1. Introduction
El Niño–Southern Oscillation (ENSO) is the dominant mode of climate variability in the instrumental record, affecting climate around the globe via atmospheric teleconnections. ENSO teleconnections are the basis for seasonal prediction and have been studied extensively. With the accumulation of observations, evidence is emerging that some ENSO teleconnections, for example, for summer climate over India (Kumar et al. 1999) and the Indo–western Pacific (Xie et al. 2010; Huang et al. 2010), are not stable in time but experience substantial interdecadal modulations. Many studies of such low-frequency modulations of interannual variability are limited to recent decades after 1950 (Nitta and Yamada 1989; Trenberth and Hurrell 1994; Terray and Dominiak 2005; Annamalai et al. 2005; Xie et al. 2010), owing to the lack of high-quality observations over tropical oceans.
A few studies have examined ENSO-induced SST variability over the Indo–western Pacific region prior to World War II. Allan et al. (1995) studied the mean conditions over the Indian Ocean region during four 21-yr epochs since 1900 and suggested that epochal change is in phase with ENSO-related correlation pattern. Their study focuses mainly on the south Indian Ocean during the mature phase of El Niño. Using composites of pronounced ENSO events from the late nineteenth century to 1989, Reason et al. (2000) examined anomalies of SST, mean sea level pressure, and cloudiness over the Indian Ocean. Allan et al. (2003) studied interference of variability on quasi-biennial to quasi-decadal time scales for 1870 to 2000 in forming “protracted” ENSO episodes and affecting climate around the Indian Ocean rim.
This study examines interdecadal modulation of ENSO teleconnections into the Indo–western Pacific during the period 1870–2007, which has not been systematically studied in the literature. In particular, the present study overcomes the limitation of sparse high-quality data before 1950s with the use of observations along frequently traveled shipping lanes to examine long-term variations in ENSO teleconnections. Specifically, the opening of the Suez Canal in 1869 substantially shortened shipping routes from Europe to East Asia, channeling the traffic along a narrow track across the north Indian Ocean (NIO) from the tip of the African horn through the Malacca Strait and across the South China Sea to the Luzon Strait. In general, surface meteorological observations are well sampled along this busy shipping lane (Fig. 1), offering a rare opportunity to study interannual variability over NIO/South China Sea for more than 100 years since 1870. Hereafter, we denote seasons during the developing and decay years of El Niño with (0) and (1), respectively. Seasons are defined by those of the Northern Hemisphere.
Distribution of sea surface temperature observations from the ICOADS for (a) 1870–99, (b) 1900–39 (before World War II), (c) 1940–49 (during and shortly after World Wars), and (d) 1950–2007 (after World War II). Color shading indicates the percentage of months with at least one measurement in a 2° × 2° longitude and latitudinal box.
Citation: Journal of Climate 25, 5; 10.1175/JCLI-D-11-00070.1
The tropical Pacific exerts strong influence over tropical Indian Ocean variability during and after the mature phase of El Niño via teleconnections (Allan et al. 1995; Klein et al. 1999; Reason et al. 2000; Alexander et al. 2002; Kinter et al. 2002; Shinoda et al. 2004; Ashok et al. 2007; Schott et al. 2009; Du et al. 2009; Xie et al. 2009, 2010). The tropical Indian Ocean basinwide warming, induced by El Niño at its mature phase, proves to have important effects on the atmospheric circulation elsewhere. Previous studies have shown that the tropical Indian Ocean sea surface temperature (SST) anomalies induce significant changes in the atmospheric circulation over the Northern Hemisphere (e.g., Watanabe and Jin 2002; Annamalai et al. 2005, 2007). Basinwide warming as a “capacitor” persists from spring to summer after the mature phase of ENSO and substantially impacts atmospheric circulation over South Asia (Yang et al. 2007). The persistent tropical Indian Ocean warming also exerts influence on northwest Pacific climate during summer after El Niño {June–August [JJA](1)} through the Kelvin wave–induced Ekman divergence mechanism–capacitor effect (Xie et al. 2009), described as follows. In response to the tropical Indian Ocean warming during JJA(1), tropospheric temperature anomalies display a Matsuno (1966)–Gill (1980) pattern. A warm Kelvin wave in tropospheric temperature propagates into the equatorial western Pacific, inducing northeasterly surface wind anomalies on the northern flank of the Kelvin wave. The surface friction–induced subtropical divergence suppresses convection over the northwest Pacific, and the resulting feedback between convection and circulation gives rise to an anomalous anticyclone in the lower troposphere. Chowdary et al. (2011) test this Kelvin wave–induced Ekman divergence mechanism with forecast model experiments. By decoupling the tropical Indian Ocean variability, their study finds that without tropical Indian Ocean influence, the northwest Pacific atmospheric anomalies weaken by 50% in JJA(1).
