Inter-decadal change in El Niño-Southern Oscillation examined with Bjerknes stability index analysis

Soon Il An, Hayoung Bong

Research output: Contribution to journalArticle

13 Citations (Scopus)

Abstract

Characteristics of El Niño-Southern Oscillation (ENSO) have changed since the late 1970s as it synchronized with the Pacific Decadal Oscillation (PDO). In order to investigate the primary feedback process responsible for the interdecadal change in ENSO characteristics according to the PDO, using the ocean assimilation data (SODA) and the reanalysis data (NCEP/NCAR), we performed Bjerknes linear stability index (BJ index) analysis of two decadal periods: one before the late 1970s (the nPDO period) and the other after the late 1970s (the pPDO period). The BJ index for the pPDO period (−0.07 year−1 for the growth rate of the eastern Pacific SST anomaly) is significantly larger than that for the nPDO period (−0.25 year−1). The larger BJ index value is primarily due to the enhanced zonal advection feedback (ZA; +0.44 year−1), thermocline feedback (TH; +0.33 year−1), and the reduced damping by the mean meridional current (MD; +0.16 year−1). The increases in ZA and TH are mainly attributed to the shoaling of the mean thermocline depth, which increased the sensitivity of the ocean dynamic fields to the wind forcing; and the reduced MD is related to the reduced mean meridional current associated with the weakened trade wind. The enhanced positive feedback is partly compensated by the enhanced thermodynamic damping including the shortwave, sensible heat flux and latent heat flux (collectively, −0.88 year−1). Interestingly, the change in air–sea coupling strength from the nPDO to the pPDO period was small. Without the two extreme El Niño events (1982–1983 and 1997–1998) in the pPDO period (pPDO_noBIG), the difference in BJ index between nPDO and pPDO_noBIG periods became smaller (~0.07 year−1), indicating that the two extreme El Niño events largely contribute to the larger ENSO variability of the pPDO period, possibly due to nonlinear feedback processes. Nevertheless, qualitative similarity in each of the feedback and damping components of BJ index exists between the pPDO and pPDO_noBIG periods, which suggests that the tropical climate states of the pPDO period provided more favorable conditions for the emergence of extreme El Niño events by intensifying the linear feedback processes.

Original languageEnglish
Pages (from-to)967-979
Number of pages13
JournalClimate Dynamics
Volume47
Issue number3-4
DOIs
Publication statusPublished - 2016 Aug 1

Fingerprint

Southern Oscillation
damping
Pacific Decadal Oscillation
thermocline
trade wind
wind forcing
ocean
latent heat flux
sensible heat flux
data assimilation
advection
sea surface temperature
thermodynamics
index
analysis
anomaly

All Science Journal Classification (ASJC) codes

  • Atmospheric Science

Cite this

@article{3362928c3f2e458b998eea78b881ddbe,
title = "Inter-decadal change in El Ni{\~n}o-Southern Oscillation examined with Bjerknes stability index analysis",
abstract = "Characteristics of El Ni{\~n}o-Southern Oscillation (ENSO) have changed since the late 1970s as it synchronized with the Pacific Decadal Oscillation (PDO). In order to investigate the primary feedback process responsible for the interdecadal change in ENSO characteristics according to the PDO, using the ocean assimilation data (SODA) and the reanalysis data (NCEP/NCAR), we performed Bjerknes linear stability index (BJ index) analysis of two decadal periods: one before the late 1970s (the nPDO period) and the other after the late 1970s (the pPDO period). The BJ index for the pPDO period (−0.07 year−1 for the growth rate of the eastern Pacific SST anomaly) is significantly larger than that for the nPDO period (−0.25 year−1). The larger BJ index value is primarily due to the enhanced zonal advection feedback (ZA; +0.44 year−1), thermocline feedback (TH; +0.33 year−1), and the reduced damping by the mean meridional current (MD; +0.16 year−1). The increases in ZA and TH are mainly attributed to the shoaling of the mean thermocline depth, which increased the sensitivity of the ocean dynamic fields to the wind forcing; and the reduced MD is related to the reduced mean meridional current associated with the weakened trade wind. The enhanced positive feedback is partly compensated by the enhanced thermodynamic damping including the shortwave, sensible heat flux and latent heat flux (collectively, −0.88 year−1). Interestingly, the change in air–sea coupling strength from the nPDO to the pPDO period was small. Without the two extreme El Ni{\~n}o events (1982–1983 and 1997–1998) in the pPDO period (pPDO_noBIG), the difference in BJ index between nPDO and pPDO_noBIG periods became smaller (~0.07 year−1), indicating that the two extreme El Ni{\~n}o events largely contribute to the larger ENSO variability of the pPDO period, possibly due to nonlinear feedback processes. Nevertheless, qualitative similarity in each of the feedback and damping components of BJ index exists between the pPDO and pPDO_noBIG periods, which suggests that the tropical climate states of the pPDO period provided more favorable conditions for the emergence of extreme El Ni{\~n}o events by intensifying the linear feedback processes.",
author = "An, {Soon Il} and Hayoung Bong",
year = "2016",
month = "8",
day = "1",
doi = "10.1007/s00382-015-2883-8",
language = "English",
volume = "47",
pages = "967--979",
journal = "Climate Dynamics",
issn = "0930-7575",
publisher = "Springer Verlag",
number = "3-4",

}

Inter-decadal change in El Niño-Southern Oscillation examined with Bjerknes stability index analysis. / An, Soon Il; Bong, Hayoung.

