ESDEarth System DynamicsESDEarth Syst. Dynam.2190-4987Copernicus PublicationsGöttingen, Germany10.5194/esd-8-1009-2017 Atmospheric torques and Earth's rotation: what drove the millisecond-level length-of-day response to the 2015–2016 El Niño?LambertSébastien B.sebastien.lambert@obspm.frMarcusSteven L.https://orcid.org/0000-0002-5763-6961de VironOlivierSYRTE, Observatoire de Paris, PSL Research University, CNRS, Sorbonne Universités, UPMC Univ. Paris 06, LNE, Paris, FranceIndependent Researcher, Santa Monica, California, USALittoral, Environnement et Sociétés (LIENSs), Université de La Rochelle and CNRS – UMR7266, La Rochelle, FranceSébastien B. Lambert (sebastien.lambert@obspm.fr)14November2017841009101730May201718July201711October201711October2017This work is licensed under the Creative Commons Attribution 3.0 Unported License. To view a copy of this licence, visit https://creativecommons.org/licenses/by/3.0/This article is available from https://esd.copernicus.org/articles/8/1009/2017/esd-8-1009-2017.htmlThe full text article is available as a PDF file from https://esd.copernicus.org/articles/8/1009/2017/esd-8-1009-2017.pdf
El Niño–Southern Oscillation (ENSO) events are classically associated with
a significant increase in the length of day (LOD), with positive mountain
torques arising from an east–west pressure dipole in the Pacific driving a
rise of atmospheric angular momentum (AAM) and consequent slowing of the
Earth's rotation. The large 1982–1983 event produced a lengthening of the day
of about 0.9 ms, while a major ENSO event during the 2015–2016 winter season
produced an LOD excursion reaching 0.81 ms in January 2016. By evaluating the
anomaly in mountain and friction torques, we found that (i) as a mixed
eastern–central Pacific event, the 2015–2016 mountain torque was smaller than
for the 1982–1983 and 1997–1998 events, which were pure eastern Pacific events,
and (ii) the smaller mountain torque was compensated for by positive friction
torques arising from an enhanced Hadley-type circulation in the eastern
Pacific, leading to similar AAM–LOD signatures for all three extreme ENSO
events. The 2015–2016 event thus contradicts the existing paradigm that
mountain torques cause the Earth rotation response for extreme El Niño events.
All the authors have agreed to the licence and copyright agreement.
Introduction
Earth rotation fluctuates with time, as a response to the interaction of the
solid Earth with celestial bodies, the liquid core, and the fluid layers of
the climate system. This interaction results in changes of the orientation of
the Earth rotation vector in space, of the orientation of the Earth around
its rotation axis, and of the Earth rotation angular velocity associated with
changes in the length of the day (LOD). Variations in the LOD can reach a few
milliseconds on the timescale of a few tens of years, due to core–mantle
interaction and references therein, and a few
tenths of a millisecond on the timescale of some days to several years, due
mostly to solid-Earth–atmosphere interaction , though
the solid-Earth–ocean interaction does play a small role
.
A major atmospheric impact on Earth rotation occurs on the annual timescale
, due to the hemispheric asymmetry of the
seasonal cycle , with El Niño–Southern
Oscillation (ENSO) events
dominating interannual (2–7 year) variability.
Extreme El Niño events such as those that occurred in the winters
of 1982–1983, 1997–1998 and 2015–2016 (e.g., http://ggweather.com/enso/oni.htm),
can generate LOD anomalies reaching amplitudes of nearly a millisecond with
respect to the climatological seasonal cycle. Previous studies found that the
creation of the large LOD anomaly during the 1982–1983 event was mainly due to
mountain torque on the American and Eurasian orography
. In general, the paradigm has
emerged that mountain torques (defined more precisely below) generate the
rotational anomalies associated with the El Niño cycle, while friction
torques play a more passive role by damping these anomalies back towards
their climatological norms.
Considering two types of El Niño events defined in the recent literature
(see below), showed that central Pacific (CP) events
are associated with smaller LOD anomalies than eastern Pacific (EP) events,
due to the position and amplitude of the pressure anomaly over the Pacific
Ocean that generates a weaker mountain torque. While 1982–1983 and 1997–1998 are
cited as examples of classical EP events, noted the
different nature of the extreme 2015–2016 episode, finding it to be the
strongest mixed EP–CP event ever recorded. In this paper we seek to document
and understand how the different atmospheric torques active during the recent
mixed event raised the atmospheric angular momentum (AAM) and consequently
the LOD anomalies to values similar to those reached during the previous
extreme EP events. As an extreme event of a unique nature, the 2015–2016 mixed
EP–CP episode offers a chance to gain further insights into how atmospheric
dynamics link Earth rotation anomalies to different types of El Niño .
