OS1.7

Using the climate model MIROC4m, we simulate self-sustained oscillations of millennial-scale periodicity in the climate and Atlantic meridional overturning circulation under glacial conditions. We show two cases of extreme climatic precession and examine the mechanism of these oscillations. When the climatic precession corresponds to strong (weak) boreal seasonality, the period of the oscillation is about 1,500 (3,000) years. During the stadial, hot (cool) summer conditions in the Northern Hemisphere contribute to thin (thick) sea ice, which covers the deep convection sites, triggering early (late) abrupt climate change. During the interstadial, as sea ice is thin (thick), cold deep-water forms and cools the subsurface quickly (slowly), which influences the stratification of the North Atlantic Ocean. We show that the oscillations are explained by the internal feedbacks of the atmosphere-sea ice-ocean system, especially subsurface ocean temperature change and salt advection feedback with a positive feedback between the subpolar gyre and deep convection.

How to cite: Kuniyoshi, Y., Abe-Ouchi, A., Sherriff-Tadano, S., Chan, W.-L., and Saito, F.: Effect of Climatic Precession on Dansgaard-Oeschger-like oscillations, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-6750, https://doi.org/10.5194/egusphere-egu22-6750, 2022.

Dominant Euro-Atlantic climate modes such as the North Atlantic Oscillation (NAO), the Eastern Atlantic pattern (EA), the Eastern Atlantic Western Russian pattern (EAWR), and the Scandinavian pattern (SCA) significantly affect interannual-to-decadal Euro-Mediterranean climate fluctuations, especially in winter.

In this contribution, we will present and discuss results from a CMIP6 multi-model analysis performed to investigate the robustness of historical and projected state and variability of such modes under the historical and ssp585 future scenario of anthropogenic forcing (fossil-fueled development with 8.5W/m² forcing level) simulations, focusing on the winter season.

Toward this goal, we first search for a reliable box-based index definition for each of the abovementioned observed climate modes and, then, we perform a comparative assessment of the temporal, spectral and distributional properties of the so-defined indices during the historical (1850-2014) and ssp585 future scenario (2015-2099) time periods, with a special focus on the two interdecadal periods 1960-1999 and 2060-2099.

Results show overall good skills of the historical ensemble to reproduce the observed temporal, spectral and distributional properties of all considered modes. At the end of the 21st Century the ssp585 ensemble yields non-significant distributional changes for NAO, EAWR, and SCA indices and a transition to a stronger baroclinic structure for EA, with persistent positive anomalies in the mid-troposphere enhancing globally-driven warming over the Euro-Mediterranean region. The hemispheric spatial correlation patterns with temperature and precipitation significantly change for all modes, that is, we observe a significant modulation of the teleconnections associated with each index.

How to cite: Cusinato, E., Rubino, A., and Zanchettin, D.: Winter Euro-Atlantic Climate Modes: Future Scenarios From a CMIP6 Multi-Model Ensemble, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-3596, https://doi.org/10.5194/egusphere-egu22-3596, 2022.

The 20th century “early warming” (1910-1940) and cooling (1940-1970) of the Northern Hemisphere offer an interesting contrast of periods with opposite temperature trends, similar hemispheric temperature anomalies, yet very different temperature anomaly patterns. These contrasts are particularly clear in the North Atlantic sector, which exhibits large climate variability over a range of time scales, from short (weather regimes) to long (Atlantic Multidecadal Variability). In this study, we explore the role of the atmospheric circulation (North Atlantic jet stream) in determining the temperature anomaly patterns over the 20^th century. While different jet configurations are associated with distinct synoptic temperature patterns in the North Atlantic sector, only some are found to contribute substantially to longer term temperature trends. Notably, the southern jet configuration has the strongest temperature anomalies, with a dipole signal that is opposite from the one under the tilted jet configuration. At the same time, these two jet configurations exhibit relatively large decadal variations in frequency (days of occurrence in given winter seasons), with trends that are almost the opposite. In fact, changes in the frequency of southern and tilted jet “days” alone account for much of the North Atlantic and Arctic temperature variability on decadal time scales, including the differences between the early warming and cooling periods (e.g., the flipped warming versus cooling patterns are associated with fewer southern jet days and more tilted jet days). However, the reconstruction skill of the 30-year mean temperature anomaly in the North Atlantic sector using jet frequency exhibits decadal variability, with high skill scores interestingly coinciding with the positive phases of the Atlantic Multidecadal Variability. The lower reconstruction skill especially during the global warming period from the1980s onwards is likely due to the impact from the warming hole in the North Atlantic, which dominates the temperature patterns in the North Atlantic. Overall, the evolution of Northern Hemisphere surface temperature over the 20^th century is found to be influenced by North Atlantic jet variability, with lower frequency ocean effects contributing more in recent decades.

