Description: Numerical modeling enables a comprehensive understanding not only of the Earth's system today, but also of the past. To date, a significant amount of time and effort has been devoted to paleoclimate modeling and analysis, which involves the latest and most advanced Paleoclimate Modelling Intercomparison Project phase 4 (PMIP4). The definition of seasonality, which is influenced by slow variations in the Earth's orbital parameters, plays a key role in determining the calculated seasonal cycle of the climate. In contrast to the classical calendar used today, where the lengths of the months and seasons are fixed, the angular calendar calculates the lengths of the months and seasons according to a fixed number of degrees along the Earth's orbit. When comparing simulation results for different time intervals, it is essential to account for the angular calendar to ensure that the data for comparison are from the same position along the Earth's orbit. Most models use the classical calendar, which can lead to strong distortions of the monthly and seasonal values, especially for the climate of the past. Here, by analyzing daily outputs from multiple PMIP4 model simulations, we examine calendar effects on surface air temperature and precipitation under mid-Holocene, Last Interglacial, and pre-industrial climate conditions. We came to the following conclusions. (a) The largest cooling bias occurs in boreal autumn when the classical calendar is applied for the mid-Holocene and Last Interglacial, due to the fact that the vernal equinox is fixed on 21 March. (b) The sign of the temperature anomalies between the Last Interglacial and pre-industrial in boreal autumn can be reversed after the switch from the classical to angular calendar, particularly over the Northern Hemisphere continents. (c) Precipitation over West Africa is overestimated in boreal summer and underestimated in boreal autumn when the classical seasonal cycle is applied. (d) Finally, month-length adjusted values for surface air temperature and precipitation are very similar to the day-length adjusted values, and therefore correcting the calendar based on the monthly model results can largely reduce the artificial bias. In addition, we examine the calendar effects in three transient simulations for 6–0 ka by AWI-ESM, MPI-ESM, and IPSL-CM. We find significant discrepancies between adjusted and unadjusted temperature values over continents for both hemispheres in boreal autumn, while for other seasons the deviations are relatively small. A drying bias can be found in the summer monsoon precipitation in Africa (in the classical calendar), whereby the magnitude of bias becomes smaller over time. Overall, our study underlines the importance of the application of calendar transformation in the analysis of climate simulations. Neglecting the calendar effects could lead to a profound artificial distortion of the calculated seasonal cycle of surface air temperature and precipitation.

Description: Numerical modeling enables a comprehensive understanding not only of the Earth's system today, but also of the past. To date, a significant amount of time and effort has been devoted to paleoclimate modeling and analysis, which involves the latest and most advanced Paleoclimate Modelling Intercomparison Project phase 4 (PMIP4). The definition of seasonality, which is influenced by slow variations in the Earth's orbital parameters, plays a key role in determining the calculated seasonal cycle of the climate. In contrast to the classical calendar used today, where the lengths of the months and seasons are fixed, the angular calendar calculates the lengths of the months and seasons according to a fixed number of degrees along the Earth's orbit. When comparing simulation results for different time intervals, it is essential to account for the angular calendar to ensure that the data for comparison are from the same position along the Earth's orbit. Most models use the classical calendar, which can lead to strong distortions of the monthly and seasonal values, especially for the climate of the past. Here, by analyzing daily outputs from multiple PMIP4 model simulations, we examine calendar effects on surface air temperature and precipitation under mid-Holocene, Last Interglacial, and pre-industrial climate conditions. We came to the following conclusions. (a) The largest cooling bias occurs in boreal autumn when the classical calendar is applied for the mid-Holocene and Last Interglacial, due to the fact that the vernal equinox is fixed on 21 March. (b) The sign of the temperature anomalies between the Last Interglacial and pre-industrial in boreal autumn can be reversed after the switch from the classical to angular calendar, particularly over the Northern Hemisphere continents. (c) Precipitation over West Africa is overestimated in boreal summer and underestimated in boreal autumn when the classical seasonal cycle is applied. (d) Finally, month-length adjusted values for surface air temperature and precipitation are very similar to the day-length adjusted values, and therefore correcting the calendar based on the monthly model results can largely reduce the artificial bias. In addition, we examine the calendar effects in three transient simulations for 6–0 ka by AWI-ESM, MPI-ESM, and IPSL-CM. We find significant discrepancies between adjusted and unadjusted temperature values over continents for both hemispheres in boreal autumn, while for other seasons the deviations are relatively small. A drying bias can be found in the summer monsoon precipitation in Africa (in the classical calendar), whereby the magnitude of bias becomes smaller over time. Overall, our study underlines the importance of the application of calendar transformation in the analysis of climate simulations. Neglecting the calendar effects could lead to a profound artificial distortion of the calculated seasonal cycle of surface air temperature and precipitation.

