We’ve just seen a titanic Madden Julian oscillation episode in the West Pacific tropics and this regime shifted east across the equatorial East Pacific and into the North Atlantic tropics the past week or so. During that transition the trade winds in the tropical East Pacific eased and the ocean surface warmed slightly although still within the La Nina threshold. During La Nina the West Pacific tropics tend to be tropically active while the eastern Pacific tropics are void of significant convection. However, the developing La Nina for late 2017 will not be able to generate the usual convection west/clear skies east tropical Pacific regime due to the lack of a cool PDO regime to the north. The general record warmth of the global oceans is preventing that regime from forming. Therefore the Bureau of Meteorology/Australia correctly projects La Nina ahead as marginal.
Pictured: The 12-week trend of Nino SSTA indicates a choppy descent toward La Nina while the subsurface equatorial East Pacific has plenty of cool fuel to initiate/sustain La Nina. The Bureau of Meteorology/Australia forecasts a weak La Nina ahead.