Thursday, October 28, 2010

okay, i finally got it figured out. here is the text that goes with the graph from a few posts ago. take the time to make the connections to the different posts, and also read snobots summary. plus read the new post about conditions on the hill in the past two days. a GRREAT start.

In the context of recent plunge of the MEI into strong La Niña conditions, this section features a comparison figure with strong La Niña events that all reached at least minus one standard deviations by June-July, and a peak of at least -1.4 sigma over the course of an event. The most recent bigger La Niña events of 1998-2001 and 2007-09 did not qualify, since they either did not reach the required peak anomaly (the first one) or became strong too late in the calendar year (both).

The most recent (August-September) MEI value shows a continued drop from earlier this year, reaching -1.99, or 0.18 sigma below last month's value, and 3.39 standard deviations below February-March, a record-fast six-month drop for any time of year, while slowing down a bit at the shorter time scales. The most recent MEI rank (lowest) is clearly below the 10%-tile threshold for strong La Niña MEI rankings for this season. One has to go back to July-August 1955 to find lower MEI values for any time of year.

Negative SST anomalies are covering much of the eastern (sub-)tropical Pacific in the latest weekly SST map. Many of these anomalies are in excess of -1C. For an alternate interpretation of the current situation, I highly recommend reading the latest NOAA ENSO Advisory which represents the official and most recent Climate Prediction Center opinion on this subject. In its latest update (7 October 2010), La Niña conditions are expected to last at least into the Northern Hemisphere spring of 2011.

There are several other ENSO indices that are kept up-to-date on the web. Several of these are tracked at the NCEP website that is usually updated around the same time as the MEI, in time for this go-around. Niño regions 3 and 3.4 showed persistent anomalies above +0.5C from June 2009 through April 2010, with a peak of +1.6C for Niño 3 and +1.8C for Niño 3.4 in December 2009, only to drop to about -0.5C or lower in both regions by early June 2010, reaching just shy of -1.0C for the month of July, and below -1.5C for September Niño 3.4 anomalies and below -1.2C for Niño 3. For extended Tahiti-Darwin SOI data back to 1876, and timely monthly updates, check the Australian Bureau of Meteorology website. This index has often been out of sync with other ENSO indices in the last few years, including a jump to +10 (+1 sigma) in April 2010 that was ahead of any other ENSO index in announcing La Niña conditions. After a drop to +2 in June, July rebounded to +20.5, August continued at an impressive +18.8, only to be followed by an even more impressive +25.0. The last time that this index showed higher values in September was back in 1917, which was also the only time on record that this happened for this month. An even longer Tahiti-Darwin SOI (back to 1866) is maintained at the Climate Research Unit of the University of East Anglia website, however with less frequent updates (currently through March 2010). Extended SST-based ENSO data can be found at the University of Washington-JISAO website, currently updated through May 2010 (which ended up just slightly below the long-term mean value).

Stay tuned for the next update (by November 5th) to see where the MEI will be heading next. After peaking seven months ago at +1.5, it has dropped just about as fast as it can, and continues to correlate highest with 1970, of the 'analog' cases shown here. Given the continued drop in the MEI into exceptionally strong territory, La Niña conditions are guaranteed well into 2011.

issued by
7 October 2010
ENSO Alert System Status: La Niña Advisory
Synopsis: La Niña is expected to last at least into the Northern Hemisphere spring 2011.
La Niña continued during September 2010 as reflected by the large expanse of below-average sea surface temperatures (SSTs) across most of the equatorial Pacific Ocean (Fig. 1). All weekly Niño SST index values were between –1.3°C and –1.8°C at the end of the month (Fig. 2). In addition, the subsurface heat content (average temperatures in the upper 300m of the ocean, Fig. 3) remained below-average, reflecting a shallower-than-average thermocline in the central and eastern Pacific (Fig. 4). Convection remained enhanced over Indonesia and suppressed over the western and central equatorial Pacific (Fig. 5). This pattern was linked to a continuation of enhanced low-level easterly trade winds and anomalous upper-level westerly winds over the western and central equatorial Pacific. Collectively, these oceanic and atmospheric anomalies reflect the ongoing La Niña.
Consistent with nearly all of the forecast models (Fig. 6), La Niña is expected to last at least into the Northern Hemisphere spring 2011. Just over half of the models, as well as the dynamical and statistical averages, predict La Niña to become a strong episode (defined by a 3-month average Niño-3.4 index of –1.5°C or colder) by the November-January season before beginning to weaken. Even though the rate of anomalous cooling temporarily abated during September, this model outcome is favored due to the historical tendency for La Niña to strengthen as winter approaches.
Likely La Niña impacts during October-December 2010 include suppressed convection over the central tropical Pacific Ocean, and enhanced convection over Indonesia. The transition into the Northern Hemisphere fall means that La Niña will begin to exert an increasing influence on the weather and climate of the United States. Expected U.S. impacts include an enhanced chance of above-average precipitation in the Pacific Northwest, and below-average precipitation across the southern tier of the country. Also, La Niña can contribute to increased Atlantic hurricane activity by decreasing the vertical wind shear over the Caribbean Sea and tropical Atlantic Ocean (see the August 5th update of the NOAA Atlantic Seasonal Hurricane Outlook). Conversely, La Niña is associated with suppressed hurricane activity across the central and eastern tropical North Pacific.
This discussion is a consolidated effort of the National Oceanic and Atmospheric Administration (NOAA), NOAA’s National Weather Service, and their funded institutions. Oceanic and atmospheric conditions are updated weekly on the Climate Prediction Center web site (El Niño/La Niña Current Conditions and Expert Discussions). Forecasts for the evolution of El Niño/La Niña are updated monthly in the Forecast Forum section of CPC's Climate Diagnostics Bulletin. The next ENSO Diagnostics Discussion is scheduled for 4 November 2010. To receive an e-mail notification when the monthly ENSO Diagnostic Discussions are released, please send an e-mail message to:
Climate Prediction Center National Centers for Environmental Prediction NOAA/National Weather Service Camp Springs, MD 20746-4304
Figure 1. Average sea surface temperature (SST) anomalies (°C) for the week centered on 29 September 2010. Anomalies are computed with respect

No comments:

Post a Comment