ACG06-01

Japan Geoscience Union Meeting 2016

The Early-1990s Climate Shift in the Pacific and the ENSO Diversity*Jin-Yi Yu1
1.University of California Irvine
There is substantial evidence that significant changes occurred in broad areas of the Pacific in
the early 1990s, including the shift of the location of El Niño events from the eastern Pacific tothe central Pacific (CP). Observational analysis and coupled model experiments are conducted toshow that the early-1990s climate shift is linked to a phase change of the Atlantic Multi-decadal
Oscillation (AMO) that occurred at about the same time. The recent emergence of the CP El Niño canbe attributed to this AMO phase change via the following chain of events: a switch in the AMO toits positive phase in the early 1990s led to an intensification of the Pacific Subtropical High.
The intensified High resulted in stronger-than-average background trade winds that enhanced theWind-Evaporation-SST feedback mechanism, strengthening the subtropical Pacific coupling between theatmosphere and ocean, making the subtropical Pacific precursors more capable of penetrating into
the deep tropics, and ultimately leading to increased occurrence of the CP El Niño events.
Associated with the change of the El Nino type, the El Nino teleconnection is found to become
different after the early-1990s. A changing relationship between El Nino and Southern Hemisphereclimate will be presented in the talk. Evidence is also found that the typical drought pattern in
Eastern China diminished after the early-1990s climate shift and is replaced by a new pattern thatis produced by the AMO via a Eurasian wave train emanating from North Atlantic to China. This study
indicates that the early 1990s is a time when the Atlantic began to exert a stronger influence on
climate over East Asia and a large part of the Pacific.
Keywords: early-1990s climate shift, ENSO diversity, Atlantic Multi-decadal Oscillation©2016. Japan Geoscience Union. All Right Reserved.

- ACG06-01 -ACG06-02

Japan Geoscience Union Meeting 2016

ENSO diversity caused by mean state-dependent ENSO modes resulting from an intermediate
coupled model*Ruihuang Xie1,2, Fei-Fei Jin3, Mu Mu1,2
1.Key Laboratory of Ocean Circulation and Waves, Institute of Oceanology, Chinese Academy of
Sciences, Qingdao 266071, China, 2.Laboratory for Ocean and Climate Dynamics, Qingdao National
Laboratory for Marine Science and Technology, Qingdao 266237, China, 3.Department of Atmospheric
Sciences, School of Ocean and Earth Science and Technology, University of Hawaii at Manoa, HI
96822, USA
ENSO diversity is referred to the event-to-event differences in the amplitude, longitudinallocation of maximum sea surface temperature (SST) anomalies and evolutional mechanisms, as
manifested in both observation data and climate model simulations. Previous studies argued that
ENSO diversity is associated with westerly wind burst (WWB) or subtropical forcing in thenortheastern Pacific. Here, we bring evidences, from a modified intermediate complexity Zebiak-Cane(MZC) coupled model, to illustrate that the ENSO diversity is also determined by the mean states.
Stabilities of the linearized MZC model reveal that the mean state with weak (strong) wind stressand deep (shallow) thermocline prefers ENSO variation in the equitorial eastern (central) Pacificwith a four-year (two-year) period. Weak wind stress and deep thermocline make the thermocline (TH)feedback the dominant contribution to the growth of ENSO SST anomalies, whereas the opposite meanstate favors the zonal advective (ZA) feedback as the key one. Different leading dynamical
contributor and pacemaker make ENSO display its diversity in spatial pattern and period. In a meanstate that resembles the tropical Pacific climate after 2000, the four-year and two-year ENSO
variations coexist with similar growth rate. Even without WWB forcing, the nonlinear integration
results with adjusted parameters in this special mean state also present at least two types of El
Niño, in which the maximum warming rates are contributed by either TH or ZA feedback. The
consistency between linear and nonlinear model results indicates that the ENSO diversity depends onthe mean state.
Keywords: ENSO diversity, ENSO modes, mean states, favorable feedbacks©2016. Japan Geoscience Union. All Right Reserved.

