No Consistent Simulated Trends in the Atlantic Meridional Overturning Circulation for the Past 6,000 Years

The Atlantic Meridional Overturning Circulation (AMOC) is a key feature of the North Atlantic with global ocean impacts. The AMOC's response to past changes in forcings during the Holocene provides important context for the coming centuries. Here, we investigate AMOC trends using an emerging set of transient simulations using multiple global climate models for the past 6,000 years. Although some models show changes, no consistent trend in overall AMOC strength during the mid-to-late Holocene emerges from the ensemble. We interpret this result to suggest no overall change in AMOC, which fits with our assessment of available proxy reconstructions. The decadal variability of the AMOC does not change in ensemble during the mid- and late-Holocene. There are interesting AMOC changes seen in the early Holocene, but their nature depends a lot on which inputs are used to drive the experiment

Vegetation and soil feedbacks at the last glacial maximum

Vegetation feedback at the Last Glacial Maximum (LGM, about 21,000 calendar years ago) remains an unresolved question. A global atmospheric general circulation model (AGCM) is asynchronously coupled with an equilibrium terrestrial biosphere model in the present study. The coupled model is then used to investigate the influences of vegetation and soil feedbacks on the LGM climate. It is found that the simulated geographical distribution of vegetation at the LGM differs from the present pattern dramatically, and glacial vegetation cover tends to be reduced on average. Vegetation feedback alone leads to an annual Surface temperature decrease of 0.31 degrees C over the LGM ice-free continental areas. Additional soil feedback reinforced vegetation-induced cooling over high latitude Eurasia and from the eastern Middle East eastward to the Indian Peninsula significantly. In the tropics, a terrestrial annual surface cooling of 0.45 degrees C is produced by vegetation and soil feedbacks. it is shown that vegetation and soil feedbacks partly reduce data-model discrepancy as produced by the AGCM alone in some regions such as Central Africa, the Indian Peninsula, South China, and North Australia. (C) 2008 Elsevier B.V. All rights reserved. [References: 51

Last Glacial Maximum over China: Sensitivities of climate to paleovegetation and Tibetan ice sheet

With the boundary conditions appropriate for the Last Glacial Maximum (LGM), including ice sheets, sea surface temperatures, sea-ice distribution, atmospheric CO2 concentration, the Earth’s orbital parameters, topography, and coastline, the atmospheric general circulation model of the Institute of Atmospheric Physics (IAP-AGCM) computes colder and drier conditions than for present day. Global annual-average surface temperature decreased by 5.3°C, and terrestrial precipitation was down by 29%. It is shown that IAP-AGCM LGM simulation compares favorably to results from other AGCMs, and/but generally shows a weak terrestrial cooling when compared to paleoclimatic reconstructions in tropics. The 21 ka (ka: thousands of years ago) vegetation reconstruction is introduced into the model to study the regional climate response to the changes in vegetation and associated soil characteristics over China. The additional cooling due to these two changes reduces, to a certain degree, the model-data discrepancies. In addition, under the precondition of continental ice existing over part of the Tibetan Plateau at the LGM, the authors examine the regional climate response to the continental ice. It follows that the glacial-age environment over the Tibetan Plateau is a very important factor for 21 ka climate simulation in East Asia

Transient response of the Atlantic Meridional Overturning Circulation to enhanced freshwater input to the Nordic Seas–Arctic Ocean in the Bergen Climate Model

The transient response of the climate system to anomalously large freshwater input to the high latitude seas is examined using the newly developed Bergen Climate Model. A 150-yr twin-experiment has been carried out, consisting of a control and a freshwater integration. In the freshwater integration, the freshwater input to the Arctic Ocean and the Nordic Seas is artificially increased by a factor of 3, or to levels comparable to those found during the last deglaciation. The obtained response shows a reduced maximum strength of the Atlantic Meridional Overturning Circulation (AMOC) over the first 50 yr of about 6 Sv (1 Sv =106 m3 s−1), followed by a gradual recovery to a level comparable to the control integration at the end of the period. The weakened AMOC in the freshwater integration is caused by reduced deep-water formation rates in the North Atlantic subpolar gyre and in the Nordic Seas, and by a reduced southward flow of intermediate water masses through the Fram Strait. The recovery of the AMOC is caused by an increased basin-scale upwelling in the Atlantic Ocean of about 1 Sv, northward transport of saline waters originating from the western tropical North Atlantic, and a surface wind field maintaining the inflow of Atlantic Water to the Nordic Seas between the Faroes and Scotland. Associated with the build-up of more saline waters in the western tropical North Atlantic, a warming of ∼0.6 ◦C over the uppermost 1000 m of the water column is obtained in this region. This finding is consistent with paleo records during the last deglaciation showing that the tropics warmed when the high latitudes cooled in periods with reduced AMOC. Furthermore, the results support the presence of a coupled North-Atlantic-Oscillation-like atmosphere–sea-ice–ocean response mode triggered by the anomalous freshwater input. Throughout most of the freshwater integration, the atmospheric circulation is characterized by anomalously low sea level pressure in the Nordic Seas and anomalously high sea level pressure over Spain. This forces the North Atlantic Drift to follow a more easterly path in the freshwater integration than in the control integration, giving an asymmetric sea surface temperature response in the northern North Atlantic, and thereby maintaining the properties of the AtlanticWater entering the Nordic Seas between the Faroes and Scotland throughout the freshwater integration