1. Introduction
El Niño–Southern Oscillation (ENSO) extremes have been linked to global climate modifications (e.g., Ropelewski and Halpert 1987, 1989; Trenberth et al. 1998; Diaz et al. 2001) and substantial precipitation impacts over portions of the United States (e.g., Ropelewski and Halpert 1986; McCabe and Dettinger 2002), southwest Asia (Barlow et al. 2002; Mariotti 2007; Syed et al. 2006, 2010), and eastern Africa (Farmer 1988; Indeje et al. 2000; Camberlin et al. 2001; Camberlin and Okoola 2003; Kijazi and Reason 2005). While the pattern and magnitude of ENSO-related tropical Pacific SST varies between individual El Niño and La Niña events [i.e., El Niño Modoki (EMI) (Ashok et al. 2007) versus canonical El Niño (Rasmusson and Carpenter 1982)], which in turn may modify the global circulations (Trenberth and Smith 2009; Ratnam et al. 2011), all ENSO events are accompanied by an anomalous zonal gradient in SST over the west-central Pacific Ocean (Figs. 1 and 4). Concomitant daily extremes in tropical central Pacific SST and an anomalous tropical zonal SST gradient over 140°–150°E have been linked to regional circulation and precipitation modifications (Hoell et al. 2013). Here, we group ENSO events in the aggregate according to the magnitude of the anomalous west Pacific Ocean SST gradient (WPG) and examine observationally the related atmospheric and oceanic responses. Specifically, we investigate the 1) temporal and spatial evolution of tropical Pacific SST, 2) tropical circulations, and 3) Northern Hemisphere circulations and precipitation during winter associated with varying magnitudes of the WPG during ENSO events.
(a) REOF1 and (d) REOF2 of SST anomaly and (c) REOF1 and (f) REF2 time series calculated over the plotted domain using a correlation matrix for 1948–2011. The zonal distribution of meridionally averaged (b) REOF1 and (e) REOF2 between 5°S and 5°N. REOF1 explains 32% of the variance and REOF2 explains 21% of the variance. Also plotted as the black line in (c) is the WPG, defined as the standardized difference between average SST over the Niño-4 domain [green box and shading in (a),(b), respectively] and over the west Pacific [brown box and shading in (a),(b), respectively].
Citation: Journal of Climate 26, 23; 10.1175/JCLI-D-12-00344.1
ENSO events have been commonly classified through the distribution and magnitude of tropical Pacific SST over specific regions, though it has long been known that no two ENSO events are the same (Wyrtki 1975). Many widely used SST-based measures of ENSO, such as the Niño-3.4 (N34) index, are related to extratropical modifications (e.g., Trenberth 1997); however, these indices do not account for varying SST patterns (Trenberth and Stepaniak 2001). Different flavors of ENSO that account for the spatial variation of tropical Pacific SST, such as the eastern Pacific (Rasmusson and Carpenter 1982), central Pacific (Kao and Yu 2009), dateline (Larkin and Harrison 2005), and El Niño Modoki (Ashok et al. 2007), have been explored. However, Takahashi et al. (2011) has argued that two flavors of ENSO, canonical ENSO and El Niño Modoki, are actually part of the nonlinear evolution ENSO and do not describe different phenomena.
Since regional teleconnections stemming from ENSO depend on not only the magnitude of SST but also their gradients, Trenberth and Stepaniak (2001) developed a zonal SST gradient-based evaluation of ENSO to be used alongside the Niño-3.4 index (5°N–5°S, 170°E–120°W). This gradient-based index, called the trans-Niño index (TNI), is computed as the difference in standardized anomalies between central [Niño-4 (N4); 5°N–5°S, 160°E–150°W] and eastern Pacific SST (Niño-1+2; 0°–10°S, 90°–80°W). While the TNI and Niño-3.4 more completely describe the evolution and magnitude of ENSO, these indices and others do not incorporate the WPG (Fig. 1) common to ENSO. In this work, we develop a WPG gradient index and, in addition to the definition of ENSO developed by Trenberth (1997), investigate ENSO-related SST and tropical circulation variations.
Both SST gradients (Lindzen and Nigam 1987) and SST anomalies (Neelin and Held 1987) are major contributors to the near-surface tropical circulation convergence. While meridional SST gradients contribute most to convergence over the eastern tropical Pacific, zonal SST gradients are the major contributor to convergence over the central and western tropical Pacific (Lindzen and Nigam 1987). Furthermore, zonal SST gradients are the major contributor to the meridional wind (Chiang et al. 2001) associated with ENSO conditions and have been shown to influence the Walker circulation (Merlis and Schneider 2011). Here, we show observationally that varying magnitudes of the WPG during ENSO events do, indeed, modify the low-level circulation and convergence over the tropical Pacific Ocean beyond the convergence caused by SST anomalies alone.
