Expansion of Low-Productivity Regions in Mid Ocean Gyres Tied to Global Warming

New research at PIFSC has confirmed that in all major, mid-ocean subtropical gyres, warming of surface waters is occurring and is accompanied by expansion of areas with low biological productivity.

The research was conducted in the Ecosystem and Oceanography Division (EOD) by PIFSC scientists Jeffrey Polovina and Evan Howell and Melanie Abecassis of the University of Hawaii Joint Institute for Marine and Atmospheric Research.

Building on earlier studies by NASA scientist Charles McClain and others, the EOD team determined that regions of low biological production within the world's subtropical gyres have been expanding in concert with rising sea surface temperatures. The phenomenon appears to varying degrees in subtropical gyres of the northern and southern hemispheres.

The conclusions were based on a statistical analysis of chlorophyll concentrations (specifically, chlorophyll-a) in surface waters as measured by the SeaWiFS color sensor on the Seastar satellite and mean sea surface temperature levels derived from in situ measurements and NOAA/AVHRR satellite data. Chlorophyll is a pigment produced during photosynthesis by phytoplankton in sunlit surface waters of the ocean, so its concentration is a measure of biological production at the base of the ocean food chain.

The central regions of mid-ocean subtropical gyres are characterized by low levels of phytoplankton production. Biological oceanographers define these "oligotrophic" regions — essentially open ocean deserts — as waters with chlorophyll concentrations of 0.07 milligrams per cubic meter or less. The PIFSC study showed that during the 9-year period 1998—2006, oligotrophic waters within subtropical gyres of the North Pacific, South Pacific, North Atlantic, and South Atlantic expanded. The low-chlorophyll regions increased at rates ranging from 0.8% per year in the South Atlantic to 4.3% per year in the North Atlantic. In the North Pacific subtropical gyre, the oligotrophic region grew by 2.2% per year, and expansion of low-chlorophyll surface waters was particularly evident in a wide band of the central Pacific east and west of the Hawaiian Archipelago.

In the North Pacific subtropical gyre (Panel a), most areas that were oligotrophic (i.e., had low levels of chlorophyll) in 1998/1999 also were oligotrophic in 2005/2006 (gray region). A few areas were oligotrophic in 1998/1998 but not in 2005/2006 (blue region). And many areas not oligotropic in 1998/1999 had become so by 2005/2006 (red region). Similar results were calculated for the North Atlantic gyre (Panel b) and the South Pacific gyre (Panel c).

The expansion of oligotrophic surface waters in the subtropical gyres appears to be directly linked to increasing temperatures. Warming of the gyres, whether induced by natural processes or human activities, strengthens vertical stratification of these open-ocean regions, reducing the likelihood that nutrient-rich deep waters will be transported into the surface layer where photosynthetic activity occurs. The expected result is lower rates of primary production in these waters, with cascading effects through higher levels of the food chain. Ocean models and other observations show warming and increased vertical stratification is occurring in all oceans, thus continued expansion of oligotrophic regions within the gyres is predicted.

Published earlier this month in the peer-reviewed journal Geophysical Research Letters, the PIFSC research has generated widespread interest in the media, with articles appearing on major news networks, in newspapers around the world and in ScienceNOW.