Methods and data
When studying the impact of the atmosphere on Earth rotation, two different
approaches can be used. First, one can consider that the atmosphere is
included in the Earth system, compute the variation of the AAM, consider that
the angular momentum of the system is conserved – that what is lost by the
atmosphere is gained by the solid Earth – and estimate from there the Earth
rotation change: this is known as the angular momentum approach. The other
approach considers the atmosphere as an external forcing on the solid Earth,
computes the torque exerted by the atmosphere on the solid Earth, and
estimates the Earth rotation changes using the angular momentum budget equation.
Note that the angular momentum approach (applied below in Fig. 2) is
preferred for explicating (or predicting) LOD anomalies, which can serve as
external “ground truth” for validating model-specified (or predicted) AAM
under the severe conditions associated with extreme ENSO episodes. Conversely, the torque approach (applied below in Fig. 4) can provide
dynamical insight into the mechanisms generating the near-millisecond LOD
anomalies that accompany these events and can also be used for internal
consistency checks of the model AAM budgets under the strong perturbations involved.
As shown in , the total torque exerted by the
atmosphere on the Earth is composed of three effects: the gravitational
attraction of the mass anomalies inside the Earth by those inside the
atmosphere, the atmospheric pressure acting over the topography, and the
friction of the wind on the surface. The first two contributions are
classically merged into the so-called mountain torque while the latter
is known as friction torque.
We used standard formulations of the AAM, and mountain and friction torques
that can be found in , for example. The AAM is
composed of two parts, a mass term corresponding to the angular
momentum associated with the rigid rotation of the atmosphere with the solid
Earth and a motion term corresponding to the relative angular momentum
of the atmosphere with respect to the solid Earth. The Z component of the
AAM was estimated from
HZmass=a4Ωg∫02π∫0πPssin3θdθdλ,HZmotion=a3g∫02π∫0π∫0Psusin2θdpdθdλ,
where a is the mean Earth radius, g is the mean gravity acceleration, u is
the zonal wind, Ps is the surface pressure, θ and
λ are the colatitude and longitude, respectively, and Ω is the
Earth mean angular velocity. The axial torques were estimated using
ΓZmountain=a3∫02π∫0π∂Ps∂λhsinθdθdλ,ΓZfriction=-a3∫02π∫0πτλsin2θdθdλ,
where h is the orography and τλ is the zonal friction drag.
The time rate of change of the total AAM is given by the sum of the mountain
and friction torques e.g.,:
ddtHZmass+HZmotion=ΓZmountain+ΓZfriction.
In what follows, the time-integrated form of Eq. () was
used to evaluate the sources of AAM maxima associated with recent extreme
El Niño events. Given the AAM variation, the induced change in the LOD is
estimated by
ΔLODLOD‾=Δ0.7HZmass+HZmotionCΩ,
where LOD‾ is the nominal length of the solar day (86 400 s)
and C is the axial mean moment of inertia of the Earth; the mass term is
evaluated using the inverted barometer assumption to
account for the quasi-static response of the oceans to atmospheric pressure
loading, and the factor of 0.7 accounts for the compensating changes in the
moment of inertia arising from the elastic deformation of the solid Earth in
response to the surface loading .
Our computations of AAM and torques were based on 2∘× 2∘
surface pressure, zonal momentum flux, and zonal wind speed data from daily
and monthly values from the European Center for Medium-range Weather Forecasts (ECMWF)
ERA-Interim model spanning 1979–2017.
Wind speeds were taken at 17 pressure levels between 10 and 1000 hPa. The
longitudinal gradients of the pressure field were computed with a five-point stencil.
For computation of the mountain torque, we used the model orography at its
native 2∘× 2∘ resolution, thereby ensuring consistency
between the wind, pressure, and zonal momentum flux data sets and the derived
AAM and torque quantities. A recent study by
found that resolved mountain torques in the Met Office United Model with free
atmospheric wind and temperature relaxed to ERA-Interim reanalyses are
relatively insensitive to increasing model resolution (see, e.g., their
Fig. 7), although they are more strongly impacted by large-scale (synoptic)
processes than are the parameterized sub-grid scale torques (not considered
in our study).