How to cite: Tao, D., Madonna, E., and Li, C.: Using atmospheric variability to understand the wintertime regional warming and cooling patterns in the North Atlantic Sector, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-5057, https://doi.org/10.5194/egusphere-egu22-5057, 2022.

Climate model biases in the North Atlantic (NA) low-level tropospheric westerly jet are a major impediment to reliably representing variability of the NA climate system and its wider influence, in particular over western Europe. We highlight an early-winter equatorward jet bias in Coupled Model Inter-comparison Project (CMIP) models and assess whether this bias is reduced in the CMIP6 models in comparison to the CMIP5 models. Historical simulations from the CMIP5 and CMIP6  are further compared against reanalysis data over the period 1862-2005.  

The results show that an equatorward bias remains significant in CMIP6 models in early winter. Almost all CMIP5 and CMIP6 model realizations exhibit equatorward climatological jet latitude biases with ensemble mean biases of 3.0° (November) and 3.0° (December) for CMIP5 and 2.5° and 2.2° for CMIP6. This represents an approximately one-fifth reduction for CMIP6 compared to CMIP5. The equatorward jet latitude bias is mainly associated with a weaker-than-observed frequency of poleward daily-weekly excursions of the jet to its northern position. A potential explanation is provided.  Our results indicate a strong link between NA jet latitude bias and systematically too-weak model-simulated low-level baroclinicity over eastern North America in early-winter.  

Implications for model representation of NA atmosphere-ocean linkages will be presented. In particular CMIP models with larger equatorward jet biases tend to exhibit weaker correlations between temporal variability in jet speed and sea surface conditions over the NA sub-polar gyre (SPG). This has implications for the ability of climate models to represent key aspects of atmospheric variability and predictability that are associated with atmosphere-ocean interactions in the SPG region.  

How to cite: Bracegirdle, T., Lu, H., and Robson, J.: Equatorward North Atlantic jet biases in CMIP models and implications for simulated regional atmosphere-ocean linkages, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-6401, https://doi.org/10.5194/egusphere-egu22-6401, 2022.

Record low surface temperatures were observed in the subpolar North Atlantic during 2015, despite the majority of the global ocean experiencing higher than average surface temperatures. We compute mixed layer temperature budgets in the ECCO Version 4 state estimate to further understand the processes responsible for the North Atlantic cold anomaly. We show that surface forcing was the cause of approximately 75% of the initial cooling in the winter of 2013/14, after which the cold anomaly was sequestered beneath the deep winter mixed layer. Re-emergence of the cold anomaly during the summer/autumn of 2014 was primarily driven by a strong temperature gradient across the base of the mixed layer. Vertical diffusion resulted in approximately 70% of the re-emergence, with entrainment of deeper water driving the remaining 30%. In the summer of 2015, surface warming of the mixed layer was then anomalously low, resulting in the most negative temperature anomalies. Spatial patterns in the budgets show that the initial surface cooling was strongest in the south of the region, due to strong westerly winds related to the positive phase of the East Atlantic Pattern. Subsequent anomalies in surface fluxes associated with the North Atlantic Oscillation were stronger in the north, but the impact on the average temperature of the mixed layer was largely masked by anomalously high winter mixed layer depths.

How to cite: Sanders, R., Jones, D., Josey, S., Sinha, B., and Forget, G.: Using mixed layer heat budgets to determine the drivers of the 2015 North Atlantic cold anomaly in ocean state estimates, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-1682, https://doi.org/10.5194/egusphere-egu22-1682, 2022.

