Climate Modelling Group
School of Earth and Ocean Sciences

Climate Research Network

Collaborative Research Agreement at the University of Victoria

on Behalf of the Canadian Institute for Climate Studies and Environment Canada

(CICS—Arctic Ocean; and subcomponent of CICS- Variability)

Progress Report:

October 1, 1999

This progress report is available on the world wide web at:

http://climate.uvic.ca/projects/CCC-Arctic-Progress1.html

1: Principal Investigator

Andrew Weaver

2: Co-Investigators

Ed Carmack, Greg Flato, Lawrence Mysak

3: Institution

School of Earth & Ocean Sciences, University of Victoria

PO Box 3055, Victoria, BC, V8W 3P6

4: Research Progress

Financial support for the new AES/CICS Arctic Node of the Canadian Climate Research Network arrived in late spring. This new node follows on from earlier projects funded under the CICS Variability and Global Oceans network. As such, some of the research discussed here was initiated under prior CICS funding and completed under the CICS Arctic funding.

The CICS Arctic funding at UVic is being used to support G. Arfeuille (a Phd student co-supervised by Ed. Carmack and A. Weaver); F. McLaughlin (a PhD student); L. Waterman (an MSc student who arrived on September 1, 1999 to work with A. Weaver and G. Flato); D. Stone (a PhD student). M. Eby is also currently being partially funded under this project and he is involved in upgrading and parallelising the UVic coupled model, including its sea ice component. Two postdocs will shortly arrive to work on this project: Harper Simmons, a PhD student working under the supervision of Doron Nof at Florida State University, will undertake sensitivities studies with the CCCma coupled model as well as develop new parameterisations of flows over deep ocean sills. Andreas Schmittner, a PhD student working under the supervision of Thomas Stocker in Switzerland, will examine the role of the Arctic Ocean and its sea ice cover in climate/climate variability using low order models.

4.1: Low-frequency climate variations in the coupled North Atlantic Ocean and Arctic sea ice system

The simulated influence of Arctic sea ice on the variability of the North Atlantic climate was analysed in the context of a global coupled ice/ocean/atmosphere model. This coupled system incorporates a general circulation ocean model, an atmospheric energy moisture balance model and a dynamic/thermodynamic sea ice model. Under steady seasonal forcing, an equilibrium solution was obtained with very little variability. To induce variability in the model, daily varying stochastic anomalies were applied to the wind forcing of the northern hemisphere sea ice cover. These stochastic anomalies had observed spatial patterns but were random in time. Model simulations were run for 1000 years from an equilibrium state and the variability in the system was analyzed. The sensitivity of the system to the ice/ocean coupling of both heat and fresh water was also examined.

Under the stochastic forcing conditions, low amplitude (approximately 10% of the mean) variability in the thermohaline circulation (THC) occurred. This variability had enhanced spectral power at interdecadal timescales which was concentrated at approximately 20 years. It was forced by fluctuations in the export of ice from the Arctic into the North Atlantic. Large changes in sea-surface temperature and salinity were related to changes in the overturning circulation and the sea ice coverage in the northern North Atlantic. Additionally, the THC variability influenced the Arctic basin through heat transport under the ice pack.

Results from sensitivity studies suggest that the fresh water exchange from the variable ice cover is the dominant process for forcing variability in the overturning. The simulated Arctic ice export appears to provide stochastic forcing to the northern North Atlantic which excites a damped oscillatory ocean-only mode. The insulating capacity of the variable sea ice has a negligible effect on the THC. Ice/ocean thermal coupling acts to preferentially damp THC variability with periods greater than approximately 30 years, but has little influence on variability at higher frequencies.

The impact of rising atmospheric CO2 levels on this low frequency variability of the North Atlantic climate was also examined. In particular, we focused on THC variability induced by fluctuations in ice export from the Arctic basin. Under 2 x CO2 conditions, the thermohaline circulation variance was reduced to 7% of its simulated value under present day forcing. This decrease was caused by relatively low ice export variability and changes in the primary ice melt location in the northern North Atlantic under 2 x CO2 conditions.

4.2 Sea ice model sensitivity analyses

Climate simulations in the UVic coupled model were investigated using a dynamic-thermodynamic sea ice and snow model with sophisticated thermodynamics and a sub-grid scale parameterisation for multiple ice thicknesses. In addition to the sea ice component, the model included a full primitive-equation ocean and a simple energy-moisture balance atmosphere. We introduced a new formulation of the ice thickness distribution that is Lagrangian in thickness-space. The method is designed to use fewer thickness categories because it adjusts to place resolution where it is needed the most, and it is free of diffusive effects that tend to smooth Eulerian distributions. Simulations showed that the model did well in capturing the mean Arctic climate. Compared to simulations without an ice-thickness distribution, we found widespread changes in the Arctic and northern North Atlantic climate. The ice-thickness distribution caused ice export through Fram Strait to be more variable and more strongly linked to meridional overturning in the North Atlantic Ocean.

