low frequency modulation of annual and sub-annual cycle precipitation and temperature in the
DESCRIPTION
Low Frequency Modulation of Annual and Sub-annual Cycle Precipitation and Temperature in the Western United States. Balaji Rajagopalan and Subhrendu Gangopadhyay. Objective. How does the low frequency components of the climate system ( e.g., ENSO, PDO, etc.) modulate high - PowerPoint PPT PresentationTRANSCRIPT
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Low Frequency Modulation of
Annual and Sub-annual Cycle
Precipitation and Temperature in the
Western United States
Balaji Rajagopalan and Subhrendu Gangopadhyay
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Objective
How does the low frequency components of the climate
system (e.g., ENSO, PDO, etc.) modulate high
frequency components of the climate system, namely,
regional annual and sub-annual cycles of precipitation
and temperature.
Study Area : Western United States
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Study Area and Data
• 11 states (AZ, CA, CO, ID, MT, NV, NM , OR, UT, WA, WY)
• total, 84 climate divisions
• 106 years (1895-2000) of monthly precipitation and
temperature data for each climate division
NM
CA
NV
ID
MT
WY
UT
AZ
CO
OR
WA
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Methodology
• Uses the technique MTM-SVD (multi-taper method using singular-
value decomposition).
• Identify significant frequencies from the MTM spectrum.
• Do space-time reconstruction at these significant frequencies.
• Use the space-time reconstructions to analyze temporal evolution
of these significant frequencies, and phase shifts.
• Finally, use spectral-coherence to relate low frequency components
with precipitation and temperature at the annual and sub-annual
frequencies.
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MTM Spectrum – Joint Precip. and Temp.
• Low Frequency
-0.0 to 0.5 cycles/yr
• High Frequency
-0.5 to 6.0 cycles/yr
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MTM Spectrum – Only Precipitation
Significant frequencies (cy/yr) at 95% confidence level
• 0.0674
• 0.1875 (~ 5 yr cycle)• 0.3721 (~ 3 yr cycle)• 1.0000 (annual cycle)• 2.0000 (sub-annual cycle)• other harmonics
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Temporal Evolution of Frequencies – Joint P-T
Moving Window MTM-SVD at 90%
Observations:-
- ENSO and decadal
oscillations are patchy
- enhanced ENSO post
1980
- annual and sub-annual
cycles significant all
throughout
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Spatial Reconstruction at Significant Frequencies – LOW Frequencies, P and T
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Spatial Reconstruction at Significant Frequencies – HIGH Frequencies, P and T
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Spatial Reconstruction of ANNUAL Cycle
Using a 20-year Moving Window – Precipitation
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Spatial Reconstruction of ANNUAL Cycle
Using a 20-year Moving Window – Temperature
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Phase Shift of ANNUAL Cycle P and T
1950-1975; 1975-2000
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Phase Shift of SUB-ANNUAL Cycle P and T
1950-1975; 1975-2000
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Diagnosis of Space-time Reconstructions and
Phase Shifts
• For precipitation, Pacific Northwest and Arizona are out of phase
( we know that these two regions have the opposing ENSO signal
for winter precipitation), the shifts are of the order of +/- 50 days
(positive is early and negative is late).
• With temperature, all the shifts in temperature are positive and in
the mid-latitudes this implies for example an early Spring. Such
shift in temperature for example in the Pacific Northwest we can
expect that winter precipitation comes more as rain-on-snow type
events.
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Diagnosis of Space-time Reconstructions andPhase Shifts
• Also in case of precipitation, there are very little shifts in the rest
of the study region.
• Wherever the annual cycle is weak, primarily in the desert regions
(in particular, California-Nevada border, southern Utah, western
Montana) there seems to be a shift in the annual cycle close to 100
days. This apparently is a very large shift but to some extent makes
sense because these are desert regions; a little precipitation goes a
long way to show significant shifts.
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Diagnosis of Space-time Reconstructions andPhase Shifts
• With the 6-month cycle, we observe that for precipitation,
the shifts are nearly halved and for temperature the shifts
are nearly double that of the annual cycle.
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Spectral Coherence
Spectral coherence between January through April SST (sea surface temperature) first principal components (refereed in the figures as PC1B1; PC1B2 and PC1B3 are the extra-tropic SST PCs) and spatially averaged MTM projections time-reconstructed for both the annual and sub-annual cycles
Next plots :-
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Spectral Coherence – Annual Cycle
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Spectral Coherence – Annual + Sub-annual
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Low Frequency Modulation of
Annual and Sub-annual Cycle
Precipitation and Temperature in the
Western United States
Balaji Rajagopalan and Subhrendu Gangopadhyay