The 2014/15 El Niño – Part 13 – More Mixed Signals

A few interesting things have happened since the July Update last week. On the ocean side, weekly sea surface temperatures in the NINO3.4 region have dropped (just) below the threshold of El Niño conditions (using the standard NOAA base years of 1971-2000 for their Reynolds OI.v2 data). On the atmospheric side, the 30-day running average of the BOM Southern Oscillation Index (SOI) has finally neared the threshold of El Niño conditions. But the SOI does not reflect what’s going on along the equator. And there is evidence that the trade winds are slightly stronger than normal across most of the equatorial Pacific.

SEA SURFACE TEMPERATURE-BASED ENSO INDICES

Sea surface temperature anomalies for the NINO3.4 region of the equatorial Pacific (5S-5N, 170W-120W) are a commonly used index for the strength, timing and duration of El Niño and La Niña events. (See the map here from the BOM for the location of the NINO3.4 region.) They indicate the surface temperature response of the equatorial Pacific (a part of the ocean processes) to variations in El Niño-Southern Oscillation. NOAA considers there to be El Niño conditions (not a full-blown official El Niño, though) in the equatorial Pacific when the sea surface temperature anomalies for the NINO3.4 region reach and exceed a threshold of +0.5 deg C. El Niño conditions had (past tense) existed in the equatorial Pacific for the past 9 weeks. Those elevated sea surface temperatures were a response to the strong downwelling Kelvin wave that had carried warm subsurface waters from the western to the eastern equatorial Pacific. (For more information about the Kelvin wave see Part 1 of this series.) But that warm water has been rising to the surface over the past few months, releasing heat to the atmosphere primarily through evaporation, and the supply of warm water has dwindled drastically. So drastically, the sea surface temperature anomalies of the NINO3.4 region have recently dropped (slightly) below the threshold of an El Niño. See Figure 1. For the week centered on July 9^th, the Reynolds OI.v2 sea surface temperature anomalies for the NINO3.4 region are at 0.43 deg C.

Figure 1 (NINO3.4 SSTa)

The NINO1+2 region (10S-0, 90W-80W) is in the eastern equatorial Pacific, south of the equator, just south and east of the Galapagos Islands. It had been showing the warmest sea surface temperature anomalies in response to that Kelvin wave. While not as low as the NINO3.4 values, the sea surface temperature anomalies of the NINO1+2 region are also falling, and falling quite rapidly. See Figure 2.

Figure 2 (NINO1+2 SSTa)

Sea surface temperature anomaly data illustrated in the above graphs are available from the NOAA NOMADS website.

SEA LEVEL PRESSURE-BASED ENSO INDEX

The Southern Oscillation Index (SOI) is a product of Australia’s Bureau of Meteorology. The SOI is another commonly used metric for the strength, duration and timing of El Niño and La Niña events. It captures a portion of the atmospheric components of the El Niño-Southern Oscillation processes. We discussed the Southern Oscillation Index in more detail in Part 8 of this series. The Southern Oscillation Index is calculated from the sea level pressures of Tahiti and Darwin, Australia. El Niño events are strong negative values and La Niñas are strong positive values, which is the reverse of what we see with sea surface temperatures. El Niño conditions, according to the BOM, are a SOI value equal to or lower than -8.0 and a SOI value equal to or greater negative number than +8.0 indicates La Niña conditions.

Because surface winds are associated with sea level pressures, a drop in the Southern Oscillation Index reflects a weakening of the trade winds in the tropical South Pacific and an increase in the SOI reflects a strengthening of the trade winds there. To reinforce (provide positive feedback to) the evolution of an El Niño, the trade winds have to weaken. After April, this has not happened so far in 2014. According to the BOM’s Recent (preliminary) Southern Oscillation Index (SOI) values, however, the 30-day running average of the SOI is at the -8.0 threshold of El Niño conditions.

Because El Niño events take place along the equatorial Pacific, and because the Southern Oscillation Index is based on the sea level pressures off the equator (which can be effected by weather noise unrelated to El Niño processes), it’s difficult to tell whether this an indication that the equatorial trade winds are finally going to provide the necessary positive feedback and allow the El Niño to develop. So we have to look somewhere else.

THE LOW LEVEL ZONAL WIND ANOMALIES ALONG THE EQUATOR

In their weekly ENSO update, NOAA includes a Hovmoller diagram of low level wind anomalies along the equator (5S-5N) for the eastern Indian Ocean and the Pacific Ocean. (See their page 16.) I’ve added some notes to it in my Figure 3.

Figure 3

Don’t be intimidated by the Hovmoller. The vertical axis is time, with January 2014 at the top and July 2014 at the bottom. The horizontal axis are longitudes, starting on the left at 60E, which is in the western Indian Ocean, and ending on the right at 80E  80W, which is at the coast of South America. For the sake of discussion, I’ve added a fine highlight at 120E to separate the eastern equatorial Indian Ocean from the western equatorial Pacific. The color-coding is such that westerly wind anomalies are in shades of red and easterly wind anomalies are in blues. The latitudes for the wind anomaly data are 5S-5N, so they capture the equatorial Pacific, which is where El Niño processes take place. And the units are trade wind anomalies (not absolutes) and the trade winds normally blow from east to west. NOAA has provided three arrows to point out the westerly wind (anomaly) bursts, which indicate severe weakening and possible reversals of the normal east to west trade winds along the western equatorial Pacific.

And I’ve also circled their note and the corresponding place on the Hovmoller:

In the last week, weak low-level easterly wind anomalies have been evident across most of the Pacific.

“Weak low-level easterly wind anomalies” indicate the trade winds are slightly stronger than normal…not weaker than normal. And that indicates that the equatorial trade winds are not providing the feedbacks necessary to help evolve the El Niño. Note also that there have not been any additional westerly wind bursts since early April 2014, contradicting all of the proclamations from around the blogosphere that there have been additional wind bursts.

Unless there are additional westerly wind bursts and/or unless the trade winds weaken, it looks like this El Niño is going to disappear before it really got started.

EARLIER POSTS IN THIS SERIES

• The 2014/15 El Niño – Part 1 – The Initial Processes of the El Niño.
• The 2014/15 El Niño – Part 2 – The Alarmist Misinformation (BS) Begins
• The 2014/15 El Niño – Part 3 – Early Evolution – Comparison with 1982/83 & 1997/98 El Niño Events
• The 2014/15 El Niño – Part 4 – Early Evolution – Comparison with Other Satellite-Era El Niños
• The 2014/15 El Niño – Part 5 – The Relationship Between the PDO and ENSO
• The 2014/15 El Niño – Part 6 – What’s All The Hubbub About?…
• The 2014/15 El Niño – Part 7 – May 2014 Update and What Should Happen Next
• The 2014/15 El Niño – Part 8 – The Southern Oscillation Indices
• The 2014/15 El Niño – Part 9 – Kevin Trenberth is Looking Forward to Another “Big Jump”
• The 2014/15 El Niño – Part 10 – June 2014 Update – Still Waiting for the Feedbacks
• The 2014/15 El Niño – Part 11 – Is the El Niño Dying?
• The 2014/15 El Niño – Part 12 – July 2014 Update – The Feedbacks Need to Kick in Soon

And for additional introductory discussions of El Niño processes see:

• An Illustrated Introduction to the Basic Processes that Drive El Niño and La Niña Events
• El Niño and La Niña Basics: Introduction to the Pacific Trade Winds
• La Niñas Do NOT Suck Heat from the Atmosphere

ENSO Basics: Westerly Wind Bursts Initiate an El Niño