Presentation on theme: "Jamie Morison Polar Science Center, University of Washington Bob Dickson CEFAS, U.K. Presentation NOClim/ProClim/AIO."— Presentation transcript:
Jamie Morison Polar Science Center, University of Washington Bob Dickson CEFAS, U.K. Presentation NOClim/ProClim/AIO Cruise Meeting Norwegian Coast Wednesday, October 1, 2003 Change in the Arctic Ocean and Regions South
Point 1: The climatic forcing of Arctic and sub-Arctic seas in recent decades has been extreme.
Variations in Global Temperature and their uncertainties,1861- April 2001 (land air and sea surface temperature) The context (i): during the past century, the global mean temperature has increased in two main episodes of warming
When we plot air temperature as a function of latitude and time, two things become clear: 1) the World is warmer. Including 2002, all ten of the warmest years since records began in 1861 have occurred since 1990; Jones and Moberg, ) in the last two decades the distribution of warming has become global. Courtesy Tom Delworth, GFDL
….and our instrumental and proxy records suggest that the NAO in the 1990s may have been at a 600 year extreme positive state. Phil Jones CRU, in press
Point 2: Change can be imposed on the Arctic Ocean from the Nordic seas.
NAO Change in winter slp, (NAO+) minus (NAO-) from Dickson et al Since the change from NAO-negative (1960s) to NAO- positive (1990s) is associated with a strengthened southerly airflow west of Norway…
….it is unsurprising that the amplifying NAO was accompanied by a warming of the 2 main Atlantic water inflows to the Arctic Ocean - -- in this case the upper 500m of the WSC west of Svalbard. Dickson et al 2000
…and the same irregular warming has affected the inflow branch flowing through the western Barents Sea. Harald Loeng IMR Bergen, pers comm T °C
Temperature Changes, Pargo ‘93 - Climatology Looking to the Arctic Ocean and comparing the 1993 SCICEX data with the EWG climatology, we see warm cores over ridges indicating a shoaling and 1.5° warming of the temperature maximum where the Atlantic Water inflow subducts to spread through the Eurasian basin. This picture that started SEARCH! Atlantic Water 1.5° warmer From Morison, et al.,, 2000,, Arctic, 53, 4.
…and Karcher et al have pieced together the spread of warming around the boundary of the Arctic Ocean ……in 1980, 1984, 1987 & 1991, from Karcher et al 2003.
… and its continued spread in 1993, 1995, 1996 & 1999, from Karcher et al 2003.
Point 3: However, the most important change in the Arctic Ocean is in the salinity distribution, and this appears to be caused by hemispheric changes in the atmosphere.
Comparing the 1993 SCICEX data with the EWG climatology, we see a salinity increase in the upper 200 m of the Makarov Basin, indicating a shift in front between Atlantic and Pacific waters. Salinity Changes: Pargo ‘93 - Climatology From Morison, et al.,, 2000,, Arctic, 53, 4. Saltier Atlantic-Derived Water Frontal Shift Salinity Increase Fresher Pacific-Derived Water
Salinity Differences, SHEBA 1997 to Climatology Comparing the 1993 SCICEX data with the EWG climatology, we see the salinity increase (1.5) extends to the upper 200 m over Chukchi Cap - Mendeleyev Ridge with slight increase at 150 m over Northwind Rise. We see a freshening near surface of Beaufort Sea.
We see a trend of decreasing surface salinity in the Beaufort Sea; the fall 1997, SHEBA salinities were lowest ever. The Spring-Fall salinity difference has also been increasing implying increased summer ice melt and river input was a record minimum summer ice extent. Spring and Summer Surface Salinity at the Start Site of the SHEBA Drift
Ice thickness decreased 42% in last 30 years (Rothrock et al, 1999) Beaufort High decreased and shifted east in 1990s Transpolar Drift of ice shifted axis counterclockwise producing a more cyclonic motion in 1990s Ice extent decreased 3%/decade (Parkinson et al.) Extent negatively correlated with Arctic Oscillation (Rigor et al., 2002) Ice and Atmospheric Pressure Changes
Connection to the Polar Vortex and Global Climate Low pressure spins up Polar Vortex, brings warm air to Greenland Sea & Russian Arctic Warm air over Greenland Sea allows warmer Atlantic Water in Arctic Ocean Warm air advection increases SAT, warms permafrost Increase in Polar Vortex - More cyclonic ocean circulation - Shift in front and Transpolar Drift - Russian shelf water to Beaufort Increase in Polar Vortex - Increases open water - Decreases Albedo - Increases radiative heating & melt - Freshens upper Beaufort Sea Rising AO means lowers SAP over the Arctic. Thompson and Wallace, 1998)
Point 4: There has been a freshening of the sub-Arctic seas over the past 4 decades. Point 4
freshening of the Nordic Seas There has been a broadscale freshening of the Nordic Seas which over the past 4 decades has reached to depths of >1 km, so is accessible to the two main overflows which cross the Greenland-Scotland Ridge. Salinity at OWS M, Norwegian Sea, , Courtesy Svein Østerhus, UiB
variety of reasons Increased precipitation (+14 cm/ winter) Decreased local sea-ice formation There are a variety of local reasons associated with the amplifying NAO for the increase in fresh water accession to the Nordic Seas.
