Poleward Decrease Research Articles

Mesoscale Variability in the Atlantic Ocean from Geosat Altimetry and WOCE High-Resolution Numerical Modeling

Characteristic of the mesoscale variability in the Atlantic Ocean are investigated by analyzing the Geosat altimeter signal between 60°S and 60°N. The rms sea-surface variability for various frequency bands is studied, including the high-frequency eddy-containing band with periods <150 days. Wavenumber spectra and spatial eddy characteristics are analyzed over 10° by 10° boxes covering both hemispheres of the Atlantic Ocean. A comparison, with solutions of a high-resolution numerical experiment, developed as the Community Modeling Effort of the World Ocean Circulation Experiment, aids interpretation of the Geosat results in the tropical and subtropical Atlantic and provides a test of the model fluctuating eddy field. Results from Geosat altimetry show a wavenumber dependence close to k1−5 (k1 being the alongtrack wave-number) over almost the entire Atlantic Ocean except for areas in the tropical and subtropical Atlantic where the rms variability in the eddy-containing band is less than 5 cm, that is, not significantly different from the altimeter noise level. Characteristic eddy length scales inferred from Geosat data are linearly related with the deformation radius of the first baroclinic mode over the whole Atlantic Ocean, except for the equatorial regime (10°S to 10°N). The data-model comparison indicates that the high-resolution model with horizontal grid size of ⅓° and ° in latitude and longitude is quite capable of simulating observed eddy characteristics in the tropics and subtropics. In mid- and high latitudes, however, the model fails to simulate the pronounced poleward decrease in eddy scales. This leads to systematic discrepancies between the model and Geosat observation, with model scales being up to 50% larger than deduced from altimetry.

Stratospheric Response to Trace Gas Perturbations: Changes in Ozone and Temperature Distributions

The stratospheric concentration of trace gases released in the atmosphere as a result of human activities is increasing at a rate of 5 to 8 percent per year in the case of the chlorofluorocarbons (CFCs), 1 percent per year in the case of methane (CH(4)), and 0.25 percent per year in the case of nitrous oxide (N(2)O). The amount of carbon dioxide (CO(2)) is expected to double before the end of the 21st century. Even if the production of the CFCs remains limited according to the protocol for the protection of the ozone layer signed in September 1987 in Montreal, the abundance of active chlorine (2 parts per billion by volume in the early 1980s) is expected to reach 6 to7 parts per billion by volume by 2050. The impact of these increases on stratospheric temperature and ozone was investigated with a two-dimensional numerical model. The model includes interactive radiation, wave and mean flow dynamics, and 40 trace species. An increase in CFCs caused ozone depletion in the model, with the largest losses near the stratopause and, in the vertical mean, at high latitudes. Increased CO(2) caused ozone amounts to increase through cooling, with the largest increases again near 45 kilometers and at high latitudes. This CO(2)-induced poleward increase reduced the CFC-induced poleward decrease. Poleward and downward ozone transport played a major role in determining the latitudinal variation in column ozone changes.

A preliminary report on the numerical simulation of the three‐dimensional structure and variability of atmospheric N2O

A numerical simulation of atmospheric N2O using the GFDL 3‐D tracer model with a small uniform surface source (15 Mton yr−1) and stratospheric destruction (150 yr lifetime) has been run to a state near transport and chemical statistical equilibrium. The resulting N2O tropospheric distribution is relatively uniform with a slight excess in the Southern Hemisphere. In the model stratosphere there is a sharp poleward decrease in N2O mixing ratio away from high values in the tropics with pronounced winter minimums at 50°S and 60°N. Even with the small uniform surface source, relative standard deviations of N2O in the surface layer range from 0.1% to 0.8%, well within the range of recent measurements. Additional experiments suggest that motions acting upon N2O accumulation in the source region boundary layer and upon the mixing ratio gradient between the troposphere and lower stratosphere are the major sources of tropospheric N2O variability.

Buried seed populations in the subarctic forest east of Great Slave Lake, Northwest Territories

Buried seeds were studied in the subarctic forest east of Great Slave Lake, N.W.T., where 62 surface samples were collected in upland vegetation of different ages and compositions. No seeds germinated from any of the samples after cold treatment and exposure to warm, moist environment. Seeds were present in the soil, but most of them proved non-viable by the tetrazolium chloride test.Lack of viable buried seeds is consistent with a poleward decrease in the buried viable seed populations under forests. Possibly the gradient parallels a decrease in length of growing season and its effect on germination strategies. The lack of viable buried seeds provides a partial explanation of observed slow vegetational recovery after fire in subarctic forests.

Taxonomic Diversity of Recent Bivalves and Some Implications for Geology

A compilation of worldwide taxonomic diversity data for Recent Bivalvia shows a consistent poleward decrease in numbers of species, genera, and families. Spherical harmonic surfaces fitted to the raw data of bivalve diversity accurately predict the position of the present equator and rotational poles at each of the three taxonomic levels examined. These findings suggest that diversity studies of fossils at the specific, generic, or familial levels will prove useful for determining ancient equator and pole positions. In addition, residuals from the fitted surfaces show promise of indicating local areas of anomalous geographic or climatic history.