Northwest Atlantic Ocean may get warmer, sooner

Northwest Atlantic Ocean may get warmer, sooner
Northwest Atlantic upper-ocean (0-300 meters) and bottom temperature change (figure below) after a doubling of global atmospheric carbon dioxide (CO2) among the four GFDL climate models of varying resolution. Credit: Saba et al, NOAA

A new study by NOAA researchers suggests future warming of ocean waters off the Northeastern U.S. may be greater and occur at an even faster rate than previously projected.

Their findings, based on output from four global climate models of varying and atmospheric , indicate that ocean temperature in the U.S. Northeast Shelf is projected to warm twice as fast as previously projected and almost three times faster than the global average. The models were developed at NOAA's Geophysical Fluid Dynamics Laboratory (GFDL) in Princeton, New Jersey.

"We looked at four GFDL models and compared their output to ocean observations in the region. The highest resolution GFDL model, CM2.6, matched the Northwest Atlantic circulation and water mass distribution most accurately," said Vincent Saba, a NOAA fisheries scientist and lead author of the study. "Prior projections for the region may be far too conservative."

Over the past ten years, the Gulf of Maine has warmed faster than 99% of the global ocean. Recent studies indicate that the enhanced warming is associated with a northerly shift in the Gulf Stream. Changes in the distribution and species composition are already evident, but existing climate change projections are based on warming scenarios from coarse resolution models. Warming of 3 to 4 degrees C (as much as 5.4 to 7.2 degrees F), projected by NOAA GFDL's CM2.6, will likely cause more extreme effects on the ecosystem.

Northwest Atlantic Ocean may get warmer, sooner
Bottom temperature change among the four models. Credit: Saba et al, NOAA

Global climate models used to project global and regional climate change generally have coarse ocean and atmospheric resolution. The higher resolution model better reflects the ocean circulation and sea floor bathymetry in smaller, complex areas like the Gulf of Maine and the U.S. Northeast Shelf. According to the study, the models project that ocean warming will be even more pronounced than suggested by coarser models under increasing concentrations of atmospheric CO2.

The study appears in the Journal of Geophysical Research - Oceans, published by the American Geophysical Union.

The global climate models assessed by the Intergovernmental Panel on Climate Change (IPCC), which are used to project global and regional climate change, are coarse resolution models based on a roughly 100-kilometer or 62-mile grid, to simulate ocean and atmospheric dynamics. NOAA GFDL's CM2.6 offers ten times more resolution by using a roughly 10-kilometer or 6.2-mile ocean grid.

Animation of Northwest Atlantic Ocean and Shelf monthly salinity/temperature change under an atmospheric CO2 doubling scenario from GFDL CM2.6. Monthly salinity and temperature from the model are averaged between 150-200 meters (roughly 500 - 650 feet) depth to represent Slope Water intrusions into the Northeast Channel. Ocean depths greater than 150 meters are shown. CM2.6 monthly salinity and temperature are animated under an 80-year run of atmospheric CO2 increasing 1% per year (at bottom) such that it doubles at year 70 and continues increasing by 1% per year until year 80.

"It is like comparing an old standard definition television screen to today's ultra high definition screens," said Saba, a member of the Northeast Fisheries Science Center's Ecosystem Assessment Program who works at GFDL. "There aren't many high resolution global climate models available due to their prohibitive cost. For much of the global ocean the coarser resolution is okay, but when you are studying a unique location like the Gulf of Maine, with its complex bathymetry of deep basins, channels, and shallow banks combined with its location near the intersection of two major ocean current systems, the output from the coarser models can be misleading."

A warm bias in sea surface temperature in most is due to a misrepresentation of the coastal separation position of the Gulf Stream, which extends too far north of Cape Hatteras, North Carolina. The model bias, known as the "Gulf Stream separation problem," is a result of the models' coarse resolution. As a consequence of that bias, existing climate change projections for the Northeast U.S. Shelf and the Gulf of Maine are based on unrealistic regional ocean circulation patterns. NOAA GFDL's highest resolution model, CM2.6, significantly reduces that bias.

The study also found that the warming of the upper 300 meters (roughly 1,000 feet) of the Northwest Atlantic increases salinity due to a change in water mass distribution related to a retreat of the colder, fresher Labrador Current and a northerly shift of the warmer, saltier Gulf Stream. Observations and the high-resolution climate model CM2.6 show a strong relationship between a weakening Atlantic Meridional Overturning Circulation (AMOC) and an increase in the proportion of warm-temperate slope water entering the U.S. Northeast Continental Shelf, primarily through the Gulf of Maine's Northeast Channel.

"These results show the need to improve simulations of basin and regional-scale ocean circulation," said Saba, who will use the CM2.6 findings for a variety of studies on living marine resources in the ecosystem. In addition to Saba and Jonathan Hare from NOAA Fisheries' Northeast Fisheries Science Center, other study authors are affiliated with NOAA's Geophysical Fluid Dynamics Laboratory and the Earth System Research Laboratory in Boulder, Colorado.

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More information: Journal of Geophysical Research - Oceans, … 02/2015JC011346/full
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Jan 15, 2016
A kinder gentler admission that previous models were - wrong - again. I would like to hear more about what forces have shifted the Gulf Stream's impact points on the Gulf of Maine channel? Is this the result of long cycle gyre oscillations further south - which could have little to do with warming? Also, if the miscalculation problem was with the older coarser (erroneous) model, then the actual warming data should have shown that almost immediately (clearly we are focusing too much on the models more than real data) and consequently, any warming won't be sooner, just the new model showing the warming earlier than the previous erroneous models failed to show. Once again we see why that the 26 major climate models are not actually in agreement and that we have a lot more to learn about climate changes and what drives them locally and globally.

Jan 15, 2016
I would like to hear more about what forces have shifted the Gulf Stream's impact points on the Gulf of Maine channel

According to other studies I've seen, the shutdown (or vast slowing) of the North Atlantic Oscillation would cause this same effect as well as raising the water levels on the coast. Europe also cools down in this scenario.

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