Point 5: Changes in the Arctic Ocean freshwater balance and pathways could be partly responsible for the decreased salinity in the sub-Arctic seas.
variety of reasons Weak, shallow Greenland Sea convection But some of the reasons for the freshwater increase in the Nordic Seas may lay with the Arctic Ocean A.O. ice melt Increased ice & freshwater flux through Fram Strait Increased precipitation (+14 cm/ winter) Decreased local sea-ice formation
Possible Arctic Ocean Contributions to Increased Freshwater Flux Enhanced Runoff Over the 64 years, discharge from the 6 largest Eurasian arctic rivers has increased 7% (128 km 3 /y or Sv) [Petersen et al., Nature 2002] Global warning scenarios predict enhanced P-E. [ Manabe and Stouffer, Quarter. Sci. Rev., 2000] ? But atmospheric reanalyses suggest decreased P-E. [ Serreze, personal communication] ? Gauging uncertainties exist for runoff and precipitation.
Possible Arctic Ocean Contributions to Increased Freshwater Flux Increased Pacific Water More direct pathway and arguably shorter residence times for Pacific Water in recent years.
Connection to the Polar Vortex and Global Climate Low pressure spins up Polar Vortex, brings warm air to Greenland Sea & Russian Arctic Warm air over Greenland Sea allows warmer Atlantic Water in Arctic Ocean Warm air advection increases SAT, warms permafrost Increase in Polar Vortex - More cyclonic ocean circulation - Shift in front and Transpolar Drift - Russian shelf water to Beaufort Increase in Polar Vortex - Increases open water - Decreases Albedo - Increases radiative heating & melt - Freshens upper Beaufort Sea Cyclonic Circulation - Increases export of fresh water and sea ice - Decreases salinity and increases stratification of the sub-Arctic seas - Inhibits global ocean overturning Thompson and Wallace, 1998) Rising AO means lowers SAP over the Arctic. Thompson and Wallace, 1998) AO update: Decreased in mid 90s but on average still higher than before 1989
Salinity Differences, NPEO 2000 & Climatology Fresher at Coast Saltier Atlantic-derived Pacific-derived Morison et al., 2002, EOS, in press. Front still shifted counterclockwise near Pole Pacific-derived surface waters moving east and appearing off Ellesmere Island
Salinity Differences, NPEO 2002 & Climatology Front still shifted counterclockwise near Pole, but Freshening of surface layer in Makarov section to near climatology Increased salinity near Ellesemere due to haline convection. pIFHChemistry indicates surface layer is Pacific derived.
Possible Arctic Ocean Contributions to Increased Freshwater Flux Increased Pacific Water More direct pathway and arguably shorter residence times for Pacific Water. ? But has there been an increase in Bering Strait inflow relative to Fram Strait inflow?
Possible Arctic Ocean Contributions to Increased Freshwater Flux More Ice Export Simulation suggests yes, in concert with shorter residence time and observed decrease thickness.
Zhang Zhang, Rothrock and Steele, 2000,, J. Clim., 13, Simulated sea ice changes by Zhang et al. (2000) show shift in drift axis, increased drift speeds, increased lateral melt (a) mean These lead to reduced residence time, reduced average thickness in the basin, but increased ice export from the Basin to the North Atlantic. Ice Budget Differences in the Basin* (89 to 96) - (79 to 88) Vert. Growth 0.0 Lateral Melt- 0.6 Export- 0.7 Ice Production-1.3 * (10 12 m 3 yr -1 ) (b) mean (c) = (b) - (a)
Possible Arctic Ocean Contributions to Increased Freshwater Flux More Ice Export Simulation suggests yes, in concert with shorter residence time and observed decrease thickness. ? But no net input of freshwater to system, distillation produces stratification. ? But if salinized deeper water has a longer residence time or escapes through an ungauged path, freshened output results. And repeated cycles of freezing and melting with advection may result in freshening of halocline.