Earth rotation data were provided by the International Earth Rotation And
Reference Systems Service (IERS) Earth Orientation Parameters (EOP) 14 C04
series available via the IERS Earth Orientation Centre website
(http://iers.obspm.fr/eop-pc). This
combination of very-long-baseline interferometry (VLBI), global navigation
satellite systems (GNSSs), Doppler Orbitography and Radiopositioning Integrated by Satellite (DORIS), and satellite laser
ranging (SLR) data provides daily estimates of the LOD with an accuracy of
about 0.05 ms. To isolate the anomalous changes due to
episodic events like ENSO as much as possible, we subtracted modeled zonal tides
, a multidecadal trend estimated with a 4-year running
mean and a 5.9-year periodic term – attributed to secular tidal braking and post-glacial
rebound and variations in the fluid core
angular momentum – and a mean
seasonal cycle estimated over 1979–2017. The residual LOD contains
essentially the fluctuation associated with anomalous AAM and oceanic
currents, with the latter being less than 5 %.
As measurements of ENSO activity, we used monthly series of Niño 1 + 2,
Niño 3, Niño 4, and Niño 3.4 indices retrieved from the Climate
Prediction Center (CPC) of NOAA. We used the Niño 1 + 2, Niño 3, and
Niño 4 series to compute the indices relevant to EP and
CP events considered in their Eqs. 3 and 4
and their Eq. 1. For comparison, we also used corresponding EP–CP indices
provided by at https://www.ess.uci.edu/~yu/2OSC
and the PT′ indices of private communication.
The December–February Niño 3.4 (N34), Niño 3 (N3), and Niño 4 (N4)
indices and other various eastern and central Pacific projection
indices for each of the three events. Eastern (central) Pacific indices are
represented right (left) of the Niño 3.4 bar. EK and CK: E and C indices from
; P and T: P and T′ indices from ; ET and CT:
E and C indices from ; NEP and NCP:
eastern Pacific and central Pacific indices as defined by .
We removed seasonal composites and linear trends from all grids and time
series and we formed 2-month “winter” values of all the time-variable
quantities above preceding the respective AAM–LOD maxima, by averaging over
December–January for the EP events and over November–December for the mixed
EP–CP event (see below). For computation of regional torques, we used a
modified version of the land–sea masks, whose geographical limits can be seen
in Fig. 3 of , with limits of the equatorial
zone set to ±15∘ from the Equator. Moreover, we separated Greenland
from North America and the Pacific Ocean into eastern and western zones,
respectively, east and west from the International Date Line.
Analysis and results
For the three extreme El Niño events that occurred in the last 40 years (winters of 1982–1983, 1997–1998, and 2015–2016), the Niño 3.4 index
reached values of more than 2 times its standard deviation over the winter
season. A feature that makes the 2015–2016 ENSO event unique is the hybrid
aspect mentioned by , who showed that EP–CP indices
defined by were of comparable magnitudes, in contrast to
their highly positive EP index and small or negative CP index in 1997–1998.
Similar conclusions can be drawn on the basis of projections provided by
and , the former
seeing the 2015–2016 event as more of a CP and the latter as more of an EP type
(Fig. 1). , who considered a methodology similar to
but also considering extratropical
Pacific variability, derived indices of equal magnitudes by including
contributions of higher-latitude sea surface temperature (SST).
For the three events, anomalous LOD excursions reached the level of nearly a
millisecond, consistent with the AAM variation (Fig. 2). The Niño 3.4
index reached its maximum values between 1 and 3 months before AAM and
LOD . Differences between AAM and LOD anomalies
might be partly due to a small, variable contribution from the ocean and
hydrology and to local biases or side effects induced by the
filtering and smoothing method used to separate the nonseasonal LOD from its
multidecadal and interannual trends. We found that the LOD for the
2015–2016 event peaked at 0.81 ms on 6 January 2016. Figure 2 also suggests that the
1982–1983 event was the strongest from the Earth rotation point of view,
generating an LOD anomaly of 0.91 ms, which is about 3.5 times the standard
deviation of the mean seasonal cycle. The 1997–1998 event was somewhat less
active with an LOD anomaly of only 0.76 ms. Our values are consistent with
analyses of , who found comparable excursions of the LOD in 1997–1998 and 2015–2016 of
about 0.75 ms, based on VLBI
data; interestingly, however, the maximum rotational anomalies for
the two earlier EP events occur nearly a month later in the season than for
the 2015 mixed event.
