Kaz Higuchi

The relationship between the North Atlantic Oscillation and El Niño‐Southern Oscillation

We have applied a multiresolution cross-spectral analysis technique to resolve the temporal relationship between the NAO and ENSO. The study shows significant coherence between NAO and Nino3 SST in about 70% of the warm ENSO events from 1900 to 1995, of which 33% and 37% are associated with a 5- to 6-year period (E1) and a 2- to 4-year period (E2) oscillation terms in the spectral decomposition, respectively. The dominant teleconnection pattern associated with changes in the mean atmospheric circulation during the initial winter of a typical E1 and E2 events is the positive phase of the Pacific/North American (PNA) pattern. Non-coherence between the NAO and ENSO occurs during relatively weak Nino3 SST anomaly, with a teleconnection pattern which shows a strong negative phase of the NAO and a pattern which resembles a weak eastward shifted negative phase of the PNA pattern.

TWO CURVE FITTING METHODS APPLIED TO CO2 FLASK DATA

Digital filtering and harmonic regression curve fitting techniques are applied to CO2 flask data from four stations in North America (Pt. Barrow, Alert, Sable Island and Cape St. James) to evaluate these two different methodologies in terms of growth rate and seasonal cycle in the atmospheric CO2 concentration. Both methods agree relatively well in producing long-term atmospheric CO2 trend at each of the monitoring stations, as well as in capturing relatively large interannual variations in the annual growth rate. Furthermore, they both agree in indicating the dependency of the variation in the seasonal amplitude on the seasonal minimum concentration. The digital filtering technique is able to capture the local temporal variation in CO2 measurements much better than the harmonic regression method, although in some cases this variability is exaggerated in the digital filtering approach. The harmonic regression approach tends to smooth out the data, with much of the power in the very long period oscillations. Also, the timing of the occurrence of the seasonal minimum calculated by the digital filtering method tends to be earlier than that calculated by the harmonic regression method, although both methods do not indicate any major secular change in the timing. The overall assessment of the two methods applied to the CO2 flask data underscores the importance of using more than one curve fitting method before any conclusions can be drawn from the flask data about the interannual variability in the trend and seasonal cycle of the atmospheric CO2 concentration.

The association between the BWA index and winter surface temperature variability over eastern Canada and west Greenland

Since about 1970, winter surface temperature data from stations on coastal eastern Canada and western Greenland have shown detectable decadal cooling. In this study, we attempt to understand some aspect of this surface cooling trend by relating it to the variability of the Canadian Polar Trough (CPT). In order to facilitate the relationship, we introduce a new 50 kPa index called the Baffin Island–West Atlantic (BWA) index which, although reflecting the variability of the western structure of the North Atlantic Oscillation (NAO), is found to explain temperature variability better in north-eastern North America than the structure characterized by the NAO index. The decadal variability in the winter surface temperature is found to be associated with the BWA index at a statistically significant correlation of 0·85. Two distinctive winter climate regimes are found to exist in the climate record from 1947 to 1995, one before and one after about 1970. Although the magnitude of the variance does not change significantly from one regime to the next, the two regimes are characterized by statistically significantly different means and by two distinct spectral signatures. Variability before 1970 is dominated by interannual fluctuations, whereas afterwards much of the contribution to the variability comes from interdecadal fluctuations. Subtraction of the 1947–1969 winter 50 kPa mean height field from the 1970–1995 mean field shows that the change in the height field over the Northern Hemisphere is reflected in the enhancement of the negative phase of the NAO mode (which corresponds to a strong jet stream over the western Atlantic and a strong Icelandic low) and of the positive phase of the Pacific/North America (PNA) mode.

Modeling and Scaling Coupled Energy, Water, and Carbon Fluxes Based on Remote Sensing: An Application to Canada's Landmass

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 ...

Regional source/sink impact on the diurnal, seasonal and inter-annual variations in atmospheric CO2 at a boreal forest site in Canada

Time series of in-situ CO2 data from Fraserdale (50°N, 81°W) in the northern Ontario boreal forest is described, together with an analysis of observed variations occurring on daily to interannual timescales. Atmospheric CO2 measurements at Fraserdale reflect a complex interaction between the daily cycle of the vegetative carbon flux and the daily evolution of the boundary layer mixing dynamics. This is particularly evident during the growing season, when CO2 concentrations are influenced strongly by local and regional biospheric activities. In addition, the atmospheric CO2 measurements at Fraserdale are greatly influenced by the direction of atmospheric transport, and reflect the complex heterogeneous distribution of ecosystem types around the site. Averaged over the 7-yr period from 1990 to 1996, the seasonal cycle associated with air from west and northwest of the site shows an amplitude of ~19 ppm, while that associated with air from the south and southwest shows an amplitude of ~23 ppm; the seasonal minimum, on average, occurs about a week earlier in the latter case than in the former. This is reflective of the fact that many of the deciduous trees are located to the south and southwest of Fraserdale. Furthermore, its location in the boreal forest causes the seasonal minimum to occur on average in early August at Fraserdale, compared to late August observed at Alert and at many other background stations in the high-latitude Northern Hemisphere. At the Fraserdale site there is no statistically significant indication, during the 1990–1996 study period, of changes in the length of the growing season (as measured by zero crossing points in the seasonal cycle).

