Length of astronomical seasons, total and average insolation over seasons

Abstract

Daily insolation at a given latitude is often used in the explanation of the long-term variations of paleoclimate records. However, the length of astronomical seasons, which has been paid less attention, could also be important. This paper provides an original calculation of the length of these seasons using the eccentric anomaly. The past and future long-term variations as well as the spectral characteristics of the length of astronomical seasons, of the total and average insolation over the astronomical seasons and of the caloric insolation are shown and compared. The length of astronomical seasons is only a function of climatic precession. The total irradiation for a given latitude and an astronomical season is mainly a function of obliquity except that it is mainly a function of eccentricity in a latitudinal band that is season dependent and from where the phase between obliquity and total irradiation reverses (e.g. ∼11.5°N for the astronomical northern hemisphere summer half-year). The spectral characteristic of the mean insolation over the astronomical summer half-year varies in latitude and time. It contains stronger obliquity signal in high latitudes than in low latitudes, but the obliquity signal already gets very weak at 45°N/S leaving the dominance of climatic precession between 45°N and 45°S. The variation of the mean summer half-year insolation over the last 1 Ma is characterized by a Mid-Brunhes transition at ∼450 ka BP, with a dominant role of climatic precession at all latitudes before ∼450 ka BP but significantly enhanced role of obliquity in high latitudes after this time. The simultaneous weakening of the variations of climatic precession and obliquity around 800 ka ago, a time corresponding to the Mid-Pleistocene Transition, leads to weak variations in both total and mean seasonal insolation. The insolation of a specific case, the Holocene, is finally discussed. Depending on the sensitivity to different types of insolation, different climate variables and regions could show different evolution and trend during the Holocene.