional gradient operators in x, y and z directions. The prime denotes interannual anomaly, and the bar is the climatological mean state. The term $\:{Q}_{net}$ denotes the net surface heat flux including surface shortwave and longwave radiation (SW and LW) and sensible and latent heat fluxes (SH and LH). All these types of surface radiation and fluxes are positive downward, indicating that the ocean receives heat. The residual term R includes contributions from diffusion, nonlinear and sub-grid-scale processes as well as potential errors in computation (Li et al. 2002; Hong et al. 2008; Chen et al. 2015). $\:\rho\:$ is the density of sea water (10³ kg/m³) and $\:{C}_{p}$ is the specific heat of sea water (4000 J/(kg·K)). H denotes the climatological mixed layer depth, defined as the depth where ocean temperature is 0.5℃ below the SST, taking the zonal variation into account. When analyzing EP (CP) ENSO events, the diagnostic region is the Niño3 (Niño4) region.

Anomalies for all variables are defined as the deviation from the long-term climatological mean. The linear trend is first removed to avoid possible influences of global warming. Also, a 10-year high-pass filter is applied to avoid the disruption of decadal and longer low-frequency signals when we examine interannual variability; and a 10-year low-pass filter is applied when we focus on the modulation effect of the AMO. All statistical significance tests are performed using the two-tailed Student’s t test.

3.1 Asymmetric ENSO amplitude during different AMO phases

Figure 1 shows the 21-year low-pass-filtered AMO index and 21-year sliding standard deviation (STD) of the 10-year high-pass-filtered Niño3, Niño3.4 and Niño4 indices in the previous winter. According to the AMO time series, we select 1900–1904, 1925–1962 and 1998–2015 as the positive AMO phases, and take 1905–1924 and 1963–1997 as the negative AMO phases. The STD of the Niño3.4 index, as a measure of ENSO amplitude, exhibits prominent interdecadal variation, with stronger ENSO amplitude during the negative AMO phase, which is consistent with the published studies (Dong et al. 2006; Kang et al. 2014; Gong et al. 2020; Xu et al. 2023). Similarly, the STD of the Niño3 index (i.e., the SST variability in the eastern equatorial Pacific) shows the interdecadal variation, although with smaller amplitude. Although the STD of the Niño4 index in the central equatorial Pacific maintains an overall negative correlation with the AMO, this relationship is less significant and becomes unstable compared to that for the Niño3.4 and Niño3 indices. It should be noted that even though the 21-year sliding STD can reflect the interdecadal variability of SST amplitude, it may overlook the potential impacts from the ENSO cycle and the different strength between El Niño and La Niña. Therefore, to better understand the modulation effect of the AMO on ENSO amplitude, we will comprehensively consider different phases and zonal positions of the ENSO. The asymmetric ENSO amplitude during different AMO phases and the potential causes will be explored through composite analysis.

Next, we investigate different characteristics of various ENSO events during different AMO phases. Figure 2 shows the composite differences of SSTA in the boreal winter for CP-La and EP-El events. It is clearly seen that the cold center of CP-La is close to the dateline (Figs. 2a and b), while the warm center of EP-El is located in the eastern equatorial Pacific (Figs. 2c and d). These two types of ENSO events are significantly stronger during the negative AMO phase (Figs. 2b and d). To better illustrate the development of the ENSO, we use Figure. 3 to exhibit the evolution of composite zonal SSTA and the minimum/maximum values within the equatorial Pacific during different ENSO events. We can see that both the distribution of SSTA in the equatorial Pacific and the associated extreme values indicate that the intensity of CP-La and EP-El is stronger during the negative AMO phase. This larger SSTA extends beyond the winter of the mature ENSO phase and almost covers the entire developing period. Moreover, there exists a notable longitudinal deviation (~ 50 degrees) between the SSTA centers of CP-La and EP-El. This confirms that the classifications of CP-ENSO and EP-ENSO in our study are reasonable and reliable. Interestingly, the ENSO amplitude differences during different AMO phases are not observed in EP-La and CP-El events (Supplementary Fig. 1). This indicates that the modulation of the AMO on ENSO amplitude is likely to be asymmetric between different phases and zonal positions of the ENSO, which has not been addressed previously.