Recent studies point to the strengthening of ENSO teleconnections to the northwest Pacific in JJA(1) (B. Wang et al. 2008) as evidenced in tropical Indian Ocean SST after the 1970s (Xie et al. 2010). The latter study suggests that a slowly decaying El Niño induces a more robust and persistent response over the tropical Indian Ocean and northwest Pacific after the 1970s regime shift. Furthermore, Xie et al. (2010) show that through the mechanism of Kelvin wave–induced Ekman divergence, a strong tropical Indian Ocean response translates into a pronounced development of atmospheric anomalies over the northwest Pacific in JJA(1) after the 1976/77 regime shift. As the tropical Indian Ocean SST warming does not persist through JJA(1) before the 1976/77s, atmospheric anomalies over the northwest Pacific weaken and are not well organized in space. Epochal changes in the spatiotemporal structure of the tropical Indian Ocean SST response to ENSO are also documented in their study. The SST warming over the NIO displays a peculiar double-peak structure, one at the mature phase of the El Niño and one during early summer following the El Niño peak (Du et al. 2009). An equatorially antisymmetric pattern of wind anomalies is key to the NIO warming during summer after El Niño (Du et al. 2009). Northeasterlies to the north of the equator and northwesterlies to the south, opposing the prevailing southwest monsoon mean winds, reduce surface evaporation and thereby warm the NIO. The antisymmetric wind pattern is anchored by the persistent SST warming over the southwest tropical Indian Ocean due to downwelling Rossby waves from the east (Xie et al. 2002; Du et al. 2009). This double-peak pattern in NIO warming is strong in the recent epoch after the 1970s but absent in the three decades before the 1970s (Xie et al. 2010). The second peak in NIO warming in turn plays an important role in maintaining the northwest Pacific atmospheric anomalies in JJA(1) and in reducing the frequency of tropical cyclones to the east of Philippines (Du et al. 2011). The strength of the summer northwest Pacific anticyclone is highly correlated with NIO SST (Xie et al. 2010; Huang et al. 2010).
The present study examines Indo–western Pacific climate variability in summer and its relationship with ENSO using the NIO–South China Sea ship track data and island meteorological observations dating back to late nineteenth century (1870–2007). The analysis of the historical SST is critical to our understanding of past, present, and future climate change. Following upon the recent studies of Xie et al. (2010) and Du et al. (2009), we examine interannual and interdecadal variability over the Indo–western Pacific associated with ENSO decay phase for different epochs. A major challenge in investigating the nature of epochal change in teleconnections is the lack of long-term observed data. Most studies of interdecadal ENSO teleconnections are based on atmospheric reanalysis, which are known to suffer spurious changes across the 1970s (Wu and Xie 2003) most likely because of the substantial increase and assimilation of satellite data. Our analysis of frequently traveled ship lanes data and land–station observations show that there are strong epochal changes in the ENSO’s influence on the Indo–western Pacific during JJA(1) for the period of 1870–2007. These results are supported by atmospheric model simulations forced with observed SST for the nearly 140-yr period. Our results suggest that the natural variability and in association with changes in ENSO variance is the key for epochal changes in ENSO’s influence on the Indo–western Pacific.
The paper is organized as follows. The following section outlines the details of different datasets used in the study. Section 3 presents dominant modes and trends in NIO ship SST. Section 4 discusses ENSO influence on the NIO in different epochs. Underlying mechanisms responsible for epochal differences in NIO warming and air–sea interactions are outlined in section 5. Section 6 describes epochal change in ENSO teleconnections to the South China Sea and northwest Pacific climate. Section 7 provides atmospheric general circulation model (AGCM)-simulated teleconnection changes in different epochs. Section 8 discusses possible causes for change in epochal teleconnections over the Indo–western Pacific linked to ENSO. The major results and conclusions of this study are summarized in section 9.
2. Data, model, and analysis methods
Weather observations are essential for global and regional climate studies and prediction. We use frequent ship lanes surface marine data over the Indo–western Pacific region from the mouth of Gulf of Aden, through Malacca Straits and the South China Sea and the Luzon Strain to East Asia for the period of 1870–2007 (Fig. 1). Dense data along such frequent ship lanes give strong evidence for the long-term climate variability and Indo–western Pacific teleconnections to ENSO, whereas widely used gridded SSTs tend to be affected by large-scale SST variability away from targeted regions because of their spatial interpolation methods such as the empirical orthogonal functions (EOFs) (Kaplan et al. 1998; Smith et al. 1998). We apply a simple linear average along the frequently traveled ship lanes over three regions, the NIO, South China Sea, and northwest Pacific, which are defined as 2° grid boxes that contain monthly mean SSTs for more than 80% of the total months from 1870 to 2007 (Fig. 2). Because the data coverage of each northwest Pacific ship lane is 50%–70% for the entire period, we average the data zonally around 130°–150°E by taking advantage of zonally uniform characteristics of ocean–atmosphere anomalies over this region.
Data averaged along the ship tracks (gray shading) over the NIO, South China Sea, and northwest Pacific are used for analysis during period of 1870–2007.
