The influence of El Niño and Southern Oscillation (ENSO) on global climate has been studied extensively since the Tropical Ocean – Global Atmosphere (TOGA) era (e.g.
Trenberth et al. 1998; Alexander et al. 2002). About the regional influence of warm ENSO on East Asian monsoon variation, a particular important feature is the low-level anomalous anticyclone (AAC) over the western North Pacific (WNP) that develops from the ENSO developing fall (SON(0)) through the following spring (MAM(1)) /summer (JJA(1)).(e.g. Tanaka 1997) (The seasons are referred to Northern Hemisphere in this article.) The prolonged existence of the AAC is attributed to a Rossby wave response to cold sea surface temperature anomaly (SSTA) that is maintained by local air-sea heat exchanges (Wang 2000, 2002). The Matsuno-Gill response of the enhanced convection activity over Central Pacific causes anomalous northeasterly winds on WNP, and cools the local SST through Wind-Evaporation-SST (WES) feedback by strengthening the winter monsoon. The cold SSTA over WNP stabilizes lower atmosphere, suppresses convection, and induces atmospheric anticyclonic flow in the northwest via Rossby Wave response. As the anomalous northeasterly in the southeast quadrant of the AAC enhances the trade wind speed, the evaporation rate increases and feeds back to cold SSTA.
The above air-sea coupling requires the existence of equatorial heating and background northeasterly monsoon flow to enhance evaporation and cooling SSTA. It is criticized that this required condition is absent in the decaying summer when monsoon flow reverses and the trade winds retreat eastward away from the WNP, so the cold SSTA often weakens and vanishes (Xie et al. 2009). On the other hand, Wang et al. (2013) showed that even in JJA, mean northeasterly still existed in subtropical WNP, and supported the WES feedback between cold SST and AAC. Taking into account of the persistent Indian Ocean (IO) warming following the El Niño development till summer,
some studies proposed that the IO warming in El Niño peaking DJF acted like a discharging capacitor to maintain the WNP AAC in the following summer (Yang et al.
2007, Xie et al. 2009, 2016). The anomalous Walker circulation excites oceanic downwelling Rossby wave in the southeastern IO during the peak winter of El Niño, causing SST warming over the southwestern IO in the following spring and summer. The IO SSTA pattern induces a basin-wide SST warming in next summer via decreasing Indian monsoon wind speed. The easterlies associated with the atmospheric Kelvin wave response to the IO SST warming then maintain the AAC over WNP in JJA (1).
In addition to the IO warming, Terao and Kubota (2005) indicated that the interbasin SST anomaly contrast between warm IO and cold WNP was also an important feature that maintained the WNP AAC in the post El Niño summer. The easterlies of Kelvin wave response to the SSTA contrast result in off-equatorial low-level divergence, which forms the eastern part of the AAC. The suppressed rainfall over the low-level divergence further emanates anticyclonic Rossby waves and the AAC is thus extended westward. Yun et al.
(2013) addressed respective delayed impacts of SSTA from IO warming, West Pacific cooling and East Pacific cooling. They showed that the enhanced WNP subtropical high was a result of combined effect of IO warming and WP cooling, and that concurrent EP cooling could strengthen North Pacific high.
In view of the interannual oscillations of WNP AAC, Chung et al. (2011) found that the WNP AAC followed ENSO evolution at two distinct time scales, 2-3 year and 3-5 year. The AAC in 2–3 year oscillation exhibits an eastward propagating feature in corresponding SST anomaly, residing in the sinking branch of the local Hadley circulation due to enhanced convection over the Maritime Continent. The AAC possesses a barotropic vertical structure and is maintained by radiative cooling. The AAC in 3–5 year oscillation shows a quasi-stationary feature with enhanced convection over Central
Pacific (CP) and central IO. The maximum sinking motion is located southeastern to the WNP AAC center. The AAC acquires a baroclinic vertical structure. The overall features suggest the AAC with a 3-5 year period is a Rossby wave response to a persistent local cold SSTA associated with negative latent heat fluxes.
From the perspective of predictability of East Asia Monsoon, both Wang et al. (2013) and Kosaka et al. (2013) proposed that WNP AAC was a part of atmosphere-ocean coupled system over Indo-Pacific warm pool. Identifying from the first EOF mode of JJA 850hPa geopotential height over the Asian-Australian monsoon area, Wang et al. (2013) attributed the WNP AAC to broad SSTA contrast between warm northern IO and cold WNP, and the WNP AAC reinforced SSTA gradient by latent heat flux exchange anomalies related to strengthened WNP northeasterlies and relaxed IO southwesterlies.