Tropical SST and global teleconnections associated with ENSO can vary considerably between seemingly similar El Niño events, termed inter–El Niño variations (e.g., Kumar and Hoerling 1997), and between El Niño and La Niña events, termed as the nonlinearity of ENSO (Hoerling et al. 1997; Livezey et al. 1997). Inter–El Niño variability has been observed globally (Kumar et al. 2005; Trenberth and Smith 2009), throughout the Northern (Annamalai et al. 2007) and Southern (Vera et al. 2004) Hemispheres, as well as over Australia (Wang and Hendon 2007). The sources of inter–El Niño variations in global teleconnections are thought to be primarily related to internal variability of the atmosphere (e.g., Kumar and Hoerling 1997; Hoerling and Kumar 1997, 2002), variability in the distribution of tropical Pacific SST (e.g., Ratnam et al. 2011), the entire Indo-Pacific basin (e.g., Lee et al. 2002; Annamalai et al. 2007), or a combination of both internal atmospheric and SST variability (e.g., Mathieu et al. 2004). Here, we group ENSO events according to an index of WPG and examine the associated inter–El Niño variability.
The influences of ENSO on the global climate, particularly over North America, East Africa, and southwest Asia have been examined in great detail. ENSO teleconnections influence East Africa through modifications of the Walker circulation (Nicholson 1996), which impact precipitation during two important rainy seasons, the long rains during March–May (Indeje et al. 2000; Camberlin and Okoola 2003; Kijazi and Reason 2005) and the short rains during October–December (Farmer 1988; Indeje et al. 2000; Camberlin et al. 2001). ENSO teleconnections influence southwest Asian climate through modifications of the regional circulation by way of exciting baroclinic Rossby waves (Barlow et al. 2002) and possibly through a global eastward propagating barotropic Rossby wave teleconnection mechanism similar to the results shown in Shaman and Tziperman (2005). ENSO-related precipitation modifications over southwest Asia are substantial and have been primarily examined during the boreal winter cold season (Barlow et al. 2002; Mariotti 2007; Barlow and Tippett 2008). Over North America, ENSO-induced teleconnections oftentimes result in the Pacific–North America pattern (Horel and Wallace 1981), which can result in strong regional (e.g., Cayan et al. 1999) and large-scale (e.g., Ropelewski and Halpert 1986; Piechota and Dracup 1996) precipitation impacts. The inter–El Niño variations over these sensitive regions can be large and therefore the potential predictability of the regional climate may be compromised. Here, we use the WPG index to assess ENSO-related Northern Hemisphere circulation and precipitation during January–March to preliminarily assess whether the WPG can enhance the potential predictability of ENSO-related conditions.
While considerable effort has been spent on the characterization of ENSO events through tropical Pacific SST distributions, none of the previous analyses have examined the WPG that exists during ENSO events nor have they attempted to group ENSO events using the WPG. Here, we identify the location of the WPG, investigate its interannual and decadal time variability, and examine the distribution of Pacific SST and their time evolution associated with varying WPG magnitudes during ENSO in section 3. Inter–El Niño variability remains an important question and has large implications for the potential predictability of the tropical climate in addition to the weather and climate over sensitive regions, including eastern Africa, southwest Asia, and North America. Therefore, we separate ENSO events according to the WPG and perform an analysis of the tropical circulation (section 4) and of Northern Hemisphere temperature and precipitation during January–March (section 5). In section 6, we provide a summary and discuss the implications of this work.
2. Data
a. SST
Monthly SST were drawn from the Extended Reconstructed Sea Surface Temperature (ERSST) analysis version 3b (Smith et al. 2008) on a fixed 2.0 × 2.0 latitude–longitude grid. ERSST version 3b is based upon the International Comprehensive Ocean–Atmosphere Data Set version 2.4 and does not incorporate satellite SST data, as satellite SST data introduces residual biases. SST with the seasonal cycles removed relative to the 1900–2011 and 1948–2011 periods were analyzed.
b. Precipitation
Monthly precipitation, with the seasonal cycle removed, was drawn from the Climate Prediction Center Merged Analysis of Precipitation (CMAP) version 1201 (Xie and Arkin 1997) on a fixed 2.5 × 2.5 latitude–longitude grid for 1979–November 2011. The CMAP dataset incorporates satellite-based precipitation estimates and global station data. Recent changes in input data to the CMAP dataset may have resulted in discontinuities between the pre- and post-2009 periods. All precipitation calculations were performed using the Global Precipitation Climatology Project (GPCP) version 2 by the World Climate Research Programme (Adler et al. 2003) for the period from 1979 to 2009, and all results were similar to the results obtained using the CMAP dataset.
c. Atmospheric circulation, temperature, and divergence
Monthly circulation, temperature, and divergence, all with the seasonal cycle removed, were drawn from the National Centers for Environmental Prediction–National Center for Atmospheric Research (NCEP–NCAR) reanalysis fields (Kalnay et al. 1996) on a fixed 2.5° × 2.5° latitude–longitude grid for 1948–2011. Temperature and circulation from the NCEP–NCAR reanalysis dataset were compared and show strong similarities with the Modern-Era Retrospective Analysis for Research and Applications (MERRA) (Rienecker et al. 2011) on a fixed 1.25 × 1.25 latitude–longitude grid with 42 levels for 1979–2011.