Citation: Journal of Climate 25, 5; 10.1175/JCLI-D-11-00070.1
Surface wind velocity, SST, cloudiness, specific humidity, and sea level pressure (SLP) data along ship lanes are extracted from the International Comprehensive Ocean–Atmosphere Dataset (ICOADS) 2.5 (Woodruff et al. 2011). All data have been trimmed using the ICOADS quality-control flag that identifies potential outliers based on the climatological 4.5 standard deviation limits. This is basically the same quality control as used for the enhanced product of the ICOADS Monthly Summary Groups available at the Research Data Archive (RDA) at the National Center for Atmospheric Research (NCAR). The record length of ~140 years (1870–2007) allows the investigation of the Indo–western Pacific climate variability associated with ENSO in different multidecadal epochs. Discontinuity in ship meteorological parameters is identified during World War II and a few years thereafter, which is due to the Suez Canal being closed several times during armed conflicts. This causes significant data gaps in 1940s along ship lanes. To avoid such a discontinuity in our SST analysis, we have filled the gap with Hadley Centre Global Sea Ice and Sea Surface Temperature (HadISST1) for the 1940s period. Global mean SSTs drop in the 1940s owing to differences in SST measurement techniques (Thompson et al. 2008). SST data were sampled on board ships using a variety of measurement techniques, including measurements taken from insulated and uninsulated buckets and engine room intakes (e.g., Folland and Parker 1995; Rayner et al. 2006). Corrections for the discontinuity or jumps in SST data in 1940s are expected to alter the character of midtwentieth-century temperature variability but not the estimate of the century-long trend in global-mean temperatures (Thompson et al. 2008) and teleconnections. Our results discussed in the paper are unaffected by the sudden drop and data gap in the 1940s.
Several other SST products are used to compare with ship track SST data. We have extracted monthly SST data along the ship tracks over the Indo–western Pacific region from different SST reanalysis products. These datasets include HadISST1 on a 1° latitude–longitude grid from 1870 to date (Rayner et al. 2003), the National Oceanic and Atmospheric Administration (NOAA)–National Climatic Data Center (NCDC) Extended Reconstruction SST (ERSST) version 2 (Smith and Reynolds 2004), the extended Kaplan SST dataset (Kaplan et al. 1998) and Centennial in situ Observation-Based Estimates of SSTs (COBE-SST; Ishii et al. 2005) from the Japan Meteorological Agency (JMA).
Observed station (e.g., Seychelles) rainfall data for twentieth century is obtained from NOAA. In addition, we make use of the gridded dataset of land monthly precipitation for the 1850–1995 of Dai et al. (1997) and the Climate Prediction Center (CPC) Merged Analysis for Precipitation (CMAP; Xie and Arkin 1997). Using the distance-weighted scheme, Dai et al. (1997) interpolated historical land station precipitation data onto a 2.5° × 2.5° grid. The spatial sampling error for the gridded precipitation anomalies is large (up to 45%) for areas having only a few stations per 2.5° × 2.5° grid box. Thus signals in precipitation fields covering only a few 2.5° × 2.5° grid boxes are generally not important, especially in sparsely covered areas (Dai et al. 1997). To overcome this problem, rain gauge–based precipitation data are constructed for the Philippines and Guam for the period of 1900 to 2007. A simple linear average for 37 Philippine stations is used from the Monthly Bulletins of Philippine Weather Bureau (Kubota and Chan 2009).
In addition to the observational data described above, we analyze a five-member ensemble integration with the NCAR Community Atmosphere Model, version 3 (CAM3; Collins et al. 2006) forced by observed monthly SSTs over the tropical oceans (20°S–20°N) during 1871–2001 from the dataset of Hurrell et al. (2008). Outside the tropics, climatological monthly SSTs are prescribed, with a linear transition between 20° and 30°. The horizontal resolution of the model is T85, equivalent to 1.4° latitude 1.4° longitude. The ensemble members differ only in their initial conditions. This integration is referred as Tropical Ocean–Global Atmosphere (TOGA) simulation. The CAM3 TOGA run has been conducted by the NCAR Community Earth System Model Climate Variability Working Group and made available to the community (http://www.cesm.ucar.edu/working_groups/Variability/experiments.html). Both observational and model results are further compared with twentieth-century reanalysis (20CR) (Compo et al. 2011). Tropospheric temperature is represented by the model geopotential height difference between 200 and 500 hPa.
