Low Frequency Modulation of Annual and Subannual Cycle

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 frequency components of the climate system, namely, regional annual and sub-annual cycles of precipitation and temperature. Study Area : Western United States

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 WA OR NV CA MT ID UT AZ WY CO NM

Methodology • Uses the technique MTM-SVD (multi-taper method usingularvalue 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.

MTM Spectrum – Joint Precip. and Temp. • Low Frequency -0. 0 to 0. 5 cycles/yr • High Frequency -0. 5 to 6. 0 cycles/yr

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

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

Spatial Reconstruction at Significant Frequencies – LOW Frequencies, P and T

Spatial Reconstruction at Significant Frequencies – HIGH Frequencies, P and T

Spatial Reconstruction of ANNUAL Cycle Using a 20 -year Moving Window – Precipitation

Spatial Reconstruction of ANNUAL Cycle Using a 20 -year Moving Window – Temperature

Phase Shift of ANNUAL Cycle P and T 1950 -1975; 1975 -2000

Phase Shift of SUB-ANNUAL Cycle P and T 1950 -1975; 1975 -2000

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.

Diagnosis of Space-time Reconstructions and Phase 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.

Diagnosis of Space-time Reconstructions and Phase 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.

Spectral Coherence Next plots : - Spectral coherence between January through April SST (sea surface temperature) first principal components (refereed in the figures as PC 1 B 1; PC 1 B 2 and PC 1 B 3 are the extra-tropic SST PCs) and spatially averaged MTM projections time-reconstructed for both the annual and sub-annual cycles

Spectral Coherence – Annual Cycle

Spectral Coherence – Annual + Sub-annual

Low Frequency Modulation of Annual and Sub-annual Cycle Precipitation and Temperature in the Western United States Balaji Rajagopalan and Subhrendu Gangopadhyay