3. Anomalous west Pacific SST gradients
The objective of this section is to identify the location of anomalous SST gradients over the west-central Pacific Ocean on interannual time scales associated with ENSO and on decadal to multidecadal time scales associated with long-term trends.
a. Identification
The leading two modes of monthly Indo-Pacific tropical SST variability for 1948–2011 over the domain 20°S–20°N, 60°E–80°W (shown in Fig. 1) are extracted using correlation-based rotated empirical orthogonal function (REOF) analysis. The procedure for calculating the REOF is the same as Kawamura (1994). A correlation-based REOF calculation was chosen in favor of a covariance-based calculation so that areas such as the Indo–west Pacific contribute to the SST leading patterns as much as the central Pacific. REOF1 (referred to as the leading SST pattern) explains 32% of the variance while REOF2 (referred to as the SST trend pattern) explains 21% of the variance. The top panels of each REOF display the spatial loadings, the middle panels display the zonal equatorial loadings meridionally averaged between 5°S and 5°N, and the bottom panels display the time series.
The leading SST pattern shown in Fig. 1a is similar to the ENSO mode displayed in Kawamura (1994). The primary difference between the leading pattern displayed here and that of Kawamura is that the strongest loadings are over the central Pacific as opposed to the eastern Pacific. When calculated using a covariance matrix, the leading SST pattern strongly resembles canonical ENSO (Rasmusson and Carpenter 1982). The leading SST pattern and the Niño-3.4 index (area-averaged SST over the domain 5°S–5°N, 120°–170°W) are correlated to 0.93.
A prominent feature of the leading SST pattern is a strong zonal loading gradient between the west and central Pacific Ocean over the region 150°–170°E (Fig. 1b). This gradient will hereafter be referred to as the WPG. A gradient-based definition of the WPG was calculated as the area-averaged zonal SST gradient at all grid points within the region 5°S–5°N, 150°–170°E. The relationships between the zonal SST gradient-based index of the WPG and a subset of SST-based ENSO indices, including the trans-Niño index (TNI), are shown in Table 1. The gradient-based WPG index is well correlated (r = 0.93) with the standardized difference between area-averaged central Pacific SST over the Niño-4 region (5°S–5°N, 160°–210°E) and area-averaged west Pacific SST over the region (0°–10°N, 130°–150°E). The gradient-based WPG index is not as strongly correlated with individual SST-based ENSO indices.
Correlation (R) of the zonal SST gradient averaged over the region 5°S–5°N, 150°–170°E and SST-based ENSO indices.
Takahashi et al. (2011) showed that individual SST-based ENSO indices can be closely reproduced using two other SST-based ENSO indices as predictors in a linear regression model even if the predictors are not well correlated with the predictant. Table 2 shows the correlation of the gradient-based WPG index to the two-predictor linear regression fit of the gradient-based WPG index using the specified SST-based ENSO indices as predictors. The linear combinations that include the Niño-4 and El Niño Modoki indices (EMI) are correlated with the gradient-based WPG index to 0.80–0.85. However, the linear combination of Niño-4 and the western Pacific indices (N4–WP) has a much larger correlation to the gradient-based WPG index (r = 0.95) than do the other linear combinations. Therefore, the standardized difference between the Niño-4 and WP indices is the best expression of the WPG and provides information beyond each of the other ENSO indices and combinations of ENSO indices. As such, we adopt this measure to define the WPG.
Correlation of the zonal SST gradient averaged over the region 5°S–5°N, 150°–170°E and the combination of a two-predictor linear regression fit of SST-based ENSO indices of the zonal SST gradient.
The SST trend pattern shown in Fig. 1d displays a warming Indo–west Pacific and a cooling central Pacific (Zhang et al. 2010), similar to the ENSO-free west Pacific warming patterns described by Compo and Sardeshmukh (2010) and Solomon and Newman (2012). The resulting long-term SST trends resemble La Niña–like conditions (Zhang et al. 2011); however, recent work by An et al. (2012) has argued that the long-term La Niña–like changes may change to an El Niño–like pattern, with the central Pacific warming faster than the west Pacific. In their argument, evaporative cooling at the surface will increase over the west Pacific, while warming via vertical thermal advection will decrease. Though the results presented here indicate that the secular trend of west Pacific SST can enhance the WPG during La Niña periods and decrease the WPG during El Niño periods, Collins et al. (2010) has argued that it is not yet possible to comment on whether ENSO activity will be modified in a changing climate.
b. Interannual and multidecadal variability
The monthly WPG (black line) and the leading SST pattern time series (colored bars) are plotted in Fig. 1c. The leading SST pattern and the WPG index follow one another, but the magnitude of the WPG does not closely follow the magnitude of the leading pattern time series during individual ENSO events. The strongest El Niño event occurred during 1997–98, but the WPG was relatively weak compared to other El Niño events. A similar statement can be made for the 1998–2001 La Niña event, as a relatively moderate La Niña, according to the leading SST pattern, occurred simultaneously with the strongest and longest-lasting WPG. Therefore, while the west Pacific SST gradient is largely ENSO dependent in terms of its sign, the strength of the gradient throughout an individual ENSO event is not necessarily related to the magnitude of SST during such event.
