Semiannual Time Scales Research Articles

A spectral analysis of biosphere–atmosphere trace gas flux densities and meteorological variables across hour to multi-year time scales

The advent of long-term studies on CO 2 and water vapor exchange provides us with new information on how the atmosphere and biosphere interact. Conventional time series analysis suggests that temporal fluctuations of weather variables and mass and energy flux densities occur on numerous time scales. The time scales of variance that are associated with annual time series of meteorological variables, scalar flux densities and their covariance with one another, however, remain unquantified. We applied Fourier analysis to time series (4 years in duration) of photon flux density, air temperature, wind speed, pressure and the flux densities of CO 2 and water vapor. At the daily time scale, strong spectral peaks occurred in the meteorological and flux density records at periods of 12 and 24 h, due to the daily rising and setting of the sun. At the synoptic time scale (3–7 days) the periodic passage of weather fronts alter available sunlight and temperature. In turn, variations in these state variables affect carbon assimilation, respiration and transpiration. At the seasonal and semi-annual time scales, a broad spectral peak occurs due to seasonal changes in weather and plant functionality and phenology. In general, 21% of the variance of CO 2 exchange is associated with the annual cycle, 43% of the variance is associated with the diurnal cycle and 9% is associated with the semi-annual time scale. A pronounced spectral gap was associated with periods 3–4 weeks long. Interactions between CO 2 flux density ( F c) and sunlight, air temperature and latent heat flux density were quantified using co-spectral, coherence and phase angle analyzes. Coherent and in-phase spectral peaks occur between CO 2 exchange rates and water vapor exchange on annual, seasonal and daily time scales. A 180° phase shift occurs between F c and photon flux density ( Q p ) on seasonal and daily time scales because the temporal course of sunlight corresponds with the withdrawal of CO 2 from the atmosphere, a flux that possesses a negative sign. Covariations between F c and T air experience a 180° phase shift with one another at the seasonal time scale because rising temperatures are associated with more carbon uptake. At daily time scales the phase angle between F c and T air is on the order of 130°. This phase lag can be explained by the strong dependence of canopy photosynthesis on available light and the 2–3 h lags, which occur between the daily course of sunlight and air temperature. Spectral analysis was used to investigate the performance of a biophysical model (CANOAK) across a spectrum of time scales. By varying meteorology, leaf area index and photosynthetic capacity, the model was able to replicate most of the spectral gaps and peaks that were associated with CO 2 exchange, when soil moisture was ample.

Northern Arabian Sea variability from TOPEX/Poseidon altimetry data: an extension of the US JGOFS/ONR shipboard ADCP study

Sea-level anomalies (SLA) derived from the TOPEX/Poseidon (T/P) altimetry and inferred geostrophic currents within the northern Arabian Sea were examined for the period from March 1993 through November 1996. The primary objective of this study was to confirm and extend our understanding of the upper-ocean mesoscale variability observed in the shipboard acoustic Doppler current profiler (ADCP) data collected during the US JGOFS/ONR Arabian Sea Expedition (September 1994–January 1996). The accuracy of the T/P altimetry data (∼3 cm rms) results in an uncertainty in the altimeter-derived velocities comparable to the ADCP measurement error. Thus the T/P data provide a reasonable method for extending studies of the mesoscale dynamics for the region. Comparison of the T/P-derived geostrophic velocities with concurrent ADCP data showed good correlation, with an r 2 between 0.7 and 0.9 and rms differences of 10 cm s −1. The T/P data confirm both the overall spatial and seasonal current patterns observed by the ADCP. The monsoonally averaged rms sea-level anomalies indicate a high degree of intraseasonal variation due to the generation of squirts, jets and eddies all along the coast, the variability of which increases in both intensity and areal extent during the Southwest Monsoon. The SLA data indicate a much reduced degree of variability over the shelf. The SLA-derived eddy kinetic energy (EKE) fields are consistent with those derived earlier from the ADCP data in both distribution and magnitude. There is a large increase in EKE to the west and southwest and to a lesser extent to the south and a large area of relatively reduced eddy activity over much of the eastern and northern Arabian Sea. The area of reduced eddy activity coincides with the location of the most intense portions of the oxygen minimum zone found in the northern Arabian Sea. The spatial scales of the eddies responsible for the EKE distribution over the study area range between 200 and 500 km in the nearshore region, decreasing to 100–200 km offshore. While there is significant energy variability in annual and semi-annual time scales, a substantial portion of the energy is found between 50 and 120 days, and the relative importance of this frequency band increases offshore. Spectra indicate a distinct break in the frequency content of the eddy field at about 15°N, with little energy at less than annual periods south of this latitude.

