No abstract

Abstract. This study presents linear trends of coastal upwelling intensity in the later part of the 20th century employing various indices of upwelling, derived from meridional wind stress and sea surface temperature. The analysis was conducted in the four major coastal upwelling regions in the world, which are off North-West Africa, Lüderitz, California and Peru. The trends in meridional wind stress showed a steady increase of intensity from 1960-2001, which was also reflected in the SST index calculated for the same time period. The steady cooling observed in the instrumental records of SST off California substantiated this observation further. It was also noted that the trends in meridional wind stress obtained from different datasets differ substantially from each other. Correlation analysis showed that basin-scale oscillations like the Atlantic Multidecadal Oscillation (AMO) and the Pacific Decadal Oscillation (PDO) could not be directly linked to the observed increase of upwelling intensity off NW Africa and California respectively. The relationship of the North Atlantic Oscillation (NAO) with coastal upwelling off NW Africa turned out to be ambiguous due to a negative correlation between the NAO index and the meridional wind stress and a lack of correlation with the SST index. Our results give additional support to the hypothesis that the coastal upwelling intensity increases globally because of raising greenhouse gas concentrations in the atmosphere and an associated increase of the land-sea pressure gradient and meridional wind stress.

Abstract. The Last Glacial Maximum (LGM, ∼ 21 000 years ago) has been a major focus for evaluating how well state-of-the-art climate models simulate climate changes as large as those expected in the future using paleoclimate reconstructions. A new generation of climate models has been used to generate LGM simulations as part of the Paleoclimate Modelling Intercomparison Project (PMIP) contribution to the Coupled Model Intercomparison Project (CMIP). Here, we provide a preliminary analysis and evaluation of the results of these LGM experiments (PMIP4, most of which are PMIP4-CMIP6) and compare them with the previous generation of simulations (PMIP3, most of which are PMIP3-CMIP5). We show that the global averages of the PMIP4 simulations span a larger range in terms of mean annual surface air temperature and mean annual precipitation compared to the PMIP3-CMIP5 simulations, with some PMIP4 simulations reaching a globally colder and drier state. However, the multi-model global cooling average is similar for the PMIP4 and PMIP3 ensembles, while the multi-model PMIP4 mean annual precipitation average is drier than the PMIP3 one. There are important differences in both atmospheric and oceanic circulations between the two sets of experiments, with the northern and southern jet streams being more poleward and the changes in the Atlantic Meridional Overturning Circulation being less pronounced in the PMIP4-CMIP6 simulations than in the PMIP3-CMIP5 simulations. Changes in simulated precipitation patterns are influenced by both temperature and circulation changes. Differences in simulated climate between individual models remain large. Therefore, although there are differences in the average behaviour across the two ensembles, the new simulation results are not fundamentally different from the PMIP3-CMIP5 results. Evaluation of large-scale climate features, such as land–sea contrast and polar amplification, confirms that the models capture these well and within the uncertainty of the paleoclimate reconstructions. Nevertheless, regional climate changes are less well simulated: the models underestimate extratropical cooling, particularly in winter, and precipitation changes. These results point to the utility of using paleoclimate simulations to understand the mechanisms of climate change and evaluate model performance.

[1] We produced gridded monthly sea-surface boundary conditions for the Atlantic Ocean at the Last Glacial Maximum (LGM) based on the sea-surface temperature reconstruction of the GLAMAP project. We used an ocean general circulation model (OGCM), subject to these sea-surface boundary conditions and a corresponding wind stress field from an atmospheric general circulation model, to study the differences in the distribution of the main water masses between the LGM and the present. Our global OGCM is characterized by high vertical resolution, low vertical diffusion, and isopycnal mixing and hence allows for a realistic representation of the hydrology and circulation of the modern Atlantic Ocean. According to a series of LGM experiments with an increasing sea-surface salinity anomaly in the Weddell Sea, the ventilated thermocline was colder than today by 2-3°C in the North Atlantic Ocean and, in the experiment with the largest anomaly (1.0 beyond the global anomaly), by 4-5°C in the South Atlantic Ocean. Its depth was reduced by 50 m on average, most notably in the tropics. In the North Atlantic Ocean the outcrop locations of the thermocline isopycnal surfaces migrated southward by 5°-10°, and the ventilation increased. In the South Atlantic Ocean the mixed layer and thermocline water masses were dominated by cold water originating from Drake Passage, and the import of warm water from the Indian Ocean was reduced to about 4 Sv or 40% of its modern value. Antarctic Intermediate Water was colder by 3-4°C and could be traced as far as 10°N. The meridional overturning rates of North Atlantic Deep Water (NADW) and Antarctic Bottom Water (AABW) in the Atlantic Ocean were similar to those of the present-day experiment (9-10 Sv and 4 Sv, respectively). However, NADW cooled by 2.5°C and AABW by 1°C. AABW was near the freezing point of seawater at the surface and the saltiest water mass in the Atlantic Ocean, even saltier than NADW. We show that the differences between the LGM and the present-day experiments can be traced back to the changes in the subpolar and interhemispheric sea-surface density gradients.

Changes in heat transport associated with fluctuations in the strength of the Atlantic meridional overturning circulation (AMOC) are widely considered to affect the position of the Intertropical Convergence Zone (ITCZ), but the temporal immediacy of this teleconnection has to date not been resolved. Based on a high‐resolution marine sediment sequence over the last deglaciation, we provide evidence for a synchronous and near‐linear link between changes in the Atlantic interhemispheric sea surface temperature difference and continental precipitation over northeast Brazil. The tight coupling between AMOC strength, sea surface temperature difference, and precipitation changes over northeast Brazil unambiguously points to a rapid and proportional adjustment of the ITCZ location to past changes in the Atlantic meridional heat transport.

No abstract

No abstract

No abstract