The 12-month running averages of the WPG as well as its contributing terms, from the central Pacific and west Pacific SST (Fig. 2), are used to assess interannual time variability from 1948 to 2011. The west Pacific SST gradient closely follows central Pacific SST, but is modified by west Pacific SST during El Niño and La Niña events. Prior to 1998, west Pacific SSTs were cool, which resulted in an enhanced (weakened) west Pacific SST gradient during El Niño (La Niña) events. After 1998 and lasting through 2011, the west Pacific SST warmed, resulting in an enhanced (weakened) west Pacific SST gradient during La Niña (El Niño) events.
One-year running averages of the monthly Niño-4 index (red), west Pacific SST index (blue), and WPG index (green) for 1948–2011.
Citation: Journal of Climate 26, 23; 10.1175/JCLI-D-12-00344.1
Seven-year running averages of the west Pacific SST gradient, central Pacific, and west Pacific SST (Fig. 3) are used to assess decadal time variability for 1900–2011. Similar to interannual variations discussed previously, the west Pacific SST gradient closely follows central Pacific SST and is modified by the west Pacific during ENSO events. There was considerable warming of the west Pacific and central Pacific SST throughout the twentieth century (Karnauskas et al. 2009; Compo and Sardeshmukh 2010; Solomon and Newman 2012). However, since 1985, 7-yr averages of the west Pacific have warmed much faster than the central Pacific, causing an increasingly negative gradient. Therefore, the long-term trend is resulting in an enhanced west Pacific SST gradient during La Niña periods, consistent with the results of the trend pattern in Fig. 1.
Seven-year running averages of the monthly Niño-4 index (red), west Pacific SST index (blue), and the WPG index (green) for 1900–2011.
Citation: Journal of Climate 26, 23; 10.1175/JCLI-D-12-00344.1
4. ENSO and the anomalous west Pacific SST gradient
The objective of this section is to examine the distribution and time evolution of Pacific SST as well as atmospheric teleconnections during ENSO events separated by the magnitude of the WPG. Since we examine solely interannual variability, we removed the secular trend in SST for all of the following analyses of the 1948–2011 period. Therefore, all indices used to define ENSO events (e.g., WPG and Niño-3.4) and SST analyses hereafter account for the removal of the secular trend. The secular SST trend over the Pacific for 1948–2011 was nearly linear (Fig. 1f), and this trend was removed through simple linear regression at all grid points.
a. Events
The method of Trenberth (1997) is used to define ENSO events. El Niño (La Niña) events occurred when the 5-month running average SST anomaly over the Niño-3.4 region exceeded (fell below) 0.4°C for longer than 5 months. ENSO events were separated according to the zonal location of the maximum Pacific SST anomaly, similar to the method of Kug et al. (2009). The zonal location of the maximum SST anomaly occurred over the eastern Pacific (EP), the central Pacific (CP), or between the EP and CP (MX). ENSO events were classified as EP when the largest standardized magnitude SST anomaly occurred over the Niño-3 region (5°S–5°N, 90°–150°W). ENSO events were classified as CP when the largest standardized magnitude SST anomaly occurred over the Niño-4 region (5°S–5°N, 160°E–150°W). ENSO events were classified as MX when the largest standardized magnitude SST anomaly occurred over the Niño-3.4 region. The duration of El Niño and La Niña events and the zonal location of the maximum SST anomaly over the Pacific Ocean are displayed in Tables 3 and 4, respectively. ENSO events were categorized as having a strong (weak) WPG if the event-averaged magnitude of the WPG index exceeded (fell within) one standard deviation from the mean.
Strength of the WPG and zonal location of the maximum standardized tropical Pacific SST anomaly for 1948–2011 El Niño events. Strong (weak) WPG events occurred when the WPG index value was greater (less) than 1.0. Eastern Pacific (EP) ENSO events occurred when the maximum standardized SST anomaly was located over the Niño-3 region. Central Pacific (CP) events occurred when the maximum standardized SST anomaly was located over the Niño-4 region. Mixed (MX) ENSO events occurred when the maximum standardized SST anomaly was located over the Niño-3.4 region, falling between the CP and EP regions.
Strength of the WPG and zonal location of the minimum standardized tropical Pacific SST anomaly for 1948–2011 La Niña events. Strong (weak) WPG events occurred when the WPG index value was greater (less) than 1.0. Eastern Pacific ENSO events occurred when the minimum standardized SST anomaly was located over the Niño-3 region. Central Pacific events occurred when the minimum standardized SST anomaly was located over the Niño-4 region. Mixed ENSO events occurred when the minimum standardized SST anomaly was located over the Niño-3.4 region, falling between the CP and EP regions.
By construction, ENSO events display similar zonal slopes over the region 150°–170°E (Fig. 4) when grouped according to strong and weak WPG. Since the magnitude of SST over the central Pacific is used to quantify the WPG, each strong WPG event contains at least a modest SST anomaly over that region. However, some strong WPG events, despite having a central Pacific signal, were classified as EP or MX ENSO events (Tables 3 and 4) because the zonal location of maximum SST anomaly was farther to the east, as shown, for example, in Fig. 5c for El Niño and Fig. 5d for La Niña. Conversely, some CP ENSO events were not necessarily characterized by a strong WPG, as shown, for example, in Fig. 5b for El Niño and Fig. 5a for La Niña. Therefore, strong WPG and CP ENSO are not mutually inclusive. Since the mid-1980s the frequency of strong WPG El Niño and La Niña events have increased, as larger magnitude SST anomalies have been observed over the central Pacific during ENSO (Kug et al. 2009). Prior to the mid-1980s strong WPG events occurred nearly every five years for El Niño and less often for La Niña.