In order to analyze the synoptic features giving rise to these rotational
anomalies, we formed global maps of the surface pressure and surface friction
drag anomalies for the 2 months that preceded them, averaging over December–January
for the 1982–1983 and 1997–1998 winters and over November–December for the 2015–2016 winter
(Fig. 3). The two EP surface pressure maps (1982–1983 and 1997–1998) reveal the
classic east–west dipole for this type of event noted by
, with the low-pressure areas in proximity to the
American coast generating substantial positive mountain torques on the
atmosphere and thereby increasing the LOD. For the mixed EP–CP event in 2015,
however, the surface pressure gradients have a substantial meridional
component, with a low-pressure area in the equatorial east–central Pacific
flanked by anomalous high-pressure zones in the northeastern (NE) and
southeastern (SE) Pacific. This Hadley-type pattern gives rise to anomalous
easterlies in the eastern equatorial (EE) Pacific, generated as inflow to the
equatorial low-pressure area near 120∘ W, and also in the NE and
SE Pacific, generated as enhanced easterly flow on the equatorward flanks of the
anomalous high-pressure Pacific areas. The result is a significant
enhancement of positive friction torque over the eastern tropical and
midlatitude Pacific, denoted by the orange shaded areas in the right-hand
column of Fig. , for the mixed EP–CP event as compared with the
earlier EP events. This comparison is highlighted in panels (g and h) of
Fig. 3, which show the difference of the surface pressure and friction drag
anomalies between the 2-month means for the mixed event and the average of
the two EP events. The pressure difference (Fig. 3g) shows that the change
between the mixed and EP events takes the form of a strengthened Hadley-type
circulation in the east–central Pacific, with the stronger and more
equatorward response in the winter (northern) hemisphere. This is reflected
in the friction difference between the mixed and EP events (Fig. 3h),
which shows a strong enhancement of the surface drag in the NE Pacific and
the EE Pacific; a weaker enhancement is also seen over the midlatitude SE Pacific,
compensated for by enhanced westerlies over the Antarctic Circumpolar Current.
The time series of daily AAM (blue) and LOD (black) values around
the three extreme events. The red dashed line represents the scaled monthly
Niño 3.4 index. The shaded area represents 1 standard deviation around
the climatological mean. The x-axis ticks indicate the first day of each
month.
(a, c, e) The surface pressure and (b, d, f) zonal
friction drag anomalies averaged over December–January for the
1982–1983 and 1997–1998 EP events and averaged over November–December for
the 2015–2016 EP–CP event. Panels (g) and (h) show, respectively, the difference
in pressure and zonal friction drag anomaly between the 2015–2016 situation and the average of the 1982–1983 and 1997–1998 situations.
The main features observed in the maps are reflected by the values of the
2-month averaged mountain and friction torque anomalies for the various
land and ocean areas given in Table 1. The lower net 2015–2016 mountain torque
compared to 1982–1983 and 1997–1998 is consistent with the lack of a pronounced
east–west pressure dipole in the Pacific in late 2015, resulting in a
substantial negative torque over North America and Greenland during that time,
while the low-pressure anomaly in the equatorial Pacific generated similar
positive mountain torques over South America as for the EP events. A
significant portion of the remaining difference in global mountain torque
between the three events is generated over the European continent, as a
consequence of changes in the relative positions of large-scale North
Atlantic features during the winter, possibly influenced by El Niño
, that modify the direction and the intensity
of the downstream pressure gradients over the continental orography
(particularly the Caucasus and Zagros mountains). The lack of a net positive
mountain torque leading up to the 2015–2016 rotational maximum, however, is
compensated for by the presence of positive friction torques during that time,
particularly over the eastern Pacific, with the NE and EE regions making the
largest contributions, relative to their corresponding values during the EP
events; the intense high-pressure area in the SE Pacific, reminiscent of the
November 2009 feature discussed by , makes a smaller
positive contribution to the CP–EP axial torque difference due to its higher latitude.
Contributions to the mountain and friction torques exerted by the
solid Earth onto the atmosphere in Hadley (i.e., 1018 Nm), averaged over
December–January for the 1982–1983 and 1997–1998 EP events (columns
D82–J83 and D97–J98, respectively) and averaged over November–December
for the 2015–2016 EP–CP event (column ND15).