3.2 Asymmetric responses of ENSO-related air-sea variables during different AMO phases

It has been widely known that both convection and precipitation play crucial roles in tropical regions. Figure 4 shows the time evolution of anomalous precipitation and outgoing longwave radiation (OLR) composites for CP-La and EP-El during different AMO phases. Corresponding to the SSTA mentioned above, stronger negative precipitation anomalies and positive OLR anomalies can be seen in the central-western equatorial Pacific from the developing summer to the decaying spring for CP-La events during the negative AMO phase (Figs. 4a and b), indicating significantly suppressed convection in the region. Similarly, for EP-El events, positive precipitation anomalies and negative OLR anomalies in the central-eastern equatorial Pacific are larger during the negative AMO phase (Figs. 4c and d), suggesting stronger convective activity. Additionally, all the precipitation anomaly centers are located in the west of the SSTA center (Figs. 3 and 4), which reflects a Gill response to the anomalous heat source related to SSTA. This also indicates the warmer and more moist mean-state background in the western and central tropical Pacific (near the warm pool) would favor a more sensitive response of anomalous convection (and also precipitation) to SSTA. During the developing period of the ENSO, persistent local precipitation anomalies in the tropical Pacific often change vertical motion through a positive feedback mechanism (i.e., upward motion - increased precipitation - latent heat release - further enhancement of upward motion), thereby altering the Walker circulation and low-level zonal wind. Through the Bjerknes positive feedback, the ENSO is continuously intensified. Therefore, the intensity of tropical Pacific precipitation during the ENSO developing phase may be coupled with ENSO intensity, and the zonal position of the anomalous precipitation corresponds to the type of an ENSO event.

To verify the hypothesis mentioned above, we use Figs. 5 and 6 to exhibit the composite sea level pressure (SLP) and 850-hPa wind anomalies for CP-La and EP-El events during the developing stage of different AMO phases, respectively. During the negative AMO phase, there exist significant positive SLP anomalies over the central tropical Pacific and negative anomalies over the Maritime Continent and the eastern tropical Indian Ocean in the developing autumn and peaking winter of CP-La events. Thus, this enlarged zonal SLP gradient results in stronger anomalous easterly winds over the central tropical Pacific (Fig. 5). Moreover, these SLP anomalies are stronger during the negative AMO phase with a wider spatial range and a longer duration, as compared with those during the positive AMO phase. Therefore, the easterly wind anomalies are more remarkable during the negative AMO phase, favoring the upwelling of cold water in the central equatorial Pacific and thus strengthening CP-La events. Meanwhile, there is a stronger downward motion in the Niño4 region (Supplementary Figs. 2a and b), indicating an adjustment of the local Walker circulation. Similarly, for EP-El events, stronger high (low) SLP anomalies appear in the central-eastern tropical Pacific (the Maritime Continent and tropical Indian Ocean) during the negative AMO phase. The resultant westerly wind anomalies in the central-eastern tropical Pacific continuously transport warm water from the warm pool to the east, thus strengthening EP-El events. In addition, the tropospheric upward motion become stronger over the Niño3 region, suggesting a weakening of the Walker circulation (Supplementary Figs. 2c and d).

As mentioned previously, the anomalous lower-level zonal winds can change the subsurface ocean layer through the Bjerknes positive feedback. Figures 7 and 8 illustrate the evolution of the composites of subsurface ocean temperature and current anomalies with thermocline depth in the equatorial Pacific for CP-La and EP-El events, respectively, during different AMO phases. Significant westward current anomalies are found in the central and western tropical Pacific during the developing spring to autumn of CP-La under the negative AMO phase. In the early spring, the anomalous cold water at a depth of 100–200 m in the warm pool continuously migrates eastward and upward along the thermocline, which moves to about 100 m deep near 120°W in winter when the CP-La event matures, with the deeper parts of the warm pool replaced by anomalous warm water Figs. 7g-l). In contrast, CP-La-related subsurface ocean temperature and current anomalies are noticeably weaker during the positive AMO phase. During early spring, the anomalous cold water is located in the central tropical Pacific at a depth of around 100 m, which moves eastward and upward slowly above the thermocline (Figs. 7a-f).

During EP-El events of the negative AMO phase, significant and persistent eastward current anomalies appear in the central and western tropical Pacific, which continuously transport warmer water (located in the central tropical Pacific at a depth of 100–200 m in the early spring) eastward and upward along the thermocline. When the EP-El peaks, the anomalous warm water moves to a depth of about 50 m near 100°W, which is shallower and more eastward than the colder water during CP-La events (Fig. 8g-l). In contrast, the subsurface current and temperature anomalies during EP-El events are much weaker during the positive AMO phase, with no significant warmer water in the developing spring. Similar to CP-La events, the anomalous warm water during the developing period stays primarily above the thermocline (Fig. 8a-f).

3.3 The role of AMO in influencing ENSO amplitude asymmetry

To gain a deeper understanding of the dynamic and thermodynamic processes that contribute to ENSO amplitude asymmetry, we conduct a mixed layer heat budget analysis, with a focus on the ENSO developing period. Based on the time evolution of ENSO-related SSTA (Figs. 3c and f), we define the developing period of CP-La as from July to the following January, and the EP-El developing period as from May to December, consistent with the previous research (Gong et al. 2020). Note that these selections do not affect our main conclusions.