Zonal distribution of SST anomaly (°C) over the tropical Pacific Ocean averaged meridionally between 5°S and 5°N for 1948–2011 (a) El Niño and (b) La Niña events. Thick solid lines (thin dashed lines) denote strong (weak) WPG events.
Citation: Journal of Climate 26, 23; 10.1175/JCLI-D-12-00344.1
Average SST anomaly (°C) during the specified (top) La Niña and (bottom) El Niño events in which (left) the WPG was weak and the maximum SST anomaly was located over the central Pacific and (right) the WPG was strong and the maximum SST anomaly was not located over the central Pacific.
Citation: Journal of Climate 26, 23; 10.1175/JCLI-D-12-00344.1
The distributions of Pacific SST during strong and weak west Pacific SST gradient ENSO events are investigated, in Fig. 6, for 1948–2011. Strong gradient La Niña events are associated with cold equatorial central Pacific SST and warm west Pacific SST within a boomerang of warm SST that extends into the central North Pacific. Weak gradient La Niña events are associated with a cold SST tongue, with maximum departures over the eastern tropical Pacific, extending from the western coast of South America into the central Pacific Ocean. The difference between strong and weak gradient La Niña events yields a cold tropical central Pacific SST and warm western and eastern tropical Pacific, similar to patterns observed during El Niño Modoki events (Ashok et al. 2007); however, as shown in Table 1, the correlation between the WPG and El Niño Modoki index is not large. Another striking feature observed in this difference is the warm area of extratropical central Pacific SST, a pattern that strongly resembles the mode of North Pacific SST variability mode described by Barlow et al. (2001).
Average SST anomaly (°C) during (top) strong WPG and (middle) weak WPG and (bottom) the difference between strong and weak WPG for (left) La Niña and (right) El Niño events for 1948–2011. All plots are significant to p < 0.05 according to a Monte Carlo test.
Citation: Journal of Climate 26, 23; 10.1175/JCLI-D-12-00344.1
Strong gradient El Niño events are associated with warm equatorial central Pacific SST and cool west Pacific SST while the SST departures over the extratropical northern Pacific are marginal. Weak gradient El Niño events are associated with a tongue of warm SST that extends from the western coast of South America toward the central Pacific Ocean with largest magnitudes over the eastern tropical Pacific. The difference, like the La Niña case, resembles an ENSO Modoki pattern of the Pacific basin.
b. Temporal evolution
The temporal evolution of Pacific SST associated with strong and weak WPG ENSO events are examined for 1948–2011 through composites of SST averaged during seven phases prior to, during, and after an event. The evolution of Pacific SST during strong and weak WPG La Niña and El Niño events are displayed in Figs. 7 and 8, respectively. SST for each ENSO event was averaged and placed into seven bins: 1) 4–6 months before an ENSO, 2) 1–3 months before and ENSO, 3) the first three months of an ENSO, 4) all months but the first 3 and last 3 of an ENSO, 5) the last 3 months of an ENSO, 6) 1–3 months of an ENSO, and 7) 4–6 months after an ENSO. Thereafter, each of the seven bins for each ENSO event were averaged according to strong and weak WPG El La Niño and La Niña events.
Average temporal evolution of (left) strong WPG and (right) weak WPG La Niña events for 1948–2011. See text for further information. All plots are significant to p < 0.05 according to a Monte Carlo test.
Citation: Journal of Climate 26, 23; 10.1175/JCLI-D-12-00344.1
As in Fig. 7, but for El Niño events.
Citation: Journal of Climate 26, 23; 10.1175/JCLI-D-12-00344.1
Strong WPG La Niña events (left panels of Fig. 7), on average, are preceded by strong canonical El Niño events (Rasmusson and Carpenter 1982). The El Niño decays quickly, and within 6 months strong east and central Pacific cold SST anomalies consistent with La Niña develop. As the La Niña matures, a cold SST spread westward toward the central Pacific while the west Pacific warms. During the decay phase of the La Niña, as the west and eastern Pacific moderate, cool SSTs over the central Pacific resemble the El Niño Modoki pattern (Ashok et al. 2007). Weak WPG La Niña events (right panels of Fig. 7) are preceded by average SST throughout the tropical and subtropical Pacific. Cool eastern Pacific SST spread westward toward the central Pacific Ocean leading up to the mature phase of the La Niña while west Pacific SSTs remain neutral. Cool central Pacific SSTs diminish and are followed by neutral conditions.
Strong WPG El Niño events (left panels of Fig. 8), on average, are preceded by La Niña conditions over the eastern Pacific. Warm SST develop in the eastern Pacific and spread westward toward the central Pacific while cool west Pacific SST develop into a boomerang pattern. As the El Niño decays, warm SST over the central Pacific weaken and shift slightly westward similar to the El Niño Modoki pattern (Ashok et al. 2007). After the strong WPG El Niño decays, La Niña conditions develop over the eastern Pacific. Weak WPG El Niño events (right panels of Fig. 8), on average, are preceded by weak La Niña conditions over the east-central Pacific. Warm eastern Pacific SST spread westward into the central Pacific Ocean, during which time the west Pacific SST remain neutral during the maturation of the weak WPG El Niño. Warm central Pacific SST diminish and are followed by neutral conditions.
