Showing papers by "Manfred Mudelsee published in 2001"

Strong coherence between solar variability and the monsoon in Oman between 9 and 6 kyr ago

Abstract: Variations in the amount of solar radiation reaching the Earth are thought to influence climate, but the extent of this influence on timescales of millennia to decades is unclear. A number of climate records show correlations between solar cycles and climate1, but the absolute changes in solar intensity over the range of decades to millennia are small2 and the influence of solar flux on climate is not well established. The formation of stalagmites in northern Oman has recorded past northward shifts of the intertropical convergence zone3, whose northward migration stops near the southern shoreline of Arabia in the present climate4. Here we present a high-resolution record of oxygen isotope variations, for the period from 9.6 to 6.1 kyr before present, in a Th–U-dated stalagmite from Oman. The δ18O record from the stalagmite, which serves as a proxy for variations in the tropical circulation and monsoon rainfall, allows us to make a direct comparison of the δ18O record with the Δ14C record from tree rings5, which largely reflects changes in solar activity6,7. The excellent correlation between the two records suggests that one of the primary controls on centennial- to decadal-scale changes in tropical rainfall and monsoon intensity during this time are variations in solar radiation.

Note on the bias in the estimation of the serial correlation coefficient of AR(1) processes

Abstract: We derive approximating formulas for the mean and the variance of an autocorrelation estimator which are of practical use over the entire range of the autocorrelation coefficient ρ. The least-squares estimator 1 $$\Sigma _{i = 1}^{n - 1} \in _i \in _{i = 1} /\Sigma _{i = 1}^{n - 1} \in _i^2 $$ is studied for a stationary AR(1) process with known mean. We use the second order Taylor expansion of a ratio, and employ the arithmetic—geometric series instead of replacing partial Cesaro sums. In case of the mean we derive Marriott and Pope’s (1954) formula, with (n - 1)-1 instead of (n)-1, and an additional term ∝ (n - 1)-2. This new formula produces the expected decline to zero negative bias as ρ approaches unity. In case of the variance Bartlett’s (1946) formula results, with (n — 1)-1 instead of (n)-1. The theoretical expressions are corroborated with a simulation experiment. A comparison shows that our formula for the mean is more accurate than the higher-order approximation of White (1961), for ¦ρ¦> 0.88 and n ≥ 20. In principal, the presented method can be used to derive approximating formulas for other estimators and processes.