Role of the strengthened El Niño teleconnection in the May 2015 floods over the southern Great Plains

Abstract

The climate anomalies leading to the May 2015 floods in Texas and Oklahoma were analyzed in the context of El Niño teleconnection in a warmer climate. El Niño tends to increase late-spring precipitation in the southern Great Plains and this effect has intensified since 1980. There was a detectable effect of anthropogenic global warming in the physical processes that caused the persistent precipitation in May of 2015: Warming in the tropical Pacific acted to strengthen the teleconnection towards North America, modification of zonal wave-5 circulation that deepened the anomalous trough to the west of Texas, and an enhanced Great Plains low-level southerlies increasing moisture supply from the Gulf of Mexico. Attribution analysis using the CMIP5 single-forcing experiments and the CESM Large Ensemble Project indicated a significant increase in the El Niño- induced precipitation anomalies over Texas and Oklahoma when increases in the anthropogenic greenhouse gases were taken into account.

Full text

Role of the strengthened El Niño teleconnection in the May
2015 floods over the southern Great Plains
S.-Y. Simon Wang
1,2
, Wan-Ru Huang
3
, Huang-Hsiung Hsu
4
, and Robert R. Gillies
1,2
1
Utah Climate Center, Utah State University, Logan, Utah, USA,
2
Department of Plants, Soils, and Climate, Utah State
University, Logan, Utah, USA,
3
Department of Earth Sciences, National Taiwan Normal University, Taipei, Taiwan,
4
Research
Center for Environmental Changes, Academia Sinica, Taipei, Taiwan
Abstract The climate anomalies leading to the May 2015 floods in Texas and Oklahoma were analyzed in
the context of El Niño teleconnection in a warmer climate. A developing El Niño tends to increase late-spring
precipitation in the southern Great Plains, and this effect has intensified since 1980. Anthropogenic global
warming contributed to the physical processes that caused the persistent precipitation in May 2015: Warming
in the tropical Pacific acted to strengthen the teleconnection toward North America, modification of zonal
wave 5 circulation that deepened the stationary trough west of Texas, and enhanced Great Plains low-level
southerlies increasing moisture supply from the Gulf of Mexico. Attribution analysis using the Coupled Model
Intercomparison Project Phase 5 single-forcing experiments and the Community Earth System Model Large
Ensemble Project indicated a significant increase in the El Niño-induced precipitation anomalies over Texas and
Oklahoma when increases in the anthropogenic greenhouse gases were taken into account.
1. Introduction
In May 2015, an El Niño had developed (Figure 1a) an d as a consequen ce—at least in part—precipitation
ano malies in Texas an d Oklahoma were off the scale reaching over 200 mm above norm al (Figure 1b); this
was accompanied by dry anomalies in Kentucky and Tenne ssee. As the Texas news media echoed “enough
rain fell in May to cover the entire state 8 inches deep” (CNN 1 June 2015) and in Houston alone, the fl oo d
damage was estimated to “top $45 million” (Associate Press 31 May 2015). While seasonal predictions,
issued as early as March, had indicated increased May precipitation for the southern Great Plains (http://
www.cpc.ncep.noaa.gov/products/NMME/), the extreme magnitude of the rainfall was not indicated nor
anticipated—a challenge that is yet to be realized.
It is known that either the onset or a persistent El Niño can increase spring precipitation in the southern Great
Plains at the same time reducing precipitation in the southeast U.S. [e.g., Ropelewski and Halpert, 1986, 1987;
S.-K. Lee et al., 2014]. Previous studies [e.g., Meehl and Teng, 2007; Stevenson et al., 2012; Wang et al., 2014]
have found that in a warmer climate, the teleconnection that underlies the El Niño–Southern Oscillation
(ENSO) and its associated impact on North America would change in terms of intensification and/or a posi-
tional shift of the resultant climate anomalies; this is regardless of the direction of future change in frequency
and intensity that ENSO might take. Climate modifications that involve ENSO have forecast implications at
both seasonal [Mo, 2010] and decadal [Meehl et al., 2014] timescales, and it is reasonable to question to what
extent, if any, the rather extreme May 2015 precipitation event that occurred during an El Niño was induced
through a warming climate. It is therefore worthwhile to conduct a climate diagnostics and an attribution
analysis of the May 2015 high-precipitation event that occurred over Texas and Oklahoma.
2. Data
We adopted 17 models from the Coupled Model Intercomparison Proj ect Phase 5’s (CMIP5) historical
single-forcing experiments that were driven by (1) natural-only forcing including solar and volcano (NAT),
(2) greenhouse gas (GHG)-only forcing, and (3) all these historical forcings (ALL) including anthropogenic
aer osols [Taylor et al., 2011]. Each experiment prod uced multip le members initialized from a long-stable
preindustrial (1850) control run up to 2005. Table S1 in the supporting information provides the full name,
institute, ensemble size, and spatial resolution of these models. (To date, these are the only models that
provide outputs for both GHG and NAT experiments.) In addition, we utilized 30 members produced by
SIMON WANG ET AL. ATTRIBUTION OF 2015 TEXAS FLOODS 1
PUBLICATION
S
Geophysical Research Letters
RESEARCH LETTER
10.1002/2015GL065211
Key Points:
• Increased GHG affects the May 2015
floods in Texas
• Climate warming intensifies the El Niño
teleconnection and climate impacts
• Amplified synoptic waves and
strengthened LLJ are part of
climate trends
Supporting Information:
• Text S1, Figures S1–S3,
and Tables S1 and S2
Correspondence to:
S.-Y. Simon Wang,
simon.wang@usu.edu
Citation:
Simon Wang, S.-Y., W.-R. Huang, H.-H.
Hsu, and R. R. Gillies (2015), Role of the
strengthened El Niño teleconnection in
the May 2015 floods over the southern
Great Plains, Geophys. Res. Lett., 42,
doi:10.1002/2015GL065211.
Received 4 JUL 2015
Accepted 5 AUG 2015
Accepted article online 7 AUG 2015
©2015. The Authors.
This is an open access article under the
terms of the Creative Commons
Attribution-NonCommercial-NoDerivs
License, whic h permits use and distri-
bution in any medium, provided the
original work is properly cited, the use is
non-commercial and no modifications
or adaptations are made.

