Friday, May 1, 2015

Traversing the Anomalously Warm Pacific

Abnormally high Sea Surface Temperatures (SST) anomalies in the North Pacific have been implicated in the wild weather over the winter in the US midwest and northeast as well as the drought in California and highlighted in the press, recently.  The region of the North Pacific containing this warm water has been dubbed the "Blob" by Washington State Climatologist Nick Bond.  The region is more than 1000 square kilometers in size and has been warming since 2013.  The maximum anomaly occurred in January, 2014, but the warm SSTs are still around today.  These anomalies are being reported as 3-4 standard deviations above the mean - which is like expecting an anomalous event to occur less than once in a millennia!

There is speculation that the state of the ocean may be changing and we may be seeing the emergence of a new dominant mode of climate variability in the Pacific, the North Pacific Mode - look out El Nino, the "Blob" may become the new black.

In addition to the presence of the "Blob" in the North Pacific,  we are transitioning into a positive phase of the Interdecadal Pacific Oscillation (IPO), a positive phase of the Pacific Decadal Oscillation (PDO), as well as an El Nino state. The IPO is one of the least well understood climate oscillations as it has a long period of variability (~15-30 years), so it is difficult to observe frequently.   It differs from the PDO in that it also influences the SSTs in the South Pacific as well.  The last positive phase of the IPO occurred between 1978 and 1999, during which time one of the largest El Nino events was observed in 1997-1998. While the "Blob” currently makes up part of the pattern comprising the transition from positive phase of the PDO and the IPO, the origins of that transition will be a research subject for the scientific community in the coming years. We are excited to be sampling the water column during such a dynamic time.

P16N leg 1 cruise track between Tahiti and Hawaii mostly following 150 degrees W.
The current warm SST anomaly, as identified from satellite data (see below), crosses our path on P16N as we sample along 150 degrees W (see map of our cruise track above) between Tahiti and Hawaii.  We are sampling the surface through our underway system - the ship intakes water as we steam across the Pacific through the bow about five meters below the surface, and we are continuously measuring temperature and salinity on that water.  In addition, we are measuring temperatures on our CTD casts happening every station on that cruise track.   On those stations the temperature sensor obtains a profile from the surface all the way down to about 10 meters off the bottom (which on our track has been between 4500 and 5000 meters, for the most part). We are sampling a particularly high resolution section across the equator (meaning the distance between stations is smaller than it has been in the past) on this cruise.  This means that we are armed with multiple different ways to identify this anomalously warm water.
SST anomaly for April 28, 2015.

One of our CTD watchstanders, Annie Foppert has been analyzing that data on board, looking out for anomalously warm waters. So far, we have been in anomalously warm surface waters nearly our entire time around the equator.  The plot below shows the raw SST from the underway system during the cruise from 17 deg S through 2 degrees north of the equator. In addition, the climatological SSTs from NOAA are also plotted for reference.  A climatology is a long term average over time.  It gives us a reference point, or a mean state, for comparison to what is happening now.  Most people refer to the climatological state as "normal."  So if you wanted to know if the SSTs were warmer than normal - you would reference what you are measuring to the climatology and see if it was warmer or cooler.

SST from the underway system (the TSG) of the Ron Brown in grey and black dots from the surface CTD measurements at our stations between Tahiti (~17 S) and our current position north of the equator.  Two different climatologies for the longitudes specified in the legend are also shown.  The shading around the climatologies corresponds to a standard deviation around each climatology.  The equatorial region is much warmer than the climatology, which is consistent with the El Nino conditions. 

The warm anomaly - in that the water is much warmer than the climatology - can be clearly seen by comparing the climatology to the underway temperatures.  The underway temperature is indeed warmer than even the three standard deviations of variability around the climatology, for the period of time around the equator - from 2 degrees S until about our present position north of the equator.  We are only a few degrees north at the moment, but the SSTs are still climbing.

In addition, we are noticing the developing El Nino conditions along our equatorial transect.  The thermocline has deepened -when compared to our prior occupation in 2006.  Also, the mixed layer temperatures between 3 degrees S and 3 degrees N are 2.87 deg C warmer than 2006.  So the warm water is not isolated to the surface.

The warm waters have made an impression on our P16N scientists and crew in one way or another.  One of the night watch scientists pointed out how pleasant it made sampling from the niskins - "The warm water feels nice over my rubber gloves".   One of the crew even went so far as to describe it as "hot".  The rosette frame for the CTD and bottles was warm to the touch when it came out of the water and back on deck, such that both of the individuals recovering the rosette commented on its warmth upon touching it. Science you can feel - priceless.

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