The second EOF mode identified in Wang et al. (2013) is linked to a La Niña-like SSTA pattern and enhanced precipitation over the Maritime Continent, which is similar with the 2-3 year oscillating AAC reported by Chung et al. (2011). Kosaka et al. (2013) interpreted that the Kelvin Waves excited from warm northern Indian Ocean into WNP induced off-equatorial divergence and maintained the WNP AAC, whose westward extension into northern IO is, as suggested by Terao and Kubota (2005), a downwelling Rossby response to the suppressed WNP convection. They reasoned that northern IO is warmed by enhanced insolation due to clear sky under AAC and relaxation of mean southwesterlies by anomalous easterlies in the southern flank of AAC. Both studies (Wang et al., 2013;
Kosaka et al., 2013) conducted atmosphere-ocean coupled experiments to confirm the concept of the large-scale unstable coupled system, which could exist without but strongly excited by ENSO.
In a review article, Li et al. (2017) suggested that the above studies of interbasin atmosphere–ocean interaction across the Indo-Pacific warm pool is a new mechanism for
maintaining the summertime AAC over the western North Pacific. In addition, Li et al (2017) also added the moist enthalpy advection/Rossby wave modulation to the WES feedback mechanism as essential for the initial development and maintenance of the WNP AAC during El Niño mature winter and subsequent spring. As discussed above, the maintenance of WNP AAC through El Niño decaying seasons is complexed with several mechanisms and involves SST distributions in both Pacific and Indian Ocean. Hence, in order to clarify the roles of each process in observation, it is straightforward to investigate the maintenance of WNP AAC with respect to classified conditions of ENSO in decaying JJA. However, to the best of knowledge, only Yun et al. (2013) did separate different ENSO events, yet their classification accorded to the SSTA in El Niño peaking DJF instead of the subsequent JJA, during which signals could be mixed up potentially. The objective of this study is to analyze the evolution of SST distribution and associated climate anomalies for different types of El Niños that show fast decay, slow decay, or persistence in Niño3.4 SSTA during post-El Niño summer.
Chapter 2 Data and Classification of ENSO 2.1 Data
The data set used in this study covers a 59-year time span from 1958 to 2016. We adopt reanalysis data from the European Centre for Medium-Range Weather Forecasts (ECMWF) including wind, p-velocity, geopotential height, temperature and specific humidity fields for each pressure level and mean sea-level pressure for the surface level.
In order to analyze as many events as possible, the dataset is combined from monthly data of ECMWF ERA-40 (Uppala et al. 2005) and ERA-Interim (Dee et al. 2011). The temporal span is September 1957 to August 2002 for ERA-40, and January 1979 to present for Interim. First, the horizontal grid size of ERA-Interim is linearly interpolated
from 1.5° by 1.5° to 2.5° by 2.5° to be consistent with that of ERA-40. In the overlapping period (January 1979 to August 2002) of two datasets, values of two datasets are averaged to obtain a combined dataset of longer time span. The two data sets are compared as discussed in the appendix.
The abnormal large-scale circulation induced by El Niño is represented by 850hPa stream function (ψ) anomaly. The monthly data of 850hPa stream function is derived from the monthly data of 850hPa horizontal wind and density according to the following of air, 𝜓 is the stream function, and 𝜒 is the velocity potential. Because the large-scale circulation is quasi-geostrophic, we choose 850hPa stream function to represent the anomalous anticyclone in western north Pacific.
To represent the large-scale atmospheric heating before satellite era, vertically integrated heat source (〈Q1〉) and moisture sink (〈Q2〉) are both calculated from 6-hourly data of ERA-40 and ERA-interim from surface to 100hPa according to the following
〈 〉 =1
𝑔∫𝑃𝑃𝑠𝑓𝑐( )𝑑𝑝
𝑡𝑜𝑝 (8) The sea surface temperature data is from National Oceanic and Atmospheric Administration (NOAA)’s Extended Reconstructed Sea Surface Temperature (ERSST) version 4 (Huang et al. 2015).
Anomaly values are calculated as the deviation from monthly means of 1958-2016 climatology. The linear trend is removed from anomalies, and then three-month running mean is applied.
2.2 Classification of El Niño
The Oceanic Niño Index (ONI) from NOAA’s Climate Prediction Center is considered to represent El Niño evolution. ONI is defined as 3-month running-mean SSTA over Niño3.4 region (170°-120°W, 5°S-5°N).
