Variations in El Nino Activity

1

• Variations in El Nino Activity
• F-F Jin
• Workshop on Western Tropical Pacific
• 11/26-28, 2007
• Collaborators
• J.-S. Kug, A. Timmermann,
• L. Bejarano, S.-T. Kim
• Florida State University

2
Two types of El Niño events Warm-pool El
Niño Cold-tongue El Niño
3
(No Transcript)
4
(No Transcript)
5
Zonal Mean Sea Level (140-270E)
Warm Pool El Nino
Cold Tongue El Nino
El Nino Decaying
Mature Phase
time
El Nino Developing
6
(No Transcript)
7
Some possibilities
(1) Internal dynamics different ENSO modes
coexist and interact ---gt intermittency? (2)
Climate state changes e.g. trade winds in
weakening due to global warming, climate regime
shift etc? (3) Enhanced Noise activity?
8
Persistent warming in NINO4 Region
Time Series of NINO4 SST (1-yr moving )
9
Bursting behavior of ENSO in QQ-QB regime
10
According to AR4 simulations ,Mean-trade wind
stress reduce about 10 for every degree warming
in the tropics. Thus for CO2 doubling with 3
degree warming, the trade can reduce about 30
Held et al 2006, Vechi et al 2006, 2007
11
Changes in instability due to mean wind weakening
BJ instability index for ENSO (Jin et al 2006,7)
A B dynamic and thermodynamic damping C, D, E
three positive feedbacks, each depending on three
factors (i) The natures of the basic state,
(ii) The sensitivity of the atmospheric surface
wind responses to SSTA in the ENSO
region. (iii) The sensitivity of the oceanic
responses to surface winds.
12

• Growth rate (left) and period (right) computed
  from SSTA time series (red dashed line) and from
  BJ index (black line) as a function of coupling
  coefficient for reference simulation

13
The correlation between NINO3 and anomalous
running variance of the equatorial surface
intraseasonal wind (15-90 days)anomalies
14
(3) ENSO-MJO interaction as modeled by
multiplicative noises
ENSO-MJO/WWB interactions play an important
role G1BT

Jin, Lin, Timmermann, Zhao, 2007, GRL
Noise-induced destabilization effect for
ENSO
15
B factor doubled !
16
Summary

• There are two-types of El Niño events, likely
  related to two different coupled modes, one is
  recharge oscillator mode, the other is mode
  mainly owing to the advective feedback.
• BJ index formula provides an useful estimation of
  the growth rate for leading ENSO modes.
• There is an strong interaction between ENSO and
  the intraseasonal surface zonal wind variability
  and it destabilizes both warm-pool and cold
  tongue El Niño through the noise induced
  instability.
• Future ENSO activity may depend on changes both
  only in the climate state and the intensity of
  the ENSO-MJO/WWB interaction

17
(No Transcript)
18
Changes in ocean basic state due to mean wind

• (a) Mean SST, (b) zonal currents, (c) upwelling,
  and (d) sea level height along the equator
  obtained from forced runs with mean winds of
  different intensity. Intensity of mean wind is
  reduced or increased by 20 (aW0.8, or aW1.2),
  and 10 (aW0.9, or aW1.1) from the reference
  wind (aW1.0).

19

• Dependence of El Niño amplitude on growth rate

• Growth rate (left) and period (right) computed
  from SSTA time series (red dashed line) and from
  BJ index (black line) as a function of coupling
  coefficient for reference simulation