Tuesday, July 22, 2014

Weak Upwelling, Hostile Environment Could Spell Weak El Nino Ahead

A weak episode of upwelling in the eastern Pacific, combined with a continued hostile environment with respect to El Nino formation, could indicate that a weak El Nino may be in store for the future, rather than previous projections of moderate and strong El Ninos.

Michael Ventrice
Shown above is what the concepts of upwelling and downwelling look like. Imagine, for a moment, the eastern Pacific is completely flat, no perturbations or disruptions in the surface or underwater currents. Imagine that the western portion of the water begins to rise. In the Pacific, Kelvin Waves can propagate from west to east along the Equator, bringing about a rise in sea surface temperatures and in the actual water height. As a result, we see water levels in the east drop ever so slightly, likely not even recognizable. As time progresses, in the Pacific, the Kelvin Wave will eventually push east, and the warm waters will push to the surface. As the third panel shows, this sort of motion is referred to as "upwelling", where the subsurface waters push to the surface. In response, a body of water nearby (in this case, the Central Pacific) will exhibit "downwelling" characteristics, where water temperatures will either warm or cool, the opposite temperature anomaly as the anomaly involved in the upwelling incident.

If that was confusing, don't worry, we can explain it better below.

The image above shows the anomalous depth of the 20ยบ Celsius isotherm below the surface in the eastern Pacific. In layman terms, positive anomalies on the chart above mean warmer than normal waters, while negative anomalies mean colder than normal waters. Check out how we've seen a series of cold and warm episodes across the Pacific in the last year. In October 2013, we saw cooler than normal water temperatures shift east with time (hence the slanting down and east with time (left legend) and direction (bottom legend)). In an interesting correlation of how upwelling episodes seem to determine the strength of the following downwelling episode, we saw warmer than normal waters follow quickly in its footsteps around November 2013. This was a classic example of upwelling and downwelling. We saw upwelling occur with the below normal waters in October, as cooler waters were brought from underwater to the surface, and the resultant downwelling episode occurred in November, when surface-originated waters were forced underwater and eastward. We saw an even stronger occurrence of this in January 2014, when upwelling occurred, and then our historic Kelvin Wave induced the upwelling in February and March 2014.

So, it would only be natural to expect an even stronger upwelling phase now, right? Wrong. Looking at that chart above, now that our Kelvin Wave has passed, we see barely any evidence of sustained upwelling. If we consider that the strength of the upwelling episodes (cool anomalies) could actually predict the strength of the downwelling episodes (warm anomalies), one might think that the upcoming downwelling episode may result in more of a weak El Nino than one of a stronger magnitude. Although the correlation discussed isn't exactly how the upwelling/downwelling episodes work, it's worth seeing if such a correlation might be even remotely successful at predicting the upcoming El Nino.

The El Nino has been difficult to come by. This most recent Kelvin Wave, expected to bring us that strong El Nino, couldn't hold its ground and ended up dissipating. Add to that the atmosphere never exhibited El Nino characteristics, and it's all-around bad luck. Some change is on the way, however.

Kyle MacRitchie
The image above shows the long range forecasted MJO phase from the CFS model, from Kyle MacRitchie. This forecast has the Madden Julian Oscillation moving into Phases 8, 1 and 2 by the early and middle parts of August. In simpler terms, this forecast suggests that we will see enhanced convection over the western portion of Oceania and around the Indian Ocean in August. Why is this important? When we see tropical convection in those areas, the atmosphere can respond by pushing this convection eastward into the open Pacific, possibly as far as into South America. If this convection can reach the Indian Ocean and around Oceania, it can create westerly winds from that area into the east Pacific, setting up a favorable environment for El Nino formation. If the convection can actually move into South America, the potential of El Nino-like conditions forming greatly rises. The gist of all of this is, the tables could be turning in favor of an El Nino, after a long time of suffering quite an uphill battle.

To summarize, the upwelling-downwelling pattern we discussed earlier tells us that only a weak El Nino could be in the cards down the road. However, after examining the projected placement of tropical convection in the Indian Ocean, El Nino formation could actually be favored, even if it's only favoring a weak or possibly moderate El Nino (I would place my bets on the former option, however).