The contributions of these processes to the LOD maxima generated during the
three extreme events can be illustrated by integrating the daily torques over
preceding intervals in the time domain to reconstitute the AAM. The
difference between the integrated friction and mountain torques and the AAM
arising from gravity-wave drag and other torques related to the sub-grid-scale orography is generally considered to be negligible on these timescales
. As starting epochs, we
chose the beginning of the rise of each AAM curve towards its peak value. The
resulting reconstituted AAM components – consistent with Figs. 2 and 3 of
for the 1982–1983 event – are shown in Fig. 4. They
reveal that a positive friction episode occurred in 2015–2016 about 15 to
20 days before the AAM peak. Such a positive friction episode, occurring about
10 to 15 days before the AAM peak, was totally absent from the 1997–1998 event
and was much smaller in the 1982–1983 event, when the integrated friction
torque remains positive during a few days before turning back to negative
values. Note that choosing starting epochs and an integration period consistent
with the 2-month intervals considered above leads to similar conclusions
but to less consistent closures of the budget due to small biases accumulated
in the integration, as already mentioned in . The lower
panel of Fig. 4 demonstrates the importance of the eastern Pacific
contribution to the overall positive friction torque in the last 2 weeks
of 2015 and highlights absolute contributions from the EE and SE Pacific
regions at this time. The NE Pacific also contributes positively but to a
lesser extent; its contribution relative to the corresponding (negative)
NE Pacific values for the EP events, however, is greater than those for the
EE and SE regions combined over the 2 months preceding these events (Table 1).
A Hovmöller (time–latitude) plot of the eastern Pacific frictional drag
contributing to the 2015–2016 LOD maximum (Fig. 5) highlights its three-belt
structure and shows the EE Pacific contribution (spanning
15∘ N–15∘ S) to arise from two areas: one in the Southern
Hemisphere originating from inflow to the westwardly displaced boreal winter
Hadley circulation and one in the Northern Hemisphere originating from
enhanced easterly flow on the equatorward flank of the NE Pacific high-pressure area (similar to the November–December 2015 pressure anomalies seen in Fig. 3e).
(a) The integrated torques compared to AAM during the three
events and (b) the integrated friction torque with contribution from
the different regions of the Pacific Ocean during the
2015–2016 event.
Time–latitude (Hovmöller) diagram of the zonal friction drag
anomaly between 11 December 2015 and 20 January 2016 and averaged over
longitudes between 180 and 270∘.
Discussion and conclusion
Surface pressure and friction torque anomaly maps for the last 2 months
before each extreme AAM–LOD peak (Fig. 3) and a time–latitude plot of
frictional stress during the last event (Fig. 5) suggest that the
2015–2016 positive friction torque arose from three zones: in the NE Pacific between
latitudes of 0 and 40∘ N, in the EE Pacific off Peru, and in the
SE Pacific between 40 and 60∘ S, showing rough symmetry about
an enhanced (boreal winter) Hadley-type circulation in the east–central
Pacific. The positive (LOD-lengthening) EE Pacific contribution to the
friction torque can be understood in the context of a CP event in which the
ENSO-driven convection is displaced towards the central Pacific; the incoming
winds that supply the convection are westward in the EE Pacific. From the
momentum point of view, this convection also strengthens the Hadley
circulation and the subtropical jets that carry the bulk of the AAM signal.
The SE Pacific positive torque contribution results from a strengthening of
the extratropical South Pacific anticyclone similar to that documented in
November 2009 during a CP event , with the
NE Pacific positive contribution stemming from a similar high-pressure response
in the subtropical winter hemisphere. Note, however, that the weaker
2009–2010 CP event, which lacked the NE Pacific circulation center found during the
extreme 2015–2016 episode, did not produce a significant anomaly in AAM or LOD.