The North Atlantic Jet Stream is well known to leave an imprint on the North Atlantic SST in the form of a tri-polar pattern.  The majority of the existing research has focused on the winter jet stream position or strength of the jet stream.  Here we look at the response of the North Atlantic SSTs to the strength and position of the North Atlantic Jet Stream across all seasons in the CMIP6 piControl simulations.  For the case of both the strength and position of the jet stream the multi-model mean response is a tripolar SST pattern, with the response to the changes in strength showing a slight horseshoe pattern with the northern and southern most anomalies connected on the east and most evident in the summer.  The SST response to winter and spring jet stream changes persist the longest with the northern most imprint on the SSTs lasting up to 2 years.  The response to changes in the jet stream in the summer and fall leave an imprint on the SSTs lasting atmost into the following year.   Furthermore, we investigate at how these responses vary among the CMIP6 models and potential mechanisms leading to the persistence.

How to cite: Mecking, J., Sinha, B., Harvey, B., Robson, J., and Bracegirdle, T.: Seasonal differences in the persistence of SST’s Response to the North Atlantic Jet Stream, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-5829, https://doi.org/10.5194/egusphere-egu22-5829, 2022.

How to cite: Hellmich, L., Matei, D., Suarez-Gutierrez, L., and Müller, W. A.: Contribution of the Atlantic Ocean to European Heat Extremes, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-5694, https://doi.org/10.5194/egusphere-egu22-5694, 2022.

The Atlantic meridional overturning circulation (AMOC) is an important part of our climate system, which keeps the North Atlantic relatively warm. It is predicted to weaken under climate change. The AMOC may have a tipping point beyond which recovery is difficult, hence showing quasi-irreversibility (hysteresis). Although hysteresis has been seen in simple models, it has been difficult to demonstrate in comprehensive global climate models.

We present initial results from the North Atlantic hosing model intercomparison project, where we applied an idealised forcing of a freshwater flux over the North Atlantic in 9 CMIP6 models. The AMOC weakens in all models from the freshening, but once the freshening ceases, the AMOC recovers in some models, and in others it stays in a weakened state. We discuss how differences in feedbacks affect the AMOC response.  

How to cite: Jackson, L., Alastrue-De-Asenjo, E., Bellomo, K., Danabasoglu, G., Hu, A., Jungclaus, J., Meccia, V., Saenko, O., Shao, A., and Swingedouw, D.: AMOC thresholds in CMIP6 models: NAHosMIP, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-2778, https://doi.org/10.5194/egusphere-egu22-2778, 2022.

Global warming since the industrial revolution has led to a series of changes in the atmosphere and ocean. As a key indicator of global ocean circulation, AMOC has shown a weakening in recent decades from both the observed and simulated results. This process which is not only affected by the local variation of the Arctic, but also by the ocean and atmosphere circulation changes in the middle and lower latitudes, might have important implications for future global climate changes. We employ the Alfred Wegener Institute Climate Model (AWI-CM 1.1 LR) and a method of perturbing coupled models to quantify and understand the impact of anthropogenic warming on the slowdown of AMOC. Conducted one control (CTRL) experiment and three sensitivity experiments (60N, 60NS, and GLOB) in which CO2 concentration were abruptly quadrupled either regionally (60N-north of 60°N, 60NS-south of 60°N) or globally (GLOB). The goal of our research is to identify the response of AMOC weakening to the quadrupling of CO2 concentration in different regions and provide future insight into ocean circulation changes in the context of climate warming. Our results show that CO2 forcing outside the Arctic dominates the weakening of AMOC. In a warming climate, the poleward heat transport increased due to the extra-Arctic CO2 forcing, which enhanced the upper ocean average stratification within the mixed-layer depth over Nordic Seas and Labrador Sea and thus weakens the AMOC to a large extent. The warming in upper-layer also lead to the dominant role of temperature contribution to stratification. However, in both the deep convection regions, the mechanism resulting in the strengthening of stratification might be quite different.

How to cite: Chen, J., Wang, X., and Wang, X.: The weakening of AMOC highly linked to climate warming outside the Arctic, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-4440, https://doi.org/10.5194/egusphere-egu22-4440, 2022.