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El Niño and La Niña Redefined

11:21 am in Uncategorized by WeatherDem

This is the week to publish lots of interesting events and articles apparently.  I have a number of things I would love to post about, but only so much time.  Here is one that relates directly to something I posted on earlier: warmest La Niña years.  Just a few short weeks after NOAA operations wrote that 2012′s La Niña was the warmest on records, NOAA researchers announced they recalculated historical La Niñas because of warming global temperatures.  NOAA confirmed something that occurred to me while I was writing that post: eventually, historical El Niños will be cooler than future La Niñas.  How then will we compare events across time as the climate evolves?  The answer is simple: redefine El Niño and La Niña.  Instead of one climate period of record, compare historical ENSO events to their contemporary climate.  In other words, “each five-year period in the historical record now has its own 30-year average centered on the first year in the period”: compare 1950-1955 to the 1936-1965 average climate; compare 1956-1960 to the 1941-1970 average.  This is different from the previous practice in which NOAA compared 1950-1955 to 1981-2010 and compared 2013 to 1981-2010.  The 1950-1955 period existed in a different enough climate that it cannot be equitably compared to the most recent climatological period.

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Figure 1. “The average monthly temperatures in the central tropical Pacific have been increasing. This graph shows the new 30-year averages that NOAA is using to calculate the relative strength of historic El Niño and La Niña events.”

I want to point out something on this graph.  Is long-term warming evident in this graph?  Yes, there is.  But note they plot the breakdown by month.  The difference between 1936-1965 and 1981-2010 in October is >1°F.  Meanwhile, the same difference in May is ~0.5°F.

Here is the effect of NOAA’s change:


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Figure 2.  3-month temperature anomalies in the Nino-3.4 region.   (Top) Characterization of ENSO using 1971-2000 data.  (Bottom) Same as top, but using 1981-2010 data.

NOAA’s updated methodology resulted in the identification of two new La Niñas: 2005-06 and 2008-09.  The reason is warmer temperatures in the most recent decade than the 1970s (it sounds obvious when you say it like that).  That warming masked La Niñas with the old methodology.  It also means that the 2012 La Niña is no longer the warmest La Niña, as I related from the National Climatic Data Center last month:


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Figure 3. Anomalies of annual global temperature as measured by NOAA.  Blue bars represent La Niña years, red bars represent El Niño years, and gray bars represent ENSO-neutral years.

That record will now go down as a tie between 2006 and 2009, with 2012 coming in a close third.  This situation is analogous to the different methodologies that NOAA and NASA use to compute global temperatures and where they rank individual years.  Records might differ because of methodological differences, but the larger picture remains intact: the globe warmed in the 20th and so far in the 21st centuries.  That signal is apparent in many datasets.  Within the week, I’m sure we’ll hear from GW skeptics that La Niña years have been getting cooler since 2006.  Here is what is most important: 2000s La Niñas were warmer than 1990 Niñas, which were warmer than 1980 Niñas, etc.

NASA & NOAA: 2012 Was In Top-10 Warmest Years For Globe On Record

10:12 am in Uncategorized by WeatherDem

According to data released by NASA and NOAA this week, 2012 was the 9th or 10th warmest year (respectively) globally on record.  NASA’s analysis produced the 9th warmest year in its dataset; NOAA recorded the 10th warmest year in its dataset.  The two agencies have slightly different analysis techniques, which in this case resulted in not only different temperature anomaly values but somewhat different rankings as well.

The details:

2012’s global average temperature was +0.56°C (1°F) warmer than the 1951-1980 base period average (1951-1980), according to NASA, as the following graphic shows.  The warmest regions on Earth (by anomaly) were the Arctic and central North America.  The fall months have a +0.68°C temperature anomaly, which was the highest three-month anomaly in 2012 due to the absence of La Niña.  In contrast, Dec-Jan-Feb produced the lowest temperature anomaly of the year because of the preceding La Niña, which was moderate in strength.  And the latest 12-month period (Nov 2011 – Oct 2012) had a +0.53°C temperature anomaly.  This anomaly is likely to grow larger in the first part of 2013 as the early months of 2012 (influenced by La Niña) slide off.  The time series graph in the lower-right quadrant shows NASA’s 12-month running mean temperature index.  The recent downturn (2010 to 2012) shows the effect of the latest La Niña event (see below for more) that ended in early 2012.  During the summer of 2012, ENSO conditions returned to a neutral state.  Therefore, the temperature trace (12-mo running mean) should track upward again as we proceed through 2013.

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Figure 1. Global mean surface temperature anomaly maps and 12-month running mean time series through December 2012 from NASA.

According to NOAA, 2012’s global average temperatures were 0.57°C (1.03°F) above the 20th century mean of 13.9°C (57.0°F).  NOAA’s global temperature anomaly map for 2012 (duplicated below) reinforces the message: high latitudes continue to warm at a faster rate than the mid- or low-latitudes.

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Figure 2. Global temperature anomaly map for 2012 from NOAA.

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2012: Hottest Year On Record For United States

3:13 pm in Uncategorized by WeatherDem

After a brief hiatus (10 graduate school credits & TA-ing leaves no time for blogging), I’m back posting on FDL.  I expect to post much more regularly in 2013 as school activities ramp down.  More of my writing will also include a policy angle.  I want to do more to bridge the science and policy worlds in my blogging as well as in my future career.