Sub-arctic affect the system of cold dense overflows Point 6: The freshening of the subarctic affects the system of cold dense overflows from the Nordic Seas that ventilate the N. Atlantic, and thus freshens the deep and abyssal layers of the Northern North Atlantic …..
freshwater flux array Schematic of the northern loop of the ocean’s “Great Conveyor”. McCartney et al 1996 The elements of an ASOF freshwater flux array are funded and partly in place. Few results yet….....but we do have one proxy measure of freshwater flux from the AR7W Line Osterhus/Vinje Dickson/Meincke Falkner Melling Prinsenberg Lee Yashayaev Meincke SFB 512
offshore density gradient The offshore density gradient in the 0-150m layer from Labrador Shelf to the Central Lab Sea is our only (proxy) measure of the changing freshwater flux to the NW Atlantic. This gradient has progressively steepened with the NAO over the past 4 decades (equivalent to a 20% increase in southward transport) largely through freshening of shelf & upper- Slope waters. Yashayaev’s analysis of AR7W Density gradient Density
overflows that renew and ventilate the deep ocean have also Tapping-off the freshening upper layer of the Nordic Seas, the two dense overflows that renew and ventilate the deep ocean have also freshened over the past 4 decades. Dickson et al, Nature 2002.
pathways …so that if we construct salinity time series at intervals along the spreading pathways of both overflows from their sills to the deep Labrador Sea…
entire system undergone freshening … we find that the entire system of overflow and entrainment that ventilates the deep Atlantic has undergone a remarkably rapid and remarkably steady freshening over the past four decades. [‘-15’ = fresher by 0.015/decade, ] A change in the ocean-climate of sub- arctic seas has thus been transferred to the deep and abyssal ocean at the headwaters of the “Great Conveyor” Dickson et al 2002
change in the Labrador Sea salinity NAO- NAO+ The resulting full-depth change in the Labrador Sea salinity is believed to be the largest change in the oceanographic record. [By 1992, equal to adding an extra 6 m of fresh water at the sea surface; Lazier 1995].
shift in the -S relation The result has been a dramatic shift in the -S relation for waters of the NW Atlantic. Igor Yashayaev, unpublished
effects on water column density …and effects on water column density (so, potentially, on circulation) are already being observed. e.g.the deepest densities in the Labrador Sea & in the Newfoundland Basin (above) have decreased between the 1960s and 1990s.... Igor Yashayaev, pers comm
Point 7: The Atlantic Water input to the Arctic is increasingly fresh.
The Atlantic Water inflow through Fram Strait has also been getting fresher. Salinity and density of the upper 500m of WSC, from Dickson et al 2000
Conditions at the North Pole EWG: 0.5° AW core at 350 m & 31 o/oo surface salinity 1990s: 1.5° AW 250 m & 32.5 o/oo surface salinity : AW temp slightly less than 1995 max Surface salinity < 1990’s EWG Climatology, 1950s, 60s, 70s, 80s Oden ‘91, SCICEX, ’93-’99 NPEO : Atlantic Water slightly fresher
Arctic temperature change can be imposed on the Nordic Seas and on the deep/abyssal Atlantic.
Denmark Strait Temperature Variation Current-meter thermistor data from 2000m depth in the core of the Denmark Strait Overflow show a dramatic decadal variation in mean temperatures. These values are not smoothed. Each 30-day mean is simply the average of 720 hourly values.
Fram Strait Origin We believe these temperature changes originate in the upper waters of the eastern Fram Strait, 2500 km upstream and 3 years earlier
Origins …and the temperature and salinity of the overflow core at 2000m off Angmagssalik clearly determine the density of the DSOW-derived layer of the abyssal Labrador Sea a further one year later.
Versus rudels ….so despite the 18 possible contributory sources of DSOW shown in Rudel’s scheme, Recirculating Atlantic Water (RAW) from the Fram Strait seems to have been the dominant control on DSOW variability in recent years. Map from Rudels et al, 2003
The 1990s revealed Arctic change involving atmosphere, ocean, and land. Appears connected to changes in the atmospheric circulation of the Northern Hemisphere. Ocean changes potentially affect global climate through changing albedo and weakening global thermohaline circulation Motivates the Study of Environmental Arctic Change (SEARCH) North Pole Environmental Observatory (NPEO) is an example of the type of long-term observation effort needed by SEARCH NPEO suggests that consistent with changes in atmospheric circulation, the central Arctic Ocean is relaxing towards climatology but is still in a changed state. Outline
AO Update Polar Vortex increased in strength ca 1989 Relaxed toward climatology in mid 1990’s However still still tends to be higher than