Association of wet and dry periods in northern California and SSTs Dave Reynolds Meteorologist in Charge WFO San Francisco Bay Area October 2009
Motivation My experience over 35 years in water year rainfall in northern and central California has been a perception of feast or famine. Extended dry periods (three consecutive years below normal) become critical for adequate water supply. Winter weather briefings given to county officials each fall. Looking to provide more definitive guidance on possibility of continued dry or drought relief as this is what most are interested in.
Analysis of Water Year Rainfall for Three Independent Data Sets Keyed on long term San Francisco downtown rainfall. Smoothing applied to highlight wet and dry periods. Compared results to two independent rainfall sets to see if wetdry periods were consistent. 8 station index and WRCC Central Coast Climate region using same smoothing. Correlations were.91 to.93. Analyzed smoothed WY rainfall in comparison to various indices related to ENSO cycle or other SST classifications. PDO SSTs off coast CA in fall etc. Results presented. Box and whiskers plots presented based on fall MEI ENSO classification which had highest correlation to WY rainfall. Global Reanalysis composites for most significant wet and dry periods using MEI classifications will be presented.
Water Year Anomaly Calculation Normalized Anomaly used for this study: Average WY Rainfall SFD = 20 in Say observed WY rainfall SFD = 40 in Anomaly = 40/20-1 = 2-1 or 1 Thus 0 = average rainfall 1 = twice normal -.5 = half normal
8 Station Index Gages and San Francisco Downtown
WRCC WestMap Climate Regions Central Coast Drainage
Crude approximation SST extracted SST Data From Hadley Center
Hadley SST data 5x5 box at 37.5, -132.5
Anomaly PDO &MEI FFT Wet-Dry Periods in Water Year Rainfall SFD 8 STA Cntrl Coast WY Anomaly PDO and MEI R=.91-.93 0.5 0.4 0.3 0.2 0.1 0-0.1-0.2-0.3-0.4-0.5 1896 1902 1908 1914 1920 1926 1932 1938 1944 1950 = 1956 1962 1968 1974 1980 1986 1992 1998 2004 Water Year (WY) Cntrl Coast 8 Sta WY SFD Anomaly FFT used for smoothing using ITSM Software (Brockwell and Davis, 2002)
SST & MEI FFT SFD WY Wet-Dry Periods in Water Year Rainfall: Relation to MEI and SSTs SFD WY Anomaly 1.5 1 0.75 0.5 0.5 0-0.5-1 -1.5 1950 1944 1938 1932 1926 1920 1914 1908 1902 1896 = 1956 Water Year (WY) 2004 1998 1992 1986 1980 1974 1968 1962 0.25-1E-15-0.25-0.5-0.75 MEI Oct-Nov Coastal Waters SST SFD Anomaly
PDO &MEI FFT Cntrl Coast SFD 8 STA WY WY Wet Dry Periods and MEI/SSTs/PDO SFD 8 STA Cntrl Coast WY Anomaly PDO and MEI 1.5 1 0.5 0-0.5-1 -1.5 1944 1938 1932 1926 1920 1914 1908 1902 1896 1950 1956 = Water Year (WY) 2004 1998 1992 1986 1980 1974 1968 1962 0.5 0.4 0.3 0.2 0.1 0-0.1-0.2-0.3-0.4-0.5 3 mo PDO MEI ASON FFT Cntrl Coast SFD Anomaly 8 Sta WY
Box and Whiskers Plots Small sample sizes given 1950-2008 period of record. Will show plot with number of cases per category
Strong La Nina Predominately below normal rainfall when MEI Oct-Nov is <~ -1.25 Median Highest 25% Max Observed Median Mean Median Lowest 25% Min Observed
Moderate to Weak La Nina Mean and Median just below average and skewed to below normal- typical of last 3 years
Near Neutral Conditions Slightly skewed to below normal rainfall MJO Impacts?
Weak to Moderate El Nino Mean is near normal rainfall but more than 50% of cases < 85% of normal.
Strong El Nino When OCT-NOV MEI is > ~1.25 almost all years above normal rainfall. Skewed to 140 to 160% of normal. Two years below normal.
ENSO strength vs SFD WY rainfall and sign of PDO Sept-Nov. Strong La Nina Weak/ Mod La Nina Neutral Weak/ Mod El Nino Strong El Nino
Ranking 1-60 Where are we now? Years with closest fit MEI Ranking 72-73 WY = +.59 65-66 WY = -.25 57-58 WY = +.67 70 60 50 40 30 20 1972-73 2009-10 1965-66 1957-58 10 0 Monthly Pairs
Global Reanalysis Composite SST for Strong El Nino and Wet
Corresponding 200 mb Jet Anomaly
2 Dry Years with Strong El Nino based on MEI index Split Flow
200mb Zonal Wind Anomaly Weak to Mod El Nino More focused in So Cal
Years since 1950 with >140% rainfall Extended East-Asian Jet into Central and So Cal with maximum just offshore
Strong La Nina Years 5 of 6 years occurred during negative PDO cycle
200 mb zonal wind anom MEI < -1.2
Years with less than 60% rainfall Note signal in Nino 1+2 Region Jet displaced well to the north
Summary Wet Dry cycles observed and highly correlated in three separate WY rainfall datasets Examined various climate indices and SST datasets and noted general relationship to WY rainfall Generally La Nina/-PDO and cooler SSTs off West Coast trend to below normal WY rainfall. Strong El Nino/+PDO and warmer SSTs off central coast associated with above normal WY rainfall. Did not investigate lead or lag correlations but appears this might be useful. Individual WY analysis showed: If one isolates values of MEI for strong El Nino or La Nina, the correlation rises to.65 Impact of strong El Nino is extension of East-Asian jet to the West Coast of the US. Long fetch of westerlies accounts for large waves, coastal flooding and