Climate indices are essential for examining the interdecadal modulations in global teleconnection patterns. Therefore in this study, various climate indices are obtained from different sources. Southern Oscillation index (SOI) based on marine surface observations is used (Allan et al. 1991). The Niño-3.4 index is computed using the ICOADS 2.5 SST anomalies. Note that the density of observations in the Niño-3.4 region may differ from the NIO in late 1800s. However, we have compared our Niño-3.4 index with different datasets (e.g., HadISST1 and ER SST) and found that our Niño-3.4 index is highly correlated with other datasets. Correlation coefficient between November–January (NDJ) Niño-3.4 index and the December–February (DJF) SOI is −0.88 for epoch 1 and is −0.83 for the entire study period. The Pacific decadal oscillation (PDO; Mantua et al. 1997) and the interdecadal Pacific oscillation (IPO; Power et al. 1999) indexes are calculated from HadISST1. EOF, linear correlation, and regression analyses are used to quantify the relationship between ENSO and climate anomalies of the Indo–western Pacific region. A season-reliant EOF (S-EOF) analysis method (Kim and Wu 1999; Wang and An 2005) is used to objectively identify the ENSO-related mode in different epochs. The advantages of the S-EOF as compared to the conventional EOF analysis are described in detail in B. Wang et al. (2008). Latent heat flux (LHF), obtained from ship meteorological data, is decomposed as the oceanic response that arises from the SST dependence on evaporation (Newtonian cooling) and atmospheric forcing (AtF-L) due mostly to changes in wind speed, relative humidity, and air–sea temperature difference (de Szoeke et al. 2007; Du and Xie 2008; Du et al. 2009). Shortwave (SW) radiation is computed based on the bulk formula of Gill (1982). Monthly varying cloudiness from ship track data and clear-sky radiation from the International Satellite Cloud Climatology Project (ISCCP) are used for the SW radiation calculation. Ship data anomalies are calculated based on the monthly mean climatology for the period of 1870–2007. A 9-yr running mean is removed to suppress the decadal and long-term variations in the datasets. To reduce the effect of pronounced intraseasonal variability over the Indo–western Pacific Oceans, a 3-month running average is applied. The typical decorrelation time for such bandpass-filtered time series is slightly more than half a year. We estimate the total degree of freedom as (analysis period in years)/1.5. For a 21-yr (27-yr) time series, a correlation of 0.43 (0.38) and 0.5 (0.44) reaches the 90% and 95% significance level, respectively, based on a two-tailed Student’s t test.
3. Dominant modes and trend in NIO SST
The 137-yr unfiltered monthly SST anomaly time series averaged along the NIO ship track is shown in Fig. 3a. A clear warming trend with ~1.4°C over the 137-yr period (thin dotted line) occurs in the NIO SST. The warming trend of ~0.1°C per decade is consistent with many other regional warming trends of different tropical oceans (e.g., Wang and Mehta 2008). There are interannual warming and cooling events evident in the detrended ship SST anomaly over the NIO (Fig. 3b). Bandpass-filtered NIO SST exhibits large variations with significant peaks at 1.5 and 3.5 years in the Fast Fourier Transform analysis (not shown), which are associated with ENSO. The correlation between NIO ship SST and other SST products along the ship track is high (r > 0.8) for the entire period of 1870–2007 (Fig. 4a), indicating a consistency with other analyzed datasets. The standard deviations of the NIO SST anomalies along the ship track are low over most of the Bay of Bengal but high over the southern tip of India and western Arabian Sea, a pattern that resembles the leading EOF pattern (Figs. 4b,c). The leading EOF explains 63.62% and 76.66% of the variance for ship track and HadISST data, respectively. The explained variance is larger for the spatial-smoothed HadISST data. High standard deviation over the southern tip of India is associated with ocean dynamics and thermodynamics during both summer and winter monsoon seasons, respectively (McCreary et al. 1993; Rao et al. 2008). The western Arabian Sea SST variability is strongly influenced by ocean dynamics such as upwelling and horizontal advection (McCreary et al. 1993; Murtugudde et al. 2000; Behera et al. 2000). Interannual variance is higher in the ship track data than in the HadISST, as the latter is heavily smoothed in space. The correlation between the leading principal components of ship data and HadISST is high over the NIO (Fig. 4d). The principal component lagged correlation with the Niño-3.4 index, where the Niño-3.4 index leads by 3 months, for both SST data exceeds 0.6 for the 137-yr period, which is highly significant at more than the 99% confidence level (Fig. 4d).
(a) Time series of monthly mean SST anomaly (°C) averaged along the NIO ship track for 1870–2007 and (b) time series after removing trend. Dotted line denotes the linear trend in (a).
Citation: Journal of Climate 25, 5; 10.1175/JCLI-D-11-00070.1
(a) Correlation between ship NIO SST anomaly time series and that of different SST products (HadISST1, ERSST, Kaplan SST, ICOADS 2.0, and COBE-SST), (b) standard deviation (°C) of ship data and HadISST1 SST, (c) first EOF of NIO SST, and (d) principal component correlation between ship data (SPC) and HadISST1 (HPC) and correlation of first principal component with Niño-3.4 index, where the Niño-3.4 index led by 3 months.