Previous works have indicated a nonlinear evolution of SST patterns between El Niño and La Niña (Takahashi et al. 2011). Here, we have observed considerable similarities between El Niño and La Niña in terms of their SST evolution when separated according to the strength of the WPG in the aggregate (Figs. 7 and 8). Also, both strong and weak WPG ENSO events appear to begin similarly over the eastern Pacific, but diverge within the last six months centered around the end of the event. Near the end of strong WPG events, SST anomalies remain over the central Pacific as SST elsewhere moderates. However, near the end of weak WPG events, central and eastern Pacific SST moderate similarly.
The central Pacific SST component during the mature and decay phases of strong WPG El Niño and La Niña events resemble the El Niño Modoki pattern (Ashok et al. 2007). Therefore, throughout their life cycle, strong WPG ENSO appear to demonstrate both canonical ENSO and El Niño Modoki SST patterns, which may lend credence to the hypothesis of Takahashi et al. (2011), where it was stated that El Niño Modoki may be part of the same nonlinear evolution of ENSO.
c. Tropical Indo-Pacific circulations and precipitation
The model of Lindzen and Nigam (1987) showed that SST gradients play an important role in driving low-level tropical convergence. While meridional SST gradients contribute most to convergence over the eastern tropical Pacific, zonal SST gradients are the major contributors over the central and western Pacific. In addition to low-level convergence forced by SST gradients, convergence is also produced by the thermodynamic effects of SST anomalies described by Neelin and Held (1987). Consistent with the aforementioned works, Zhang et al. (2011) has indicated that a warming west Pacific relative to the central Pacific has led to a La Niña–like trend that is subsequently increasing the ascending branch of the Walker circulation. Here, we show observationally that varying magnitudes of the WPG during ENSO are linked to modifications of the low-level circulation and convergence over the tropical Pacific Ocean. Though in this work the secular SST trend was removed and the focus is placed on the anomalous SST gradient associated with ENSO, the results presented in the following indicate increased tropical divergence and modifications to the western branch of the Walker circulation associated with strong WPG ENSO events.
Displayed in Fig. 9 are vertical sections of equatorial temperature (shading), zonal wind (vectors), and divergence (filled contours) anomalies across the Indo-Pacific basin during strong and weak WPG El Niño and La Niña events. Contours filled with horizontal lines denote convergence, while contours filled with dots denote divergence. During strong WPG La Niña events a lower troposphere temperature gradient is present over the WPG region 150°–170°E, which separates cold temperatures over the central Pacific and warm temperatures over the west Pacific. The temperature gradient leads to a gradient of density in the lowest 300 hPa of the equatorial atmosphere, resulting in increased easterly flow between 140° and 200°E. Increased easterly flow results in convergence (divergence) in areas where wind speeds slow (increase). The slowing of the wind and convergence occurs between 130° and 150°E, to the west of the WPG, and the acceleration of the wind and convergence occurs between 170° and 220°E, to the east of the WPG. The convergence and divergence patterns are observed throughout the low to middle troposphere. Strong WPG El Niño events display nearly the mirror image, yet slightly weaker, in terms of temperature, zonal, and wind divergence vertical profiles of strong WPG La Niña events.
Meridional average difference between 5°S and 5°N of temperature anomaly (°C, shaded), zonal wind anomaly (m s−1, vectors), and divergence anomaly (s−1, filled contours) during (left) strong WPG and (right) weak WPG for (top) La Niña and (bottom) El Niño events. The contour interval for divergence is 0.5 × 10−7 s−1, and contours filled with dots refer to divergence while contours filled with hatching refer to convergence.
Citation: Journal of Climate 26, 23; 10.1175/JCLI-D-12-00344.1
Weak WPG La Niña events are related to different vertical profiles of equatorial Pacific zonal wind, divergence, and temperature. In the absence of a strong zonal SST gradient over 150°–170°E, the vertical distribution of temperature throughout the lower troposphere does not vary strongly from east to west. The zonal wind anomalies across the central and western Pacific are constant in the zonal direction and do not produce enhanced low-level convergence. Over the central and eastern Pacific there is convergence, but this convergence does not appear to be produced by the zonal wind. Weak convergence over the Maritimes in the low to middle troposphere is likely a result of the mass balance effects of the zonal wind. Weak WPG El Niño events mirror weak WPG La Niña events.
In recent decades, there has been an increase in the frequency of strong WPG ENSO events associated with MX and CP ENSO (Tables 3 and 4). The increased frequency of strong WPG ENSO events has resulted partially from an increased frequency of ENSO-related SST anomalies over the central Pacific, even when the long-term warming trend of west Pacific SST was removed. If the recent trend of strong WPG ENSO events continues into the future, the increased magnitude of tropical circulations (Fig. 9) will also continue, which would support a stronger interannual variability in the magnitude of western branch of the Walker circulation. What is not clear is how the long-term changes in tropical Pacific SST would influence the tropical circulations associated with ENSO partitioned by the WPG, as many works have argued for (e.g., Meng et al. 2012) and against (e.g., Vecchi et al. 2006) an increased magnitude Walker circulation in recent decades.