Figure 9 shows composite differences of each heat budget term between the negative and positive AMO phases for CP-La and EP-El events, respectively. A negative (positive) T’t indicates the CP-La (EP-El) growing tendency is greater during the negative AMO phase. We can see that meridional advective feedback (term 5), thermocline feedback (term TH) and net heat flux (term Qnet) play significant roles in strengthening CP-La during its developing period in the negative AMO phase (Fig. 9a), while zonal advective feedback (term ZA) acts to suppress the intensification of CP-La. For EP-El events, the strengthening is primarily driven by zonal advective feedback, meridional advective feedback and thermocline feedback, with an opposite effect from the net heat flux Fig. 9b). Chen et al. (2016) demonstrated that the meridional advective feedback mainly serves as an amplifier that relies on the net effect of the other terms. Moreover, as EP-El events are recognized as the traditional ENSO events, diagnostic results for these events are consistent with previous findings (e.g., Gong et al. 2020). Therefore, we will mainly focus on the differences in zonal advective feedback, thermocline feedback and net heat flux for CP-La events during different AMO phases next.

Previous studies illustrated that the tropical Pacific thermocline would flatten under the negative AMO phase (Dong et al. 2006; Levine et al. 2017; Xu et al. 2023), that is, the thermocline shoals (deepens) in the central (eastern) equatorial Pacific, corresponding to upward (downward) background vertical current deviation. Compared to the positive AMO phase, the low-frequency background eastward current in the central equatorial Pacific accelerates significantly during the negative AMO phase (Fig. 10a). Such a current speed divergence in this region can trigger upward motion that prompts the upwelling of colder water from deeper layers, thereby facilitating the emergence and growth of CP-La events. Conversely, in the eastern equatorial Pacific, the eastward current becomes weaker and the associated convergence induces downward motion to warm the subsurface sea water, favoring the development and intensification of EP-El events. Given this, we also show the differences of Ekman pumping velocity (EPV), calculated by using wind stress, in the tropical Pacific between the two AMO phases (Fig. 10b). In contrast to those in the positive AMO phase, there are notable positive EPV anomalies in almost the entire western and central equatorial Pacific and negative EPV anomalies around 145°-85°W. This would favor the development of CP-La and EP-El events.

Figure 11a presents the low-frequency SST and 850-hPa wind regressed onto the AMO index. Note that regression results are reversed in sign to effectively represent the background conditions during the negative AMO phase. During the negative AMO phase, relatively colder SSTs occur in the western tropical Pacific (i.e., the warm pool), while the SSTs in the east are narrowly and slightly warmer. Concurrently, anomalous westerly (easterly) winds prevail over the central-western (eastern) tropical Pacific, indicating a weaking (strengthening) of the trade wind. These results are consistent with the previous findings (Dong et al. 2006; Gong et al. 2020). Moreover, these low-frequency anomalies of SST and low-level wind suggest that during the negative AMO phase, the zonal mean-state SST gradient is weaker than that during the positive AMO phase, thus increasing the air-sea coupling instability and thus favors the development of the ENSO (Dong et al. 2006; Gong et al. 2020). In addition, this weaker zonal SST gradient would enhance the zonal advection feedback during the developing period of EP-El, but suppress it during the development phase of CP-La, consistent with the diagnostic results of the ocean heat budget.

Finally, we investigate the difference of Qnet term in Eq. (1) for CP-La events (subplots in Fig. 9). Generally, for La Niña events, the central tropical Pacific SST gradually decreases during the developing period. We can see that the Qnet term is positive near the cold SSTA center, indicating that the ocean receives heat from the atmosphere to suppress the development of CP-La events. Interestingly, the Qnet difference between the two AMO phases is negative (Fig. 9a), favoring the development of CP-La during the negative AMO phase. Furthermore, this Qnet difference mainly comes from the LH difference (subplot in Fig. 9a) with a strong upward LH anomaly in the Niño4 region during the negative AMO phase, but shows little LH anomaly during the positive AMO phase (Supplementary Fig. 3).

As LH is closely related to air-sea humidity difference, we show the regression map of 2-m specific humidity onto the AMO index (Fig. 11b). Clearly, the entire tropical Pacific region becomes significantly drier during the negative AMO phase, except for weak moist anomalies in the central tropical Pacific. When combined with the low-level winds shown in Fig. 11a, it can be observed that both the westerly wind anomaly in the central and western tropical Pacific and the easterly wind anomaly in the east jointly produce a pronounced dry advection toward the central tropical Pacific. As the ocean stays in a saturated state, this strong dry advection in the atmosphere would enlarge the air-sea humidity difference in central tropical Pacific. Consequently, during the developing period of CP-La, the ocean releases more LH to the atmosphere in the central tropical Pacific to further decrease the SST there, which promotes the strengthening of CP-La during the negative AMO phase.