Citation: Journal of Climate 25, 5; 10.1175/JCLI-D-11-00070.1
4. NIO warming associated with El Niño
El Niño–induced SST warming over the tropical Indian Ocean–NIO generally starts in [September–November (SON)] SON(0) and persists for 1–3 seasons. Figure 5 shows the correlation in 21-yr sliding windows between NIO SST averaged along the ship track and 
Correlation (shaded) in 21-yr sliding windows between NIO SST anomalies (averaged along ship track) and 
Citation: Journal of Climate 25, 5; 10.1175/JCLI-D-11-00070.1
Figure 6 displays the correlation of SST and surface winds with the 
Correlation of SST (shaded) and surface winds (vectors) along the NIO ship track, with the 
Citation: Journal of Climate 25, 5; 10.1175/JCLI-D-11-00070.1
Prior to examining the mechanism associated with differences in warming among four epochs, we conduct S-EOF analysis from DJF(0) (a year before the mature phase of El Niño) to DJF(2) (a year after the mature phase of El Niño) to objectively identify SST modes over the NIO. Figure 7 shows the ENSO-related S-EOF SST modes, which are very similar to the SST anomalies associated with ENSO obtained via correlation analysis. The time series of the principal component corresponding to the ENSO-related S-EOF is significantly correlated with 
ENSO-related S-EOFs of NIO ship track SST for the interval from DJF(0) (a year before the mature phase of El Niño) to DJF(2) (a year after the mature phase of El Niño) for (a) epoch 1, (b) epoch 2, (c) epoch 3, and (d) epoch 4. Surface wind anomalies (vectors, m s−1) regressed upon the principal component corresponding to the spatial pattern (shaded) in each epoch. (top) Percentage of variance explained by S-EOF is shown at the top of each panel.
Citation: Journal of Climate 25, 5; 10.1175/JCLI-D-11-00070.1
5. Underlying mechanisms: Role of air–sea interactions
a. Local air–sea interactions
The ENSO-induced large-scale atmospheric teleconnections alter the near-surface air temperature, humidity, and wind, as well as the distribution of clouds over tropical Indian Ocean. The resulting variations in the surface heat, momentum, and freshwater fluxes force SST anomalies in the NIO during ENSO (Klein et al. 1999; Alexander et al. 2002; Du et al. 2009). Many studies emphasize the role of heat fluxes in this region based on observational analyses (Yu and Rienecker 1999; Reason et al. 2000; Tokinaga and Tanimoto 2004; Chowdary and Gnanaseelan 2007; Du et al. 2009) and models (Behera et al. 2000; Venzke et al. 2000).
Ship reports include observations of cloudiness, wind speed, surface air temperature and relative humidity, permitting calculation of surface heat flux along ship tracks starting from the 1870s. Figure 8 shows the regression of the atmospheric component of LHF (AtF-L), surface wind speed, and SW radiation upon the 
Regression coefficients of AtF-L (shaded; W m−2), surface winds (vectors; m s−1), and shortwave radiation (contours; W m−2) along the NIO ship track, upon the 
Citation: Journal of Climate 25, 5; 10.1175/JCLI-D-11-00070.1
In epoch 3, positive SST anomalies decay rapidly after [January–February (JF)] JF(1) in the Arabian Sea (Fig. 6c). During this epoch, AtF-L appears to be more dominant in controlling the SST than SW radiation. Warming during epoch 3 decreases rapidly compared to other epochs because of a sudden change in heat flux components (Fig. 8c). Wind speed variations are consistent with AtF-L and hence the SST pattern. Unlike other epochs, during epoch 4 northeasterly wind anomalies persist for 3–4 seasons over the NIO. Figure 8d shows that AtF-L is consistent with the double-peak SST warming pattern.
The warming associated with the first peak during SON(0) is mostly confined to the Arabian Sea where the anomalous northeasterlies weaken the mean southwesterlies (Lau and Nath 2000; Du et al. 2009), and the resulting AtF-L warms the SST there. SST cooling in the late winter and early spring is due to the AtF-L response to wind anomalies. Strong easterly wind anomalies intensify the northeast monsoon mean winds, cooling the SST owing to increased latent heat fluxes over the NIO. The asymmetric mode in wind anomalies with northeasterlies north of the equator and northwesterlies south of the equator attains its peak phase at the same time (Kawamura et al. 2001; Wu et al. 2008; Wu and Yeh 2010; Du et al. 2009). As the mean wind turns southwesterly in May over the NIO, the northeasterly anomalies oppose the mean southwesterlies and reduce the LHF, giving rise to the second peak of the SST warming in JJA(1). Over the Bay of Bengal, SST warming is delayed by about a season and SW radiation also seems to play a role in maintaining the SST warming in late spring and early summer (Fig. 8d). Overall in most of NIO during JJA(1), northeasterly wind anomalies reduce the southwesterly mean flow to warm the SST because of the reduced AtF-L (Du et al. 2009) in both epochs 1 and 4.
b. Role of southwest tropical Indian Ocean
During El Niño, atmospheric teleconnection forces downwelling Rossby waves in the southern tropical Indian Ocean, which propagate westward (Masumoto and Meyers 1998) and induce SST warming in the southwest tropical Indian Ocean (Xie et al. 2002; Huang and Kinter 2002). At the El Niño mature phase, strong downwelling Ekman pumping associated with anomalous wind curl in the southeast tropical Indian Ocean forces a downwelling Rossby wave. As the downwelling Rossby waves propagate into the thermocline ridge over the southwest tropical Indian Ocean, they induce positive SST anomalies via thermocline feedback (Xie et al. 2002). The SST warming propagates slowly westward with the oceanic Rossby waves and persists through spring and early summer (Du et al. 2009; Xie et al. 2010). Warm SST anomalies over the southwest tropical Indian Ocean drive an antisymentric wind pattern with northeasterly anomalies over the NIO. These northeasterly anomalies in north of the equator help to warm SST over the NIO as discussed in the previous section by adjusting the heat flux during JJA(1).
