Mariano S. Alvarez1, C. S. Vera1, G. Kiladis2 and B.
Liebmann2
1
Centro de Investigaciones del Mar y la Atmósfera, CONICET-UBA, DCAO/FCEN, UMI
IFAECI/CNRS
2 Earth System Research Laboratory/NOAA, CIRES/University of Colorado
[email protected]
OBJECTIVES
• Document the principal characteristics that IS variability (as described by
filtered OLR anomalies) exhibits during winter in South America.
• Analyze its relation to the variability observed in the circulation in the
Southern Hemisphere.
• Explore the modulation that the IS variability produces in the frequency of
wet spells.
DATA
• 28 austral cold seasons (151 days centered in JJA) from 1979 to 2006
were used.
• NCEP/NCAR Reanalysis I of daily mean geopotential heights and winds
(Kalnay et al., 1996).
• Daily precipitation station data from CLARIS-LPB database (available
online).
• Daily gridded precipitation data in South America (Liebmann and Allured,
2005).
METHODOLOGY
• All variable anomalies were calculated with respect to the seasonal cycle.
• OLR anomalies were filtered (FOLR) using a Lanczos 10-90 days
band-pass filter.
Standard deviation of 1010-90 day bandpass filtered OLR
anomalies (FOLR)
Cold Season
Warm season
5N
5N
15 S
15 S
35 S
35 S
65W
45W
5
8
11
65W
14
17
20
23
45W
26
29
32
Standard deviation of 1010-90 day bandpass filtered
anomalies
250 hPa geopotential heights
90W
250 hPa meridional winds
90W
0
180
90E
90E
EOF1 of FOLR: Cold Season IntraS
ntraSeasonal Pattern
5N
CSIS index: standardized Principal
component of EOF1
Positive values of CSIS index are
associated to negative FOLR in
central SA.
15 S
Positive (negative) CSIS events :
periods of at least 5 consecutive days
associated to CSIS index > (<) 1 (-1)
35 S
65W
45W
Regression maps
against CSIS
index of FOLR
(contours) and
precipitation
(shaded)
Negative (positive)
FOLR regressed
anomalies are
associated to positive
(negative)
precipitation
regressed anomalies
FOLR anomalies
are able to describe
the main features of
precipitation IS
variability
Day -8
Day -6
Day -4
Day -2
Day 0
Day +2
Day +4
Day +6
Day +8
5N
15 S
35 S
5N
15 S
35 S
5N
15 S
35 S
4
80
3
60
2
40
1
20
0
0
-1
-20
-2
-40
Evolution of daily precipitation amounts
(blue bars) in southeastern South America
and CSIS index (solid line) during cold
season of 1986
mm day-1
Normalized units
Modulation of daily precipitation by CSIS pattern
activity
CSIS Pattern
Wet spells in southeastern South America
Wet spell
(at least 2 consecutive days with
pp>1mm)
P75 wet spell
(at least 2 consecutive days with
pp>18.5mm (75th percentile))
Magenta: Climatological distributions of wet spell frequency in southeastern South
America
Blue: Percentage of rainy days explained by the wet spells
Modulation of wet spells in southeastern South
America by CSIS Pattern activity
As related to the sign of CSIS index
Larger wet spell frequency
associated with positive CSIS index
values
As related to the evolution stage of
the positive CSIS index
Growing stage of positive CSIS
phases is the most likely period for
wet spell occurrence
Modulation of wet spells P75 in southeastern South
America by CSIS Pattern activity
As related to the sign of CSIS index
Larger wet spell frequency associated
with positive CSIS index values
As related to the evolution stage of
the positive CSIS index
Growing stage of CSIS index and positive
events is the most likely period for wet spell
P75 occurrence
Regression
maps against
CSIS index of
FOLR
Day -14
X Center that will
maximize at day 0
over southeastern
South America
Day -12
Day -10
positive values over
southeastern SA
MJO-like evolution
Day -8
alternate centers develop
and slowly propagate
northeastwards
Day -6
Day -4
intensification and stall of X
center
Day -2
weakened anomalies over
Indian Ocean
Day 0
maximum magnitude over
SA
Day +2
Day +4
60E
180
60W
Regression maps
against CSIS
index of 250 hPa
geopotential
heights anomalies
-20
-18
-10
-8
0
+2
Negative SAM
phase develops
Positive SAM
phase
-16
-6
+4
SAM weakens
A wavetrain develops
Y Center that will
maximize at day 0
over southeastern
South America
Maximization of
the Y center
magnitude
Y Center
weakens and
moves eastwards
-14
-4
+6
-12
-2
+8
Maximization of
the Y centerupstream
antyciclonic
anomaly
CONCLUSIONS
• IS variability as described by 10-90 day bandpass filtered OLR anomalies
(FOLR) explains a large percentage of variance in South America during the
cold season.
•The leading pattern of FOLR or Cold Season IntraS
Seasonal pattern is
characterized by a monopole extended with a NW-SE orientation over
southeastern South America. CSIS positive phase is linked to positive
precipitation anomalies in the region.
• CSIS activity induces a large modulation of daily precipitation anomalies and
specially of wet spells and P75 wet spells. Both growing and mature stages of
CSIS positive phase are the most likely period for wet spell occurrence.
•The analysis of the large-scale conditions associated to CSIS activity shows
links to MJO-like activity over the tropical Indian and west Pacific Oceans.
•Large-scale circulation anomalies along the Southern Hemisphere exhibits
evidences of both SAM activity and development of Rossby wavetrains in
association to CSIS evolution