- ACG06-02 -ACG06-03

Japan Geoscience Union Meeting 2016

ENSO diversity generated by the state dependence of westerly wind events*Michiya Hayashi1, Masahiro Watanabe1
1.Atmosphere and Ocean Research Institute, the University of Tokyo
A coupled dynamics between westerly wind events (WWEs) and El Niño-Southern Oscillation (ENSO) is
examined using an atmosphere-ocean coupled model with an intermediate complexity. WWEs areshort-lived surface westerly wind anomalies over the western-central equatorial Pacific andobserved frequently at the eastern edge of the warm pool when the sea surface temperature (SST)
anomaly at the Niño4 region (160 °E-150 °W, 5 °S-5 °N) is positively large. These features of WWEs
are parameterized as a state-dependent stochastic noise to wind stresses in the model. Without thenoise (experiment NO), the model produces a periodic ENSO-like oscillation with a period of 6 years
and its variance increases with respect to a parameter that controls efficiency of the positive
thermocline feedback, γ. When additive (purely stochastic) noise is given to the model over thewestern Pacific (experiment AD), oscillations become irregular with the dominant period of about 5
years and the increase of its variance relative to NO depends on γ. When the state-dependent noiseis adopted (experiment SD), the oscillatory solution is also irregular besides its variance andasymmetry increase irrespective the value of γ.
Both the additive and state-dependent noises help to produce two types of oscillation,corresponding to the eastern-Pacific (EP) and central-Pacific (CP) El Niños, although there is nosuch diversity in NO. EP El Niño is magnified in SD due to the eastward shift of the noise location
caused by the warm pool expansion. CP El Niño is even favored by the state-dependent stochastic
noise, which enhances the zonal advection to warm the central Pacific, and in turn the warmer Niño4
SST increases the probability of occurrence of the noise. This positive feedback ensures theexistence of CP El Niño regardless of γin SD, while the number of CP El Niño declines with larger γ
in AD. The above results thereby suggest that the state dependence of WWEs may play a crucial roleon the asymmetry and diversity of ENSO in nature.
Keywords: Westerly wind event, ENSO, State dependence©2016. Japan Geoscience Union. All Right Reserved.

- ACG06-03 -ACG06-04

Japan Geoscience Union Meeting 2016

Impacts of decaying eastern and central Pacific El Niños on tropical cyclone activities
over the western North Pacific in summer*Yuxing Yang1, Ruihuang Xie1, Faming Wang1, Fei Huang2
1.Key Laboratory of Ocean Circulation and Wave, Institute of Oceanology, Chinese Academy of
Science, 2.Ocean University of China
We investigate the influences of the decaying eastern Pacific El Niño (EP - El Niño) and central
Pacific El Niño (CP - El Niño) on tropical cyclone (TC) activities in the western North Pacific(WNP) during July, August and September (JAS). During this period, TC geneses and tracks are
reduced in the central and eastern WNP. However, TC tracks reaching the Philippines increase, andmore TC geneses appear west of 145°E during EP - El Niño. During CP - El Niño, tracks reaching theSouth China Sea (SCS) and southeast coast of China increase, and positive anomalies of TC genesis
are found in the southern part of the central WNP and southern SCS. It is possible that thedifferent variation of the anomalous anticyclone over east of the Philippines in the WNP induced by
El Niños are instrumental to different TC variations in the two types of decaying El Niños duringJAS. Compared with EP - El Niño, strengthening and northward expansion of the anomalous anticyclone
during CP - El Niño cause a westward shift of the western Pacific subtropical high in summer, whichis responsible for more westward TC tracks over the SCS and southeast coast of China. This
northward expansion can cause the center of suppressed TC geneses in the central WNP to migrate
further north during CP - El Niño. A decreased magnitude of vertical shear dominates the southernpart of the central WNP and southern SCS, which enhances TC formation in these regions during CP El Niño.
Keywords: two types of El Niños, tropical cyclone, decaying summer©2016. Japan Geoscience Union. All Right Reserved.

- ACG06-04 -ACG06-05

Japan Geoscience Union Meeting 2016

The role of tropical Atlantic SST anomalies in modulating western North Pacific tropical
cyclone genesis*Dachao Jin1,2, Liwei Huo2, Saji Hameed1, Pinwen Guo2
1.University of Aizu, 2.Nanjing university of Information Science & Technology
The connection between north tropical Atlantic (NTA) sea surface temperature (SST) anomalies andtropical cyclone (TC) genesis over the western North Pacific (WNP) and associated physicalmechanisms are investigated in this study.We demonstrate a remarkable negative correlation ofWNP TC
genesis frequency with the (preceding) boreal spring NTA SST anomalies. Our analysis suggests thatmajor factors for TC genesis including distributions of large-scale vorticity and midtropospherichumidity are rendered unfavorable by remote teleconnections while barotropic energy conversion fromthe large-scale flow is suppressed. As shown in recent studies, the remote teleconnection from theAtlantic is sustained and enhanced throughout the typhoon season through local air-sea
interactions. These results suggest that boreal spring NTA SST anomaly could be a new predictor forthe seasonal WNP TC activity.
Keywords: Climate, tropical Atlantic SSTA, western North Pacific, tropical cyclone genesis©2016. Japan Geoscience Union. All Right Reserved.