Hoell et al. (2013) showed that Maritime Continent precipitation extends farther westward into the Indian Ocean during individual days when both the Niño-3.4 index and the zonal SST gradient over 140°–150°E were strong. These results were consistent with those of Barlow et al. (2002), where it was shown that concomitant, but opposing, monthly SST extremes between the west Pacific and Niño-3.4 region were linked to a westward extension of Maritime Continent precipitation into the eastern Indian Ocean. Here, we assess whether strong WPG El Niño and La Niña events in the aggregate, as opposed to individual months or days, are related to modified Maritime Continent precipitation (Fig. 10) in addition to possible precipitation changes over the entire tropical Indo-Pacific domain.
Average precipitation anomaly (mm day−1) during (top) strong WPG and (middle) weak WPG and (bottom) the difference between strong and weak WPG for (left) La Niña and (right) El Niño events for 1948–2011. All plots are significant to p < 0.05 according to a Monte Carlo test.
Citation: Journal of Climate 26, 23; 10.1175/JCLI-D-12-00344.1
The magnitude of the WPG during La Niña periods has a large impact on Maritime Continent precipitation as well as precipitation over the central Pacific. Maritime Continent precipitation departures during strong WPG La Niña episodes strongly exceed precipitation departures observed during weak WPG La Niña episodes. Furthermore, enhanced Maritime Continent precipitation departures extend westward into the eastern Indian Ocean during strong WPG La Niña episodes unlike the weak WPG La Niña case. Over the central Pacific strong WPG La Niña episodes are related with stronger precipitation decreases than during the neutral WPG La Niña events.
The magnitude of WPG during El Niño periods impacts central Pacific precipitation departures but does not appear to have a large impact on Maritime Continent precipitation. While precipitation during strong WPG episodes relative to weak WPG episodes is slightly diminished over the Moluccas (5°S, 130°E), precipitation is mixed or slightly enhanced over Sumatra and Borneo. Enhanced precipitation over the west-central Pacific Ocean during strong WPG El Niño periods exceeds enhanced precipitation during weak WPG El Niño periods. Conversely, diminished precipitation over the east-central Pacific Ocean during strong WPG El Niño periods trails enhanced precipitation during weak WPG El Niño periods.
d. Northern Hemisphere wintertime circulations and precipitation
We perform a preliminary assessment of circulation and precipitation impacts over the Northern Hemisphere during January–March (JFM) associated with WPG ENSO event occurrences. ENSO events have been shown to influence precipitation over North America (e.g., Ropelewski and Halpert 1986; Cayan et al. 1999; Piechota and Dracup 1996), central and southwest Asia (Barlow et al. 2002; Mariotti 2007; Syed et al. 2006, 2010) as well as over the Horn of Africa (Indeje et al. 2000; Camberlin and Okoola 2003; Kijazi and Reason 2005). Here, we examine whether the WPG is linked to ENSO-related 200-hPa circulation (Fig. 11) and precipitation (Fig. 12) throughout the Northern Hemisphere during JFM, as JFM encompasses the rainy seasons over all of the aforementioned regions.
Average January–March 200-hPa streamfunction anomaly (m2 s−1) during (left) strong and (right) weak WPG for (top) La Niña and (bottom) El Niño occurrences for 1948–2011. All plots are significant to p < 0.05 according to a Monte Carlo test.
Citation: Journal of Climate 26, 23; 10.1175/JCLI-D-12-00344.1
Average January–March standardized precipitation anomaly during (left) strong and (right) weak WPG for (top) La Niña and (bottom) El Niño occurrences for 1948–2011. All plots are significant to p < 0.05 according to a Monte Carlo test.
Citation: Journal of Climate 26, 23; 10.1175/JCLI-D-12-00344.1
Substantial Northern Hemisphere circulation (Fig. 11a) and precipitation (Fig. 12a) departures are associated with strong WPG La Niña events during JFM. The average 200-hPa streamfunction and precipitation of all strong WPG La Niña events closely resemble those same patterns for each WPG La Niña event (Table 4). Positive streamfunction over Asia is associated with a baroclinic circulation, as evidenced by streamfunction centers of the opposing sign between the top and bottom of the troposphere (not shown in the lower troposphere). The circulation throughout the rest of the Northern Hemisphere is equivalent barotropic and appears to be part of the same teleconnection mechanism that extends from the North Pacific Ocean through North America across the Atlantic Ocean and into Europe. Large standardized precipitation departures are associated with all positive streamfunction departures at 200 hPa across the entire Northern Hemisphere. Precipitation departures are particularly strong over southern North America, Europe, the Middle East, western Asia, and eastern Africa. Strong WPG La Niña events have consistently been linked to substantial extratropical impacts, including severe drought over the Northern Hemisphere. These results imply at least some potential predictability for Northern Hemisphere conditions based upon the strength of the WPG during La Niña.