the Community Earth System Model version 1 (CESM1) through the Large Ensemble Project (LEP) [Kay et al.,
2014]. The CESM1 ensemble simulations covered two periods: 1920–2005 with “ALL” forcing and 2006–2080
with RCP8.5 forcing (the total increase of radiative forcing at 8.5 W/m
2
/yr since preindustrial values). The
CESM1 was used here because it simulates well the ENSO cycle and associated teleconnection in North
America [Wang et al., 2014, 2015]. All model outputs were regridded to 2.5° longitude × 2.5° latitude resolution
before averaging, to be comparable with the National Centers fo r Environmental Predic tion (NCEP)/
National Center for Atmospheric Research Rean alysis (R1) data [Kalnay et al., 1996]. Although the R1 is an
older-generation reanalysis, it is the only data set that covers the presatellite era (before 1979) and yet is still
updated operationally. Meanwhile, we utilized four satellite era reanalyses to establish a consensus for the
trend analysis, including MERRA [Rienecker et al., 2011], CFSR [Saha et al., 2010], ERA-Interim [Dee et al.,
2011], and the JRA-25 [Onogi et al., 2007], detailed in Table S2. These reanalyses were averaged with equal
weighting to form an ensemble, following Wang et al. [2013]. Other data sets included were the Extended
Reconstructed Sea Surface Temperature (v3b) derived from the International Comprehensive Ocean-
Atmosphere Data Set [Smith and Reynolds, 2003] and global precipitation produced by NOAA’s
Precipitation Reconstruction over Land (PREC/L) and historical observations over ocean (PREC/O); these
commence from 1948 [Chen et al., 2002]. The definition of ENSO was determined by the monthly mean
Niño3.4 index provided by the Climate Prediction Center (CPC).
3. Results
3.1. El Niño Factor
As shown in Figure 1c, the May 2015 anom aly of the 250 hPa stream function ( as a departure from the
1981–2010 mean) depicts a standing trough o ver the southwest U.S.; this trough directed a series of
Figure 1. (a) SST anomaly (°C) of 6–15 May 2015 obtained from http://www.ospo.noaa.gov/Products/ocean/sst/anomaly/.
(b) May 2015 precipitation anomaly in millimeters obtained from http://water.weather.gov/precip/. (c) 250 hPa stream
function anomaly (ψ; shadings) and 850 hPa anomalous winds (vectors, m/s) of May 2015 derived from NCEP1 Reanalysis;
the black contours outline ±12 × 10
6
m
2
s
1
.
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SIMON WANG ET AL. ATTRIBUTION OF 2015 TEXAS FLOODS 2

