Wednesday, September 3, 2014

What a long, strange trip it's been

by Kim Cobb

Today we are on our way to the middle of the Pacific Ocean, where we will install a large number of temperature and salinity sensors around Christmas Island (2°N, 157°W), in the hopes of capturing the dramatic environmental shifts associated with the strong El Niño event this coming winter. But will it actually materialize?

Many months have passed since my blog post about the potential for a very strong El Niño event this winter. Two months later I sealed my fate by tweeting the following:  “Going on record for a strong El Niño this winter, amplitude of 2.5+/- 0.3°C as measured by NIÑO3 SST. Anybody else in? #nogutsnoglory” In late spring, the Atmospheric and Geospace Science and Biology divisions at NSF funded myself and four other PIs to gather a suite of physical oceanographic and ecological data from Christmas and Palmyra Islands. These remote coral atolls lie near the equator in the central tropical Pacific, in the very heart of El Niño action. In doing so, NSF took a fairly big gamble that the climate models forecasting a significant El Niño for this winter were accurate. They hoped that our group would document the impacts, both physical and biological, of a large El Niño event in unprecedented detail. I refer readers to our inaugural post on the project to read about our research goals and their broader relevance.

Months of planning and many tens of thousands of dollars later, our entire science team is now deployed on the equator for the first of three large, coordinated expeditions. In our “divide and conquer” strategy, Mark Merrifield’s team is currently on Palmyra to install moored arrays of sensors that will log temperature, salinity, and currents with incredible accuracy and precision. Samantha Stevenson (a climate modeler turned field buff for the month) is setting up the seawater and rainwater collection program at Palmyra, so that we can monitor the El Niño’s effects on the atmospheric and oceanic distributions of oxygen isotopes. This isotope system forms the basis of our reconstructions of paleo-El Niño events in corals from these sites. Meteorological stations on both islands will log a suite of atmospheric variables, including winds, rainfall, and pressure. On Christmas Island, we will join Julia Baum’s team of reef ecologists in surveying up to a dozen sites that she’s been monitoring for the last 5 years. We are excited to learn their elaborate sampling protocols, even if it means signing on to their 4-dives-a-day, 9-hr boat operations. And we thought fossil coral scavenging on sun-baked beaches was hard work…

So here’s the million dollar question:  Will there be a strong El Niño event this winter? Note that most climatemodels do forecast a moderate El Niño event. But we are really banking on (quite literally) The Big One. Admittedly, much of the wind has gone out of the El Nino sails this summer, when favorable conditions in mid-June (see Figure 1) gave way to a near death experience by late July. However, a modest westerly wind burst in mid-August has breathed new life into the tropical Pacific, with a prominent depression of the thermocline making its way eastward. The only remaining hope for a strong El Niño event rests on the subsequent surface warming that will appear in the next weeks in the eastern equatorial Pacific, and a concomitant relaxation of the trade winds. Whether the trades weaken enough to sustain the depression of the thermocline and associated surface warming is unknown. I will say that the persistent warm SST anomalies near the dateline throughout the spring and summer (see Figure 1) have not helped, as they weaken the atmospheric punch of the intermittent surface warmings in the eastern equatorial Pacific. Upon waking today, however, I noted that anomalies of >1°C are now visible across a broad swathe of the eastern equatorial Pacific. So there is still a chance for a major event to take shape, but that window is rapidly closing. These next few weeks are critical.

Maps of 5-day averaged SST and wind anomalies across the equatorial Pacific, ending in the date specified. The region of westerly wind burst activity is indicated in the red box. "Favorable" winds blow from the west towards the east, and drive equatorial warming in the eastern Pacific in the next months, furthering the development of El Niño conditions. The blue 'X' marks the location of Christmas and Palmyra Islands. The top plot shows average SST and winds for the 5-day period ending on June 15, 2014, for reference. Plots from
For the purposes of our research at Christmas and Palmyra, the ~0.5-1°C warming that has characterized the last 6 months at these sites may have already had significant impacts on the coral reef ecosystem.  So in terms of El Niño impacts, we might be perfectly placed to capture the peak of what might be a relatively strong ‘central Pacific El Niño’. In that sense, our research would be an important contribution to ecological and climate science (and specifically, their intersection), as many models project a shift towards such events in coming decades.

Stay tuned for updates from the field over the next two weeks!

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