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Amir Shabbar, Kaz Higuchi, and John L. Knox

Abstract

In Knox et al., the interannual variation of the Northern Hemisphere 50 kPa geopotential height field averaged between 30° and 80°N was investigated for the 40-year period from 1946 to 1985. We presented strong statistical evidence supporting the notion that a rather abrupt transition in the climate system took place during the early 1960s. There was no attempt to compare the spatial distribution of the 50 kPa height difference between Regime 1 (1946–62) and Regime 2 (1963–85).

As a sequel to the first paper, we investigate the spatial characteristics of the transition height field. We find that the difference in the 50 kPa height field between Regime 1 and Regime 2 is characterized by low frequency circulation modes of the Pacific/North American (PNA) teleconnection pattern, the North Atlantic Oscillation (NAO), and an Arctic oscillation. There was an increase (in the residual sense) of the frequency and amplitude of the positive phase of the PNA in Regime 2 relative to Regime 1.

Fourier analysis is applied to interpret the regime changes in terms of planetary and long waves during the winter season. The change in the Arctic circulation is primarily associated with an amplification of the wave 2 component in its normal phase location, while in the midlatitudes the primary contributor is wave 1, again in its normal location.

We also examine the 40-year time series of 50 kPa height at the three centers of the winter PNA and confirm a strong negative correlation between the first two centers and a significant positive correlation between the first and third.

To assess the current trend, the 50 kPa anomaly field averaged over the 1981–87 period is examined. The winter season shows an eastward shift of the North Pacific Ocean cooling pattern and amplified warming over most of North America, the maximum centered over western Canada. The NAO phase changed to negative.

Our results are discussed in relation to the interregime sea surface temperature change over the North Pacific Ocean and to the increase in frequency and amplitude of ENSO events during Regime 2. Overall, there is a reinforcement of the earlier evidence for the two subclimate regimes.

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Douglas Chan, Kaz Higuchi, and Charles A. Lin

Abstract

Atmospheric sensible and latent heat fluxes constitute an important component of the total poleward energy transport in the climate system. The authors investigate the relative role of these heat fluxes in normal and enhanced C02 warming scenarios, using a two-dimensional latitude-height multilayer energy balance climate model. The model uses a diffusive scheme to parameterize the heat transports, where the diffusion coefficients are calculated as a function of the temperature gradient.

Results of various numerical experiments show that changes in the diffusion coefficients of both the latent and sensible heat fluxes can significantly affect the present (1×C02) equilibrium climate. The difference between the 2×C02 and 1×C02 climate, however, as measured by the simulated difference temperature field, is much more sensitive to changes in the latent heat rather than the sensible beat diffusion coefficients, particularly in the Tropics. The parameterization scheme with temperature-dependent diffusion coefficients is able to resolve the water vapor feedback in the 2×C02 warming, thus enabling simple energy balance climate models to produce results comparable to those obtained by more complex climate models such as general circulation models.

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John L. Knox, Kaz Higuchi, Amir Shabbar, and Neil E. Sargent

Abstract

There is accumulating evidence in the literature that different short-period climate regimes (subclimates) may have characterized the Northern Hemisphere during the past 40 years. We, therefore, investigate the 40-yr record of 50 kPa height (1946–85) and analyze the time series of zonal anomalies stratified by season. We find that there appears to be two contiguous regimes-with a rather abrupt transition during the early 1960s—which had significantly different means, trends and degrees of variability. The results are compared with those from recent investigations of Northern Hemisphere surface and/or tropospheric temperature variation. The possibility of a “climatic jump” during the early 1960s is discussed. Our results raise the question of an appropriate period to use for determining “normals,” whether for standard level surfaces or, more generally, for calculating the statistics of the general circulation, both in the horizontal and vertical.

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Baozhang Chen, Jing M. Chen, Gang Mo, Chiu-Wai Yuen, Hank Margolis, Kaz Higuchi, and Douglas Chan

Abstract

Land surface models (LSMs) need to be coupled with atmospheric general circulation models (GCMs) to adequately simulate the exchanges of energy, water, and carbon between the atmosphere and terrestrial surfaces. The heterogeneity of the land surface and its interaction with temporally and spatially varying meteorological conditions result in nonlinear effects on fluxes of energy, water, and carbon, making it challenging to scale these fluxes accurately. The issue of up-scaling remains one of the critical unsolved problems in the parameterization of subgrid-scale fluxes in coupled LSM and GCM models.

A new distributed LSM, the Ecosystem–Atmosphere Simulation Scheme (EASS) was developed and coupled with the atmospheric Global Environmental Multiscale model (GEM) to simulate energy, water, and carbon fluxes over Canada’s landmass through the use of remote sensing and ancillary data. Two approaches (lumped case and distributed case) for handling subgrid heterogeneity were used to evaluate the effect of land-cover heterogeneity on regional flux simulations based on remote sensing. Online runs for a week in August 2003 provided an opportunity to investigate model performance and spatial scaling issues.

Comparisons of simulated results with available tower observations (five sites) across an east–west transect over Canada’s southern forest regions indicate that the model is reasonably successful in capturing both the spatial and temporal variations in carbon and energy fluxes, although there were still some biases in estimates of latent and sensible heat fluxes between the simulations and the tower observations. Moreover, the latent and sensible heat fluxes were found to be better modeled in the coupled EASS–GEM system than in the uncoupled GEM. There are marked spatial variations in simulated fluxes over Canada’s landmass. These patterns of spatial variation closely follow vegetation-cover types as well as leaf area index, both of which are highly correlated with the underlying soil types, soil moisture conditions, and soil carbon pools. The surface fluxes modeled by the two up-scaling approaches (lumped and distributed cases) differ by 5%–15% on average and by up to 15%–25% in highly heterogeneous regions. This suggests that different ways of treating subgrid land surface heterogeneities could lead to noticeable biases in model output.

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