Oceanography of the Beaufort Gyre: state and problems

A. Proshutinsky, Woods Hole Oceanographic Institution

Science and Education Opportunities for an Arctic Cabled Seafloor Observatory

An NSF-Supported Community Meeting, Barrow, Alaska 7 – 8 February, 2005

• Oceanographic conditions of the Beaufort Gyre (BG) are regulated by the BG system (atmosphere, sea ice, and ocean) mechanisms and interactions and will be discussed in the context of the entire Beaufort Gyre system variability. • The major goal of this talk is to show how a long-term observational program specifically designed for the Barrow Cabled Observatory (BCO) will contribute to our understanding and prediction of state and variability of the Beaufort Gyre (BG) system, its regulating mechanisms, and impact on Arctic climate.

Beaufort Gyre region:Beaufort Gyre region:

Aagaard and Carmack,1994.

BG

BG in the Arctic climate system

Arctic Ocean: vertical stratification

Greenland

Kara SeaLaptev Sea

Barents Sea

Baffin Bay

Siberia

Alaska

1

23

1 – Beaufort Gyre

2 – Transpolar Drift

3 – West Greenland current

Toporkov, 1970.

Barrow and Barrow Canyon

Atmosphere

Sea Ice

Ocean Mixed Layer

Pacific Halocline

Atlantic Layer

Deep Waters

Coupling Diagram of the Beaufort Gyre System:Coupling Diagram of the Beaufort Gyre System:

Each component of the system stores and exchanges mass and energy differently during different climate regimes. Quantifying and describing the state and variability of these components and their coupling is essential to understand the state and fate of present day Arctic climate.

SOURCES OF INFORMATIONSOURCES OF INFORMATION:

1. Environmental Working Group (EWG) Atlas of the Arctic Ocean, 1997,1998 (water temperature and salinity for 1950s, 1960s, 1970s, 1980s)

2. 1990-present hydrographic surveys in the Beaufort Sea (submarines, icebreakers, buoys, airborne expeditions, drifting stations)

3. International Arctic Buoy Program (IABP): (sea level pressure, 2-m air temperature, ice drift vectors for 1979-present)

4. NCAR/NCEP reanalysis project (6-hourly SLP and SAT, 1948 - present)

5. Satellite based sea ice concentration, drift, surface temperature and other products (1978-present)

6. Atlases and reference books

• Atmospheric system:

• Atmospheric system of the BG is regulated by the Arctic Oscillation processes. The origin of these processes is debatable and is beyond our discussion here. In normal oscillating arctic climate conditions the atmospheric part of the BG is responsible for:

• Forcing dynamics of anticyclonic and cyclonic circulation regimes (dynamics of AO).

• Establishing positive anomalies of air temperature during high AO and negative anomalies during low AO.

• Producing positive anomalies of precipitation during high AO and negative during low AO.

• Variability of other atmospheric parameters (cloudiness, solar radiation, humidity, wind speed) that change from regime to regime accordingly.

Characteristics of the Beaufort Gyre Climate SystemCharacteristics of the Beaufort Gyre Climate System

Over the Beaufort Gyre, large-scale atmospheric circulation changes from season to season and alternates between cyclonic (summer) and anticyclonic circulation (winter conditions). High atmospheric pressure prevails over the Beaufort Gyre in winter and low pressure dominates in summer

ATMOSPHERE and ICE DRIFTATMOSPHERE and ICE DRIFT

A. Winter SLP and wind B. Summer SLP and wind

C. Winter buoy drift D. Summer buoy drift

Seasonal variability of SLP:Seasonal variability of SLP:

Solid – Anticyclonic circulation regime

Dotted – Cyclonic circulation regime

Seasonal variability of surface winds:Seasonal variability of surface winds:

Solid – Anticyclonic circulation regimeDotted – Cyclonic circulation regime

Figure shows that the sea ice drifts anticyclonically in both winter and summer.

This is because sea ice is driven by winds and ocean currents and in the annual ice drift, the ocean currents dominate wind-driven circulation.

