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:
October 31, 1998
 
 

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

Low-frequency climate interactions in the North Atlantic have been explored with an ocean and sea ice model coupled to an energy and moisture conserving atmosphere model. We implemented an advanced sea ice model to better simulate atmosphere-ocean exchange. With annually periodic wind stress and solar forcing, the coupled atmosphere, sea ice, ocean model responded with no substantial interannual variability. However, the model simulated significant decadal variability when the wind field included interannually varying fluctuations at high latitudes (north of 60°) and over the North Atlantic Ocean. Several thousand year integrations using synthetic stochastic wind forcing based on observed spatial patterns of variability have now been conducted.

The model reproduced the timescale and patterns of the two leading modes of observed sea surface temperature (SST) in the North Atlantic described by Deser and Blackmon, 1993. The timescale of the leading mode is of the order 50 years and resembles the signature of the surface warming trend during the 1920s. The second mode occurs on decadal timescales. Both of these modes are strongly coupled to anomalies in the sea ice, sea surface salinity (SSS), and overturning circulation in the North Atlantic. We find that negative SST and SSS anomalies and enhanced sinking in the northern North Atlantic are preceded by strong negative ice anomalies in the northern North Atlantic and positive anomalies in the Siberian sector of the Arctic.

Sensitivity experiments that determine separately the role of the stochastic wind by means of anomalous wind stress forcing and turbulent latent and sensible heat flux forcing are being developed. In addition, we will explore which spatial modes in the wind field are important for forcing the ocean and sea ice variability.

3.2 Thermohaline variability in a coupled atmosphere-ocean model:

The thermohaline circulation is thought to be an important contributor to interdecadal climate variability and a key aspect in understanding the likely consequences of anthropogenic climate change. Modelling studies have shown that the stability of the thermohaline circulation is sensitive to the formulation of the surface boundary conditions and parameterizations of physical processes. As a consequence, many of the phenomena exhibited in low-order and simplified systems, such as multiple equilibria and self-sustained oscillations, are not found as models progress to more realistic degrees of complexity and coupling. We have concentrated on understanding the impact of atmospheric transports of heat and fresh water due to transient storm activity on the strength and variability of the thermohaline circulation. Box models show that the atmospheric eddy transports of fresh water can have a de-stabilizing impact on the thermohaline circulation which exceeds the stabilizing transport of heat by the atmosphere. Whether this feedback is realistic for more complex models and the real climate system remains an open question.

Aaron Brasket, a visiting PhD student from the University of Colorado is examining this question using a zonally-averaged atmosphere, with a parameterization of the eddy transports, coupled to a 3-dimensional ocean basin in an idealized model domain. With this configuration, we can study in detail the realistic coupling of the atmosphere-ocean system over a wide range of parameter values. Our results indicate that the de-stabilizing impact of the atmospheric water transports is small compared with other feedbacks and that in a coupled environment the water transport is weaker than in low-order systems. The results also seem to indicate that in our coupled system the thermohaline circulation is more robust to perturbations than in low order systems or coupled models with large flux corrections or other simplified boundary conditions. The reasons for this are not completely clear but two possible explanations have emerged so far. The first explanation involves the temperature dependence of the available moisture in the atmosphere. A perturbation of the thermohaline circulation which reduces the ocean overturning induces larger transports of heat and fresh water by the atmosphere. However, the reduction of oceanic heat transport also leads to colder ocean and air surface temperatures which reduces the moisture available for precipitation. Colder temperatures may force the increased atmospheric transports to be rained out of the atmosphere further south before they reach the convective regions in high latitudes. In effect, a short-circuit in the water transport feedback. Another possibility involves the oceanic gyre circulation and associated transports. Enhanced atmospheric transports are accompanied by an increase in the strength of the zonal jet and an acceleration of the sub-tropical and sub-polar gyres in the ocean. The tracer transports of heat and salinity by the gyre circulation act in opposition to the overturning perturbation. Whether this negative feedback of the coupled system is dominant or in addition to the other feedbacks mentioned is the current focus of our research.

3.3 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 south-eastern 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, is undertaking 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 is using the improved daily precipitation dataset produced by E. Mekis and W. D. Hogg.

Research completed thus far consists of an examination of trends in mean precipitation and correlations between seasonal precipitation anomalies and various climate indices, on a regional basis. Mr. Stone has examined the distribution of station trends for separate regions and confirmed the existence both of linear trends in the southeast and north and of periodic variations in the southwest. An examination of the relationship between seasonal precipitation and several climate indices is revealing some important correlations. It has also confirmed some previously suspected relationships, such as the influence of the Pacific/North America pattern and the El Niño-Southern Oscillation on winter precipitation from southern British Columbia to Southern Ontario and over the mainland Northwest Territories. Work on extreme precipitation events will begin upon completion of this segment.


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