In: Climate Dynamics, Vol. 47, No. 3-4, 01.08.2016, p. 967-979.

Research output: Contribution to journalArticle

TY - JOUR

T1 - Inter-decadal change in El Niño-Southern Oscillation examined with Bjerknes stability index analysis

AU - An, Soon Il

AU - Bong, Hayoung

PY - 2016/8/1

Y1 - 2016/8/1

N2 - Characteristics of El Niño-Southern Oscillation (ENSO) have changed since the late 1970s as it synchronized with the Pacific Decadal Oscillation (PDO). In order to investigate the primary feedback process responsible for the interdecadal change in ENSO characteristics according to the PDO, using the ocean assimilation data (SODA) and the reanalysis data (NCEP/NCAR), we performed Bjerknes linear stability index (BJ index) analysis of two decadal periods: one before the late 1970s (the nPDO period) and the other after the late 1970s (the pPDO period). The BJ index for the pPDO period (−0.07 year−1 for the growth rate of the eastern Pacific SST anomaly) is significantly larger than that for the nPDO period (−0.25 year−1). The larger BJ index value is primarily due to the enhanced zonal advection feedback (ZA; +0.44 year−1), thermocline feedback (TH; +0.33 year−1), and the reduced damping by the mean meridional current (MD; +0.16 year−1). The increases in ZA and TH are mainly attributed to the shoaling of the mean thermocline depth, which increased the sensitivity of the ocean dynamic fields to the wind forcing; and the reduced MD is related to the reduced mean meridional current associated with the weakened trade wind. The enhanced positive feedback is partly compensated by the enhanced thermodynamic damping including the shortwave, sensible heat flux and latent heat flux (collectively, −0.88 year−1). Interestingly, the change in air–sea coupling strength from the nPDO to the pPDO period was small. Without the two extreme El Niño events (1982–1983 and 1997–1998) in the pPDO period (pPDO_noBIG), the difference in BJ index between nPDO and pPDO_noBIG periods became smaller (~0.07 year−1), indicating that the two extreme El Niño events largely contribute to the larger ENSO variability of the pPDO period, possibly due to nonlinear feedback processes. Nevertheless, qualitative similarity in each of the feedback and damping components of BJ index exists between the pPDO and pPDO_noBIG periods, which suggests that the tropical climate states of the pPDO period provided more favorable conditions for the emergence of extreme El Niño events by intensifying the linear feedback processes.

AB - Characteristics of El Niño-Southern Oscillation (ENSO) have changed since the late 1970s as it synchronized with the Pacific Decadal Oscillation (PDO). In order to investigate the primary feedback process responsible for the interdecadal change in ENSO characteristics according to the PDO, using the ocean assimilation data (SODA) and the reanalysis data (NCEP/NCAR), we performed Bjerknes linear stability index (BJ index) analysis of two decadal periods: one before the late 1970s (the nPDO period) and the other after the late 1970s (the pPDO period). The BJ index for the pPDO period (−0.07 year−1 for the growth rate of the eastern Pacific SST anomaly) is significantly larger than that for the nPDO period (−0.25 year−1). The larger BJ index value is primarily due to the enhanced zonal advection feedback (ZA; +0.44 year−1), thermocline feedback (TH; +0.33 year−1), and the reduced damping by the mean meridional current (MD; +0.16 year−1). The increases in ZA and TH are mainly attributed to the shoaling of the mean thermocline depth, which increased the sensitivity of the ocean dynamic fields to the wind forcing; and the reduced MD is related to the reduced mean meridional current associated with the weakened trade wind. The enhanced positive feedback is partly compensated by the enhanced thermodynamic damping including the shortwave, sensible heat flux and latent heat flux (collectively, −0.88 year−1). Interestingly, the change in air–sea coupling strength from the nPDO to the pPDO period was small. Without the two extreme El Niño events (1982–1983 and 1997–1998) in the pPDO period (pPDO_noBIG), the difference in BJ index between nPDO and pPDO_noBIG periods became smaller (~0.07 year−1), indicating that the two extreme El Niño events largely contribute to the larger ENSO variability of the pPDO period, possibly due to nonlinear feedback processes. Nevertheless, qualitative similarity in each of the feedback and damping components of BJ index exists between the pPDO and pPDO_noBIG periods, which suggests that the tropical climate states of the pPDO period provided more favorable conditions for the emergence of extreme El Niño events by intensifying the linear feedback processes.

UR - http://www.scopus.com/inward/record.url?scp=84946114019&partnerID=8YFLogxK

UR - http://www.scopus.com/inward/citedby.url?scp=84946114019&partnerID=8YFLogxK

U2 - 10.1007/s00382-015-2883-8

DO - 10.1007/s00382-015-2883-8

M3 - Article

AN - SCOPUS:84946114019

VL - 47

SP - 967

EP - 979

JO - Climate Dynamics

JF - Climate Dynamics

SN - 0930-7575

IS - 3-4

ER -