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
(#7 CICS-Variability)

 

Progress Report:
April 1, 1999

1: Principal Investigator

Andrew Weaver

2. Institution

School of Earth & Ocean Sciences
University of Victoria
PO Box 3055
Victoria, B.C., V8W 3P6

3. Research Progress

3.1: 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 investigated in the context of a global coupled ice/ocean/atmosphere model. This coupled system incorporated 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 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 thermohaline circulation variability influenced the Arctic basin through heat transport under the ice pack.

Results from sensitivity studies suggested that the fresh water exchange from the variable ice cover was the dominant process for forcing variability in the overturning. The simulated Arctic ice export provided a stochastic forcing to the northern North Atlantic which excited a damped oscillatory ocean-only mode. The insulating capacity of the variable sea ice had a negligible effect on the thermohaline circulation. Ice/ocean thermal coupling acted to preferentially damp variability of the thermohaline circulation with periods greater than 30 years, but had little influence on variability at higher frequencies.

Published in Holland et al (1999a)

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

3.2: The impact of rising atmospheric CO2 levels on low frequency North Atlantic climate variability

Observations show that the North Atlantic climate system possesses pronounced interdecadal variability in its sea-ice, ocean and atmosphere components. The long timescale associated with this variability suggests that the ocean, and in particular its thermohaline component, may play an integral role in its mechanism. A question which naturally arises concerns how such variability will change under increased levels of atmospheric CO2. Several studies have examined the impact of increased atmospheric CO2 on climate variability through the use of atmospheric/ocean mixed layer model or full coupled models although they have used fairly short integrations (20-80 years) and largely focused on changes in atmospheric variability.

We used a coupled ice/ocean/atmosphere model in an attempt to quantify potential changes in North Atlantic interdecadal climate variability under increased atmospheric CO2. We focused our attention on the variability of the thermohaline circulation induced by fluctuations in Arctic Ocean ice export to the North Atlantic. Under 2 x CO2 conditions, the variance of the thermohaline circulation 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.

Published in Holland et al (1999b)

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

3.3: Extratropical subduction and decadal modulation of El Niño

An extension of the Battisti and Hirst delayed oscillator model was developed in an attempt to understand the potential effects of extratropical subduction on El Niño. This extension involved the inclusion of a meridional delay in a term parameterising the effects of extratropical subduction on eastern equatorial Pacific pycnocline anomalies. The magnitude of the eastern equatorial pycnocline displacement, and the time delay for its response to an extratropical sea surface temperature anomaly, were inferred from experiments with both ocean and coupled atmosphere-ocean GCMs. The inclusion of the delayed extratropical subduction term caused El Niño to modulate on decadal-interdecadal timescales, consistent with observations and the expectations of Gu and Philander.

Published in Weaver (1999)

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

3.4: An analysis of the Canadian precipitation record

Global climate model simulations suggest that a warmer climate may include a more dynamic hydrological cycle. That is, one in which there is more precipitation and more extreme precipitation events, both wet and dry, in mid and high latitudes (e.g. Canada). So far, analyses of observational records have detected a positive trend in global land precipitation, consistent with enhanced greenhouse projections. For example, recent studies have found precipitation increases in southeastern Canada and in the Canadian Arctic. However, despite the more forceful impact of a change in the characteristics of extreme precipitation events, only preliminary analyses have examined this for Canada, with mixed results. Daithi Stone, an MSc student, has recently completed a comprehensive analysis of the Canadian precipitation record to examine: trends in precipitation; trends in the distribution of its extremes; links between seasonal precipitation anomalies over Canada and various climate indices; relationships between the results of these three topics. He used the improved daily precipitation dataset produced by E. Mekis and W. D. Hogg. The abstract of Mr. Stone’s thesis is attached below (results will be submitted for publication this summer):

Abstract of the MSc thesis of Mr. Daithi Stone

Of all climate variables, precipitation probably impacts humanity most directly and significantly. Past research has unveiled important variations in mean precipitation accumulations, 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 accumulations.

The goal of this analysis is to uncover variations in precipitation event intensity over Canada and, furthermore, to compare the observed variations with those in mean precipitation accumulation and two dominant modes of atmospheric variability, namely the North Atlantic Oscillation (NAO) and the Pacific/North America pattern (PNA). Results are examined on both annual and seasonal bases, and with regions defined by similarities in monthly variability.

While the seasonal increasing trend in southern areas of Canada are found to result from increases in all levels of event intensity, the larger increase in the Arctic results from an abrupt increase in only heavy and intermediate events in the early 1960s. This analysis finds that significant variations in event intensity are often unrelated to variations in the mean accumulation. Consistent with these differences, the precipitation responses to the two teleconnections patterns are found to often occur only at specific levels of event intensity. The PNA strongly influences precipitation climates in many regions of the country for most months of the year. In particular, it strongly influences winter variations in southwestern regions, generally affecting most event intensities. The PNA only influences the frequency of heavier events in Autumn and Winter in the Great Lakes region. Precipitation responses to the NAO occur only in a few months of the year, generally in northeastern and Arctic regions.




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