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Normal January 200 – hPa Wind January 1998 200 – hPa Wind Fig. 3. Storm tracks at 200hPa (~39,000 feet – jet stream level): (a) Normal January circulation, (b) Southern Storm Track circulation (Jan 1998). Wind speeds are in meters per second (to convert to mph multiply by 2.25). Ref: Monteverdi, J., J. Null: El Niño and California Rainfall, NOAA Western Region Technical Attachment, No. 97-37, November 21, 1997. Figure 4 Fig. 4. Normalized Departure from Normal Rainfall at Los Angeles Civic Center. Red indicates Type 1 El Nino seasons. Note that every Type 1 El Nino had above normal rainfall, although three out of seven had normalized rainfall less than 0.5 above the mean. Ref: NOAA/NWS/Western Region Headquarters. 26 Citrograph September/October 2012 The PNA atmospheric response is strongly forced by sea surface temperature (SST) anomalies in the midlatitude Pacific. A cold SST anomaly in the east-central Pacific and a warm SST anomaly adjacent to the south or the east favors a negative phase of the PNA (west-coast trough and eastern US ridge). If the PNA pattern is sufficiently large and strong (in negative sense), it can by itself also generate a southern storm track as in Jan 1995 (Figure 5b). The negative PNA pattern that was present in Feb – May 2012 has continued until midsummer (Figure 6a). <strong>This</strong> has caused the persistent heat in the central U.S. this early- to mid-summer, and some off-season rains in northern California and unusually cool conditions to coastal California. It also contributed to the unusually wet spring in NW California (Eureka had 7 inches more rain than normal during Feb 1 – May 31). It is important to note that a southern storm track, capable of producing significant rains in California (and southern California) can be produced by either of these separate large scale patterns, El Nino and a negative PNA. These two cycles are always present in any given winter, but with different periods. Because the cycles have different time scales and are both somewhat irregular, they are often not in phase and, therefore, conflict with each other. When the contributions from El Nino and a negative PNA pattern occur in phase with each other, their contributions are additive. The result is an enhanced storm track, affecting both the subtropics and mid-latitudes, and potential for frequent and heavy rains. Troughs and fronts that occur draw energy from the southern storm track and cause stronger fronts and heavier rains in California than would otherwise occur. PNA-related rain periods and El Nino Although El Nino is a dominant factor that induces development of a southern storm track, it is not the only one. The PNA can induce a strong southern storm track (Figure 5), even if El Nino is weak (1994-95) or absent (2004-05). The winter of 2004-05 was a good example of what can happen in a very
Figure 5a weak or non-El Nino season when there is unusual wintertime warmth at high latitudes and when a PNA pattern is present (cold storms persisting in the east Pacific). Sea surface temperatures in the Gulf of Alaska were 3-4C above normal. Rainfall at Los Angeles Civic Center was the third highest of record: 37.97 inches in the season ending June 30, 2005. Moderate to strong El Nino patterns often cause a modification in the underlying SSTA pattern in the mid-latitudes west of California, and thereby maintain an active storm track through most of the spring. For a negative PNA plus a moderate El Nino: Both favor an active southern storm track (Figures 2 and 5). When both are present together, they favor a stronger and optimally active southern storm track. Both favor mild conditions in the Great Lakes and northeastern US. Fig. 5a. Pacific-North-America Pattern: (a) Negative Phase --Trough in the northwestern U.S., ridge in central Pacific south of Aleutians, ridge and warm weather in the eastern U.S.; (b) Positive Phase -- Ridge in the western and northwestern U.S., trough in the eastern U.S., and a deep trough south of the Aleutians. Ref: NOAA/NCEP/ EMC/CMB, published at http://www.emc.ncep. noaa.gov/gmb/ssaha/indices/ pna_load.gif. For this season The dominant factors will be El Nino, and a tendency for troughing near the California coast (negative PNA). Cold sea surface temperatures (cold SSTAs) have continued through midsummer off the California coast and west of northern California and the Pacific Northwest. (Figure 6a – current SST Anomaly map from 1 August 2012). Both anomalies at this time appear to be moderate intensity. Therefore, we expect that: The cold SST anomaly west of northern Figure and central California 5b will tend to force troughs to develop near the central California coast. The influence of this cold anomaly is expected to be greatest during November through February and into March 2013. The El Nino (moderate intensity) will tend to persist longer -- through April 2013. <strong>This</strong> current El Nino is of lesser magnitude than the El Nino of 1997- 98: Observed SSTAs were +3 to +4C in the region 140 to 110W for December 1997, measured by NOAA’s TOGA TAO array (Figure 1). The NOAA SSTA forecast for Nov 2012 shows forecasted SSTAs +1.5 to +2.6 C (Fig. 2). These two separate sea surface temperature anomalies (SSTAs) will tend to be of moderate intensity. For El Nino, “moderate” means an El Nino less severe, rain-wise than the El Ninos of 1982-83 and 1997-98. In 1997-98, seasonal rainfall at Los Angeles Civic Cen- Fig. 5b. Large-scale circulation features affecting the North Pacific and North America during strong negative PNA event in Dec 1994-Jan 1995. Dominant features were: (1) a strong southern storm track from the mid-latitude subtropical Pacific to the southern half of California to Texas, and (2) an eastern U.S. storm track well to the north of normal (warm in eastern U.S.). Associated with this strong PNA pattern was southern storm track, which caused heavy rains in all of California, including the south half (even in San Diego). Ref: NOAA/NCEP/Climate Analysis Center. September/October 2012 Citrograph 27
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