Here, we examine the evolution of SST and cloudiness over the southwest tropical Indian Ocean for the twentieth century. Figure 9a shows the correlation of ICOADS SST and cloudiness anomalies averaged between 55°–70°E and 5°–12°S with the 
The 21-yr sliding correlation between 
Citation: Journal of Climate 25, 5; 10.1175/JCLI-D-11-00070.1
6. South China Sea and northwest Pacific
Interannual SST anomalies in the South China Sea are highly correlated with ENSO (Xie et al. 2003; Liu et al. 2004; Wang et al. 2006; Du et al. 2009). Similar to the NIO warming, South China Sea SST also exhibits apparent interdecadal variability in responses to El Niño during the past century. Figure 10 shows 
Correlation of SST (shaded) and surface winds (vectors) along the South China Sea ship track, with the 
Citation: Journal of Climate 25, 5; 10.1175/JCLI-D-11-00070.1
At the ENSO decay phase, pronounced atmospheric anomalies are observed over the northwest Pacific (Wang et al. 2000; Xie et al. 2009; Chowdary et al. 2010, 2011). Figure 11 shows correlations of the 
Correlation of SLP (shaded) and cloudiness (contours) along the northwest Pacific ship track (east of the Philippines) with the 
Citation: Journal of Climate 25, 5; 10.1175/JCLI-D-11-00070.1
The Philippines and Guam are located in the subtropical northwest Pacific region that tends to experience rainfall deficits during the decay phase of El Niño from winter to early summer. Here we analyze historical rain gauge data for the Philippines and Guam from 1900 to 2007 to corroborate interdecadal modulation of the ENSO teleconnection. As seen in the northwest Pacific ship track cloudiness analysis, negative rainfall anomalies begin around fall [SON(0)] in response to El Niño over the Philippines (Fig. 12). This negative rainfall response ends around early summer at the beginning and end of the (epoch 4) twentieth century. This Philippine rainfall variability in the summer following El Niño in recent decades is consistent with the findings of Xie et al. (2010). Guam is located at 13.6°N and 144.8°E, which is almost at the center of negative precipitation anomalies during summer after El Niño. As shown in Fig. 12, significant negative precipitation anomalies at Guam extend to JJA(1) at the end of the twentieth century. Similar negative rainfall anomalies are seen at the beginning of the twentieth century but are significant at 90% confidence level only until early summer. Epoch 3, starting from 1950s, features low correlations in summer following El Niño.
The 21-yr sliding correlations between the 
Citation: Journal of Climate 25, 5; 10.1175/JCLI-D-11-00070.1
Furthermore, to examine the spatial pattern of summer rainfall we use land rainfall data from Dai et al. (1997) for the first three epochs and CMAP data (both land and ocean) for the most recent epoch. Figure 13 shows the spatial pattern of JJA(1) precipitation correlation with the 
The JJA(1) correlation of land precipitation with the 
Citation: Journal of Climate 25, 5; 10.1175/JCLI-D-11-00070.1
7. Analysis of TOGA CAM3 simulations
Because of its relatively slow variations, SST is generally better observed than sparse meteorological measurements prior to 1950. Forced by observed SST, atmospheric GCMs show skills in simulating ENSO teleconnections and their interdecadal modulation (e.g., Huang et al. 2010). To complement our observational analysis, we examine the ensemble mean of five simulations from an AGCM forced with TOGA SST from 1871 to 2000. Figure 14 shows the ENSO correlation with JJA(1) SST, SLP, and surface wind velocity from the ensemble mean of the CAM3 TOGA run. Note that during the most recent epoch, model data are available for a shorter period of only 24 years instead of 27 years. Five AGCM integrations (ensemble members) may be adequate for the tropical response, especially for correlation analysis. Correlation maps between 
The 
Citation: Journal of Climate 25, 5; 10.1175/JCLI-D-11-00070.1
The SST displays warming north of the Maritime Continent and over the tropical Indian Ocean mainly in epochs 1 and 4 and to some extent in epoch 2 (Fig. 14). The model reproduces the main features of the Indo–western Pacific teleconnections during the summer following El Niño well in comparison with observations (e.g., Xie et al. 2009). Chowdary et al. (2011) conducted a pair of experiments using a coupled GCM, one with an interactive air–sea coupling in the tropical Indian Ocean and one prescribing climatological SST. The AGCM circulation patterns in the present paper are consistent with coupled model experiment result. Specifically, the model shows a strong anticyclonic circulation in the northwest Pacific region both in epochs 1 and 4. Basinwide warming over the tropical Indian Ocean appears strong during epoch 2 but the northwest anticyclonic circulation seems weak. This may be due to the presence of significant warm SST anomalies east of the Philippines, which act to push the center of the northwest Pacific high pressure eastward in epoch 2 as compared to epochs 1 and 4. In addition, the equatorial Pacific warming weakens the off-equatorial high pressure anomalies via Rossby wave mechanism. During epoch 3, basinwide tropical Indian Ocean warming is very weak in JJA(1), and as noted by earlier studies (Huang et al. 2010; Xie et al. 2010), both the anticyclone and SLP high over the northwest Pacific shifted slightly to the east of 120°E and weakened in magnitude. SST warming over the NIO in different epochs is consistent with ship track data during JJA(1) (Fig. 6).
