Soon-Il An1 and Jung Choi2

1Yonsei University, 2Seoul National University (sian@yonsei.ac.kr)

Pacific Zonal Mode as a leading tropical Pacific decadal mode

and it implication for the future ENSO change

39th Climate Diagnostics Workshop

Leading mode of Pacific SST: Pacific Decadal Oscillation (PDO)

www.yonsei.ac.kr From IPCC AR4

Tropical Pacific Decadal Variability

www.yonsei.ac.kr

EOF modes of 7-yr running SSTA for NDJ

ENSO-like

Pacific Zonal Mode

EOFs of SSTA and 10-yr running SSTA

www.yonsei.ac.kr Choi, An et al. (2013)

ENSO Pacific Zonal Mode ENSO-like Mode

What causes Pacific Zonal Mode ?

www.yonsei.ac.kr

Time series of winter (November-January) NINO3m (gray-solid line), NINO4m (black-dashed line) SST anomaly indices, N34mVar index (decadal amplitude, thick solid line), and its mirror (thick dashed line).

Amplitude Modulation of ENSO from GFDL CM2.1 pre-industrial run

High Amplitude Regime

Low Amplitude Regime

Choi, An, Yeh (2012)

Composite SST anomalies (ND(0)J(1)) for (a) El Nino and (b) La Nina events during the Strong-ENSO period. (c), (d) Same as (a) and (b) but for the Weak-ENSO period. Shading indicates 95% significance level by a student t-test.

(37 events)

(45 events)

(28 events)

(24 events)

Composite for strong and weak ENSOs

EP El Nino CP El Nino

CP La Nina EP La Nina

High Amplitude Regime Low Amplitude Regime

Choi, An, Yeh (2012)

(a) The residual (El Nino+La Nina) of equatorially-averaged (5°S-5°N) temperature anomalies during the winter (ND(0)J(1)) for the strong-ENSO period. (b) The first EOF mode of 20-yr moving averaged equatorial subsurface temperature. Upper (lower) thick solid line represents the climatology of mixed layer depth (17°C isotherm depth).

Pattern correlation :(a) & (b) = 0.904

Tropical Pacific Decadal Mode

Residual due to El Nino-La Nina Asymmetry

High Amplitude Regime

20-yr running averaged temperature

Choi, An, Yeh (2012)

(a) Map of linear regression coefficient for SST with respect to the first EOF PC time series [K]. (b), (c) Same as (a) but for the 17°C isotherm depth [m] and equatorially-averaged (5°S-5°N) zonal wind stress [Pax104], respectively.

Pattern correlation (REG, residual Strong period) : SST(8a,6a) = 0.849 : TH(8b,6b) = 0.923

(El Nino + La Nina) for High Amplitude Regime period

Choi, An, Yeh (2012)

Black dashed line = The first EOF PC of 20yr SubT Red solid line = 20-yr sliding STD of SSTA<East> (ENSO amplitude)

Stability

Residual

Choi, An, Yeh (2012)

Interaction between ENSO and Mean state

Warm Eastern Pacific/Cold

Western Pacific

Strong ENSO EP El Nino/CP La

Nina

Residual accumulation

ENSO Stability

Cold Eastern Pacific/Warm

Western Pacific

Weak ENSO CP El Nino/EP La

Nina

ENSO stability

Residual accumulation

Slow transition due to slow ocean adjustment?

www.yonsei.ac.kr

Implication of PZM for the future ENSO change

(Why SST trends are similar but changes in

ENSO are different?)

Why the Twenty-first Century Tropical Pacific Trend Pattern cannot significantly

influence ENSO Amplitude?

www.yonsei.ac.kr

An and Choi (2014)

Change in ENSO SST variance in a warmer climate condition is uncertain (IPCC AR5)

CMIP5

SSTA against ENSO-Amplitude Modulation index

Decadal-to-ID ENSO amplitude modulation index

PZM mode is correlated to ENSO Amplitude

Pacific Zonal Mode

Patterns related to ENSO amplitude (PI)

Regression pattern of SSTA against the PC of the first EOF of the tropical Pacific SST anomaly and N3Var index

Control simulation (PI) from CMIP5

Air-sea coupling enhanced

PZM in RCP4.5 and 8.5 runs

SST Trend in RCP runs

Cane-Zebiak type model experiments

Modified mean SST in atmosphere heating

PZM or Trend pattern

Feedback process

PZM (E-warm, W-

cool)

ENSO

Strengthening of A-O coupling

Eastward shift of ENSO action center

Residual effect due to El Nino-La Nina asymmetry

Enhance PZM mode (Choi et al. 2009)

External factors?

Trend (basin-wide

warm) ?

Different

Conclusions

Two dominant modes of TPDV exist: ENSO-like mode and Pacific zonal mode.

Decadal modulation of ENSO is highly correlated to PZM but not to ENSO-like mode.

Asymmetry of ENSO (EP .vs. CP) induces PZM.

SST trend pattern more resembles ENSO-like mode.

www.yonsei.ac.kr

Conclusions

In CMIP5 (both PI and RCP runs) and observation, the zonal dipole pattern of SST (PZM) is closely related to change in ENSO amplitude.

PZM provides favorable conditions for the intensification of ENSO by strengthening air-sea coupling and modifying ENSO pattern.

On the other hand, the 21st century SST trend pattern is not favorable towards changing ENSO amplitude.

This talk covers the following papers

S.-I. An and J. Choi, 2014 : Why the Twenty-first Century Tropical Pacific Trend Pattern cannot significantly influence ENSO Amplitude. Climate Dynamics, accepted

J. Choi, S.-I. An, S.-W. Yeh, and J.-Y. Yu, 2013: ENSO-like and ENSO-induced Tropical Pacific Decadal Variability in CGCMs, J. Climate, 26,1485-1501.

J. Choi, S.-I. An, and S.-W. Yeh, 2012 : Decadal amplitude modulation of two types of ENSO and its relationship with the mean state, Climate Dynamics, 38, 2631-2644.

www.yonsei.ac.kr

REFs http://web.yonsei.ac.kr/climate/index.htm Any questions sian@yonsei.ac.kr

Further remark; Non-monotonic response of ENSO to

global warming global warming

www.yonsei.ac.kr

T2

T1

time

thermodynamical process

Dynamical process