short waves toward Texas and Oklahoma
(not shown). The 850 hPa wind anomalies
(vector) depict the intensified southerly
flow that conveyed moisture from the
Gulf of Mexico into the Great Plains; these
upper and lower level features indicate a
coupling of baroclinic (frontal) forcing
and moisture supply and are of climatolo-
gical importance in sustaining late-spring
rainfall in the southern Great Plains
[Helfand and Schubert, 1995; Higgins
et al., 1997; Santanello et al., 2012].
Additionally, as shown in Figure 1c, the
anomalous trough over the southwest
U.S. was accompanied by a subtropical
anticyclone in the eastern Pacific; this
formed a teleconnection pattern that
resembles the atmospheric response to
equatorial eastern Pacific sea surface tem-
perature (SST) forcing [e.g., Mo, 2010].
Daily rainfall data over Texas and
Oklahoma (not shown) indicated that
the entire month of May, with the
exception of 1–3 and 12 May, experi-
enced consecutively large rainfall totals.
The coexistence of an El Niño with large
May rainfall echoes the observation of
S.-K. Lee et al. [2014] that a positive
precipitation anomaly centered over
Texas tends to occur in late spring during
the onset of an El Niño; this is coincident
with CPC’s announcement of 2015 El
Niño advisory in March (i.e., issued when
El Niño conditions are observed and
expected to build) so that by the time May came around, the warm-tongue SST pattern (Figure 1a) had
transitioned toward an El Nino onset.
To examine the long-term change in the relationship between the ENSO teleconnection and precipitation
anomalies in the southern U.S., we regressed the May Niño3.4 index with the monthly mean precipitation
for two periods: 1948–1980 (Figure 2a) and 1981–2014 (Figure 2b, unit: mm/month/C)). Here all the data
within either period were linearly detrended to minimize the effect of any interdecadal trends or cycles.
While the east-west contrast between a wetter Great Plains and drier southeast is apparent in both periods,
the regressed precipitation over Texas and Oklahoma exhibits a distinctively stronger signal in the latter time
period. Next, the role of increased GHG in the atmosphere in the ENSO-precipitation relationship was
analyzed by conducting the regression analysis for CMIP5’s NAT-only (Figure 2c) and GHG-only (Figure 2d)
experiments, covering the period 1970–2005. Despite differences in the general precipitation pattern as com-
pared to observations (a result of inherent model biases), the GHG-only forcing produces a significantly stron-
ger precipitation response over Texas and Oklahoma. Moreover, a similar analysis performed using the CESM1
LEP data for the periods of 1940–1980 (Figure 2e) and 2010–2050 (Figure 2f; i.e., that reflects future precipitation
outcomes) reveals the same tendency, i.e., stronger precipitation anomalies over Texas and Oklahoma in
response to a transformational ENSO signature in a warmer climate. Noteworthy here is that since CESM1
is not included in the 17 CMIP5 models, this result also serves to validate the multimodel analysis.
Since the regression is linear, the corollary is valid—that is, the La Niña-induced precipitation deficit or drought
would likewise result over Texas and Oklahoma and equally so, any overtones associated with a strong La Niña
Figure 2. May precipitation regression with Niño3.4 index for (a)
1948–1980 and (b) 1981–2014; values exceeding ±9 are significant at the
95% interval (unit: mm/month/C). The high-precipitation region in Texas
and Oklahoma is outlined with the white dashed line. (c and d) Same as
Figures 2a and 2b except for the ensembles of CMIP5 historical experi-
ments from the NAT-only and GHG-only forcing experiments, respectively,
over the 1970–2005 period. Purple contours outline the values of 23. (e and
f) Similar to Figures 2c and 2d except for the 30-member ensembles of
CESM1 for the 1940–1980 and 2010–2050 periods, respectively.
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SIMON WANG ET AL. ATTRIBUTION OF 2015 TEXAS FLOODS 3