The warm phase of ENSO, or the periods of El Niño from 1958-2016 are identified based on ONI following NOAA´s definition that periods with ONI above 0.5°C for at least of 5 consecutive months. Each boreal winter in the El Niño period is counted as an El Niño event since an El Niño always peaks at DJF. Observed El Niño events are categorized into fast-decay, slow-decay, and prolonged types, based on its ONI in the decaying summer, June-July-August, JJA(1). The El Niño events that decay quickly and have negative ONI in JJA(1) are defined as the fast-decay type events, and those events that have positive ONI in JJA(1) are the slow-decay type events. Furthermore, if ONI remains above 0.5 in the following winter, or D(1)JF(2), the event is defined as a prolonged El Niño. The only excluded event is 2002/03, because its ONI dropped below zero in the decaying April-March-June, but turned positive in JJA(1) again. The result of the classification is shown in Table 1. There are 11 fast-decay events, 5 slow-decay events
and 4 prolonged events. The ONI evolution of each El Niño type from JJA(-1) to JJA(2) is presented in figure 1.
Chapter 3 Results
The following results are based on the composite analysis of SSTA, low-level circulation and vertically integrated apparent heat source (〈Q1〉) anomalies of each El Niño type from the developing boreal falls to the following summers. All three types of El Niño events have an anomalous anticyclone over western North Pacific (WNP AAC) in JJA(1), but only fast and slow-decay types have WNP AAC during the El Niño mature winters, and the WNP AAC in the prolonged type establishes in the following springs.
3.1 Fast-decay El Niño Events
Figure 2 and figure 3 show the composite of SST anomaly, 850hPa stream function anomaly and 〈Q1〉 anomaly of the fast-decay El Niño evolution from developing falls, SON(0) to following summers, JJA(1). The fast-decay El Niño is accompanied by a positive Indian Ocean Dipole (IOD) of fair magnitude in SON(0), so the SSTA field is featured with three poles at the equator, which are heating, cooling, heating over CP, Eastern IO, Western IO, respectively. These SSTA poles drive corresponding 〈Q1〉 anomalies showed in figure 3, establish anomalous Walker Circulation, and induce a strong low-level quadrupled circulation by Gill-Matsuno type response; the quadruple symmetrical about the equator is composed of a pair of cyclones east and anticyclones west to the cooling center. The AAC is located over North IO, in the northwestern quadrant of the quadruple. It is the South IO AAC that drives oceanic downwelling Rossby Waves which deepen the thermocline and warm SST of the southwestern IO in the following seasons, as the charging stage of the IO capacitor effect.
In El Niño peaking winter (D(0)JF(1)), IOD mode change into a basin-wide warm SSTA, Indian Ocean Basin (IOB) mode while the SSTA pattern over the tropical Pacific remains El Niño condition and the quadruple circulation continues to dominates the low-level atmosphere, with its western components slightly shifting eastward. This shift is triggered by the abrupt demise of the eastern pole of IOD, and further amplified by WES feedback given by the background northeasterly. This coupling is supported by the synchronous migration of the maximum cold SSTA and atmospheric cooling from Eastern IO to WP.
In the following spring (MAM(1)), the local WES feedback may still exists over WNP remains clear, as indicated by the cold SSTA and anomalous northeasterly. Because the EP SSTA decays dramatically, the anomalously reversed Walker Circulation weakens and the suppressed convection as well as accompanying anomalous low-level divergence around WP/Maritime Continent diminishes. Since the westerly eastern to the divergence disappears, the equatorial easterly over WP, induced by Kelvin Wave response to the atmospheric heating and warm SSTA over tropical IO, is no longer counteracted, and supports the AAC vorticity. This easterly may also feedback to El Niño decay via triggering oceanic upwelling Kelvin Waves.
In JJA(1), the SSTA over Tropical Pacific turns to a La Niña condition, and the 〈Q1〉 anomaly shows strong atmospheric heating over eastern IO and Maritime Continent, and cooling over tropical Pacific and WNP. The strong enhanced convection is located around Maritime Continent and contributes to the sinking motion of WNP AAC in form of local Hadley Circulation. On the other hand, the equatorial easterly forced by zonal SSTA gradient of La Niña can also support WNP AAC by providing negative vorticity.
3.2 Slow-decay El Niño Events
Figure 4 and figure 5 show the composite of slow-decay type. For the slow-decay events, the composite fields of SSTA, circulation and 〈Q1〉 are similar to those of the fast-decay events in El Niño developing fall (SON(0)) and winter (D(0)JF(1)), but with IO feature of weaker amplitude. In SON(0), there is still a clear IOD pattern over IO but it does not turn into a strong IOB in D(0)JF(1), compared with the fast-decay type. Instead of forming a large anticyclone extended from IO to WNP like the fast-decay type, the eastward migration of the AAC from North IO to WNP in the El Niño peak winter with clear WES feedback over WNP in the decay type events. In D(0)JF(1) of the slow-decay type, the cold SSTA over WNP is even stronger than the fast-slow-decay type.
