Why are methane levels over the Arctic Ocean high from October to March?

Water temperatures at different depth

Why are methane concentrations in the atmosphere over the Arctic Ocean so high from October through to March?

The image below, replotted by Leonid Yurganov from a study by Chepurin et al, shows sea water temperature at different depths in the Barents Sea.

Above image illustrates that, while Arctic sea water at the surface reaches its highest temperatures in the months from July to September, water at greater depth reaches its highest temperature in the months from October to March. Accordingly, huge amounts of methane are starting to get released from the Arctic Ocean's seafloor in October.

Surface temperatures in October

As the image below shows, temperature at 2 meters was below 0°C (32°F, i.e. the temperature at which water freezes) over most of the Arctic Ocean on October 26, 2014. The Arctic was over 6°F (3.34°C) warmer than average, and at places was up to 20°C (36°F) warmer than average.

  

Image from 'Ocean temperature rise'

At the same time, continents around the Arctic Ocean are frozen. Surface temperatures over the Arctic Ocean were higher than temperatures on land at the end of October, due to the enormous amounts of heat being transferred from the waters of the Arctic Ocean to the atmosphere. This was the result of ocean heat content, which in 2014 was the highest on record, especially in the Arctic Ocean, which also made that at that time of year the sea ice extent was still minimal in extent and especially in volume. 

Start of freezing period

In October, the Arctic Ocean typically freezes over, so less heat will from then on be able to escape to the atmosphere. Sealed off from the atmosphere by sea ice, greater mixing of heat in the water will occur down to the seafloor of the Arctic Ocean.

Less fresh water added to Arctic Ocean

The sea ice also seals the water of the Arctic Ocean off from precipitation, so no more fresh water will be added to the Arctic Ocean due to rain falling or snow melting on the water. In October, temperatures on land around the Arctic Ocean will have fallen below freezing point, so less fresh water will flow from glaciers and rivers into the Arctic Ocean. At that time of year, melting of sea ice has also stopped, so fresh water from melting sea ice is no longer added to the Arctic Ocean either. 

Rising salt content

As addition of fresh water ends, the salt content of the water in the Arctic Ocean starts to rise accordingly, while the Gulf Stream continues to push salty water into the Arctic Ocean. The higher salt content of the water makes it easier for ice to melt at the seafloor of the Arctic Ocean. Saltier water causes ice in cracks and passages in sediments at the seafloor of the Arctic Ocean to melt, allowing methane contained in the sediment to escape. 

Pingos and conduits. Hovland et al. (2006)

The image on the right, from a study by Hovland et al., shows that hydrates can exist at the end of conduits in the sediment, formed when methane did escape from such hydrates in the past. Heat can travel down such conduits relatively fast, warming up the hydrates and destabilizing them in the process, which can result in huge abrupt releases of methane.

Heat can penetrate cracks and conduits in the seafloor, destabilizing methane held in hydrates and in the form of free gas in the sediments.

Less hydroxyl in atmosphere

Besides heat, open water also transfers more moisture to the air. The greater presence of sea ice from October onward acts as a seal, making that less moisture will evaporate from the water. Less moisture evaporating, together with the change of seasons (i.e. less sunshine) results in lower hydroxyl levels in the atmosphere at the higher latitudes of the Northern Hemisphere, in turn resulting in less methane being broken down in the atmosphere over the Arctic.

Gulf Stream

Malcolm Light writes in this and this earlier posts that the volume transport of the Gulf Stream has increased by three times since the 1940's, due to the rising atmospheric pressure difference set up between the polluted, greenhouse gas rich air above North America and the marine Atlantic Air. 

The increasingly heated Gulf Stream with its associated high winds and energy rich weather systems then flows NE to Europe where it is increasingly pummeling Great Britain with catastrophic storms, as also described in this earlier post, which adds that faster winds means more water evaporation, and warmer air holds more water vapor, so this can result in huge rainstorms that can rapidly devastate the integrity of the ice. The image below further illustrates the danger of strong winds over the North Atlantic reaching the Arctic.

Branches of the Gulf Stream then enter the Arctic and disassociate the subsea Arctic methane hydrate seals on subsea and deep high - pressure mantle methane reservoirs below the Eurasian Basin - Laptev Sea transition. This is releasing increasing amounts of methane into the atmosphere where they contribute to anomalously high local temperatures, greater than 20°C above average.

From: The Biggest Story of 2013

Emissions from North America are - due to the Coriolis effect - moving over areas off the North American coast in the path of the Gulf Stream (see animation on the right).

The Gulf Stream reaches its maximum temperatures off the North American coast in July. It can take almost four months for this heat to travel along the Gulf Coast and reach the Arctic Ocean, i.e. water warmed up off Florida in early July may only reach waters beyond Svalbard by the end of October.

Waters close to Svalbard reached temperatures as high as 63.5°F (17.5°C) on September 1, 2014 (green circle). The image below shows sea surface temperatures only - at greater depths (say about 300 m), the Gulf Stream can push even warmer water through the Greenland Sea than temperatures at the sea surface.

Since the passage west of Svalbard is rather shallow, a lot of this very warm water comes to the surface at that spot, resulting in an anomaly of 11.9°C. The high sea surface temperatures west of Svalbard thus show that the Gulf Stream can carry very warm water (warmer than 17°C) at greater depths and is pushing this underneath the sea ice north of Svalbard.

from: Causes of high methane levels over Arctic Ocean 

Through to March the following year, salty and warm water (i.e. warmer than water that is present in the Arctic Ocean) will continue to be carried by the Gulf Stream into the Arctic Ocean, while the sea ice will keep the water sealed off from the atmosphere, so little heat and moisture will be able to be transferred to the atmosphere. 

Start of melting period

This situation continues until March, when the sea ice starts to retreat and more hydroxyl starts getting produced in the atmosphere. Increased sea ice melt and glaciers melt, the latter resulting in warmer water flowing into the Arctic Ocean from rivers, will cause salinity levels in the Arctic Ocean to fall, in turn causing methane levels to fall in the atmosphere over the Arctic Ocean. Furthermore, the water traveling along the Gulf Stream and arriving in the Arctic Ocean in March will be relatively cold.  

References


- Chepurin, G.A., and J.A. Carton, 2012: Sub-arctic and Arctic sea surface temperature and its relation to ocean heat content 1982-2010, J. Geophys. Res.-Oceans., 117, C06019, DOI: 10.1029/2011JC007770. http://onlinelibrary.wiley.com/doi/10.1029/2011JC007770/abstract 

- Combination image created by Sam Carana with Climate Reanalyzer, from: Temperature Rise, http://arctic-news.blogspot.com/2014/10/ocean-temperature-rise.html 

- Submarine pingoes: Indicators of shallow gas hydrates in a pockmark at Nyegga, Norwegian Sea, by Martin Hovland and Henrik Svensen (2006) http://www.sciencedirect.com/science/article/pii/S0025322705003968

- Sea surface temperature west of Svalbard,
created by Sam Carana with
http://earth.nullschool.net

Post by Sam Carana.