The three extreme ENSO events of 1982–1983, 1997–1998, and 2015–2016 were of
comparable strengths expressed through both SST and subsurface indices
. The latest was, however, of a different
nature, as a mixed EP–CP event, as opposed to the other two pure EP events
. All three events produced
anomalous excursions of the Earth's LOD between 0.76 ms (1997–1998) and 0.91 ms
(1982–1983), the amplitude of the 2015–2016 (0.81 ms) excursion being
intermediate. We showed that, though the 1982–1983 and 1997–1998 LOD anomalies
were driven by the mountain torque, as expected with pure EP events, the LOD
excitation mechanism of the mixed EP–CP 2015–2016 event was different. The
weaker mountain torque was compensated for by a positive friction torque acting
in the eastern Pacific, both in an absolute sense and relative to the
frictional torques prevailing there during the earlier EP events. The
2015–2016 event, unique for its nature and intensity among the ENSO events recorded for
the last 4 decades, thus contradicts the existing paradigm that mountain
torques cause the Earth rotation response for extreme El Niño events.
For mixed or CP events, increasing in frequency and strength since the turn
of the century , friction torques arising form tropical
and extratropical centers of action can make a significant contribution to
the positive LOD anomalies, thereby compensating for the less efficient CP
mountain torque coupling and maintaining the
capability for a robust rotational response to this new type of event.
Interestingly, enhanced easterlies or positive
friction;, more and stronger CP events
, and the global warming hiatus
have coincided in the
early 21st century; the chain of causality among these events, however, is
far from clear.
These three extreme events also exemplify the complex relation between the
ENSO strength and the atmospheric response (AAM and torques) that leads to
variations in the Earth's rotation rate. The dominant factor is the position
and the depth of the ENSO pressure dipole that can significantly strengthen
or weaken the mountain torques exerted by the atmosphere on the Andes and the
Rocky Mountains. Nevertheless, the factors governing teleconnections between tropical
Pacific SST anomalies and the globally distributed pressure–wind response that are still being actively investigated e.g.,
may play a critical role in determining the relative rotational signatures of the events.
Surface pressure, wind, zonal momentum flux (eastward turbulent
surface stress) and orography data used in this study are made publicly available
by the European Center for Medium range Weather Forecasts (ECMWF) at
http://apps.ecmwf.int/datasets/data/interim-full-daily/levtype=sfc/. The
ENSO indices are publicly available via the Climate Prediction Center of the NOAA
at http://www.cpc.ncep.noaa.gov/data/indices/ersst4.nino.mth.81-10.ascii.
Earth rotation data are available via the IERS Earth Orientation Centre website
(http://iers.obspm.fr/eop-pc).
SL proposed the idea for this study. All authors contributed to developing the
methods and analyzing the data as well as contributed the materials and analysis tools. All
the authors jointly wrote the paper and extensively discussed the results and
the interpretations. All authors read and approved the final manuscript.
The authors declare that they have no conflict of interest.
Acknowledgements
The work of Olivier de Viron was financially supported by CNES, through the TOSCA program,
as an application of the geodesy missions.
Edited by: Valerio Lucarini
Reviewed by: Antonio Speranza and one anonymous referee
ReferencesBarnes, R. T. H., Hide, R., White, A. A., and Wilson, C. A.: Atmospheric angular
momentum fluctuations, length-of-day changes and polar motion, P. Roy. Soc.
Lond. A, 387, 31–73, 10.1098/rspa.1983.0050, 1983.Butler, A. H., Polvani, L. M., and Deser, C.: Separating the stratospheric and
tropospheric pathways of El Niño–Southern Oscillation teleconnections,
Environ. Res. Lett., 9, 024014, 10.1088/1748-9326/9/2/024014, 2014.
Carter, W., Robertson, D., Pettey, J., Tapley, B., Schutz, B., Eanes, R., and
Lufeng, M.: Variations in the rotation of the Earth, Science, 224, 957–961, 1984.
Chao, B. F.: Interannual length-of-day variation with relation to the Southern
Oscillation/El Niño, Geophys. Res. Lett., 11, 541–544, 1984.
Chen, X. and Wallace, J. M.: Orthogonal PDO and ENSO Indices, J. Climate,
29, 3883–3892, 2016.Dee, D. P., Uppala, S. M., Simmons, A. J., Berrisford, P., Poli, P., Kobayashi,
S., Andrae, U., Balmaseda, M. A., Balsamo, G., Bauer, P., Bechtold, P., Beljaars,
A. C. M., van de Berg, L., Bidlot, J., Bormann, N., Delsol, C., Dragani, R.,
Fuentes, M., Geer, A. J., Haimberger, L., Healy, S. B., Hersbach, H., Hólm,
E. V., Isaksen, L., Kållberg, P., Köhler, M., Matricardi, M., McNally,
A. P., Monge-Sanz, B. M., Morcrette, J.-J., Park, B.-K., Peubey, C., de Rosnay,
P., Tavolato, C., Thépaut, J.-N., and Vitart, F.: The ERA-Interim reanalysis:
configuration and performance of the data assimilation system, Q. J. Roy.