It’s official: 2012 was indeed the hottest year in 100+ years of record keeping for the contiguous U.S. (lower 48 states).  The record-breaking heat in March certainly set the table for the record and the heat just kept coming through the summer.  The previous record holder is very noteworthy.  2012 broke 1998′s record by more than 1°F!  Does that sound small?  Let’s put in perspective: that’s the average temperature for thousands of weather stations across a country over 3,000,000 sq. mi. in area for an entire year.  Previously to 2012, temperature records were broken by tenths of a degree or so.  Additionally, 1998 was the year that a high magnitude El Niño occurred.  This El Niño event caused global temperatures to spike to then-record values.  The latest La Niña event, by contrast, wrapped up during 2012.  La Niñas typically keep global temperatures cooler than they otherwise would be.  So this new record is truly astounding!

The official national annual mean temperature: 55.3°F, which was 3.3°F above the 20th century mean value of 52°F.

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Figure 1 – NOAA Graph showing year-to-date average US temperatures from 1895-2012.

This first graph shows that January and February started out warmer than usual (top-5), but it was March that separated 2012 from any other year on record.  The heat of July also caused the year-to-date average temperature to further separate 2012 from other years.  Note the separation between 2012 and the previous five-warmest years on record from March through December.  Note further that four of the six warmest years on record occurred since 1999.  Only 1921 and 1934 made the top-five before 2012 and now 1921 will drop off that list.

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Figure 2 – Contiguous US map showing state-based ranks of 2012 average temperature.

Nineteen states set all-time annual average temperature records.  This makes sense since dozens of individual stations set all-time monthly and annual temperature records.  Another nine states witnessed their 2nd warmest year on record.  Nine more states had top-five warmest years.  Only one state (Washington) wasn’t classified as “Much Above Normal” for the entire year.  The 2012 heat wave was extensive in space and severe in magnitude.

Usually, dryness tends to accompany La Niña events for the western and central US.  This condition was present again in 2012, as the next figure shows:

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NASA & NOAA: July 2012 Was 12th, 4th Warmest On Record

9:49 am in Uncategorized by WeatherDem

According to data released by NASA and NOAA this week, July 2012 was the 12th and 4th warmest July (respectively) globally on record.  NASA’s analysis produced the 12th warmest July in its dataset; NOAA recorded the 4th warmest July in its dataset.  The two agencies have slightly different analysis techniques, which in this case resulted in not only different temperature anomaly values but rather different rankings as well.

The details:

July’s global average temperatures were 0.47°C (0.85°F) above normal (1951-1980), according to NASA, as the following graphic shows.  The warmest regions on Earth coincide with the locations where climate models have been projecting the most warmth to occur for years: high latitudes (especially within the Arctic Circle in July 2012).  The past three months have a +0.56°C temperature anomaly.  And the latest 12-month period (Aug 2011 – Jul 2012) had a +0.50°C temperature anomaly.  The time series graph in the lower-right quadrant shows NASA’s 12-month running mean temperature index.  The recent downturn (post-2010) is largely due to the latest La Niña event (see below for more) that recently ended.  As ENSO conditions return to neutral or even El Niño-like, the temperature trace should track upward again.

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Figure 1. Global mean surface temperature anomaly maps and 12-month running mean time series through July 2012 from NASA.

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2011: 9th Warmest Calendar Year On Record, Even With A La Niña

7:37 am in Uncategorized by WeatherDem

NASA’s James Hansen and a few of his colleagues released their assessment of 2011 global temperatures recently. In short, 2011 was the 9th warmest year in the GISS dataset.

Just as importantly, this situation occurred in the midst of a continuing La Niña event that is of moderate strength. La Niña is characterized by a general cooling of the tropical Pacific waters near the surface; it is frequently referred to as being the opposite of El Niño. As La Niñas progress, global temperatures tend to cool from their normal state. This of course has implications as scientists work to differentiate the effects of natural climate processes and those brought about by humans. If one year’s temperatures are cooler than the preceding year’s (or are warmer), does that mean that global warming has stopped (as skeptics like to say) or does that mean that there are competing forcings that affect the temperatures recorded?

It is the assessment of an overwhelming majority of climate scientists that global warming has not stopped. Instead, the 2nd half of 2010 and all of 2011 were dominated by La Niña events. What does this mean? It means that if the La Niña events had not occurred (and if there were no El Niños either), in other words purely “normal” conditions, 2011 likely would have been warmer than was recorded. This should become obvious in the next 6 months to 3 years as this La Niña dissipates and conditions across the globe respond accordingly. It takes ~6 months for downstream effects to show up in observations after ENSO phases start and after they go away.

Here is Hansen et al.‘s updated figure showing global land-ocean temperatures using an index:

Figure 1. Global surface air temperature anomalies relative to 1951-1980 base period for annual and 5-year running means. Green vertical bars are 2σ error estimates (Hansen et al., 2010). (Source for all graphs: Hansen)

The last black square on the right hand side of the graph is 2011′s temperature index value: +0.51°C. You can clearly see where the 9th highest ranking comes from when viewing this graph. You can further see that 2011 was warmer than 2001, 2004 and 2008 (simply comparing the past 10 years of values), as well as every year prior to 2000 save 1998, the year when the last century’s strongest El Niño occurred. Read the rest of this entry →