The Northern Hemisphere circulation and precipitation associated with weak WPG La Niña events vary considerably for JFM, which is demonstrated by the weak signals in the average of those fields displayed in Fig. 11c and Fig. 12c, respectively. Some events, such as JFM 1985, are associated with Northern Hemisphere anticyclonic circulation in the upper troposphere and drought, while other events, such as JFM 1972, are associated with fairly weak extratropical circulation anomalies. Therefore, weak WPG La Niña events during Northern Hemisphere winter are linked to large variation in extratropical impacts in terms of circulation and precipitation.
Both strong and weak gradient El Niño events are related to similar average 200-hPa circulation and precipitation anomalies. The 200-hPa streamfunction fields for both strong and weak WPG La Niña events are similar to, yet weaker than, the strong WPG La Niña case but with opposite sign. However, the averages of weak WPG El Niño events is related to stronger circulation magnitudes, particularly over Asia, the North Pacific, and North America, owing to contributions from JFM 1983 and JFM 1997. While both strong and weak WPG El Niño events are related to pluvials over North America and Asia on average, the position and magnitude of enhanced precipitation differ. Strong WPG El Niño events are related to stronger precipitation increases over southern North America and eastern Asia, while weak WPG El Niño events are related to strong and more widespread precipitation departures over Europe and western Asia on average.
5. Summary
El Niño and La Niña events are grouped according to the magnitude of the west Pacific Ocean SST gradient and the tropical circulation and precipitation as well as an analysis of Northern Hemisphere climate associated with these events are examined for 1948–2011. When El Niño and La Niña events are separated by the strength of the WPG, the temporal evolution and distribution of Pacific SST as well as the global circulation display consistent and coherent patterns, indicating possible predictive capabilities of the WPG. Recently the magnitude of the WPG has increased during La Niña events as a result of a rapidly warming west Pacific relative to the central Pacific in addition to the increased frequency of central Pacific ENSO events (Kug et al. 2009), which both have strong implications on the future tropical and extratropical circulations.
Zonal SST gradients over the west Pacific Ocean are major contributors to the low-level tropical convergence (Lindzen and Nigam 1987). However, well-known measures of ENSO, whether they directly measure tropical Pacific SST over defined regions (i.e., Niño-3.4), the distribution of tropical Pacific SST (i.e., canonical ENSO and El Niño Modoki), or gradient-based indices over the eastern Pacific (TNI), have not considered the WPG during ENSO. In this work, the leading pattern of Indo-Pacific SST (Fig. 1), extracted using a REOF, was used to identify the WPG during ENSO over the region 150°–170°E. The WPG is defined as the standardized difference between area-averaged central Pacific SST over the Niño-4 region and an area-averaged index of west Pacific SST. While the direction of the WPG follows ENSO cycles, the magnitude of the gradient varies considerably between El Niño and La Niña events owing to the interplay of the west and central Pacific SST.
Tropical and extratropical variability can occur during similar El Niño events (e.g., Kumar and Hoerling 1997) and between El Niño and La Niña events (e.g., Hoerling et al. 1997). Therefore, the tropical and extratropical impacts prior to and during an ENSO event are oftentimes difficult to forecast. In this work, ENSO events were grouped according to the magnitude of the WPG and the temporal evolution of SST and global circulation were examined. The temporal SST evolutions of WPG ENSO events were similar in terms of both the distribution and movement of central Pacific SST anomalies as well as the persistence of the west Pacific SST leading up to and throughout the event. Additionally, the tropical and extratropical circulation variability during El Niño and La Niña events partitioned via the WPG are strikingly similar. These strong similarities imply potential predictability of tropical and extratropical circulations associated with the magnitude of the WPG.
Whether an ENSO event is associated with a strong or weak WPG may have important implications for diagnosing extratropical circulations and precipitation throughout the Northern Hemisphere, particularly over Africa, Asia, the Middle East, and North America. Separating La Niña events by the strength of the WPG proves to be an important indicator of extratropical impacts during Northern Hemisphere winter; however, this is not necessarily the case for El Niño events. Substantial extratropical impacts over the Northern Hemisphere during winter associated with El Niño appear to be distributed between strong and weak WPG cases. The 1998–2001 Northern Hemisphere drought (Hoerling and Kumar 2003), which was particularly strong over southwest Asia (Barlow et al. 2002) and portions of the United States (Seager 2007), was associated with a strong WPG La Niña event. Also, other strong WPG La Niña events, such as during the early 1970s and 2000s, were associated with similar extratropical circulations. A closer investigation between the extratropical similarities linked to these strong WPG La Niña events may prove useful in predicting widespread and devastating drought.
Acknowledgments
The authors thank two anonymous reviewers whose comments greatly improved the manuscript. The authors thank Matt Barlow, Laura Harrison, Marty Hoerling, and Amy McNally for useful discussions and for reading early versions of this manuscript. NCEP reanalysis and ERSST data were provided by the NOAA/OAR/ESRL PSD, Boulder, Colorado, from their website (at http://www.esrl.noaa.gov/psd/). This research builds upon a multiyear research project carried out under a U.S. Agency for International Development–funded Famine Early Warning System Network agreement with the U.S. Geological Survey.
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