Meteorol. Soc., 137, 553–597, 10.1002/qj.828, 2011.de Viron, O. and Dickey, J. O.: The two types of El-Niño and their impacts
on the length of day, Geophys. Res. Lett., 41, 3407–3412, 10.1002/2014GL059948, 2014.de Viron, O., Bizouard, C., Salstein, D., and Dehant, V.: Atmospheric torque on
the Earth and comparison with atmospheric angular momentum variations, J. Geophys.
Res.-Solid Ea., 104, 4861–4875, 10.1029/1998JB900063, 1999.
de Viron, O., Dickey, J., and Marcus, S.: Annual atmospheric torques: Processes
and regional contributions, Geophys. Res. Lett., 29, 44–1–44–3, 2002.Dickey, J. O., Marcus, S. L., and Chin, T. M.: Thermal wind forcing and
atmospheric angular momentum: Origin of the Earth's delayed response to ENSO,
Geophys. Res. Lett., 34, L17803, 10.1029/2007GL030846, 2007.Dickey, J. O., Marcus, S. L., and de Viron, O.: Closure in the Earth's angular
momentum budget observed from subseasonal periods down to four days: No core
effects needed, Geophys. Res. Lett., 37, l03307, 10.1029/2009GL041118, 2010.Douville, H., Voldoire, A., and Geoffroy, O.: The recent global warming hiatus:
What is the role of Pacific variability?, Geophys. Res. Lett., 42, 880–888,
10.1002/2014GL062775, 2015.
England, M. H., McGregor, S., Spence, P., Meehl, G. A., Timmermann, A., Cai, W.,
Gupta, A. S., McPhaden, M. J., Purich, A., and Santoso, A.: Recent intensification
of wind-driven circulation in the Pacific and the ongoing warming hiatus, Nat.
Clim. Change, 4, 222–227, 2014.
Gipson, J. M.: El Niño and VLBI Measured Length of Day, in: IVS 2016 General
Meeting Proceedings: New Horizons with VGOS, edited by: Behrend, D., Baver, K.
D., and Armstrong, K. L., 13–17 March 2016, Johannesburg, South Africa, p. 336, 2016.Hide, R. and Dickey, J. O.: Earth's Variable Rotation, Science, 253, 629–637,
10.1126/science.253.5020.629, 1991.Hide, R., Dickey, J., Marcus, S., Rosen, R., and Salstein, D.: Atmospheric
angular momentum fluctuations during 1979-1988 simulated by global circulation
models, Phys. Chem. Earth, 23, 1089–1090, 10.1016/S0079-1946(98)00148-7, 1998.
Hide, R., Boggs, D. H., and Dickey, J. O.: Angular momentum fluctuations within
the Earth's liquid core and torsional oscillations of the core–mantle system,
Geophys. J. Int., 143, 777–786, 2000.
Holme, R. and de Viron, O.: Characterization and implications of intradecadal
variations in length of day, Nature, 499, 202–204, 2013.Huang, H.-P., Sardeshmukh, P. D., and Weickmann, K. M.: The balance of global
angular momentum in a long-term atmospheric data set, J. Geophys. Res., 104,
2031–2040, 10.1029/1998JD200068, 1999.Jeffreys, H.: Causes contributory to the annual variations of latitude, Mon.
