NAO Shifting During The Last Glacial Maximum In PMIP2Coupled Model Simulations

The spatial shift of the centers of the North Atlantic Oscillation (NAO) is studied from the Twentieth Century Reanalysis version2dataset in modern times and the results of two ocean-atmosphere coupled models, CCSM3and HadCM3M2, in the Paleoclimate Modeling Intercomparison Project Phase2(PMIP2) during the Last Glacial Maximum (LGM,21ka BP) relative to pre-industrial (PI,0ka BP).In regard to the spatial shift of the NAO in the current climate, the spatial shift of the NAO is characterized by four NAO spatial shift indices, the zonal and meridional shifts of the NAO southern and northern action centers, by identifying NAO action centers directly on winter mean sea-level pressure (SLP) anomaly maps. It is found that the zonal and meridional shift trends of the action centers of NAO move along a path of southwest-northwest direction. Multi-Taper Method of Spectrum Analysis shows that the four NAO spatial shift indices have periodicity of2-6years, sharing the common periodicity of2-3years with NAO index in terms of high-frequency variations. On a decadal time scale, the NAO spatial shift indices are closely (positively) related to the NAO index, which is in agreement with previous studies of the relationship between the NAO index and the spatial shift of the NAO pattern. However, there is no relationship between the NAO index and the meridional shift of the northern action center on an interannual time scale. The significant relationship between the NAO index and the interannual variability of NAO spatial shift indices is very likely to be associated with synoptic-scale Rossby wave breaking, which generates surface pressure anomalies and thus affects the phase and pattern of the NAO. The correlations of winter westerly winds over90°W0°and the NAO index and the NAO spatial shift indices have a ’+-+-’ structure from the Equator to the North Pole. Although there is close correlation between the NAO spatial shift indices and the strength of the zonal winds in the North Atlantic region, the effect of the zonal winds on the NAO spatial shift differs at different latitudes. Hence, the role of the zonal winds is probably a result of the NAO spatial shifts. The zonal shift of the northern action centers of NAO variability during1978-1997is rather unusual in the context of the recent137years (1872-2008). However, it is not a special case for the past137years in terms of the whole spatial structure of interannual NAO variability.The NAO is significantly different during LGM from that at PI, showing a more negative NAO regime. In addition, the NAO pattern changes too, exhibiting a shifting of southeast-northwest (SE-NW) tilting or north-south symmetry structure at PI to southwest-northeast (SW-NE) tilting during LGM and the eastward shift of the northern center of NAO. Besides, both of the centers have a southward shift except the northern center in CCSM3. Apart from the southward shift of the NAO pattern, the other two characters resemble that in the late1970s based on observations.The climate at LGM is significantly different from today, which is characterized by lower greenhouse gas concentrations, more negative NAO regime, decreased North Atlantic storm activity and weaker mean westerly winds over the North Atlantic region. Previous studies have suggested that the eastward shift of NAO in the late1970s is probably due to the increased greenhouse gas concentrations, a higher NAO index, increased North Atlantic storm activity, stronger mean westerly winds in the North Atlantic region and the northward shift of the core of the Atlantic jet. However, these factors related to the shift of NAO pattern are significantly different as compared to late20th century except for the uncertainty of the position of the core of the Atlantic jet.Further study reveals that the spatial shift of the action centers of NAO at LGM is probably due to the sharp decline in the temperature over the North America region (120°-30°and40°-60°N), which is closely related to the presence of massive ice-sheets over the Northern Hemisphere, and the existence of larger areas of winter sea ice over the North Atlantic area (south of65°N). The colder North America Area mean air temperature is the main cause for the NAO dipole anomaly in the simulations of CCSM3. Due to the presence of massive ice-sheets over North America and increased sea ice over North Atlantic, there is an increased zonal temperature gradient between40°-60°N over North Atlantic region, which generates surface pressure anomalies and thus affects the pattern of the NAO. However in the simulations of HadCM3M2, the main factor is the existence of larger areas of North Atlantic sea ice. The North Atlantic sea ice concentration is positively related to the phase of the NAO. Besides, it also affects the spatial pattern of NAO variability. The significant relationship between the North Atlantic sea ice concentration and the NAO is very likely to be associated with the SST variability. However, the relationship between the SST and the NAO disappears in the LGM simulation with CCSM3, which results in the weak correlation between the North Atlantic sea ice concentration and the NAO.This paper also analyzes the changes of winter sea ice over the North Atlantic range. Simulation results show the increase in the winter sea ice concentration over the Greenland-Norwegian Sea and North Atlantic Ocean and the southern end of the sea ice cover reaches to about35°N during LGM as compared to PI. Further study reveals that the existence of larger areas of winter sea ice is closely related to the strong cooling of the sea surface temperature (SST) during the LGM, which is not due to the differences in insolation caused by the variability of orbital parameters, and the change in Atlantic Ocean currents. The weaker Gulf Stream and North Atlantic Drift and stronger Labrador Current during LGM could lead to the decrease in oceanic northward heat transport, resulting in the strong cooling of the SST and the increase in the sea ice concentration.