Overview

Following a rather warm and dry first two weeks of September, an apparent “fall transition” occurred in the middle of the month. If this was truly our annual transition, it occurred early: generally speaking, the change from warm-dry to cooler-wetter occurs some time in October.

Table 1 is a summary of monthly averages and totals at selected stations throughout the state. Table 2 lists daily temperatures and precipitation for most of the locations listed in Table 1. In Table 3, monthly precipitation totals throughout the state are listed. Figure 1 is a map showing the percentage of normal precipitation statewide for the Water Year.

Basin Summary

Here is a summary of water indicators at the end of the month, by river basin:

n.a. Not available
(1) Percent of normal June precipitation, from NOAA Cooperative sites
(2) Percent of normal seasonal precipitation (since Oct. 1), from NOAA Cooperative sites
(3) Percent of normal seasonal precipitation, from Natural Resources Conservation Service (NRCS) SNOTEL sites
(4) Percent of normal June stream flow, from U.S. Geological Survey (USGS)
(5) Percent of normal seasonal stream flow (since Oct. 1), from USGS
(6) Surface Water Supply Index, from NRCS (-4 = very dry, 0 = normal, +4 = very wet)

Forecasts

The Climate Prediction Center’s (CPC) forecasts for October-December appear below. There is a higher likelihood of warmer than average temperatures and below-average precipitation in Oregon.

Oregon Climate Service predicts normal temperatures and above-normal precipitation for the next three months.

ENSO Update

ENSO Summary: Australia Bureau of Meteorology (BOM), October 3, 2007

With the exception of the Southern Oscillation Index (SOI), all ENSO indicators showed an intensifying La Niña during September. Computer models forecast the La Niña to last until early 2008, making it almost certain that 2007 will be considered a La Niña year. Most critically, the near-equatorial Pacific Ocean has continued to cool both on and below the surface, the Trade Winds remain stronger than normal across the western to central Pacific, and cloudiness in the equatorial Pacific is reduced. Together, these indicators suggest the atmosphere and ocean are reinforcing each other; a critical component in sustaining La Niña conditions for any period of time. However, this La Niña has been late to develop by historical standards. In the past, most significant La Niña events were established by winter's end, with widespread above-average rain falling over Australia's eastern half. With a late-developing La Niña, this typical rainfall response is not as likely as in past episodes. Indeed it has been largely absent to date.

NOAA’s Climate Prediction Center (CPC) agreed, saying (October 11):

La Niña conditions strengthened during September 2007, as negative SST anomalies along the equator expanded westward and now extend from 170°E to the South American coast. The latest weekly analysis shows the largest SST departures (-2°C to -3°C) between 120°W and the coast, with departures of -0.5°C to -1°C centered near the date line. The magnitude of the negative SST anomalies increased in all of the Niño regions, with the Niño-3.4 index dropping to -1.2°C and the Niño-4 index dropping to -0.5°C by the end of the month. The upper-ocean heat content (average temperatures in the upper 300 m of the ocean) in the central and east-central equatorial Pacific remained below average during September, with temperatures ranging from 2°C to 4°C below average at thermocline depth (see Figure 4 below). Consistent with these conditions, the low-level easterly winds and upper-level westerly winds remained stronger than average across the central equatorial Pacific, convection remained suppressed throughout the central and eastern equatorial Pacific, and enhanced convection again covered parts of Indonesia and the far western Pacific. Collectively, these oceanic and atmospheric conditions reflect a strengthening La Niña.

The recent SST forecasts (dynamical and statistical models) for the Niño 3.4 region indicate a weak-to-moderate La Niña continuing into early 2008. Current atmospheric and oceanic conditions and recent trends indicate that La Niña will continue and may strengthen during the next 3 months.

La Niña Conditions Strengthen
George H. Taylor October, 2007

Week by week, “La Niña” conditions strengthen in the Pacific. Week by week, the likelihood of a cool, wet winter goes up, because historically our wettest, coolest winters occur when we have a La Niña. Mind you, not every La Niña is cool and wet, but most of them are. And while there are many things that influence the weather and climate in the Northwest, the tropical Pacific, with its La Niña (and La Niña) events, has the biggest influence (in my humble opinion).

To refresh:

Off the west coast of South America, waters are unusually cool, for two reasons: the cool Humboldt (Peruvian) current which moves northward along the coast, bringing water from the Antarctic regions; and the easterly trade winds, which push surface water away from the coast and cause upwelling, allowing cool water to flow upward to the surface. As the trades send the cool water away from the coast, a "cool tongue" of water forms along the equator. Gradually, as the winds push the water westward across the vast Pacific, it warms. The warmest waters in the tropical Pacific are in the western portions, near Indonesia and the Philippines; average surface temperatures there are about 90 degrees F, compared with about 68-70 degrees off South America.

At least, that’s what happens on average. But “average” doesn’t always occur. About every 3 to 7 years, the ocean temperatures off the South American coast get warmer. At the same time, temperatures in the western Pacific usually decrease. This phenomenon is known as an El Niño, or "warm event." In the Northwest, our driest and warmest winters occur when an El Niño is present.

At other times (as in Right Now!), cool water off South America becomes even cooler, and the warm water in the western Pacific even warmer than average, and we call that La Niña, or "cool event." La Niñas appear to occur when easterly trade winds increase in strength, causing greater upwelling – but there are probably other phenomena involved. Though our understanding of El Niño and La Niña – collectively called the “El Niño Southern Oscillation” or ENSO – are improving, there is still much that we don’t know. For that reason, many scientists, including many of my colleagues at OSU, are studying ENSO to increase our understanding of the phenomenon. See for example, some of the activities in the College of Oceanic and Atmospheric Sciences (http://www.coas.oregonstate.edu/).

This week we received updates on the sea surface temperatures in the four “ENSO regions” which lie along the Equator from the South American coast westward. The easternmost region, near South America, is Niño 1&2. In September the average temperature was 65.5 degrees F, which is about 3 degrees below average, indicative of La Niña. More significantly, though, last month’s temperature was the lowest ever reported in Niño 1&2 by NOAA – with records going back to 1950.

Yes, something dramatic is happening in the Pacific, and may portend “interesting” events in the Northwest.