ATMOSPHERE and ICE DRIFTATMOSPHERE and ICE DRIFT

A. Winter SLP and wind B. Summer SLP and wind

C. Winter buoy drift D. Summer buoy drift

Hydrographic station locations (blue dots) in the 1950s and 1960sHydrographic station locations (blue dots) in the 1950s and 1960s

Hydrographic station locations (blue dots) in the 1970s and 1980sHydrographic station locations (blue dots) in the 1970s and 1980s

1950s 1960s

1970s 1980s

WATER TEMPERATURE: 5 metersWATER TEMPERATURE: 5 meters

Source: EWG, 1997,1998

1950s 1960s

1970s 1980s

WATER TEMPERATURE: 250MWATER TEMPERATURE: 250M

Source: EWG, 1997,1998

WATER TEMPERATURE: 500MWATER TEMPERATURE: 500M

1950s 1960s

1970s 1980s

Atlantic water with temperatures higher than 0 C occupies water layer from 300-400 to ~1,000-1,500m in the Canadian Basin

Source: EWG Atlas, 1997,1998.

WATER SALINITY: 5 MWATER SALINITY: 5 M

1950s 1960s

1970s 1980s

Arctic surface waters occupy 30-50 meter layer with water temperatures at freezing point and relatively low salinities

WATER SALINITY: 150 MWATER SALINITY: 150 M

Salinity distribution in the upper 200-meter layer

GreenlandBarents Sea

Kara

Sea

East-Siberian

SeaAlaska Chukchi

Sea

Laptev Sea

Beaufort Sea

Beaufort Gyre

Top:

Left: water salinity (S) at 10 m

Right: Salinity section

Bottom:

Left: water salinity (S) at 100 m

Right: Dynamic topography

Data source:

EWG Atlas, 1997, 1998

Beaufort Gyre mechanism of fresh water accumulation and release

Ice Convergenceor divergence

Fresh water accumulation

or release

Wind cyclonic oranticyclonic

Beaufort Gyre

Ice and water convergence,Fresh water accumulation due to Ekman pumping and sea ice accumulation due to ridging and cooling

Downwelling in the center and upwelling along continental slope

From Proshutinsky and Johnson, 1997

Parameter Anomaly

ACCR CCR

Atmospheric vorticity over polar cap N P

SLP over the Beaufort Gyre P N

Surface wind circulation A C

Surface wind speed P N

Cloudiness N P

Precipitation N P

Air temperature N P

Sea ice extent P N

Sea ice thickness P N

Sea ice drift A C

Duration of ice melt N P

Ocean surface circulation A C

Ocean surface water temperature N P

Ocean surface water salinity P N

Ocean heat content N P

Ocean freshwater content P N

Storm activity N P

River discharge P N

Permafrost temperature N P

Interpretation of observed Interpretation of observed and simulated anomalies of and simulated anomalies of environmental parameters environmental parameters for:for: ACCR -Anticyclonic Circulation Regime, and

CCR - Cyclonic Circulation Regime in the Canadian Basin and the Beaufort Gyre.

N – negative anomaly

P – positive anomaly

A – anticyclonic (clockwise)

C – cyclonic (counterclockwise)

• Oceanic system:Oceanic system:

• The oceanic portion of the Beaufort Gyre climate system:

• Stabilizes the anticyclonic circulation of sea ice and upper ocean;

• Accumulates and releases liquid fresh water and sea ice from the BG;

• Governs the ventilation of the ocean in coastal polynyas and openings along shelf-break;

• Regulates the circulation and fractional redistribution of the summer and winter Pacific waters;

• Determines the pathways of fresh water export from the Arctic to the North Atlantic;

• WE ASSUME THAT BARROW CANYON CAN BE USED TO DETECT CHANGES IN THE BG SYSTEN BECAUSE IT AMPLIFIES DYNAMYCS OF THE OCEANIC PORTION OF THE BG

Sea Ice SystemSea Ice System

Sea ice is an intermediate link between the atmosphere and ocean and is a product of interactions between the two. The sea ice system in the BG system is responsible for:

• Regulating momentum and heat transfer between the atmosphere and ocean.

• Accumulating and releasing fresh water or salt during melting-freezing cycle.

• Redistributing fresh water sources by incorporating first year ice from the marginal seas into the convergent BG circulation, holding it there and transforming it into ridged and thick multi-year ice.

• Memorizing the previous year’s conditions, buffering variations and reducing abrupt changes.