as was the case in 2011 [Peterson et al., 2012]. In the future, as can be inferred from Figure 2f, one may anticipate
the tendency for an increased ENSO impact on the southern Great Plains precipitation regime.
Recent research [Meehl and Teng, 2007; Stevenson et al., 2012; Wang et al., 2015] indicates that ENSO teleconnec-
tion and its regional impact would intensify in response to increasing SST. Under such a premise we regressed
the 250 hPa stream function with the Niño3.4 index (Figure 3a) for the same two time periods as in Figure 2. The
ENSO-induced “great arch” teleconnection emanating from the central equatorial Pacific is visible in both
periods. However, the post-1980 time period features a noticeably stronger circulation amplitude; this includes
a deepened, standing trough west of Texas as in May 2015. The deepened trough, together with an abnormally
strong subtropical jet that extends into Baja California (not shown), is indicative of enhanced synoptic waves
directed toward the southern Great Plains. In Figures 3c and 3d we show the tropical precipitation (shaded)
Figure 3. Same as Figures 2a and 2b except for (a, b) the observed 250 hPa stream function anomalies ψ (contours for ±5)
and (c, d) global precipitation (shadings) and SSTA (contours for 0.4, 0.6, and 0.85°C) in May. Here the Niño3.4 index was
standardized so the variables reflect their native unit. (e and f) May SST means of the two periods with a single contour of
28.5°C encircling the warm pool. The yellow ovals indicate the precipitation anomaly center based upon the 1981–2014
period. Contours in Figures 3a and 3b outline 5 × 10
6
m
2
s
1
which cover the 99% confidence interval.
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SIMON WANG ET AL. ATTRIBUTION OF 2015 TEXAS FLOODS 4

and SST (contoured) regressions during the different time periods: After 1980, the El Niño-induced pre-
cipitation and SST anomalies r eveal marked increases over the equatorial central Pacific—in fact, at the
center (outlined by a yellow circle), precipitation has increased 1.7 times in the latter period while the
mean SST increased by about 1°C (Figur es 3e and 3f).
Focusing on wintertime, Zhou et al. [2014] found that the tropical Pacific precipitation anomalies associated
with ENSO would intensify in a warmer climate while extending eastward over the equatorial eastern basin.
Such an increase in precipitation, which is similar to what Figure 3d shows, leads to substantial change in
latent heating that can further intensify the teleconnection response [e.g., Branstator, 1985; Palmer, 2014;
Wang et al., 2015]. To examine further, we conducted a comparable analysis to that of Figure 3 but this time
with respect to the CMIP5 models in order to detect any change in the ENSO teleconnection between that of
NAT and GHG; this is shown in the supporting information Figure S1 for the time period 1970–2005. The results
are alike in that under CHG forcing, an intensified teleconnection wave train linked to a deepened trough in the
southwestern U.S. is observed, in association with precipitation anomaly enhancement over the equatorial cen-
tral Pacific. Altogether, the results presented here (i.e., Figures 2 and 3) as well as those in the current literature
support the strengthening of the ENSO teleconnection due to a warmer climate.
3.2. Other Factors
The cause of widespread flooding is manifold and cannot be explained solely by any single climate process.
Additional circulation features associated with the extreme rainfall of May 2015 do exist: By conducting a
power spectral analysis for zonal wave numbers in the May 2015 stream function within the 30°–50°N latitu-
dinal zone, a wave 2 regime and a wave 5 regime emerged (see Figure S2). While the wave 2 regime reflects
the ENSO-induced circulation anomaly that is inherently of longer wavelength [Wallace and Gutzler, 1981],
the wave 5 regime echoes an increasingly influential mode of the so-called circumglobal teleconnection
[Branstator, 2002; Schubert et al., 2011; Teng et al., 2013]. Focusing on the latter, we performed a zonal harmo-
nic analysis on the stream function anomaly following Wang et al. [2013]; this wave 5 component is shown in
Figure 4a. A clear short-wave train emerges encompassing the deepened trough west of the flooded region
and the anomalous ridge to the east. We next computed the linear trend (slope) of this wave 5 stream func-
tion for the time period of 1980–2014 from the ensemble of modern-era reanalyses; this is shown in
Figure 4. (a) The 250 hPa stream function anomalies of May 2015 in the wave 5 regime overlaid with the climatological jet
stream (hatched; |V| > 25 m/s); the yellow-red domain indicates the Texas-Oklahoma floods. (b) Linear trend (total change
over the 1981–2014 period) of the wave 5 regime stream function (unit: 10
6
m
2
s
1
) computed from the ensemble of four
satellite era reanalyses, with the 95% confidence interval shaded. Notice the phase coincidence between Figures 4a and 4b.
(c) Same as Figure 4b but for the column water vapor flux (formula indicated, where g is gravity, q is specifichumidity,p
is pressure, and V is horizontal winds). Southerly component is colored with the red scale.
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SIMON WANG ET AL. ATTRIBUTION OF 2015 TEXAS FLOODS 5

