The Boreal Summer Intra-Seasonal Oscillation (BSISO) is the propagation of long lasting cloudy condition from
equatorial region. The BSISO has been studied to understand the Asian summer monsoon.
Fig. 9 study region and domain for selecting
SST anomaly (black rectangle)
Effect of sea surface temperature on precipitation variability in East Asia induced by the Boreal Summer Intra-seasonal OscillationDeokyoung Han (Master course student)
Graduate School of Environmental Science, Hokkaido University
Result
BSISO index and its relationship with precipitation
Discussion
Purpose of the research
Conclusion
Hoyos and webster (2007)
Phase 1 Phase 2 Phase 3 Phase 4
Phase 5 Phase 6 Phase 7 Phase 8
Fig. 15 Difference between SST higher and SST lower composites about potential temperature anomaly (color, K) and moisture flux (line, 102 g kg-1 m s-1 )
of 25-35N latitude average associated with the BSISO phases. Solid (Dotted) lines indicate westerly (easterly) moisture flux
Phase 1 Phase 2 Phase 3 Phase 4
Phase 5 Phase 6 Phase 7 Phase 8
Fig. 16 Difference between SST higher and SST lower composites about moisture amount anomaly (color, hPa) and zonal wind anomaly
(line, m s-1 ) of 25-35N latitude average associated with the BSISO phases. Solid (Dotted) lines indicate westerly (easterly) wind anomaly
Weather
Intra-Seasonal
Oscillation
Seasonal
variation
daily scale
weekly scale
monthly scale i.e. summer
i.e. frequent
stationary front
i.e. heavy precipitation
in few days
Fig. 3 Time scale of atmospheric phenomena
Indian summer monsoon
East Asian summer monsoon
m / s
mm / day
Fig. 4 Climatology of precipitation and surface wind for MJJA during 2000-2019
Lee et al. (2017)
EEOF analysis of filtered outgoing longwave radiation can extract the signal of the BSISO.
Phase diagram of the BSISO PCs indicates the approximate location of active convection (Fig. 5, 6).
Kikuchi. (2012)
Fig. 5 Spatial and temporal pattern of 25-90-day filtered OLR for JJA.
0 day indicates the appearance of convection in left-bottom panel
The northward propagation of BSISO in Indian Ocean influenced precipitation in Indian
peninsula (Fig. 7). When the phase of 30-60-day BSISO was 3 to 5, there was positive
precipitation anomaly in East Asia with respect to non-BSISO cases (Fig. 8).
Fig. 7 Schematic diagram of the development of precipitation over the BoB associated
with the ISO; (a) genesis of the ISO and (b) northward propagation of the ISO. The
darker (clearer) shading indicates warmer (colder) SST than usual
Fig. 8 Composites of precipitation anomaly in the phase 4
of 30-60-day BSISO respect to the non-BSISO cases
Data & Method
There are few researches that SST has an effect on the precipitation variability
associated with the BSISO in East Asia.
This research is to explain the precipitation variability associated with the
BSISO would be different upon SST over East Asia.
Table. 1 Dataset of the research
Open source of PCs of BSISO (Kikuchi et al. 2012) was used to define time series and each
phase of the BSISO. Daily area-averaged SST anomalies above (under) +0.5 (-0.5) were
collected to describe the effect of SST enough.
Fig. 11 Snap shots of BSISO events
Time progresses from top to bottom
Phase 4
Phase 6
Phase 8
Phase 4
Phase 2
Fig. 12 OLR anomalies (color, unit W /m2) & UV850 anomalies (vector, m /s) for JJASO during 2000-2019 associated with each BSISO phase. Dots and vectors indicate over 95% confidence level. The number of BSISO events is given in each phase
Fig. 13 Composite of 25-90-day filtered precipitation anomalies (color, mm/day) for JJASO during 2000-2019 associated
with the BSISO phases. Dots indicate over 95% confidence level. The number of BSISO events is given in each phase
Phase 1 (246) Phase 2 (225) Phase 3 (251) Phase 4 (239)
Phase 5 (240) Phase 6 (228) Phase 7 (245) Phase 8 (216)
Phase 2 (32) Phase 4 (69) Phase 6 (65) Phase 8 (25)
Phase 2 (55) Phase 4 (40) Phase 6 (23) Phase 8 (33)
Area-averaged SST anomaly > + 0.5: enough to describe the effect of positive SST anomaly
Area-averaged SST anomaly < - 0.5: enough to describe the effect of negative SST anomaly
Fig. 14 Composite of 25-90-day filtered precipitation anomalies (mm/day) for each phase associated with SST anomalies in East China
Sea and western North Pacific. Color indicates over 95% confidence level. The number of BSISO events is given in each phase
Positive SST anomaly might induce zonally elongated positive precipitation anomaly during BSISO phase 4, 6 in East Asia (Fig. 14). BSISO phase 4, 6 are corresponding to the anti-cyclonic circulation in western North Pacific (Fig. 12)
When the anti-cyclonic circulation was located in western North-Pacific like BSISO phase 3, 4, 6 (Fig. 12), warm and humid westerly wind might trigger zonally elongated precipitation if SST is higher near East Asia (Fig. 15, 16).
