Latest Ocean Data Tilts More In Favor Of La Nina
By Drew Lerner
Kansas City, March 28 (World Weather Inc.) – The latest data from the Pacific Ocean shows a rapid decline in the depth of warm water associated with El Nino in the past few weeks and a nearly constant, but broad region of cooler than usual ocean water lingering a little deeper in the ocean. As 
noted in our last ENSO update, the month of March and April were expected to show the greatest decline in El Nino conditions and that is still a true statement. The recent progress in the ocean temperature changes is shedding more light on the prospects for La Nina and for the first time World Weather, Inc. sees a large enough body of cold water lurking to the west and down deep into the central Pacific Ocean to not only eliminate El Nino, but to possibly cool equatorial Pacific Ocean temperatures below average later this year which is needed to induce La Nina.
El Nino remains in place today and will likely continue to prevail for another few weeks. The changes we are noting today are really nothing new to what has been expected in the past except for the fact that the area of colder than normal ocean water that lurks below the warm surface water is now larger in size than the warmer than normal water region. Recent past data has kept these two anomalous water temperature regions mostly well balanced so that there was just enough cool water to neutralize El Nino, but not enough to induce La Nina. Today’s data suggests that there is now enough cool water to neutralize El Nino and possibly cool 
the surface of the ocean below average to possibly induce La Nina.
The question remaining is when will the transition to La Nina take place? Today’s data suggests sooner than we (World Weather, Inc.) were expecting and if this trend continues over the next few weeks El Nino might dissipate and the evolution toward La Nina would begin late in the second calendar quarter and more likely in the third quarter. Originally we believed the transition to La Nina would come a little later in the third quarter, but that “may” not be the case.
Of course, the implications of this change raises concern about the already anomalously dry and warm region that is expected in the central and southern Plains and western Corn Belt this summer. Translating into La Nina during the summer growing season would reinforce the already warmer and drier biased environment that is expected this summer in the central and southern Plains and portions of the western Corn Belt. Enhancing the ridge of high pressure expected in the Plains might send warmer than usual weather into the northern Plains, upper Midwest, eastern Canada Prairies and greater amounts of the central Midwest.
The charts above are subsurface ocean temperature anomalies. Each rectangular box is a cross-sectional snapshot in time of ocean temperature anomalies across the equatorial Pacific Ocean. The top of each box is the surface of the ocean and the bottom is 300 meters below the surface. The far right hand side is where the coast of South America and Central America would be if they were shown and the lower left corner of each box is where Australia is located. The heavy vertical line going through each box is the International Dateline. Blue colors represent colder than usual sub-surface ocean water and yellow, orange and red are differing degrees of warmer than usual ocean temperatures. The blue circle drawn on each box is only there for a reference so that it can be determined if the sub-surface ocean water in the eastern equatorial Pacific Ocean is trending cooler.
There are several things to note in these images on page 2. First, they are time stamped with the image on the top left from February 2 and each box shows the change in ocean temperatures at roughly 2-week intervals except the last box in the lower right of the graphic where it is just five days later than the fourth box on the left.
There are two important ocean currents to note. One is a typical west to east flow of water that moves sub-surface water from the western Pacific into the eastern Pacific Ocean. The second is an upwelling current that brings water in the far eastern Pacific Ocean from the bottom of the ocean to the top. Notice first that cooler water has been evolving at the bottom of the ocean in the far eastern Pacific Ocean – as shown in a very light blue color. This new region of cooler water deep into the ocean will eventually be lifted upward toward the surface of the ocean if the cooler bias is sustainable.
In the meantime, the broad region of cooler sub-surface ocean temperatures in the western and central Pacific Ocean is moving from the west to east. Usually, once the cooler water gets east of the International Dateline it will continue moving eastward. However, sometimes the anomalously cool 
water to the west of the Dateline will stay there for an extended period of time maybe allow small pools of cool water to break away and move into the eastern Pacific Ocean from time to time. In the past, La Nina events have occurred when a large portion or perhaps all of the cooler ocean water to the west shifts east. There is no assurance that all or just a part of the cool water will move east.
Ocean the cooler water shifts far enough to the east of the International Dateline it will be influence by the upwelling current in the far eastern Pacific Ocean and that will start lifting the colder than usual water toward the surface of the ocean. The greater the volume and intensity of the colder than usual water that moves east of the International Dateline the more significance it will have on the warm surface ocean temperatures that remain near the ocean surface. In other words if a larger enough pool of cool weather shifts east of the Dateline it will lift to the surface and dissipate El Nino and possibly leave behind a cooler bias in surface temperatures. That needs to happen before any La Nina event can evolve.
Notice on these charts how deep the warm ocean water was in the eastern equatorial Pacific Ocean on February 2. It expended down more than 150 meters and in some areas down more than 200 meters. But that same region of ocean now has warm water confined to between 25 and 30 meters. That is a substantial change in just six weeks and the greatest change in the depth of warm water has actually occurred in the past three to four weeks.
If the current trend continues much longer the El Nino event may experience a significant weakening trend over the next few weeks and could dissipate much sooner than expected. However, what is most likely 
to happen is that cooling will occur over the eastern equatorial Pacific Ocean more significantly than in central parts of the ocean and there may be a continuation of weak Modoki El Nino conditions for a period of a few weeks preventing a La Nina from evolving for a while longer.
As a review, Modoki El Nino events occur when there is a warmer-than-usual pool of ocean surface temperatures in the central Pacific while cooler-biased conditions continue to the east and west of the warm pool. Modoki El Nino would maintain a drier than usual bias in the Philippines and parts of both California and the southwestern United States for a while longer if it evolves.
So, with the latest data, the debate will be over whether this El Nino will experience continued aggressive weakening raising the potential for an earlier than usual demise or whether only the easternmost Pacific Ocean will experience the cool off leaving water in the central Pacific Ocean warmer than usual creating Modoki El Nino conditions. World Weather, Inc. believes that the Modoki solution is most likely, but only for a few weeks because of the lingering pool of cold water that remains extensive across the subsurface Pacific Ocean Basin will continue to expand to the east and be lifted upward eventually squelching the Modoki El Nino, but a little later in the spring or summer.
World Weather, Inc. believes the changes taking place are extremely important and if La Nina does evolve during the growing season it will take the already expected heat and dryness that will occur from Nebraska to Texas and into neighboring areas of the western Corn Belt and western Delta and expand it northward in Manitoba and slightly further to the east. It is too soon to make this official change in the forecast, but if there is more evidence of La Nina evolving during the growing season we will shift our outlook to a drier and warmer one over a larger part of the Plains, western two thirds of the Midwest and eastern Canada’s Prairies.
A very close watch on ocean temperature changes is imperative if we are going to get any advanced notice about worsening dryness in the middle of the nation. As noted before, the forecast is already favoring summer heat and dryness in the central and southern Plains with some neighboring areas in the western Corn Belt and western Delta to be impacted, as well. But for a more threatening broad-scale heat and dryness problem that would be a greater threat to eastern Canada’s Prairies and the heart of the Midwest La Nina is absolutely necessary and it is still not completely clear when La Nina will evolve. The sooner La Nina does evolve the greater the impact on U.S. and Canada crops. The longer it takes to evolve the more likely dryness problems in the U.S. this summer will be more regionalized and not broad based.
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