Figure 4b. The trend reveals a distinct wave pattern in the wave 5 regime, and the phase of this intensified
short-wave train is remarkably coincident with the May 2015 anomaly. This result suggests an intensification
of the short-wave circulation and is resonant with the findings of Meehl and Teng [2007] and J.-Y. Lee et al.
[2014], i.e., that increased ENSO amplitude that ensues from a warmer climate produces a prominent wave
5 pattern within the teleconnection.
The similarity between the two wave 5 circulations (Figures 4a and 4b) accompanied by a stronger low-level
jet (LLJ) in the Great Plains (Figure 1c) implies a coupling enhancement in the classic trough-LLJ setting
[Uccellini, 1980], and such coupling produces the majority of precipitation in the southern Great Plains during
the late spring [Wang and Chen, 2009]. Readers are referred to Text S1 for further explanation of the relevant
synoptic processes. The long-term change in this trough-LLJ coupling was further examined by computing
the linear trend of the column water vapor fluxes QðÞ, integrated up to 300 hPa (Figure 4c). A distinct band
of southerly Q forms over the southern Great Plains signifying an intensified LLJ that is coupled with the dee-
pened upper level trough to the west, as was noted in Barandiaran et al. [2013]. While Weaver et al. [2009]
related the springtime LLJ intensification to interdecadal variation in the North Atlantic, Cook et al. [2008]
linked the increased LLJ with the anthropogenic global warming.
Though the cause of flood is not the focus here, a final comment that should be realized is that ground con-
ditions in Texas more than likely were “preconditioned” to initiate flooding conditions (J. Meng, NOAA/NCEP/
EMC, personal communication, 2015): As Figure S3 shows, April 2015 was abnormally wet in southern Texas
(though not exceptionally) whereupon soil moisture in the Huston area was already above normal for that
time of year which carried forward into early May near saturation conditions.
4. Summary
The record precipitation that occurred over Texas and Oklahoma during the month of May 2015 was the
result of a series of climate interactions and anomalies. Foremost is the role that ENSO played: A developing
El Niño has a tendency to increase spring precipitation over the southern Great Plains, and this effect was
found to have intensified since 1980; this intensification was concomitant with a warmer atmosphere due
to anthropogenic GHG. Specifically, the intensified ENSO teleconnection appears to be triggered by
enhanced latent heating in the equatorial central Pacific and is associated with broad SST warming in the tro-
pics. In essence, there was a detectable effect of anthropogenic global warming on the teleconnection and
moisture transport leading to May 2015’s high precipitation. Previous studies as well as this one point to
the following processes: (1) long-term warming of the tropical Pacific acting to strengthen the atmospheric
response to ENSO, (2) El Niño modulating the wave 5 circulation pattern in a warmer climate and its phase
lock with the May 2015 anomaly, (3) enhancement of the Great Plains LLJ and associated moisture supply
in late spring, and (4) the LLJ’s coupling with the deepened spring trough at upper levels. All the aforemen-
tioned processes together with the attribution analyses of CMIP5 and CESM1 models analyzed here point
toward the exacerbating effect of increasing GHG on the springtime precipitation over Texas and
Oklahoma during a developing El Niño—this being so currently (i.e., 2015) and in the future. Furthermore,
the diagnostic analyses detailed here, in which increased extreme events and a warmer climate were shown
to be dynamically linked, are keys in the provision of seasonal predictions as a guide to future occurrences
and intensities of extreme weather events.
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Soil moisture information shared by Jesse
Meng is appreciated. W.-R. Huang was
supported by the Ministry of Science and
Technology of Taiwan under MOST 104-
2111-M-003-001 and MOST 103-2111-M-
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