Sensitivity of equatorial Pacific and Indian Ocean watermasses to the position of the Indonesian Throughflow

The sensitivity of the thermal structure of the equatorial Pacific and Indian Ocean pycnoclines to a model's representation of the Indonesian Straits connecting the two basins is investigated. Two integrations are performed using the global HOPE ocean model. The initial conditions and surface forcing for both cases are identical; the only difference between the runs is that one has an opening for the Indonesian Straits which spans the equator on the Pacific side, and the other has an opening which lies fully north of the equator. The resulting sensitivity throughout much of the upper ocean is greater than 0.5°C for both the equatorial Indian and Pacific. A realistic simulation of net Indonesian Throughflow (ITF) transport (measured in Sverdrups) is not sufficient for an adequate simulation of equatorial watermasses. The ITF must also contain a realistic admixture of northern and southern Pacific source water

Testing the Mann et al. (1998) approach to paleoclimate reconstructions in the context of a 1000-yr control simulation with the ECHO-G coupled climate model

Statistical reconstructions of past climate variability based on climate indicators face several uncertainties: for instance, to what extent is the network of available proxy indicators dense enough for a meaningful estimation of past global temperatures?; can statistical models, calibrated with data at interannual timescales be used to estimate the low-frequency variability of the past climate?; and what is the influence of the limited spatial coverage of the instrumental records used to calibrate the statistical models? Possible answers to these questions are searched by applying the statistical method of Mann et al. to a long control climate simulation as a climate surrogate. The role of the proxy indicators is played by the temperature simulated by the model at selected grid points.It is found that generally a set of a few tens of climate indicators is enough to provide a meaningful estimation (resolved variance of about 30%) of the simulated global annual temperature at annual timescales. The reconstructions based on around 10 indicators are barely able to resolve 10% of the temperature variance. The skill of the regression model increases at lower frequencies, so that at timescales longer than 20 yr the explained variance may reach 65%. However, the reconstructions tend to underestimate some periods of global cooling that are associated with temperatures anomalies off the Antarctic coast and south of Greenland lasting for about 20 yr. Also, it is found that in one 100-yr period, the low-frequency behavior of the global temperature evolution is not well reproduced, the error being probably related to tropical dynamics.This analysis could be influenced by the lack of a realistic variability of external forcing in the simulation and also by the quality of simulated key variability modes, such as ENSO. Both factors can affect the large-scale coherence of the temperature field and, therefore, the skill of the statistical models

Variability of the Atlantic meridional overturning circulation in the last millennium and two IPCC scenarios

International audienceThe variability of the Atlantic meridional overturning circulation (AMOC) is investigated in several climate simulations with the ECHO-G atmosphere-ocean general circulation model, including two forced integrations of the last millennium, one millennial-long control run, and two future scenario simulations of the twenty-first century. This constitutes a new framework in which the AMOC response to future climate change conditions is addressed in the context of both its past evolution and its natural variability. The main mechanisms responsible for the AMOC variability at interannual and multidecadal time scales are described. At high frequencies, the AMOC is directly responding to local changes in the Ekman transport, associated with three modes of climate variability: El Niño-Southern Oscillation (ENSO), the North Atlantic Oscillation (NAO), and the East Atlantic (EA) pattern. At low frequencies, the AMOC is largely controlled by convection activity south of Greenland. Again, the atmosphere is found to play a leading role in these variations. Positive anomalies of convection are preceded in 1 year by intensified zonal winds, associated in the forced runs to a positive NAO-like pattern. Finally, the sensitivity of the AMOC to three different forcing factors is investigated. The major impact is associated with increasing greenhouse gases, given their strong and persistent radiative forcing. Starting in the Industrial Era and continuing in the future scenarios, the AMOC experiences a final decrease of up to 40% with respect to the preindustrial average. Also, a weak but significant AMOC strengthening is found in response to the major volcanic eruptions, which produce colder and saltier surface conditions over the main convection regions. In contrast, no meaningful impact of the solar forcing on the AMOC is observed. Indeed, solar irradiance only affects convection in the Nordic Seas, with a marginal contribution to the AMOC variability in the ECHO-G runs

Robust changes in tropical rainy season length at 1.5 degrees C and 2 degrees C

Changes in the hydrological cycle are among the aspects of climate change most relevant for human systems and ecosystems. Besides trends in overall wetting or drying, changes in temporal characteristics of wetting and drying are of crucial importance in determining the climate hazard posed by such changes. This is particularly the case for tropical regions, where most precipitation occurs during the rainy season and changes in rainy season onset and length have substantial consequences. Here we present projections for changes in tropical rainy season lengths for mean temperature increase of 1.5 °C and 2 °C above pre-industrial levels. Based on multi-ensemble quasi-stationary simulations at these warming levels, our analysis indicates robust changes in rainy season characteristics in large parts of the tropics despite substantial natural variability. Specifically, we report a robust shortening of the rainy season for all of tropical Africa as well as north-east Brazil. About 27% of West Africa is projected to experience robust changes in the rainy season length with a mean shortening of about 7 days under 1.5 °C. We find that changes in the temporal characteristics are largely unrelated to changes in overall precipitation, highlighting the importance of investigating both separately.ISSN:1748-9326ISSN:1748-931

Robust changes in tropical rainy season length at 1.5 degrees C and 2 degrees C

Changes in the hydrological cycle are among the aspects of climate change most relevant for human systems and ecosystems. Besides trends in overall wetting or drying, changes in temporal characteristics of wetting and drying are of crucial importance in determining the climate hazard posed by such changes. This is particularly the case for tropical regions, where most precipitation occurs during the rainy season and changes in rainy season onset and length have substantial consequences. Here we present projections for changes in tropical rainy season lengths for mean temperature increase of 1.5 °C and 2 °C above pre-industrial levels. Based on multi-ensemble quasi-stationary simulations at these warming levels, our analysis indicates robust changes in rainy season characteristics in large parts of the tropics despite substantial natural variability. Specifically, we report a robust shortening of the rainy season for all of tropical Africa as well as north-east Brazil. About 27% of West Africa is projected to experience robust changes in the rainy season length with a mean shortening of about 7 days under 1.5 °C. We find that changes in the temporal characteristics are largely unrelated to changes in overall precipitation, highlighting the importance of investigating both separately.ISSN:1748-9326ISSN:1748-931