Questions pertaining to "The Oceanography of Jarvis Island"

We received a question by Reille related to the blog post entitled "The Oceanography of Jarvis Island" written by Jamison Gove on 3-April-2010.

Response by Jamison Gove, Oceanographer and Chief Scientist of the current expedition

Great questions Riell! I'll do my best to answer them appropriately, but if you would like more detail on the oceanographic conditions at Jarvis Island, see Gove et al (2006) Temporal variability of current-driven upwelling at Jarvis Island

Question 1: You mention that "few places on the planet have the oceanographic and coral reef environment that is found at Jarvis" could you tell me what other places in the Pacific have both the oceanographic features and the high-productivity coral reef that Jarvis does?

Due to the remote nature of the central equatorial Pacific, I imagine there may be a few islands that have similar ecosystem dynamics as those observed at Jarvis; however, it is the particular location and shape of Jarvis Island which facilitates its oceanographic and biological uniqueness, and when combined with limited human presence over the past half-century, it remains a rarity. 

Question 2: I guess something similar happens around the Galapagos and that is a result of the Cromwell Current, as well, but how does the situation there compare to the oceanographic conditions at Jarvis?

They two island ecosystems are comparable as the Equatorial Undercurrent (a.k.a. Cromwell Current) fuels the high productivity at both the Galapagos and Jarvis.  That being said, fundamentally different physical oceanographic dynamics occur between the two ecosystems.  At the latitude of Jarvis Island, the EUC is flowing incredibly fast for an open ocean current (~1 meter/second) at a depth of 100-150 meters.   When this fast moving, subsurface current interacts with Jarvis it results in a cessation of flow, and due to pressure differences, isotherms (lines of equal temperature) are forced vertically upward to the near surface.  This island-current interaction driving upwelling is a result of Bernoulli dynamics, which happens to be the very same physical mechanism which gives airplane wings lift. 

Due to the upward tilt of the EUC and the thermocline from west to east across the Pacific (see figure below), the EUC is near the surface (0 – 50 meters) at the latitude of the Galapagos Islands.  As such, the Galapagos are surrounded by nutrient-rich waters.  The productivity at the Galapagos is also enhanced (and therefore my explanation confounded) by natural iron input associated with the geological make-up of the Galapagos Islands, but that’s another question best left for another time.

 Side view of the equatorial Pacific showing the Equatorial Undercurrent flowing east along the thermocline.  Note the upward tilt of the EUC from the western to the eastern Pacific.  Colors indicate relative temperature, with warmer temperatures shown in red and cooler temperatures in blue.  Figure modified from http://www.pmel.noaa.gov/tao/

Question 3: I wonder -- does the size of the island result in differences between one side and the other; does the upwelling affect all sides equally?

The upwelling at Jarvis only occurs to the western side of the island, principally due to the fact that the EUC is an eastward flowing current.  Surprisingly, there can be a 1-3 ºC difference between the western side of the island and the eastern side (see figure below).  Given that Jarvis is only 4 x 2 kilometers, this is a rather substantial gradient in temperature over a very short distance.

Temperature at 25 meters depth around Jarvis obtained from near shore conductivity, temperature, and depth (CTD) casts (locations indicated by triangles).  Note the 2.5 degree Celsius difference between the western side and all other sides of the island.  Figure taken from Gove et al., 2006.

Question 4: Also, could you briefly describe what kind of seasonal variation you see, or variation in El Niño years, and whether you've yet observed organism behavioral adaptations in relation to any variation?

There is definitely seasonal and interannual variability in upwelling at Jarvis.  Seasonally, the strongest upwelling at Jarvis occurs during northern hemisphere spring, due to a locally shallow thermocline and shallow and strong EUC.  Year to year differences in upwelling are driven by the strength of the trade winds in the western Pacific and their impacts on flow of the EUC; intensified trade winds associated with La Niña conditions favor the shoaling and strengthening of the EUC at Jarvis, and therefore strong upwelling, while a weakening of the trade winds results in a slackening and deepening of the EUC, diminishing or all together shutting down upwelling.  Presumably, this variability would impact local fish and benthic coral reef communities; however, we have yet to analyze the data collected during the current El Niño to confirm this statement