Weather Rev., 44, 337–337, 10.1175/1520-0493(1916)44<337b:CCTTAV>2.0.CO;2, 1916.Ji, X., Neelin, J. D., and Mechoso, C. R.: Baroclinic-to-Barotropic Pathway in
El Niño–Southern Oscillation Teleconnections from the Viewpoint of a
Barotropic Rossby Wave Source, J. Atmos. Sci., 73, 4989–5002, 10.1175/JAS-D-16-0053.1, 2016.Johnson, N. C.: How Many ENSO Flavors Can We Distinguish?, J. Climate, 26,
4816–4827, 10.1175/JCLI-D-12-00649.1, 2013.Kao, H.-Y. and Yu, J.-Y.: Contrasting Eastern-Pacific and Central-Pacific Types
of ENSO, J. Climate, 22, 615–632, 10.1175/2008JCLI2309.1, 2009.Lee, T. and McPhaden, M. J.: Increasing intensity of El Niño in the
central-equatorial Pacific, Geophys. Res. Lett., 37, l14603, 10.1029/2010GL044007, 2010.Lee, T., Hobbs, W. R., Willis, J. K., Halkides, D., Fukumori, I., Armstrong,
E. M., Hayashi, A. K., Liu, W. T., Patzert, W., and Wang, O.: Record warming
in the South Pacific and western Antarctica associated with the strong
central-Pacific El Niño in 2009–10, Geophys. Res. Lett., 37, L19704,
10.1029/2010GL044865, 2010.L'Heureux, M. L., Takahashi, K., Watkins, A. B., Barnston, A. G., Becker, E. J.,
Di Liberto, T. E., Gamble, F., Gottschalck, J., Halpert, M. S., Huang, B.,
Mosquera-Vásquez, K., and Wittenberg, A. T.: Observing and Predicting the
2015/16 El Niño, B. Am. Meteorol. Soc., 98, 1363–1382, 10.1175/BAMS-D-16-0009.1, 2017.Marcus, S. L., Chao, Y., Dickey, J., and Gegout, P.: Detection and modeling of
nontidal oceanic effects on Earth's rotation rate, Science, 281, 1656–1659,
10.1126/science.281.5383.1656, 1998.Marcus, S. L., de Viron, O., and Dickey, J. O.: Abrupt atmospheric torque
changes and their role in the 1976–1977 climate regime shift, J. Geophys.
Res.-Atmos., 116, D03107, 10.1029/2010JD015032, 2011.Marcus, S. L., Dickey, J. O., Fukumori, I., and de Viron, O.: Detection of the
Earth rotation response to a rapid fluctuation of Southern Ocean circulation in
November 2009, Geophys. Res. Lett., 39, L04605, 10.1029/2011GL050671, 2012.
Munk, W. H. and MacDonald, G. J. F.: The Rotation of the Earth: A Geophysical
Discussion, Cambridge University Press, Cambridge, 1960.Paek, H., Yu, J.-Y., and Qian, C.: Why were the 2015/16 and 1997/98 Extreme
El Niños different?, Geophys. Res. Lett., 10.1002/2016GL071515, in press, 2017.Palmeiro, F. M., Iza, M., Barriopedro, D., Calvo, N., and García-Herrera,
R.: The complex behavior of El Niño winter 2015–2016, Geophys. Res. Lett.,
10.1002/2017GL072920, in press, 2017.
Petit, G. and Luzum, B.: IERS Conventions 2010, IERS Technical Note 36, Verlag
des Bundesamts für Kartographie und Geodäsie, Frankfurt am Main, 179 pp., 2010.Ponte, R. M. and Rosen, R. D.: Torques Responsible for Evolution of Atmospheric
Angular Momentum during the 1982–83 El Niño, J. Atmos. Sci., 56, 3457–3462,
10.1175/1520-0469(1999)056<3457:TRFEOA>2.0.CO;2, 1999.
Ponte, R. M., Rosen, R., and Boer, G.: Angular momentum and torques in a simulation
of the atmosphere's response to the 1982–83 El Niño, J. Climate, 7, 538–550, 1994.Ren, H.-L. and Jin, F.-F.: Niño indices for two types of ENSO, Geophys.
Res. Lett., 38, L04704, 10.1029/2010GL046031, 2011.Song, J., Wang, Y., and Tang, J.: A Hiatus of the Greenhouse Effect, Scient.
Rep., 6, 33315, 10.1038/srep33315, 2016.Takahashi, K., Montecinos, A., Goubanova, K., and Dewitte, B.: ENSO regimes:
Reinterpreting the canonical and Modoki El Niño, Geophys. Res. Lett., 38,
l10704, 10.1029/2011GL047364, 2011.van Niekerk, A., Shepherd, T. G., Vosper, S. B., and Webster, S.: Sensitivity
of resolved and parametrized surface drag to changes in resolution and
parametrization, Q. J. Roy. Meteorol. Soc., 142, 2300–2313, 10.1002/qj.2821, 2016.Wang, G. and Cai, W.: Climate-change impact on the 20th-century relationship
between the Southern Annular Mode and global mean temperature, Scient. Rep.,
3, 2039, 10.1038/srep02039, 2013.