The Lagrangian formulation of the ice thickness distribution allows for the inclusion of a vertical temperature profile with relative ease compared to an Eulerian method. We found ice growth rates and ocean surface salinity differed in our model with a well resolved vertical temperature profile in the ice and snow and an explicit brine-pocket parameterisation compared to a simulation with Semtner zero-layer thermodynamics. Although these differences are important for the climate of the Arctic, the effect of an ice thickness distribution are more dramatic and have further reaching consequences for the Northern Hemisphere. Sensitivity experiments indicate that five ice thickness categories with ~ 50 cm vertical temperature resolution captures the effects of the ice thickness distribution on the heat and freshwater exchange across the surface in the presence of sea ice in climate simulations.

 

4.3: An analysis of the Canadian precipitation record

Past research has unveiled important variations in mean precipitation, often related to large scale shifts in atmospheric circulation, and consistent with projected responses to enhanced global warming. More recently, however, it has been realised that important and influential changes in the variability of daily precipitation events have also occurred in the past, often unrelated to changes in mean accumulation. This study aimed to uncover variations in precipitation event intensity over Canada and to compare the observed variations with those in mean accumulation and two dominant modes of atmospheric variability, namely the Arctic/North Atlantic Oscillation (AO/NAO) and the Pacific/North America teleconnection pattern (PNA). Results were examined on both annual and seasonal bases, and with regions defined by similarities in monthly variability.

Seasonally increasing trends were found in southern areas of Canada that result from increases in all levels of event intensity during the 20th century. During the latter half of the century increases were concentrated in heavy and intermediate events, with the largest changes occurring in Arctic areas. Variations in precipitation intensity can, however, be unrelated to variations in the mean accumulation. Consistent with these differences, the precipitation responses to the NAO and PNA were found to often occur in northeastern regions in summer and winter with the intensity affected in both seasons. The PNA strongly influences precipitation in many regions of the country during autumn and winter. In particular, it strongly influences variations in southern British Columbia and the Prairies, affecting the intensity in only some areas. However, it only influences the frequency of heavier events in autumn and winter in Ontario and southern Quebec, where this response is actually more robust than the response in total accumulation. During these seasons a negative PNA generally leads to more extreme precipitation events.

4.4: The Canada Basin 1989—1995: Upstream change and far-field effects of the

Barents Sea Branch.

Physical and geochemical tracer measurements, collected at an oceanographic station in the southern Canada Basin from 1989 to 1995, were examined to see whether recent events in three upstream Arctic Ocean sub-basins produced change in Canada Basin waters. Upstream events included warming of the Atlantic layer, relocation of the Atlantic/Pacific water mass boundary, and increased ventilation of boundary current waters. Early signals of change appeared in Canada Basin waters in 1993 first along the continental margin and, by 1995, had spread both laterally to the basin interior and further downstream. Changes in physical and geochemical properties (nutrient, oxygen, 129I and CFCs) were observed throughout much of the water column to depths greater than 1600m. In particular, the boundary between Pacific and Atlantic-origin waters was found to be shallower and Atlantic-origin waters occupied more of the Canada Basin water column. In 1993 and 1995 these Atlantic-origin waters were colder in the lower halocline and, within the water column's Atlantic layer, Fram Strait Branch waters were colder and fresher, and Barents Sea Branch waters were warmer and fresher, than during the four previous years. Comparison with physical and geochemical data collected upstream in Nansen Basin in 1993 demonstrated that Canada Basin changes were attributable to far-field effects of Barents Sea outflow. This finding was manifested in a higher percentage of Barents Sea Branch water in the Atlantic layer.

4.5 Fresh water fluxes through the Canadian Arctic Archipelago

As part of his PhD, Gilles Arfeuille participated in an Arctic cruise which completed 184 science stations in the western part of the Canadian Arctic Archipelago from the 23rd of August to the 25th of October 1999. During these sciences stations CTD/Rosette casts were conducted to infer the fresh water flux through the Canadian Arctic Archipelago, especially in its western part (i.e. Amundsen Gulf, Prince of Wales Strait, Dolphin and Union Strait, Coronation Gulf, Dease Strait, Queen Maud Gulf, Simpson Strait, Chantrey Inlet, Rae Strait, James Ross Strait, Peel Sound, and Bellot Strait), which has not been studied in detail in previous years. Using 1995, 1997 and this 1999 data, the fresh water flux by sea ice transport and buoyancy boundary currents will be inferred. The fresh water import into the North Atlantic via Baffin Bay is estimated to be 20% of the sea-ice export through Fram Strait. The dO18 data from the water samples taken during the science cruise will reveal the origin of the fresh water forming the buoyancy boundary currents observed during the cruise using the CTD data (i.e. sea-ice melting or river runoff origin). Initial results, which are promising, reveal the relative importance of fresh water input from rivers and sea-ice melting in the Archipelago and the transport of this fresh water via buoyancy boundary currents.

