Changing Trends Of Sea Surface Temperatures In The North Atlantic During The Holocene: A Study Of Proxy Records-simulation Comparison

The Holocene is the recent warming and glacier melting of the Quaternary,has opened a new chapter in geological history,and has an important influence on the survival and development of mankind.The Holocene is a kind of "historical similarity" similar to the present environmental change.It is of great research value and significance.Therefore,the study of climate change in the Holocene is the focus of the Past Global Changes research.The North Atlantic waters have a unique geographical location,it is a key area sensitive to Holocene climate change response.It is well known that ocean circulation systems and glacier activities have a dramatic impact on the North Atlantic and its neighboring areas to a large extent,and that important factors affecting Global Climate Change are also those climate change factors.So the North Atlantic region as one of the sources of Global Climate Change.However,the study of North Atlantic sea surface temperature and climate change is the key to comprehend the global environmental changes of the past.Therefore,the study of sea surface temperature changes in the North Atlantic during the Holocene has a crucial role to understand the Holocene climate change.Changing trends of the sea surface temperatures(SSTs)in the North Atlantic were analyzed in this study through comparing proxy records and climatic simulation results.Paleo-SST proxy records,including 27 alkenone records and 12 Mg/Ca records,are from the extended global database for alkenone-derived Holocene SST records(GHOST database).Climatic simulation results are from a long-term transient simulation performed with a coupled atmosphere-ocean-sea ice general circulation model,the Kiel Climate Model(KCM),forced by the Earth's orbital variations for the last 9500 years.Other forcing factors such as Greenhouse gas and continental icesheets have been neglected.Alkenone-derived SSTs exhibit a warming trend in the low-latitude North Atlantic(0°~15°N)and a cooling trend in the middle-and high-latitude North Atlantic during the Holocene.The greatest variability in SSTs is seen in the middle-latitude western North Atlantic,with a trend reaching-7.9°C/9.5ka.Mg/Ca-based SSTs show distinct trends from alkenone records with an increasing trend over the middle-latitude eastern and high-latitude North Atlantic and a decreasing trend over middle-latitude western and low-latitude North Atlantic.SST trends derived from Mg/Ca records are much smaller than that from alkenone records.Model results suggest a longitudinal tripole pattern in the linear trends of North Atlantic SSTs during the Holocene for all four seasons,with a center of decreasing trend sandwiched between two centers of increasing trend.North Atlantic SSTs were dominated by a warming trend in winter and spring and a cooling trend in summer and autumn.The cooling trend in winter and spring is distributed only over the southwest of the Labrador Sea,while the warming trend in summer and autumn is mainly distributed in the low-and high-latitude North Atlantic.Model-data comparisons suggest that alkenone records may reflect summer and autumn SSTs in the low-and middle-latitude North Atlantic.Mg/Ca records may represent summer and autumn SSTs in the low-latitude western North Atlantic but winter and spring SST in the middle-latitude eastern North Atlantic.In addition,Mg/Ca records in the middle-latitude western and high-latitude North Atlantic may represent all four seasons' SST.Changing trends of North Atlantic SSTs during the Holocene were greatly affected by winter and summer insolation.North Atlantic SSTs tend to extend their memories from winter to spring and from summer to autumn in response to the insolation forcing.Therefore,Mg/Ca records in the subtropical and subpolar North Atlantic can reflect winter SSTs even if there were no evidence for a calcifying population of planktonic foraminifera being present in winter as planktonic foraminifera records in this area can record spring ocean conditions that receive signal from winter SSTs.In addition,North Atlantic SSTs were modulated by North Atlantic atmospheric circulation that generated a longitudinal tripole pattern in the linear trends of North Atlantic SSTs during the Holocene for all four seasons.