Forced with observed SST, AGCMs have difficulty simulating Asian–Pacific summer monsoon rainfall properly (Wang et al. 2005). By contrast, JJA(1) rainfall patterns are fairly well simulated over the Indo–western Pacific region in the current AGCM. When correlated with 
As in Fig. 14, but for precipitation (shaded) and tropospheric temperature (contours). Note that tropospheric temperature is represented by the geopotential height difference between 200 and 500 hPa.
Citation: Journal of Climate 25, 5; 10.1175/JCLI-D-11-00070.1
The results presented so far suggest that the most robust atmospheric signals are evident over the northwest Pacific in the summer following El Niño both in observations and in the model. Earlier studies reported that the northwest Pacific JJA(1) atmospheric anomalies (such as precipitation, SLP, and winds) are highly predictable in coupled models and also emerge as one of the leading modes in interannual variability (Wang et al. 2009; Chowdary et al. 2010, 2011). The leading EOF patterns corresponding to JJA(1) SLP and rainfall over the northwest Pacific are well represented in the model for all four epochs (not shown).
8. Discussion: Causes of interdecadal change in ENSO teleconnection
Our results suggest that there is clear interdecadal modulation of interannual variability in the Indo–western Pacific region in relation to ENSO over the past 130 years. What causes such interdecadal–epochal change in ENSO-induced anomalies? Are the epochal changes described in the present study part of ongoing global warming? Xie et al. (2010) hint at the possible mechanism that gives some sign of the global warming effect on the recent strengthening of ENSO teleconnections to the Indo–western Pacific at the ENSO decay phase based on data analysis of the past 60 years. The secular warming of the tropical Indian Ocean since the 1950s (Du and Xie 2008) may enhance the convective and tropospheric temperature response to interannual anomalies of the tropical Indian Ocean SST. The enhanced ocean temperature stratification helps strengthen the tropical Indian Ocean basin mode warming response to El Niño by shoaling the mixed layer and intensifying thermocline feedback in upwelling regions such as the southwest tropical Indian Ocean (Xie et al. 2010). This secular warming may have some influence on the ENSO teleconnections to the Indian Ocean in the recent epoch. Indeed, Zheng et al. (2011) show that El Niño–induced tropical Indian Ocean warming and the anomalous anticyclone in the northwest Pacific persist for a long period of time in a coupled GCM that simulates global warming in response to induced GHG.
Looking back into historical data more than a century provides a perspective about whether these teleconnection changes are related to climate change or to natural variability. A strong ENSO teleconnection to the Indo–western Pacific region occurs during JJA(1) in epochs 1 and 4. For example, Fig. 16a illustrates the 21-yr sliding correlation of JJA NIO SST (ship) and northwest Pacific SLP (CAM3 run) anomalies with the 
(a) The time series of the PDO index (scaled by 2; gray bars), the IPO index (black line), and 21-yr sliding correlations of 
Citation: Journal of Climate 25, 5; 10.1175/JCLI-D-11-00070.1
Another hypothesis for variations in ENSO impacts involves variations in the PDO (Mantua et al. 1997; Gershunov and Barnett 1998). The PDO is defined here as the leading EOF of North Pacific SST (north of 20°N) for the extended winter (November–March) from 1900 to 2009. Both the seasonal cycle and global mean SST are removed at each grid point prior to the EOF analysis. The PDO index is generated by projecting North Pacific SST for all months from 1870 to 2009 and then standardizing the resulting time series. The time series of the PDO (5-yr smoothing is applied) is in-phase with the modulation of ENSO teleconnections to the summer Indo–western Pacific in the two most recent epochs starting from 1950s (Fig. 16a). ENSO teleconnections are weak during the negative phase of PDO in epoch 3 (1950–mid-1970s) but strong during the positive phase of the PDO (epoch 4). This is consistent with some earlier studies suggesting PDO modulation of ENSO correlations with Australia precipitation (Power et al. 1999) and with the East Asia winter monsoon (L. Wang et al. 2008). However, the relationship between the PDO and the correlation between the Niño-3.4 index and the Indo–western Pacific is very weak prior to the 1950s.
The IPO (Power et al. 1999) index is plotted along with the PDO index in Fig. 16a. The IPO index is calculated based on a procedure as in Parker et al. (2007). Similar to the PDO index, IPO also displayed some coherence with ENSO teleconnections to the Indo–western Pacific after 1950s. In the late nineteenth and the first-half of the twentieth century, the IPO is not correlated with ENSO teleconnections. This shows that epochal changes of ENSO teleconnections to the Indo–western Pacific in summer is more complicated than previously thought. Apart from PDO and IPO, decadal ENSO-like mode also exhibits some impact on the climate of the Indian Ocean and adjacent landmasses on a range of decadal to multidecadal time scale (Allan and D’Arrigo 1999; Allan et al. 2003).
