MODES OF VARIABILITY IN A 1000-YEAR CLIMATE MODEL SIMULATION FOR 8.5 KA

Modes of internal climate variability are investigated for the Mid-Holocene, 8500 year ago (8.5 ka), using a 1000-year simulation with the NCAR Climate System Model (CSM). The CSM is a global coupled atmosphere-ocean-sea ice model.

ENSO variability, as measured by sea surface temperature (SST) anomalies in the Niño-3.4 region (5ºS-5ºN, 170º-120ºW), shows weaker variability (standard deviation=0.55ºC) compared to the present-day simulation (standard deviation=0.68ºC). This is consistent with the proxy data of Rodbell et al. and Tudhope et al. Warm and cold SST events in the central and eastern Pacific peak in boreal winter and wavelet decomposition shows that most of the power is contained at periods between 2 to 10 years, both similar to present. Extended warm and cold periods are also apparent in the time series.

The North Atlantic region shows considerable internal variability on decadal to centennial time scales. The North Atlantic Oscillation (NAO), defined as the normalized December-March sea level pressure difference between Iceland (64.2ºN, 22ºW) and Lisbon, Portugal (38.75ºN, 9ºW), indicates strong peaks at a period of ~7 years that occur intermittently every 100-200 years, and peaks in the 10-40 year band and the 50-80 year band that recur intermittently on longer time scales.

Time series of simulated eastern Greenland surface temperature and Greenland-Norwegian sea surface temperature exhibit strong fluctuations on multi-decadal time scales with a century long cold period that start at year 630 of the simulation. Eastern Greenland surface temperature cools abruptly by 8ºC. This cool event is also coincident with a weakening of both the North Atlantic Deep Water and Antarctic Bottom Water overturning cells and a Labrador Sea increase in sea ice extent. Proxy evidence suggests a major 100-year cooling fluctuation in the North Atlantic about 8.2 ka. It has been suggested that this event may have been caused by the response of the atmosphere-ocean-sea ice to a meltwater pulse emanating from the collapse of an ice dome in Hudson Bay. Our simulation for 8.5 ka forced only by Milankovitch orbital changes (no meltwater pulse) shows an event of similar magnitude and time scale.