strides, steps and stumbles in the march of the seasons 1.astronomical signal, processed by earth...
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Strides, steps and stumbles in the march of the seasons
1. Astronomical signal, processed by Earth2. Earthly data sets, processed by computer3. Strides: annual, semiannual4. Steps: Asian monsoon onset, Americas MSD5. Stumbles (a.k.a. “singularities”)
• the US January Thaw• Brazil rain
• Software demo (DVD available - ask)
High frequency aspects of the mean seasonal cycle
Brian Mapes
University of Miami
Seasonality: the forcing
QuickTime™ and aTIFF (Uncompressed) decompressor
are needed to see this picture.
Astronomical forcing(spherical Earth, circular orbit)
solstice equinox solsticeequinox
Temporal Fourier spectrum of annual insolation
sin(2t/365d)Annual
sin(4t/365d)Semiannual
no forcing, just internal
nonlinearitites
Annual cycle signal processing(by the Earth)
heating
Cloud, albedo
temperature
Heat capacity
pressuregravity
windrotation
evaporation, advectionhumidity
vertical motioncloud
micro-physics
precip
Annual cycle signal processing (by the computer)
1. Form mean daily climatologies: var (365 calendar days; lat, lon, level, dataset)
2. Compute mean, annual + semiannual harmonics
mean, 2 amplitudes, 2 phases
3. Residual is called “HF” HF = climatology - annual - semiannual
4. Wavelet analysis of HF {dates, periods, amplitudes, significances of HF events}
Wavelet example
Paul wavelet •Sharp time localization broad in frequency•Real part: max/min•Imaginary part: rise/fall(Torrence and Compo 1998 BAMS)
Signal processed. Now what??Fourier and wavelet coefficients form a
supplement (index) to climatology.
This doubles the size of the data set.
?? What to do with all this information ??interactive GUI visualization tool
Outline of examples1. Annual
– “Easy” (local, thermal): Tsfc
– Nonlocal: Z250 (jet streams)
2. Semiannual– Nonlinear: angular momentum and u250
– Statistical: 250mb eddy activity
3. Ter-annual and beyond– oceanic subtropical highs, monsoons
Simplest: annual harmonic of surface air T
CSM
OBS
GFDL
Cold winter low tropospheric
thickness?
annual harmonic of Z250
CSM
OBS
GFDL
Temperature thickness?
Not so simple…
Annual Harmonicof atmos. heating
Annual Harmonicof u at 200 hPa
Local Hadley cells?
Jan 1
Apr 1
Outline of examples1. Annual
– “Easy” (local, thermal): Tsfc
– Nonlocal: Z250 (jet streams)
2. Semiannual– Nonlinear: angular momentum and u250
– Statistical: 250mb eddy activity
3. Ter-annual and beyond– oceanic subtropical highs, monsoons
Semiannual harmonic of
250 hPa zonal wind
(Weickmann & Chervin 1988)
Jan maxJan min
Apr Apr
Semiannual harmonic of
250 hPa zonal wind
total
JanApr-Oct
Apr
JanApr-Oct
Apr
Apr
Apr
May+Nov
Jun+DecCSM
OBS
HF
semiannual harmonic of 250 hPa zonal wind
semiannual u in a forced/damped
barotropic model forced with annual harmonic only of
200mb div (Huang & Sardeshmukh
2000)
total
Jan-Jul
Apr-Oct
Apr-Oct
Apr-OctJan-Jul
Apr-Oct
A nonlinear overtoneA nonlinear overtone
Semiannual eddy (storm) activityMidwinter Suppression of Baroclinic Wave Activity in the PacificNakamura 1992ABSTRACT Seasonal variations in baroclinic wave activity and jet stream structure in the Northern
Hemisphere are investigated based upon over 20 years of daily data. Baroclinic wave activity at each grid point is represented for each day by an envelope function, the lowpass-filtered time series of the squared highpass-filtered geopotential height. Baroclinic wave activity over the Atlantic exhibits a single maximum in January, whereas in the Pacific it exhibits peaks in late autumn and in early spring and a significant weakening in midwinter, which is more evident at the tropopause level than near the surface. This suppression occurs despite the fact that the low-level baroclinity and the intensity of the jet stream are strongest in midwinter. Based on the analysis of 31-day running mean fields for individual winters, it is shown that over both the oceans baroclinic wave activity is positively correlated with the strength of the upper-tropospheric jet for wind speeds up to 45 m s 1. When the strength of the westerlies exceeds this optimal value, as it usually does over the western Pacific during midwinter, the correlation is negative: wave amplitude and the meridional fluxes of heat and zonal momentum all decrease with increasing wind speed. The phase speed of the waves increases with wind speed, while the steering level drops, which is indicative of the increasing effects of the mean flow advection and the trapping of the waves near the surface.