- ACG06-05 -ACG06-06

Japan Geoscience Union Meeting 2016

The Indo-western Pacific Ocean capacitor mode and coherent climate anomalies in post-ENSO
summer*Yu Kosaka1, Shang-Ping Xie2, Hisayuki Kubota3, Yan Du4, Kaiming Hu5, Jasti S Chowdary6, Gang Huang5
1.Research Center for Advanced Science and Technology, University of Tokyo, 2.Scripps Institutionof Oceanography, University of California San Diego, 3.JAMSTEC, 4.South China Sea Institute of
Oceanography, Chinese Academy of Sciences, 5.Institute of Atmospheric Physics, Chinese Academy of
Sciences, 6.Indian Institute of Tropical Meteorology
El Niño typically peaks in boreal winter, and the associated equatorial Pacific sea surface
temperature (SST) signal dissipates before subsequent summer. Its impact, however, outlasts until
boreal summer in the Indo-western Pacific, featuring basin-wide Indian Ocean warming and tropicalNorthwestern Pacific cooling accompanied by the Pacific-Japan (PJ) teleconnection pattern with ansurface anomalous anticyclone (AAC) extending from the Philippine Sea to the northern Indian Ocean.
Two formation mechanisms have been proposed for these climate anomalies in post-El Niño-Southern
Oscillation (ENSO) summer. One hypothesis invokes the wind-evaporation-SST (WES) feedback in thetropical Northwestern Pacific, while the other points to inter-basin feedback between the Indian
Ocean and tropical Northwestern Pacific. Based on a coupled model experiment, we propose an
ocean-atmosphere coupled mode that synthesizes the two mechanisms. This Indo-western Pacific Ocean
capacitor (IPOC) mode evolves seasonally from spring to summer under seasonal migration of
background state. In spring, the WES feedback is operative in association with the tropicalNorthwestern Pacific cooling, while in summer the Indian Ocean warming and the inter-basininteraction maintains the AAC. While the IPOC mode is independent of ENSO in mechanism, ENSO candrive this mode in its decay phase. This excitation, however, has undergone substantial
interdecadal modulations, depending on ENSO amplitude and persistence of Indian Ocean warming. The
ENSO-IPOC correlation is high after the mid-1970s and at the beginning of the 20th century, but low
in between.
Keywords: Air-sea interaction, Teleconnection, East Asian summer monsoon©2016. Japan Geoscience Union. All Right Reserved.

- ACG06-06 -ACG06-07

Japan Geoscience Union Meeting 2016

A sea surface salinity dipole mode in the tropical Indian Ocean*Yan DU1, Yuhong Zhang1, Tangdong Qu
1.South China Sea Institute of Oceanology, Chinese Academy of Sciences
Ocean salinity is a natural freshwater tracer in the global hydrological cycle and its changes
represent large-scale ocean-atmosphere coupled climate signals such as the El Ni?o/Southern
Oscillation (ENSO). Studies of ocean salinity are much less than those of temperature sincesalinity observations are more sparse. Based on the sea surface salinity (SSS) data from Argo andreanalysis dataset, we identified a salinity dipole mode in the tropical Indian Ocean, termed
S-IOD: a pattern of interannual SSS variability with anomalously low-salinity in the central
equatorial and high-salinity in the southeastern tropical Indian Ocean (IO). The S-IOD matures in
November-December, lagging the Indian Ocean dipole (IOD) mode derived from sea surface temperature(SST) by two months. For the period of observations, the S-IOD persists longer than the IOD, until
the following September-October. Oscillations of the two S-IOD poles are governed by different
processes. Ocean advection associated with equatorial current variability dominates the SSS
anomalies of the northern pole, while surface freshwater flux variability plays a key role in the
SSS anomalies of the southern pole, where anomalous precipitation is sustained by preformed sea
surface temperature anomalies. The S-IOD concurs with the strong IOD, reflecting an
ocean-atmosphere coupling through the SST-precipitation-SSS feedback.
Keywords: Salinity, S-IOD, tropical Indian Ocean©2016. Japan Geoscience Union. All Right Reserved.