Latitudinal distribution of atmospheric CO2 sources and sinks inferred by δ13C measurements from 1985 to 1991

Net CO 2 fluxes between the atmosphere and the ocean, and between the atmosphere and the terrestrial biosphere, were estimated by constraining a two-dimensional atmospheric transport model with the CO 2 concentration and its δ 13 C data obtained from our shipboard measurements in the western Pacific region during the period April 1984 to December 1991. The results obtained for the non-El Nino and Southern Oscillation (ENSO) years during this time period showed that the ocean acted as a CO 2 sink in middle latitudes of both hemispheres and a CO 2 source around the equator. The results also suggested that during the 1984-1991 period there were biospheric CO 2 sources in southern low and northern middle latitudes, and significant biospheric sinks in northern high latitudes. By comparing the latitudinal distributions of CO 2 source/sink for ENSO years with those for non-ENSO years, it was found that a considerable amount of CO 2 was released from the terrestrial biosphere in low latitudes in association with the 1986-1987 ENSO event. While the oceanic region from the equator to 30°N acted as an excess CO 2 sink of about 1.0 Gt C yr -1 during 1987 and 1988, the oceanic contributions to the CO 2 anomaly in the atmosphere were less pronounced in the northern high latitudes and in the southern hemisphere. Also, compared with the estimated interannual fluctuations in the atmosphere-biosphere CO 2 flux, the net CO 2 flux between the atmosphere and the ocean showed relatively less interannual variability.

Global monthly CO2 flux inversion with a focus over North America

A nested inverse modelling system was developed for estimating carbon fluxes of 30 regions in North America and 20 regions for the rest of the globe. Monthly inverse modelling was conducted using CO2 concentration measurements of 3 yr (2001–2003) at 88 sites. Inversion results show that in 2003 the global carbon sink is -2.76 ± 0.55 Pg C. Oceans and lands are responsible for 88.5% and 11.5% of the sink, respectively. Northern lands are the largest sinks with North America contributing a sink of -0.97 ± 0.21 Pg C in 2003, of which Canada’s sink is -0.34 ± 0.14 Pg C. For Canada, the inverse results show a spatial pattern in agreement, for the most part, with a carbon source and sink distribution map previously derived through ecosystem modelling. However, discrepancies in the spatial pattern and in flux magnitude between these two estimates exist in certain regions. Numerical experiments with a full covariance matrix, with the consideration of the error structure of the a priori flux field based on meteorological variables among the 30 North America regions, resulted in a small but meaningful improvement in the inverted fluxes. Uncertainty reduction analysis suggests that new observation sites are still needed to further improve the inversion for these 30 regions in North America.

Secular variation of Northern Hemisphere 50 kPa geopotential height

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.

Regional Analysis of Northern Hemisphere 50 kPa Geopotential Heights from 1946 to 1985

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 analysi...

A multi-box model study of the role of the biospheric metabolism in the recent decline of δ18O in atmospheric CO2

Abstract From around 1993 to 1997, the NOAA-CU δ18O measurements at Pt. Barrow, Mauna Loa, Cape Kumukahi, Cape Grim and the South Pole show a decrease in atmospheric CO2δ18O of about 0.5°. Recently,Gillon and Yakir (2001) have attributed this decrease to a conversion of C3 forests to C4 grasslands through anthropogenic land-use change. However, their explanation can account for only about 0.02° yr−1 decrease rate. In this paper we offer a viable alternative explanation. We have used a multi-box model of the global carbon cycle and its δ18O to show that an increase in biospheric respiration (CO2 flux from plant with lower-than-atmosphereδ18O), combined with a decrease in the amount of CO2 (with higher-than-atmosphere δ18O) diffusing back from plant leaves before being assimilated as part of the gross primary production (GPP), could produce the observed decline in the atmospheric CO2δ18O. This decrease in the CO2 back diffusion out of leaves could be interpreted as an overall increase in both biospheric activities of photosynthesis and respiration. Change in the metabolic activities of the biosphere as a possible cause for the observed decrease inδ18O is a reasonable hypothesis, since isotopic fractionations that occur during CO2 exchange processes (photosynthesis and respiration) between the atmosphere and the biosphere contribute significantly to the observed variations in atmospheric CO2δ18O, while contribution from the net air-sea CO2 exchange is negligible.