midwinter eddy
minimum
semiannual amplitude (color) of
a climatology of stdev of 9d sliding
window v250
Decently simulated by
coupled model
NCEP 1969-96
CSM 1.3
Oct+Apr maximaof eddy v variance
Outline of examples1. Annual
– “Easy” (local, thermal): Tsfc
– Nonlocal: Z250 (jet streams)
2. Semiannual– Nonlinear: angular momentum and u250
– Statistical: 250mb eddy activity
3. Ter-annual and beyond– oceanic subtropical highs, monsoons
Ter-annual variations of
sea-level pressure total
100d
10d
HF
CSM climate model
total
HF
Understanding SLP
seasonality:Zonal mean
total
100d
10d
HF
NH Winter High
Pacific ter-annual SLP schematic
NH Summer Low
Oceanic (Aleutian)~semiannual low
Monsoon-related~100d period high
Outline of examples1. Annual
– “Easy” (local, thermal): Tsfc
– Nonlocal: Z250 (jet streams)
2. Semiannual– Nonlinear: angular momentum and u250
– Statistical: 250mb eddy activity
3. Ter-annual and beyond– oceanic subtropical highs, monsoons
Linho and Wang 2002
wavelet method ISM
WNPM
EASM
ISM
WNPM
EASM
a model:decent
ISM onset, but poor at
oceanic systems ISM
WNPM
EASM
North America at same time
ISM
WNPM
EASM
ISM
WNPM
EASMNAM
ENASM?
June onset in southeast US
(from CPC .25deg 1948-98 gauge data set)
(mid-summer dip)
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are needed to see this picture.
Jun 1 Jul 1
Ter-annual midsummer
structure
high & dry in tropical
Americas
NCEP 27 year (1969-96) SLPGulf of Mexico area
July Aug
CMAP 23y mean obs. rain
1
5
dry
HF
21 May-10 June
3-27July
wet
dry
Midsummer high in W. Atlantic
SLP obs G Mexico
IPRC/ ECHAM model: decent
Key features of Asian vs. American monsoons : a model experiment
IPRC / ECHAM model: shown to have decent Asian onset, American midsummer drought
Enhance monsoons by allowing each calendar day (solar declination) to last 5x24 hours. Summer continents get hotter, atm has time to equilibrate.
(SST fixed to obs. for each calendar day)
April-Sept rainfall, exp-control:S. Asia wetter, Americas drier
• S. Asia time series shows earlier (May) onset in exp
• Stated regarding the control run,– Land-atm lags delay Asian
monsoon onset behind its seasonal forcing by ~1mo.
– Onset is the main disequilibrium of the Asian monsoon wrt seasonal forcing
May
total
July
April-Sept rainfall, exp-control:S. Asia wetter, Americas drier
• Americas time series shows mid-summer drought earlier and drier in exp
• Stated regarding the control run,– Land-atm lags prevent
American midsummer drought from developing fully
– Midsummer drought is the main disequilibrium of the American summer monsoon wrt seasonal forcing
Jun
Outline of examples1. Ter-annual and beyond
– oceanic subtropical highs– monsoons
• Asian onset
• American midsummer drought
• These things are related
Chen, Hoerling and Dole 2001
Heating
Eddy Z1000
w/o shear
Eddy Z1000
July u(y,p)
u300, zonal mean
Westerlies retreat to >30N in midsummerQuickTime™ and aTIFF (LZW) decompressorare needed to see this picture.
<0
time slice
easterlies protrudeto 30N suddenlyin mid summer
WHY, in terms of [u] budget? • Not f[v]: ~barotropic; [v](t) wrong
• [u’v’]: Tilted TUTTs, Tibetan High, Transients?
Eq
60N
J F M A M J J A S O N D
Jul-Aug time slice
Suggested story timing as seen in Key West data
Winter Wiggles
• Sub-ter-annual time scales: real ??
– ‘The January Thaw’• a statistical phantom? (BAMS Jan 2001)
– Brazilian rainfall
January Thawlit. of 1919
Boston, 1872-1965
Entire debate centerson this spike
missing this
Revisit, withmore spatialaveraging
• 1969-96 NCEP reanalysis surface air temperature
• N. America mean
• (think vdT/dy)
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100d
10d
unsmoothed
daily THF part
Jul JunJan
5K
3K
CPC USprecip1948-98
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Winter Wiggles
• Sub-ter-annual time scales: real ??
– ‘The January Thaw’• a statistical phantom? (BAMS Jan 2001)
– Brazilian rainfall
Where are those boxes? W. subtropical oceans
High HighHighHigh
CMAP rainfall climatologies:
TAIWAN 25N
CUBA 25N
SE BRAZIL 25S
MADAGASCAR 25S
May
Nov.
Nov.
May
Source: CPTEC Web site
East Brazil rain gauge data
?
SP & Rio
clim.
Source: Hotel inter-continental Web site
?
1. Semiannual jets: questions
• OK, it’s a nonlinear overtone driven by the annual harmonic of divergence. What and where is the nonlinearity?
• Superposition fails of course, but surely one could say more than ann div everywhere -> semiann u– Tropical vs. extratropical divergence?– Advection of/by divergent vs. rotational wind, u vs v?– Zonal mean vs. longitudinal features (where)?– Spring/summer/fall/winter?
(HS2000 is really just a succession of steady states)
Does using obs. divergence presuppose too much?
East Asia midsummer
drought mechanism: the
Bonin High
upper level (barotropic)
“Formation mechanism of the Bonin High in
August”(Enomoto, Hoskins, & Matsuda 2003)
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HF anomalies (total - annual - semiannual):
Yes, the Bonin High and 2 other stationary waves are evident, but also a zonally elongated u
anomaly spanning all Asia-WPAC
time slice
0 m/s 2-4 -2
u200 HF anomaly plot, July-Aug
HF wavelet analysis
100d
10d
total HF
Japan
Jul Aug
Interannual consistency of the Asian HF jet anomaly ~1 Aug
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A hemisphericadjustment?
• SLP 1969-96 from NCEP reanalysis