- ACG06-07 -ACG06-08

Japan Geoscience Union Meeting 2016Low salinity signal on the high salinity subsurface water during negative Indian Ocean
Dipole*Takanori Horii1, Wenju Cai2, Kanako Sato1, Kentaro Ando1
1.Research and Development Center for Global Change (RCGC), Japan Agency for Marine-Earth Scienceand Technology (JAMSTEC), 2.Commonwealth Scientific and Industrial Research Organisation (CSIRO)
The Indian Ocean Dipole (IOD) is a seasonal to interannual ocean-atmosphere phenomenon occurring in
the tropical Indian Ocean. During the negative phase of the IOD (nIOD), the eastern Indian Ocean ischaracterized by warmer-than-normal sea surface temperature (SST), enhanced atmospheric convection,and high-salinity anomalies advected from the west. In this study, we investigated ocean
temperature and salinity data in the south eastern Indian Ocean to understand a possible role ofthe salinity variation on the development of nIOD. We used ocean temperature and salinity data from
Argo floats and mooring buoy. We also used satellite SST and precipitation data from the TropicalRainfall Measuring Mission satellite. During the development phase of the 2010 nIOD
(July-August-September), eastward surface currents induced by westerly wind anomalies produced highsalinity anomalies in the central-eastern equatorial Indian Ocean. Observation data also showed
relatively low salinity signal around 0-10m depth together with relatively shallow mixed layer inthe south-eastern Indian Ocean. Our analysis indicated that the low salinity signal was associatedwith enhanced local precipitation that eventually formed vertical salinity gradient on the highsalinity anomalies. The upper-layer stratification due to the salinity variation could affectocean-atmosphere interaction during the nIOD by changing the mixed layer depth. A possible
contribution of the salinity variation to the mixed layer heat balance and hence an effect on SST
will be discussed.
Keywords: Indian Ocean Dipole, Precipitation, Barrier Layer©2016. Japan Geoscience Union. All Right Reserved.

- ACG06-08 -ACG06-09

Japan Geoscience Union Meeting 2016

Anomalous Walker Circulation Associated with Two Types of the Indian Ocean Dipole*Tomoki Tozuka1, Satoru Endo1
1.Department of Earth and Planetary Science, Graduate School of Science, The University of Tokyo
The Indian Ocean Dipole (IOD), an air-sea coupled phenomenon in the tropical Indian Ocean, can beclassified into two types based on sea surface temperature (SST) anomaly patterns. One type isreferred to as the canonical IOD; positive (negative) sea surface temperature (SST) anomalies cover
the eastern (central to western) tropical Indian Ocean. The other is named the IOD Modoki; it isassociated with positive (negative) SST anomalies cover the central (eastern and western) tropicalIndian Ocean. It is shown that the canonical IOD is associated with a single cell anomalous Walker
Circulation, while the IOD Modoki is accompanied by a double-cell anomalous Walker Circulation.
Implications of differences in the anomalous Walker Circulation cell will also be discussed.
Keywords: Indian Ocean Dipole, Walker Circulation, Tropical Indian Ocean©2016. Japan Geoscience Union. All Right Reserved.

- ACG06-09 -ACG06-10

Japan Geoscience Union Meeting 2016

On semiannual equatorial undercurrents in the eastern Indian Ocean*Motoki Nagura1, Michael J. McPhaden2
1.Japan Agency for Marine-Earth Science and Technology, 2.National Oceanic and Atmospheric
Administration
Unlike those in the Pacific and Atlantic, the equatorial undercurrents (EUCs) in the Indian Ocean
are transient and eastward only in early boreal spring and fall. Their dynamics is investigated inthis study using observations obtained from four acoustic Doppler current profilers (ADCPs)
deployed along the equator in the eastern Indian Ocean (78oE, 80.5oE, 83oE and 90oE). The harmonic
analysis is applied to observed zonal velocity and reveals that annual and semiannual variability
contributes to zonal velocity at the depth of the EUCs (about 100 m). Whereas the annual harmonic
does not show any consistent tendency of zonal phase propagation, the semiannual harmonic shows
eastward propagation at the depth of the EUCs. Owing to data gaps in ADCP records, the analysis isrepeatedly applied to several two-year segments, and the phase speed is estimated using results
obtained from various pairs of ADCPs. The results show that eastward phase propagation of thesemiannual harmonic is a statistically robust feature. For a further confirmation, zonal velocity
and its divergence are calculated using ADCP records. Zonal velocity leads zonal divergence, which
is another evidence for eastward phase propagation. These results suggest that the semiannualtransient EUCs in the Indian Ocean are Kelvin beams radiated from the surface to the east and to
the depth.
Keywords: Equatorial undercurrents, Indian Ocean, Kelvin beams©2016. Japan Geoscience Union. All Right Reserved.

