Summer Intraseasonal Oscillation

1
Introduction
Summer Intraseasonal Oscillation ENSO (Teng
Wang 2003)
Da-hee Choi, Daehyun Kim, Taewon Park
2
Introduction

• ISO (IntraSeasonal Oscillation)
• Winter ISO eastward propagation (MJO)
• Summer ISO Indian monsoon ? northward
  propagation
• WNP ? northwestward
  propagation
• The amplitude of ISO is stronger when local SST
  is warm. However, unable to detect influence
  from ENSO.
• The overall MJO activity is uncorrelated with
  ENSO. All these studies focused on the boreal
  winter MJO.

Active Break of monsoon rainfall
3
Interannual Variations of the Boreal Summer ISO-
Overall intensity

• Our study focuses on the boreal summer season
  from May to October.
• Filter 10 90 day
• Standard deviations are used to represent the
  amplitude of ISO.

4
Interannual Variations of the Boreal Summer ISO-
Overall intensity
Interannual variation
Correlation between ISO Intensity ENSO

• Although climatologically the Bay of Bengal is
  the most active center for the summer ISO, its
  interannual variation is weaker than in the WNP.
• The ISO intensity shows the most significant
  correlation with Nino3.4 SST in the western and
  central Pacific.
• Both convection and lower-troposphere circulation
  show pronounced interannul variation in the ISO
  intensity in the WNP, which is closely linked
  with ENSO.

5
Interannual Variations of the Boreal Summer ISO
MAY-JUL
AUG-OCT

• Period for zonally propagating wavenumber-1 OLR
  anomalies at 40E-180
• Kemball-cook Wang (2001) ISO behavior change
  within the summer
• MJJ ? equator eastward (40E-180)
• ASO ? 10-20N westward strengthen (100E-180)
• Switch of the dominance from MJO in MJJ to the
  off-equatorial westward-propagating ISO in ASO is
  associated with the monsoon annual cycle.

6
Interannual Variations of the Boreal Summer ISO-
The MJO (eastward propagation at 40E-180)

• Analysis period May-July
• MJO index is defined as the mean spectrum
  density for 20-50 day eastward propagating
  wavenumber-1(at 40E-180)
• OLR MJO index anomalies averaged at 2.5S-5N
  during 1979-2001
• ?500 MJO index at 5S-10N during 1958-2001
• The tight MJO-ENSO correlation in May-July is in
  a sharp contrast with the insignificant
  correlation previously found between the boreal
  winter MJO and ENSO.

7
Interannual Variations of the Boreal Summer ISO-
Westward-propagating ISO in the WNP

• The WNP ISO has a predominant northwestward
  propagation
• ( Lau Chan 1986)

8
Interannual Variations of the Boreal Summer ISO-
Westward-propagating ISO in the WNP (at 100E-180)

• Analysis period July - October
• ISO index is defined as the mean spectrum
  density for 10-50 day westward propagating
  wavenumber-13(at 100E-180)
• V850 ISO index anomalies averaged at 5-20N
  during 1958-2001
• Correlation between V850 ISO index Nino 3.4
  SSTA 0.62

9
Interannual Variations of the Boreal Summer ISO-
Northward-propagating ISO (at 5S-25N)
mean
Correlation between wavenumber-1 northward
propagating U850 anomaly Nino 3.4 SSTA
(19582001)
160E
Std dev

• Analysis period May October
• period for wavenumber-1 northward propagating
  U850 anomalies at 5S-25N in May-Oct
• over WNP northward propagating ISO variation is
  correlated with Nino 3.4 SSTA.

160E
10
Why is the WNP ISO intimately linked to ENSO?
11
Relationship to Mean state ISO intensity

• Compared with the ISO in the Indian monsoon
  region, the ISO in the WNP is found more
  sensitive to change in summers mean state.
• WNP ISO intensity can be explained by summer
  mean state.

Correlation between boreal summer ISO intensity
and seasonal mean state using U850 in May-Oct
12
Vertical shear ENSO
May-Oct averaged U850-U200 shear

• WP transition area between easterly and
  westerly shear
• However, El Nino
• WP easterly vertical shear strengthen
• lower troposphere westerly anomalies
• Upper troposphere easterly anomalies

May-Oct averaged U850-U200 shear difference El
Nino La Nina
13
Vertical wind shear mechanism

• The vertical shear of zonal flow has a
  remarkable impact on Rossby wave. (Wang Xie
  1996)
• During El Nino, the enhanced mean easterly
  vertical shear over WNP is responsible for the
  enhanced emanation of the moist equatorial Rossby
  waves. (Kemball-cook Wang 2001)

WNP
eq
CP
14
Relationship to Vertical shear ISO intensity
1st SVD mode between vertical wind shear V850
ISO intensity

• The dominant interannual modes of the summer ISO
  intensity in V850 (Jul-Oct) are related to summer
  mean vertical shear (May-Oct).

15
Conclusion

• May-July MJO
• July-October northwestward propagation in the
  WNP
• May-July MJO July-October northwestward
  propagating ISO mode in the WNP are sensitive to
  ENSO.
• The linkage between ENSO northwestward
  propagating ISO in the WNP can be explained by
  the vertical wind shear mechanism.