26 May 2014 – by Lead Forecaster Miles Schumacher
In the past two weeks, the Madden Julian Oscillation (MJO) weakened considerably allowing the stronger El Niño, and warmer temperatures, to spread across much of the U.S. The breakdown of the MJO was expected, based on the charts shown in my last discussion from 12 May 2014 “High Latitude Blocking Continues to Dominate”. The MJO is presently very weak and is not influencing temperature changes. As discussed in my prior paper, the MJO, combined with high latitude blocking, allowed a strong surge of cold air to penetrate into the U.S. However, the MJO is weaker at present, as is the high latitude blocking, therefore neither will have much of an effect on the weather pattern into early June. MJO will become stronger as it moves through Octant 3 and brings a surge of cooler weather around the second week of June. As the MJO progresses east into octants 4, 5 and 6, at about one sigma in strength, and combines with warming water from El Niño, the last part of the month will likely be warmer than normal.
The MJO collapsed into the central circle during the past two weeks, as shown on the Hendon-Wheel chart on page two, meaning it has little influence on atmospheric circulation at present. Moderate convection associated with MJO has shifted into the Atlantic Ocean, Africa, and is spreading into the Indian Ocean as seen on satellite imagery (not shown here) from 25 May 2014. This position is consistent with what would be expected with octants 1 and 2. The MJO is expected to remain relatively weak for the next seven days with little overall movement. After that time, it is expected to strengthen slowly and begin to progress east. Composite charts below the Hendon-Wheeler chart show, statistically, the effect MJO typically has on June temperatures. They show cooler than normal temperatures over much of the east half of the U.S. when MJO is in octants 1 and 2, with a stronger signal for cooler than normal temperatures for octant 3. The Hendon-Wheeler chart shows a plot of the location and strength of the MJO over the past 30 days. The counterclockwise progression is evident from octant 7 initially to octant 1 at present. The red part of the line indicates progression from late April through late May. The green line on the chart is the position of the MJO, forecast from the European Center for Medium range Weather Forecasting (ECMWF), for the next four weeks. The yellow shows the output from each of the 51 individual ensemble members. The MJO remains weaker than one standard deviation in strength for the next two to two and one half weeks, with strengthening indicated as it progresses into octant 3, and then maintaining moderate strength in octants 4, 5 and 6.
The developing El Niño, discussed in my prior paper, continued to strengthen as the warm pool of water caused by the progression of a Kelvin Wave began reaching the surface. On page three is a graphic of the equatorial Pacific from 130º east to 100º west showing temperature anomalies in the top 450 meters of the ocean. The progression has reached the east Pacific. Warming off the South American coast is well underway. Recall that until recently, the SST departure maps have shown an overall cool Pacific. As was expected, the SST has risen and will likely to rise through the summer, signifying the onset of an El Niño.
Below is the map showing the SST anomalies from 24 May 2014. The area along the equatorial Pacific is showing above normal temperatures along much of the region. The area of warmer than normal SST has increased off of South America and south of Baja California. The warming suggests the onset of El Niño is taking place. Given the trends, my conclusion is that El Niño will develop over the next few months.
As discussed, there are two factors that oppose each other, the MJO and the warm water off of South America. The MJO moving through octants 1, 2 and into 3 suggest a cooler than normal weather pattern across much of the U.S. At the same time, the relatively warm water off of South America in the equatorial Pacific favors warmer than normal temperatures for a large part of the U.S. The warming of the east Pacific is more dominant than the MJO at present. The result is the development of a weak mean ridge in the upper atmosphere over the central U.S. Warmer temperatures have prevailed under this ridge.
The associated trough to the west has produced a southwest flow into the severe drought areas of the southwest U.S. with the first significant rainfall of the year in many areas. The MJO will be in sync with the Pacific SST pattern later in June and is expected to deliver a period of above normal temperature to most of the U.S. with cold air penetration limited to the far north.
In conclusion for the month of June, MJO is weak now, so the temperature pattern will be dominated by the warm waters of the developing El Niño from the end of May into early June. MJO is likely to have a stronger influence by the second week of June with a cooler period of about a week. As MJO moves into octants more favorable for above normal temperatures and El Niño continues to develop, above normal temperatures are expected to round out the month.
For July and August, my prediction is that El Niño will be the more dominant force, which would make temperature and precipitation patterns quite different from the past two summers. The past two summers featured excessive heat and drought through the central U.S. with many locations near record heat and among the driest summers on record. A discussion of that will appear in my next paper.