In MAM(1), because the EP SSTA remains fairly warm, the anomalously reversed Walker Circulation over the tropical Pacific is still strong, so the divergence over Maritime Continent contributes to part of the AAC over IO. The local cold SSTA southeast to the WNP AAC also continue to support it via WES feedback. Because there is still atmospheric cooling over the Maritime continent, the AAC does not shift eastward, like the one in the fast-decay type.
In JJA(1), the SSTA pattern is still a weak El Niño condition with warm SSTA over tropical CP, EP and IO but the cold core of SSTA over WP moves from WNP to southern hemisphere. The pattern of 〈Q1〉 anomaly shows a band of heating anomaly extends from northern IO to tropical central Pacific and cooing anomaly over WNP and New Guinea and forms a pattern similar to the Pacific-Japan pattern. Compared to WNP AAC in MAM(1), WNP AAC in JJA(1) shifts to the northern subtropics (10°-30°N). The AAC over WNP can still be maintained by the Rossby response to the SSTA west to the AAC itself while the contrast heating between northern IO and WNP induces easterly anomaly that can sustain the WNP AAC by low level divergence.
3.3 Prolonged El Niño Events
Figure 6 and 7 show the composite evolution of the prolonged type events from SON(0) to JJA(1). During the developing fall (SON(0)), the SSTA pattern over the tropical IO and the maritime continent is less prominent compared to the fast-decay and slow-decay cases, so the anticyclone pairs that appear in the fast-decay and slow-decay composites are unclear. The SSTA is significant over the southwestern IO, the central and eastern Pacific.
Because AAC does not form in IO in SON(0), there is no AAC shift from North IO to WNP in D(0)JF(1) in the prolonged type. As the result, the AAC in the northwestern quarter of the quadruple barely exists in D(0)JF(1) even though the prolonged events cause negative 〈Q1〉 anomaly over WNP by WES feedback in figure 6.b. On the other hand, convection activity is suppressed over western tropic IO, and an AAC is located over south IO.
In MAM(1), the suppressed convection organizes around Eastern IO and Maritime Continent. In response to the organized atmospheric cooling over Maritime Continent and atmospheric heating over central tropical Pacific, the anomalous circulation forms a quadrupole structure with a pair of anticyclones over IO and a pair of cyclones over central tropical Pacific. It is also noted that the AAC extends northeastward, and forms a secondary maximum over extratropical North Central Pacific in MAM(1). Because of the atmospheric cooling and northeasterly anomaly located at the southeastern of WNP AAC, it is convincing that the positive stream function anomaly over WNP is maintained by WES feedback. The positive SSTA over the IO basin does not trigger significant atmospheric heating so the warn IO should be the result of the increased solar radiation caused by suppressed convections.
In JJA(1), patterns of SSTA, 〈Q1〉 anomaly and 850hPa steam function anomaly do
not change much from the ones in MAM(1). The AAC over WNP still exists because the El Niño condition persists and induces atmospheric cooling over WNP by WES feedback.
Although the warm SSTA appears across IO basin, it does not conduct significant large-scale atmospheric heating. Thus, the IO SSTA is suggested to play a passive role.
Chapter 4 Discussions and Empirical Orthogonal Function Analysis 4.1 Discussions
Our results show that the WNP AAC does exist in all three types of El Niño events in the decaying summer categorized by this study (Figure 3d, 5d, and 7d). However, the evolution of the WNP AAC reveals different features in the three types of events.
WES feedback maintains the WNP AAC in the fast-decay (Figure 2b, 3b) and slow-decay type (Figure 4b, 5b) events in the El Niño peak winter, and works in all three types in the following spring, MAM(1). In contrast to the other two types, the fast-decay type of events reveals a dramatic decay in SSTA over equatorial Pacific from MAM(1) to JJA(1), and convective heating over the Maritime Continent/eastern IO and cooling over equatorial Pacific in JJA(1) is the key forcing for the maintenance of WNP AAC.
During boreal summers, northeasterly trades still dominate background winds over subtropical WNP (Wang et al., 2013), while in the slow-decay and prolonged composite fields, atmospheric cooling and associated cold SSTA remain weak in the AAC’s southeast quadrant. It suggests that in slow-decay or prolonged events, WES feedback can partly maintain WNPAC in JJA(1). In addition, the warm SSTA over tropical CP in the slow-decay and prolonged events maintains the heating source that triggers northeasterly anomaly over subtropical CP.
In the fast-decay events, the SSTA over equatorial Pacific is in La Niña condition, and
In the fast-decay events, the SSTA over equatorial Pacific is in La Niña condition, and