SST anomaly
> + 0.5SST anomaly
< - 0.5
BSISO Phase 3-6: warmer potential temperature and strong westerly moisture flux near 120-130E
BSISO Phase 3, 4, 6: larger moisture amount and corresponding westerly wind anomaly near 120-130E
▪ Zonally elongated precipitation appeared when the anti-cyclonic circulation in western North Pacific was located and SST was higher near East Asia.
▪ Westerly wind corresponding to synoptic circulation and humid air from higher SST might contribute zonally elongated precipitation.
Fig. 1 Weather chart at 9 a.m. on June 30th, 2018
JMA
Fig. 6 Phase diagram of BSISO mode
Kikuchi. (2012)
PC1
PC2
Phase 7
Phase 8
Phase 6
Phase 4
Phase 3
Phase 1
Phase 5
Phase 2
Hypothesis
Precipitation variability associated with the BSISO would be different upon
SST condition near East Asia
Fig. 10 The phase diagram of BSISO PCs (blue) time series for JJASO
during 2000-2019. The life cycle of BSISO events (black) from June
1st to July 23th in 2004. The number of BSISO events is given
PC2
Phase 7
(245)
Phase 8
(216)
Phase 6
(228)
Phase 4
(239)
Phase 3
(251)
Phase 1
(246)
Bay of Bengal
Ind
ia
& M
ari
tim
e C
on
tin
en
t
Western North Pacific
Ea
ste
rn N
ort
h P
ac
ific
&
eq
uato
ria
l In
dia
n O
ce
an
Phase 5
(240)
Phase 2
(225)
m /s
Snap shots (Fig. 11) indicate life cycle of the BSISO, which means propagation of the
active convection (blue Color) in Asian region.
Two main characteristics of precipitation among BSISO phases (Fig. 13)▪ Significant positive precipitation anomaly in eastern China during BSISO phase 3, 5▪ Significant positive precipitation anomaly over western North Pacific in BSISO phase 2, 7, 8
Fig. 2 25-90-day filtered OLR (Color) & UV850 (vector) anomalies averaged from June 28th to July 8th in 2018. Blue Color indicates convection
m /s
W /m2
Heavy precipitation over western Japan from June 28th to July 8th in 2018 cause severe damage to human society (Fig. 1).
25-90-day active convection and southerly wind anomaly were noticeable in the period of heavy precipitation (Fig. 2).
Phase 5 (240) Phase 6 (228) Phase 7 (245) Phase 8 (216)
Phase 1 (246) Phase 2 (225) Phase 3 (251) Phase 4 (239)
Two main characteristics of synoptic circulation among BSISO phases (Fig. 12)▪ Anti-cyclonic circulation in western North Pacific in BSISO phase 3, 4, 6▪ Cyclonic circulation in western North Pacific during BSISO phase 2, 7, 8
Cyclonic circulation
in WNP Anti-cyclonic circulation
in WNP
Anti-cyclonic circulation
in WNP
Anti-cyclonic circulation
in WNP Cyclonic circulation in WNP Cyclonic circulation in WNP
Positive rainfall
anomaly in China
Positive rainfall
anomaly in WNP
Positive rainfall
anomaly in WNP
Positive rainfall
anomaly in WNP
Positive rainfall
anomaly in China
Zonally elongated precipitation
25-90-day bandpass filter on
OLR, UV850, precipitation
to examine the effect of BSISO
minus
SST anomaly
> + 0.5SST anomaly
< - 0.5minus
Background
W /m2
m /s
m /s
m /s
m /s
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