ALBERT

Description: The variability of the Atlantic Meridional Overturning Circulation (AMOC) and its governing processes during the Last Glacial Maximum (LGM) is investigated in the Kiel Climate Model (KCM). Under LGM conditions, enhanced multidecadal AMOC variability is simulated relative to a preindustrial control run and surface heat flux variability linked to the East Atlantic pattern the primary driver of AMOC variability. In contrast, the multidecadal AMOC variability in the preindustrial control simulation is mainly driven by surface heat flux variability associated with the North Atlantic Oscillation (NAO). Stand-alone atmosphere model experiments show that the difference in mechanism is tightly linked to the differences in topography. The stronger multidecadal AMOC variability suggested by the KCM may be an important additional factor to understanding abrupt climate changes over the North Atlantic sector during the LGM.

Description: Proxy data suggest that the Plio-Pleistocene transition from ~3.2 - 3.0 to 2.5 Ma featured the onset of Northern Hemisphere glaciation and enhanced climate variability on obliquity timescale. Here, we investigate the influence of the expanding Greenland ice sheet (GrIS) on the mean climate and obliquity-related variability. Special attention is given to the Atlantic Meridional Overturning Circulation (AMOC). A series of climate model simulations suggest that the expanding GrIS weakens the AMOC by ~1 Sv, which is mainly due to reduced heat loss of the Greenland-Iceland-Norwegian Sea. Moreover, the expanded GrIS amplifies the Hadley circulation response to obliquity forcing. This drives enhanced obliquity-forced variations in freshwater export from the tropical Atlantic and in turn variations of the AMOC that increase by about a factor. The stronger AMOC response to obliquity forcing in turn drives a stronger global-mean near-surface temperature response. We conclude that the AMOC response to obliquity forcing is important to understand the enhanced climate variability on obliquity timescale during the Plio-Pleistocene transition.

Description: Climate models generally underestimate the pronounced warming in the sea surface temperature (SST) over the North Atlantic during the mid-Pliocene that is suggested by proxy data. Here, we investigate the influence of the North Atlantic cold SST bias, which is observed in many climate models, on the simulation of mid-Pliocene surface climate in a series of simulations with the Kiel Climate Model. A surface freshwater-flux correction is applied over the North Atlantic, which considerably enhances simulation of North Atlantic Ocean circulation and SST under present-day conditions. Using reconstructed mid-Pliocene boundary conditions with closed Bering and Arctic Archipelago Straits, the corrected model depicts significantly reduced model-proxy SST discrepancy in comparison to the uncorrected model. A key factor in reducing the discrepancy is the stronger and more sensitive Atlantic Meridional Overturning Circulation and poleward heat transport. We conclude that simulations of mid-Pliocene surface climate over the North Atlantic can considerably benefit from alleviating model biases in this region.

Description: The early Pliocene warm phase was characterized by high sea surface temperatures and a deep thermocline in the eastern equatorial Pacific. A new hypothesis suggests that the progressive closure of the Panamanian seaway contributed substantially to the termination of this zonally symmetric state in the equatorial Pacific. According to this hypothesis, intensification of the Atlantic meridional overturning circulation (AMOC) - induced by the closure of the gateway - was the principal cause of equatorial Pacific thermocline shoaling during the Pliocene. In this study, twelve Panama seaway sensitivity experiments from eight ocean/climate models of different complexity are analyzed to examine the effect of an open gateway on AMOC strength and thermocline depth. All models show an eastward Panamanian net throughflow, leading to a reduction in AMOC strength compared to the corresponding closed-Panama case. In those models that do not include a dynamic atmosphere, deepening of the equatorial Pacific thermocline appears to scale almost linearly with the throughflow-induced reduction in AMOC strength. Models with dynamic atmosphere do not follow this simple relation. There are indications that in four out of five models equatorial wind-stress anomalies amplify the tropical Pacific thermocline deepening. In summary, the models provide strong support for the hypothesized relationship between Panama closure and equatorial Pacific thermocline shoaling.

Description: Stable oxygen isotope records from central Greenland suggest disproportionally large long-term surface-air temperature (SAT) variability during the last glacial maximum (LGM) relative to preindustrial times. Large perturbations in mean atmospheric circulation and its variability forced by extensive Northern Hemisphere ice sheet coverage has been suggested as cause for the enhanced Greenland SAT variability. Here, we assess the factors driving Greenland SAT variability during the LGM by means of dedicated climate model simulations and find remote forcing from the Pacific of critical importance. Atmospheric teleconnections from the Interdecadal Pacific Oscillation (IPO), a multidecadal oscillation of sea-surface temperature in the Pacific Ocean, strongly intensify under LGM conditions, driving enhanced surface wind variability over Greenland, which in turn amplifies SAT variability by anomalous atmospheric heat transport. A major role of the IPO in forcing Greenland SAT variability also is supported by a number of models from the Paleoclimate Modelling Intercomparison Project Phase III.

Description: Long-term predictability of the North Atlantic sea surface temperature (SST) is commonly attributed to buoyancy-forced changes of the Atlantic Meridional Overturning Circulation. Here we investigate the role of surface wind stress forcing in decadal hindcasts as another source of extratropical North Atlantic SST predictability. For this purpose, a global climate model is forced by reanalysis (ERA-interim) wind stress anomalies over the period 1979-2017. The simulated climate states serve as initial conditions for decadal hindcasts. Significant skill in predicting detrended observed annual SST anomalies is observed over the extratropical central North Atlantic with anomaly correlation coefficients exceeding 0.6 at lead times of 4 to 7 years. The skill is insensitive to the calendar month of initialization and linked to upper-ocean heat content anomalies that lead anomalous SSTs by several years.

Description: We study the two primary modes of variability associated with the East Asian summer monsoon, as identified using a multivariate Empirical Orthogonal Function (EOF) analysis. The second mode is shown to be related to changes in intensity of the South Asian High at 100 hPa while, consistent with previous work, the first mode is associated with an index for the shear vorticity of the 850 hPa zonal wind over the monsoon region. We show that a linear, dry dynamical model, when driven by the diabatic heating anomalies associated with each mode, can reproduce many of the anomalous circulation features, especially for the first EOF and in the lower troposphere. The model results indicate the importance of diabatic heating anomalies over the tropical Indian Ocean in the dynamics of both modes, especially EOF-1, and illustrate the role of local diabatic feedback for intensifying the circulation anomalies; in particular, the subtropical anticyclonic anomalies that are found in the positive phase of both modes, and the circulation anomaly associated with the Meiyu/Changma/Baiu rain band. A running cross-correlation analysis shows that the second EOF is consistently linked to both the decaying and the onset phase of El Niño/Southern Oscillation (ENSO) events throughout the study period (1958-2001). We attribute the connection in the onset phase to zonal wind anomalies along the Equator in the west Pacific associated with this mode. On the other hand, a link between the first EOF and ENSO is found only in the post-1979 period. We note also the role of sea-surface temperature anomalies in the tropical Indian Ocean in the dynamics of EOF-1, and a link to the variability of the Indian summer monsoon in the case of EOF-2. Copyright © 2010 Royal Meteorological Society