To examine if ENSO variability itself is responsible for changes in decadal teleconnection patterns, we present 
9. Summary
We have studied the centennial modulation of interannual variability over the Indo–western Pacific and its relationship to ENSO for 1870–2007 through the innovative use of surface meteorological observations along a busy ship track across the north Indian Ocean and South China Sea. Four epochs, 1883–1909 (epoch 1), 1910–36 (epoch 2), 1950–76 (epoch 3), and 1977–2003 (epoch 4) are chosen based on the seasonal evolution of NIO SST warming associated with ENSO. The period during and shortly after World War II is not examined in detail because of insufficient data.
El Niño–induced warming in the NIO is found to display a double-peak pattern in time during epochs 1 and 4 and is especially prominent in the Arabian Sea (west of 75°E). These results are supported by S-EOF analysis for different epochs. The second peak of the NIO warming in summer following El Niño is primarily driven by air–sea interactions within the tropical Indian Ocean. Warming in the southwest tropical Indian Ocean drives the northeasterly anomalies over the NIO. In the latter region easterly wind anomalies intensify the northeast monsoon mean winds and cool the SST because of latent heat fluxes in winter and early spring after El Niño in epoch 4. As the mean wind turns southwesterly in April and May over the NIO, northeasterly anomalies oppose the mean southwesterlies, and the reduced latent heat flux gives rise to the SST warming in JJA(1). This mechanism is similar to that of Du et al. (2009). The warming in the NIO during JJA(1) is of interest because it anchors the northwest Pacific anticyclonic circulation (Xie et al. 2009) and reduces the number of tropical cyclones in the latter region (Du et al. 2011).
Our analysis reveals that ENSO teleconnections to the Indo–western Pacific in the spring and summer following El Niño has strengthened in the most recent decades, which is consistent with previous studies (Xie et al. 2010). Similarly strong teleconnections are also observed in epoch 1 between the late nineteenth and early twentieth centuries. SST warming over the Indo–western Pacific, the development of northwest Pacific anticyclonic circulation anomalies, and the tropospheric temperature distribution are the most robust anomalies associated with ENSO during the summer following El Niño. During epochs 2 and 3 the ENSO teleconnection to the northwest Pacific is weak during JJA(1), a result corroborated by island observations at Guam and the Philippines. The AGCM forced with observed SST (TOGA) anomalies reproduces such epochal changes. Specifically, the northwest Pacific anticyclone is weak and shifted to east of 120°E during epochs 2 and 3. These results are also reflected in the twentieth-century reanalysis (see appendix).
The strengthening and weakening of the ENSO influence on the Indo–western Pacific region during spring and summer following El Niño in different epochs are related to variations with El Niño. The decadal changes in ENSO teleconnection coincide with that of the PDO and IPO after the 1950s, but this correlation breaks down prior to 1950s. Our results suggest that this decadal modulation of teleconnections associated with ENSO over the Indo–western Pacific is governed primarily by ENSO variance. A recent study based on North American tree rings showed that interdecadal modulation of ENSO amplitude has been common for the past millennium (Li et al. 2011). The strengthening of ENSO influence on the Indo–western Pacific in recent decades and in the late 1800s may be a part of natural variability of the ocean–atmosphere system. This shows that the most recent epochal change of strengthening in ENSO–Indo–western Pacific correlation are most likely not outside the range of natural variability. It is still interesting to ask how global warming affects ENSO teleconnection to the Indo–western Pacific (Zheng et al. 2011). Future studies need to investigate physical mechanisms for slow modulation of ENSO and its teleconnections.
Acknowledgments
This work is supported by the U.S. National Science Foundation, the National Basic Research Program of China (2012CB955600), and the Japan Agency for Marine-Earth Science and Technology. We wish to thank June-Yi Lee for the S-EOF code, Clara Deser and Adam Phillips of the CESM (Community Earth System Model) Climate Variability Working Group for making the CAM3 TOGA simulations available, the editor, and anonymous reviewers for their valuable comments that helped improve the manuscript. Kaplan SST V2 and ERSST data are provided by the NOAA/OAR/ESRL PSD, Boulder, Colorado. Figures are prepared in Grads.
APPENDIX
Twentieth-Century Reanalysis
Decadal changes in ENSO teleconnection to the Indo–western Pacific are evident in both ship observations and atmospheric model simulations. Here, we analyze the 20CR data from 1871 to 2008 to verify whether such decadal change exists in reanalysis data and is similar to our observations. Figure A1 shows correlations of JJA(1) 2-m temperature, SLP, and 850-hPa wind anomalies with the 
The 
Citation: Journal of Climate 25, 5; 10.1175/JCLI-D-11-00070.1
As in Fig. A1, but for precipitable water (shaded) and tropospheric temperature (contours). Tropospheric temperature is represented by the geopotential height difference between 200 and 850 hPa.
Citation: Journal of Climate 25, 5; 10.1175/JCLI-D-11-00070.1
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