Summer Quasi-Biweekly Oscillation over the Tropical Americas: Spatial Features and Relation with ENSO

1
Summer Quasi-Biweekly Oscillation over the
Tropical Americas Spatial Features and Relation
with ENSO Min Wen1,2, Song Yang1, R. Wayne
Higgins1, and Renhe Zhang2 1NOAAs Climate
Prediction Center, Camp Springs, MD
20746 2Chinese Academy of Meteorological
Sciences, Beijing, China
1.Objective To investigate the quasi-biweekly
oscillation (QBWO), a pronounced but relatively
under-studied oscillation of intraseasonal
variability, over the tropical Americas and
Pacific-Atlantic Oceans To identify the dominant
modes of QBWO and their spatial and temporal
features To understand the interannual
variability of QBWO, especially its relationships
with sea surface temperature To evaluate the
performance of the NCEP CFS in simulating QBWO To
seek helpful information for improving Week-2
predictions
3.2 Interannual Variability Associated with ENSO.
Simultaneous correlation between
seasonal means of variances of EOF modes time
series and SST, from 1979 to 2005. Shadings
denote significant correlation passing 90 and
95 confidence levels.
Simultaneous correlation between
seasonal means of variances of 10-20-day filtered
OLR over GOM (upper) and NEP (lower) and SST.
3.1 Features of QBWO Cycles
2. Data and Methodology 2.1Data Daily NECP/NCAR
global reanalysis Daily NOAA outgoing longwave
radiation data Daily data of CFS T126L64 free
runs, 61st -90th years used NOAA extended
reconstructed sea surface temperature 2.2Methodolo
gy Fourier filter, sum of 18th-37th harmonics
used to represent 10-20-day oscillation
Empirical orthogonal function analysis Extended
empirical orthogonal function analysis Linear
regression analysis
Gulf of Mexico (GOM) 100-70W, 20-35N Northeast
Pacific (NEP) 120-90W, 5-20N
Lag-regressed OLR (shadings) and winds at 850hPa
(vectors) against the third EOF modes time
series. Values under the 95 confidence level are
omitted.
Lag-regressed OLR (shadings) and winds at 850hPa
(vectors) against the first EOF modes time
series. Values under the 95 confidence level are
omitted.
Based on time series of filtered OLR over NEP and
GOM, strong QBWO cases with peaks exceeding one
standard deviation are selected for the QBWO
cycle composites for normal, El Nino, and La Nina
years, respectively. Centered at the peak (both
negative and positive) day, 10 lag and lead days
are chosen. (All values of negative cases are
multiplied by -1.) The composite patterns
(figures omitted) show that the 10-20-day
disturbances reaching NEP in normal years are
mostly from the eastern Atlantic, and they may
have some relations with the hurricanes
influencing the Americas. In ENSO years, most of
the disturbances are from the western Pacific.
Most of the disturbances that affect GOM are also
from the western Pacific during ENSO years, but
they become much weaker during the normal years.
That is, only in ENSO years can 10-20-day
oscillations reach the higher latitudes such as
GOM from the tropics. These results further
confirm the correlations between QBWO and SST.
Lag-regression of OLR against the first EOF
modes time series along 10º?20ºN
Lag-regression of OLR against the first EOF
modes time series along 5º?15ºN
3.Results 3.1 Features of QBWO Cycles
?The east-west mode originates over the eastern
Atlantic, moves westward along 15ºN, and enhances
over the Caribbean Sea before it disatppears over
he central Pacific. The south-north mode
originates over the western Pacific, moves
eastward, and enhances over the eastern Pacific.
It shifts northward after arriving the Caribbean
Sea.
EOF1
EOF2
?Compared to observation , the first mode of
CFS-OLR QBWO is closer to the Equator, and so the
propagation of QBWO is southeast-northwest
orientated. For the second mode, the
disturbances are too strong over the eastern
tropical Pacific. These features may also be
accounted for by the warm bias of the tropical
SST.
EOF3
EOF4
4.Conclusions
During summer, there exist vigorous convection
and pronounced quasi-biweekly oscillation (QBWO)
over the eastern Pacific, southern Mexico, and
northern South America. EOF analysis shows that
the QBWO of convection has two major modes an
east-west orientated mode and a south-north
orientated mode. The east-west mode, which is
explained by the first two EOF modes, originates
over the eastern Atlantic, moves westward along
15ºN, and enhances over the Caribbean Sea. It
disappears over the central Pacific. The
south-north mode, explained by the third and
fourth EOF modes, originates over the western
Pacific, moves eastward, and enhances over the
eastern Pacific. It shifts northward after
arriving over the Caribbean Sea. The interannual
variability of the east-west mode is related to
middle-latitude SST, while the south-north mode
is related to ENSO events. The NCEP CFS captures
many important features of observed QBWO, but not
its interannual variability.
The first four EOF modes of 10-20-day filtered
OLR during 1 June - 31 August (1979 to 2005).
First four EEOF modes of 10-20-day filtered
CFS-OLR for time t, t2 and t4 days during JJA.
?The CFS captures many important features of QBWO
over the tropical Americas, including the maximum
variance centers over the northeastern tropical
Pacific, Gulf of Mexico, and east coast of US.
However, it overestimates the intensity,
especially over the tropical Atlantic where an
exaggerated broad center appears due to
overestimation of SST.
?Lag-regression of OLR against (a) the first EOF
modes time series along 10º?20ºN and (b) the
third EOF modes time series along 5º?15ºN.
?The QBWO of convection over tropical Americas
has two major modes the east-west mode explained
by the 1st and 2nd EOF modes and the south-north
mode explained by the 3rd ant 4th EOF modes. The
two joined modes have highest correlation when
one leads the other by 3 days, implying that
these two modes are the different phases of same
QBWO cycle.