In this study, we investigate the modulation effect of the AMO on ENSO amplitude asymmetry using long-term observations and reanalysis. We found that the modulation of the AMO on ENSO amplitude exhibits significant asymmetry in both zonal positions and in terms of cold and warm events, with CP-La and EP-El being significantly strengthened during the negative AMO phase between different AMO phases, but no significant differences for EP-La and CP-El. Further analysis revealed that during the developing period of CP-La and EP-El, responses of ENSO-related sea level pressure, low-level wind, precipitation, and subsurface ocean currents are relatively more pronounced during the negative AMO phase, which reflects stronger air-sea coupling and positive Bjerknes feedback, thus favoring the enhancement of the ENSO.

Using the ocean mixed layer heat budget analysis, we found that for CP-La events, the meridional advective feedback, thermocline feedback and net surface heat flux are crucial for the enhancement during the negative AMO phase, while zonal advective feedback acts as an impediment. In the case of EP-El events, zonal advective feedback, meridional advective feedback and thermocline feedback contribute to the intensification during the negative AMO phase, while net surface heat flux is not conducive to the development. Further examination into the difference of low-frequency background state during different AMO phases showed that relatively cold and warm SSTs are observed in the warm pool and cold tongue during the negative AMO phase, accompanied by weakened and strengthened trade wind over the western and eastern equatorial Pacific. These changes in mean state SST zonal gradient and low-level wind cause variations in thermocline depth, wind stress and its curl. Both the shallower and deeper thermocline depths and upward and downward motions in the central and eastern equatorial Pacific are conducive to the intensification of CP-La and EP-El, respectively. It is worth noting that the dry mean state over the tropical Pacific combined with the zonal wind produces significant dry advection transport to the central tropical Pacific during the negative AMO, which enlarges the local sea-air humidity difference and promotes the release of LH flux from the ocean, hence strengthening CP-La during the developing period.

We mainly demonstrate the asymmetric modulation of the AMO on ENSO amplitude, with a focus on the differences between cold and warm events and between CP-type and EP-type events. Our conclusions are mainly based on observational and reanalysis datasets. Further verification through climate models is needed. Our current study mainly emphasizes the mean-state changes during different AMO phases on ENSO amplitude asymmetry, but the AMO can also modulate the high-frequency forcing to change the ENSO signal as reported previously (Wittenberg 2009; Williamson et al. 2018). Thus, the impact of AMO-related high-frequency signals on ENSO asymmetry needs further investigation. In addition, as tropical inter-basin linkages may also play a role (Cai et al. 2019), further exploration is needed to see whether the AMO can modulate interannual ENSO amplitude asymmetry by adjusting the local climate in the Atlantic and/or Indian oceans. Last, CP-ENSO events have emerged more frequently since the beginning of the 21st century, partly due to the global warming; and the cycles of the CP ENSO (with a quasi-biennial periodicity) and EP ENSO (with a quasi-quadrennial periodicity) are quite different (Timmermann et al. 2018; Zhang et al. 2019). Thus, whether the AMO can independently change the prominent periodicity in the Niño3 and Niño4 regions to cause the asymmetric modulation effect on ENSO amplitude is another urgent scientific issue to be addressed.

Acknowledgments

This work is supported by the National Natural Science Foundation of China (42192562). Jing Ma is supported by the National Natural Science Foundation of China under grant 42088101 and 42030605. Jiechun Deng is supported by the National Natural Science Foundation of China under grant 42475033.

Funding

This work is supported by the National Natural Science Foundation of China (grants 42192562, 42088101, 42030605 and 42475033)

Competing interests

The authors have no relevant financial or non-financial interests to disclose.

Author Contributions

Xingyang Guo contributed to do the analyses, made the figures and wrote the first draft of the paper. Haiming Xu formulated the main ideas, contributed to the design of the analyses, and helped review and editing the manuscript. Jing Ma and Jiechun Deng helped review and editing the manuscript.

Data availability

The HadISST v1.1 is obtained from https://www.metoffice.gov.uk/hadobs/hadisst. The NOAA-20CRv3 reanalysis dataset is available at

https://www.psl.noaa.gov/data/gridded/data.20thC_ReanV3.html, and the SODA2.2.4 data are derived from http://apdrc.soest.hawaii.edu/erddap/griddap.

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Asymmetric Modulation of the Atlantic Multidecadal Oscillation on ENSO Amplitude

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Abstract

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1 Introduction

2 Datasets and Methods

2.1 Datasets

2.2 Methods

a. Definition of AMO index

b. Definition of ENSO events and their types

c. Mixed layer heat budget

3 Results

3.1 Asymmetric ENSO amplitude during different AMO phases

3.2 Asymmetric responses of ENSO-related air-sea variables during different AMO phases

3.3 The role of AMO in influencing ENSO amplitude asymmetry

4 Summary and Discussion

Declarations

Acknowledgments

Funding

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