- ACG06-10 -ACG06-11

Japan Geoscience Union Meeting 2016

Global non-hydrostatic simulation of the Pre-YMC field campaign in 2015
*Tomoe Nasuno1, Kunio Yoneyama1, Masaki Katsumata1, Shuichi Mori1, Junko Suzuki1
1.Japan Agency for Marine-Earth Science and Technology
In order to deepen our understanding of multi-scale multi-process interactions over the Maritime
Continent, a field campaign Pre-YMC was conducted during November-December 2015 in the southwest
Sumatra by JAMSTEC. Near real-time forecasts using a global non-hydrostatic model (Nonhydrostatic
Icosahedral Atmospheric Model, NICAM) had been operated throughout the observation period atcloud-system-resolving resolutions. This approach is useful for investigating relationship between
mesoscale convective systems and large-scale disturbances, such as the Madden-Julian Oscillation(MJO), equatorial waves, and monsoon activities. The forecasts were initialized at 0000 UTC each
day using NCEP Final analysis, and integrated for 7 (30) days using the 7 (14) km mesh sizes. Twomember ensemble was made by different setups of sea surface temperature. We will discuss the model
performance in simulating the observed large-scale variabilities and the processes in them.During
the first half of the observation period, lower tropospheric westerlies and convective center
persisted over the central Indian Ocean. Over the Maritime Continent, diurnal variation of
precipitation was pronounced, with passages of westward propagating synoptic-scale vorticity
disturbances at 4-5 periodicity. After 12 December 2015, the peak of equatorial westerlies rapidly
moved eastward, with southward shift of westerly axis over the broad warm pool domain. These
correspond to intensification of MJO amplitude and eastward propagation. The 30-day forecasts
generally capture these large scale variations at the lead time of approximately two weeks. The
abrupt change in the low-level winds were accompanied with marked variation in convective
organization. The 7-km mesh simulations show that the westerly intensification started on 10-11
December as a part of a vorticity disturbance around the northwest Sumatra, followed by further
acceleration within eastward propagating Kelvin-wave like convective disturbances. In the latter
phase, convective activity was significantly enhanced, which masked diurnal variation inprecipitation. These results suggest that mesoscale convective organization was not totally passive
to the dynamical modulations, but drove the large-scale change in some ways. Possible scenarios arebeing searched.
Keywords: Global high-resolution model, Maritime Continent, Madden-Julian Oscillation, equatorial
waves, diurnal cycle©2016. Japan Geoscience Union. All Right Reserved.

- ACG06-11 -ACG06-12

Japan Geoscience Union Meeting 2016

A Regional Climate Mode Discovered in the North Atlantic: Dakar Niño/Niña*Pascal Oettli1, Yushi Morioka1, Toshio Yamagata1
1.Japan Agency for Marine-Earth Science and Technology
The interrannual variability of coastal sea surface temperature (SST) anomalies confined off
Senegal is explored from a new viewpoint of the ocean-land-atmosphere interaction. The phenomenon
may be classified into "coastal Niño/Niña" in the North Atlantic as discussed recently in the
Northeastern Pacific and Southeastern Indian Oceans. The interannual variability of the regional
mixed-layer temperature anomaly that evolves in boreal late fall and peaks in spring is associatedwith the alongshore wind anomaly, mixed-layer depth anomaly and cross-shore atmospheric pressure
gradient anomaly, suggesting the existence of ocean-land-atmosphere coupled processes.
The coupled warm (cold) event is named Dakar Niño (Niña). The oceanic aspect of the Dakar Niño (Niña) may be basically explained by anomalous warming (cooling) of the anomalously thin (thick)mixed-layer, which absorbs shortwave surface heat flux. In the case of Dakar Niña, however,enhancement of the entrainment at the bottom of the mixed-layer is not negligible.
The atmospheric aspect is a warming (cooling) of the lower atmosphere, in response to the warming(cooling) of the upper ocean. Locally, this modifies the cross-shore pressure gradient and helps tomaintain weaker (stronger) than normal wind along the coast. This can be viewed as a "coastal
Bjerknes feedback".

©2016. Japan Geoscience Union. All Right Reserved.

- ACG06-12 -