Description: Orbital forcing is a major driver of climate variability on timescales of 10,000 to 100,000 years. The orbital parameters responsible for these changes are Eccentricity, Obliquity and Procession. For comparison between simulated paleo and present climate, biases in seasonality may emerge without the use of the angular calendar, as in the today’s classical calendar, the start/end of one season may correspond to different angles between the vernal equinox and the earth. On the other hand, model resolution is also an important factor for resolving some small-scale process in the simulated world. Hence, the Alfred Wegener Institute has established and developed the state-of-the-art high-resolution Earth system models AWI-ESM1 and AWI-ESM2, with the ice-ocean component being based on finite element/volume formation. In this study, we take advantage of the simulation results from AWI-ESM1 and AWI-ESM2. Climate variables such as surface temperature and precipitation are analyzed and compared between paleo and modern climate conditions. The simulations for mid-Holocene (MH, 6k B.P.) and Last Inter-glacial (LIG, 127k B.P.) were designed to examine the climate responses to changes in orbital forcings and greenhouse gases. The continental configuration remains unchanged with respect to the pre-industrial (PI) condition. Due to the insolation anomaly induced by orbital configuration, the seasonality for the MH is larger than today resulting from the positive anomaly of solar insolation in boreal summer and autumn, and negative anomalies during winter time. The Antarctic was colder in DJF and MAM and warmer in JJA and SON in the MH as compared to present. There was more precipitation in the MH over the tropical rain belt as a result of a northward shift of the inter-tropical convergence zone (ITCZ). Additionally, there is an enhanced seasonality in the LIG as compared to present-day for the simulated surface temperature with a cooling of up to 5 K in boreal winter and warming of more than 5 K for boreal summer. Furthermore, a stronger summer monsoon over the Northern Hemisphere monsoon domains, including the North America, South Asia, and Western Africa, is observed in the LIG when compared to PI which is accompanied by a pronounced increase in precipitation, which mimics the pattern of the MH but much stronger in magnitude. However, seasonality definition of paleo-climate based on present-day fixed calendar can lead to biases for the seasonal changes. In the present study, we performed calendar correction onto the simulated PI, MH and LIG climate. The results show that, generally the classical calendar tends to underestimate the warming over MH/LIG northern hemisphere especially during SON. The temperature anomalies in MH/LIG SON with regards to PI can even flip its sign after calendar conversion. Moreover, the classical calendar overestimate and underestimate the MH and LIG Africa summer monsoon precipitation for JJA and SON respectively. Our study indicates the importance of seasonality definition on investigating the past climate. In addition, we also explored the calendar effect on surface air temperature and precipitation in a transient simulation for the past 6000 years, in which we varied the greenhouse gases and orbital parameters while keep other boundary conditions unchanged. Our main focus was NH continents as they appear to have the most pronounced response to calendar conversion. Anomalies in NH temperatures (between angular and classical means) over land tend to have different trends in different seasons. For DJF and MAM, there is a gradual decline in the temperature differences from 6K to present associated with calendar correction. The temperature bias is not strong owing to the fact that, the beginning and end of summer in angular calendar for 0k is similar as in our fixed-length calendar used today. The mean angular-minus-classical temperature in SON present a pronounced warming from 6k to 3k, and an obvious cooling from 3k to 0k. The strong bias around 6k in SON results from the large shift in autumn days (6 days) between angular and classical calendars. Similar trend is noticed over the Northern Hemisphere ocean, though with a smaller magnitude. On the other hand, we found a noticeable calendar effect on precipitation, especially over the Africa Monsoon domain in summer and autumn months. Under present fixed-length calendar, the Africa monsoon rainfall is largely underestimated, and such bias becomes weaker from 6k to 0k. Another topic of the present thesis is the characteristic of El Nino and the Southern Oscillation (ENSO) in a transient study which was carried out using AWI-ESM2. In-order to investigate the evolution of ENSO we examined the Nino3.4 index, which is calculated as the sea surface temperature anomalies over the region 5N-5S, 170W-120W. In this new study, inter-annual variability and seasonality of the Nino3.4 index was investigated. In addition, composite analysis is performed to further investigate the climate response to ENSO through time (i.e 6k - 0k) regionally and globally. Our results indicate an increase in both inter-annual variability and seasonality of ENSO through time. We also found a negative correlation between the Nino3.4 index and the precipitation over all Northern Hemisphere monsoon domains. Considering the rising trend in Nino3.4 index and its strengthened variability from 6k to 0k, we could conclude that extreme dry events in present-day very likely occur more often than in the mid-Holocene. Seasonality on the other hand has shown an increase which has peak around 2k before attaining steady increase. However, the changes in seasonality can result from ENSO magnitude and the timing of ENSO development. While the composite analysis show no noticeable change.