Contribution of the atmosphere to the seasonal changes in earth's rotation

Using new data of atmospheric angular momentum for the period 1975–1995, the contributions of the atmosphere to the changes in LCD and polar motion on the seasonal time scale are investigated. The results show that, when the effects of wind and atmospheric pressure are considered, the atmosphere's contribution on the annual and semi-annual time scales may reach, respectively, 95% and 88%. We also give some quantitative results of the atmosphere's excitation of polar motion. On the annual time scale, the contribution to the X-component of polar motion is 16% and that to the Y-component is 43%. On the semi-annual time scale, the contributions to the X- and Y-components are, respectively, 9% and 30%. From the above results, it is clear that the contribution of the ocean should also be included in a more complete solution of the problem of excitation of the earth's rotation.

Geophysical contributions to satellite nodal residual variation

Even‐degree zonal gravitational variations due to mass redistribution within the Earth's system, especially in the atmosphere, hydrosphere, and oceans will lead to variations in the nodal precession rate of satellite orbit. The accurately measured nodal variation for LAGEOS 1 provides a means to study planetary‐scale mass redistributions and gravitational variations from the space. In this paper, we investigate atmospheric and hydrologic contributions to the LAGEOS 1 nodal changes using barometric pressure, soil moisture, and snow accumulation values from data‐assimilating numerical models. Oceanic effects are estimated from nonsteric sea level change determined by TOPEX/Poseidon satellite radar altimeter observation and a simple model for steric sea level changes. The results are compared with the LAGEOS 1 nodal change time series observed by satellite laser ranging. At annual and semiannual time scales, the atmosphere and hydrosphere provide significant contributions. The atmosphere provides broadband excitation of nodal changes at intraseasonal timescales. Seasonal and intraseasonal nontidal oceanic effects are also significant. General agreement between predicted and observed nodal precession rate residuals is improved relative to earlier studies, in part because of the better estimation of hydrological effects and new assessment of nontidal oceanic effects.

Contents of volume 17

Wavelet analysis of altimeter sea level in the Indian Ocean shows regions of high variability (maxima) and low variability (minima) at all time scales. At interannual time scales, i.e., at periods of 17 months or more, minima are seen at several places: in the central equatorial Indian Ocean; in the Arabian Sea along the south and west coasts of India and Sri Lanka, along the northern boundary, in the Gulf of Aden, and in patches along the coast of Oman; and in the Bay of Bengal along the east coasts of Sri Lanka and India south of ∼10°N, and in the southern bay east of the Sri Lanka thermal dome. We investigate the cause of these interannual minima using a linear, continuously stratified numerical model, which is able to simulate the observed minima. We separate the forcing into a set of processes: direct forcing by winds in the interior ocean, forcing by winds blowing along continental boundaries, and forcing by Rossby waves generated by the reflection of equatorial Kelvin waves at the eastern boundary. At interannual periods, minima (maxima) of interannual variability occur where the direct wind forcing and reflected Rossby waves interfere destructively (constructively). At interannual periods within the tropics, the adjustment time scale of the system is less than that of the forcing, leading to a quasi-steady balance, a property that distinguishes the interannual minima from those at annual and semiannual time scales. Idealized solutions show that the presence of India causes the minimum along the Indian west coast, and that it extends around the perimeter of the Arabian Sea into the Gulf of Aden.