• Protecting the ocean from overcooling or overheating (the latter is extremely important for polar biology).

• Sea ice plays an important role in the storage and redistribution of energy in arctic climate (Overland and Turet, 1994).

SurfaceSurface Water:Water:

• Along with sea ice, the surface water is the most active oceanic part of the BG. It is assumed that the surface water follows sea ice drift however its circulation patterns have not been measured directly.

• It is important to investigate the processes and mechanisms of heat transformation and variability of FW in the upper layer for a better understanding of the FW role in stabilizing the BG system.

• Recent changes in surface water structure were reported by Macdonald et al. (2002). The unusually fresh surface layer and thin ice observed in the Canada Basin interior appear to be manifestations of a complex interaction between wind fields, runoff and ice.

PacificPacific Water:Water:

- The circulation of Pacific water may be coherent with the surface currents but its pathways are not known from direct observations. Recently the vertical structure of this layer and its properties have been revised by Shimada et al., (2001) and Steele et al., (2004) where the presence of two types of summer Pacific halocline water and one type of winter Pacific halocline water in the BG were reported.

- According to EWG analysis, the total thickness of the Pacific layer in the BG is approximately 150 m. This thickness is subject to temporal variability (McLaughlin et al., 2003) depending on wind stresses and circulation modes (Proshutinsky et al., 2002).

- It is important to investigate the variability of the different Pacific-origin water components, their circulation patterns and their role in stabilizing or destabilizing the BG climatic flywheel.

AtlanticAtlantic Water:Water:

• The circulation pattern of this water is probably better known, but the role that atmospheric forcing plays in the propagation and transformation of Atlantic-origin waters is poorly understood.

• The cyclonic pattern of this water propagation along the continental slope, proposed by Rudels et al. (1994) is supported by some numerical models (Holland, Karcher, Holloway, AOMIP, pers. com.). However other models (Häkkinen, Maslowski, Zhang, AOMIP, pers. com.) show anticyclonic rotation of this “wheel”.

Models with cyclonic circulation of Atlantic water

MOM high resolution POMMOM low resolution

MOM Global, OPA

Models with anticyclonic circulation of Atlantic layer

MOM high resolution Finite elements

MOM

AtlanticAtlantic Water:Water:

• McLauglin et al., (2004) showed that Atlantic water as much as 0.5C warmer than the historical record were observed in the eastern Canada Basin.These observations signaled that warm-anomaly Fram Strait waters, first observed upstream in the Nansen Basin in 1990, had arrived in the Canada Basin and BG and confirm the cyclonic circulation scheme.

• The collected data show that the Atlantic Waters are in transition and less dense than in previous decades. The impetus for such change requires further investigation.

DeepDeep Water:Water:

• There are several hypotheses for the origin of the deep water in the Canada Basin (Timmermans et al., 2003).

• One hypothesis is that deep-water renewal is episodic. Another hypothesis is that the deep water is derived from continuous renewal by shelf water. It has been suggested that brine release on Arctic continental shelves is partly responsible for the formation of water that ventilates the cold Arctic halocline in the upper 200 m. Further, if the salinity of the shelf water in the Arctic increases, then water that presently maintains the halocline can become sufficiently dense to sink into the deep basins.

• Presumably these processes were not important in the past but under new climatic conditions their role could be enhanced. This is because the reduction of sea ice extent and the enhanced formation of sea ice in winter as a result may lead to a larger volume of dense water formation (c.f. McLaughlin et al., 2003), under warmer climate conditions.

Beaufort Sea floor cable observatory (BGFO) location

The Beaufort Gyre Observing System (BGOS) is operational since August 2003.

The BGOS project is supported by NSF and current support covers 2005-2008 (including International Polar Year 2007/2008).

White starts – CTD stations; Yellow circles – moorings; red triangles – Ice-tethered profilers (drifting buoys)

Mooring equipped with MMP profiler, ULS, ADCP, sediment trap, and bottom pressure recorder (bottom tide gauge)

BGOS and BSFCOBGOS and BSFCO

Freshwater content (Freshwater content (metersmeters) and station locations in 1950s and 1960s) and station locations in 1950s and 1960s

Freshwater content (meters) and station locations

in 1970s and 1980s