4.6 McGill University

A major accomplishment has been the simulation of the interannual variability of the wind driven Arctic sea ice cover during 1958-98, in collaboration with Gilles Arfeuille (now a PhD student at UVic) and Bruno Tremblay (a PDF at LDGO, Columbia Univ.). In this study the sea ice model of Tremblay and Mysak, which is based on a granular material rheology, is driven by 41 years of NCEP winds in order to produce the year-to-year variations in the sea ice circulation in the Arctic and its thickness. Efforts are focused on analyzing the variability of the sea ice volume in the Arctic Basin and the subsequent changes in the sea ice export into the Greenland Sea via Fram Strait. The relative contributions to the Fram Strait thickness and velocity anomalies to the export are first investigated. The export anomalies are then linked to prior ice volume anomalies in the Arctic Basin. The origin and evolution of the sea ice volume anomalies are then related to the sea ice circulation and atmospheric forcing patterns in the Arctic.

Efforts are now underway (with PDF Todd Arbetter) to couple this sea ice model to the MOM 3 GFDL ocean circulation model. This coupled model will be used to study polynya formation in northern Baffin Bay and the interannual variability of the sea ice cover in this region. Also, the coupled ice-ocean model will be used to study the influence of sea ice export anomalies on the thermohaline circulation in the North Atlantic.

1999 Publications Arising from CICS Arctic and Variability Supported Research

1. Weaver, A.J., C.M. Bitz, A.F. Fanning and M.M. Holland, 1999: Thermohaline circulation: High latitude phenomena and the difference between the Pacific and Atlantic. Annual Review of Earth and Planetary Sciences, 27, 231—285.

2. Weaver, A.J., 1999: Extratropical subduction and decadal modulation of El Niño. Geophysical Research Letters, 26, 743—746.

3. Huck, T., A. Colin de Verdière and A.J. Weaver, 1999: Interdecadal variability of the thermohaline circulation in box-ocean models forced by fixed surface fluxes. Journal of Physical Oceanography, 29, 893—910.

4. Huck, T., A.J. Weaver and A. Colin de Verdière, 1999: On the influence of the parameterisation of lateral boundary layers on the thermohaline circulation in coarse-resolution ocean models. Journal of Marine Research, 57, 387—426.

5. Duffy, P.B., M. Eby and A.J. Weaver, 1999: Effects of sinking of salt rejected during formation of sea ice on results of a global ocean-atmosphere-sea ice climate model. Geophysical Research Letters, 26, 1739-1742.

6. Wiebe, E.C. and A.J. Weaver, 1999: On the sensitivity of global warming experiments to the parametrisation of sub-grid scale ocean mixing. Climate Dynamics, in press.

7. Weaver, A.J. , P.B. Duffy, M. Eby and E.C. Wiebe, 1999: Evaluation of ocean and climate models using present-day observations and forcing. Atmosphere-Ocean, in press.

8. Weaver, A.J. and E.C. Wiebe, 1999: On the sensitivity of projected oceanic thermal expansion to the parameterisation of sub-grid scale ocean mixing. Geophysical Research Letters, in press.

9. Flato, G.M., G.J. Boer, N.A. McFarlane, D. Ramsden, M.C. Reader and A.J. Weaver, 1999: The Canadian Centre for Climate Modelling and Analysis global coupled model and its climate. Climate Dynamics, in press.

10. Duffy, P.B., M. Eby and A.J. Weaver, 1999: Climate model simulations of effect of Antarctic sea ice on stratification of the Southern Ocean. Journal of Climate, submitted

11. Holland, M.M., A.J. Brasket and A.J. Weaver, 1999: The impact of rising atmospheric CO2 on low frequency North Atlantic climate variability. Geophysical Research Letters, submitted.

12. Holland, M.M., C.M. Bitz, M. Eby and A.J. Weaver, 1999: The role of ice ocean interactions in the variability of the North Atlantic thermohaline circulation. Journal of Climate, submitted.

13. Wang, H., P.B. Duffy, K. Caldeira, M. Eby, A.J. Weaver and A.F. Fanning, 1999: Importance of water vapor transport to the hydrological cycle in an atmospheric energy-moisture balance model coupled to an OGCM. Journal of Geophysical Research, submitted.

14. Stone, D.A., A.J. Weaver and F.W. Zwiers, 1999: Trends in Canadian precipitation intensity. Atmosphere-Ocean, submitted.

15. Bitz, C.M., M.M. Holland, A.J. Weaver and M. Eby, 1999: Simulating the ice-thickness distribution in a coupled climate model. Journal of Geophysical Research, submitted.

1999 McGill Publications Arising from CICS Arctic Supported Research

Arfeuille, G., L.A. Mysak and L.-B Tremblay, 1999: Simulation of the interannual variability of the wind driven Arctic sea ice cover during 1958-98. Climate Dynamics, in press.

Yi, D., L.A. Mysak and S.A. Venegas, 1999: Decadal-to-interdecadal fluctuations of Arctic sea ice cover and the atmospheric circulation during 1954-94. Atmosphere-Ocean, in press.

Venegas, S.A. and L.A. Mysak, 1999: Is there a dominant timescale of natural climate variability in the Arctic? J. Climate, submitted.


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