In the spirit of furthering collaboration and exchange, the program of the 4th km-scale hackathon hosted at the Max-Planck-Institute for Meteorology (MPI-M) earlier this year centered on intensifying interactions within and between different working groups. To expand upon these critical periods of mutual exchange, several keynote speeches were presented on overarching topics that string together the diverse themes convoluted in nextGEMS, WarmWorld, and EERIE. On Wednesday – the halfway point of the hackathon – the MPI-M’s newest member on the board of directors, Sarah Kang, gave an insightful talk, providing an outlook on her prospective work and its links to ongoing research at the institute.
From the outset, Sarah Kang noted that the research underpinning her talk, “Possible shifting mechanism for tropical Pacific surface warming pattern”, was motivated by a “passion for understanding why things do what they do and how they work”. With this indicative motto in mind, Kang’s presentation indexed the central problem tackled by her research: the discrepancy between model and observed sea surface temperatures in the Southern Pacific Ocean (SPO). This divergence was highlighted by contrasting simulations based on observational records with different model projections from 1979-2014. However, since the large observed trends cannot be explained by natural variability alone, there may be an external factor influencing the temperature of the SPO that is presently not captured by the applied models – What are the mechanisms underlying this forced response?
The real difficulty in identifying such mechanisms lies in isolating them from the many convoluted processes that induce warming and cooling patterns over the greater Pacific area. One factor impacting the cooling in the South Pacific is the relative warming trend in the Indian and Atlantic Oceans. For the former, this teleconnection can be explained by a strengthening of the so-called trade winds, the prevailing easterly winds at the equator, caused by a warming of the Indian Ocean. Another process tthought to influence the South Pacific cooling is a decrease in sea surface temperature (SST) in the Southern Ocean (SO). Here, increasing amounts of Antarctic meltwater impact the internal variability associated with deep ocean convection, thereby amplify a milder cooling effect associated natural variability. This latter point may offer an inroad to study the model-observation discrepancy as commonly used GCMs do not represent the SO cooling.
The experimental setup to test these links between the SO and the Southern Pacific is based on this insight. By using historical simulations from CMIP5 and CMIP6 runs, and comparing them to simulation scenarios adapted by Sarah Kang and her collaborators, the team could discern a substantial difference in outcome. In the latter simulations runs, termed Southern Ocean Pacemaker (SOPACE) simulations, historical radiative forcing was included and the sea surface temperature anomalies poleward of 40°S were restored to observations from 1970-2014. While global warming trends again dominated historical simulations, the SOPACE runs clearly linked the SO SST declines to the Southern Pacific cooling.
Although the experiments have detected SO SST as an external forcing of Southern Pacific cooling, trends in multi-decadal cooling are expected to be transient features that eventually subside into the dominant global warming trend. However, open questions remain attached to this line of research, particularly about the role of mesoscale processes in modulating the mechanisms at fast time scales. Therefore, one of the next steps in bringing this research to the MPI-M will be to adopt the globally coupled version of ICON (5km ocean and 10km atmosphere) in experimental setups.
Ever wondered how human-induced climate change can influence km-scale whirls in the ocean, so-called ocean eddies? The study by Beech et al. (2022) about long-term evolution of ocean eddy activity in a warming world, delves into this question by investigating the response of ocean eddy activity to anthropogenic climate change through climate modeling. Here is a summary of the key messages.
Specifically, the research employs climate change projections to analyze the variability and long-term trends of ocean currents and eddy kinetic energy (EKE) in different regions. By examining the representation of EKE in a climate model and comparing it with observed EKE from satellite altimetry data, the study aims to improve our understanding of ocean eddies – the weather of the ocean – and how they change in a warming planet.
This research field is crucial in the context of climate change because eddies are known to subsequently impact ocean systems through ventilation, volume transport, carbon sequestration and heat, and nutrient transport.
The study uses the AWI-CM-1-1-MR climate model. This model is used as part of the CMIP6 (Coupled Model Intercomparison Project Phase 6) ensemble, and it has been developed to simulate the Earth’s climate system. This model is also used in nextGEMS and one of its unique features is its variable-resolution ocean grid, which allows for a more accurate representation of eddy activity in the world’s oceans by employing enhanced resolution in dynamically active regions. The ocean component also utilizes a highly scalable and dynamical core, in addition to an unstructured mesh. This enables it to overcome the computational challenges associated with simulating long time series at sufficient resolutions needed to represent eddies.
The study projects several shifts on ocean eddy activity due to anthropogenic climate change impacts, resulting in implications for various oceanic processes and circulation patterns. For instance, EKE is expected to shift poleward in most eddy-rich regions, whereas it is expected to intensify in the Kuroshio Current, Brazil and Malvinas currents, and Antarctic Circumpolar Current. Conversely, the Gulf Stream is projected to experience a decrease in EKE, which is due to a decline of the Atlantic Meridional Overturning Circulation (AMOC). Overall, these projections of EKE in the world’s oceans show pronounced transitions on a global scale. Furthermore, these changes are linked to broader climate elements such as the decline of the AMOC; the intensification of Agulhas leakage, and the shifting Southern Hemisphere westerlies.
The study shows that it is difficult to conclude something so robust from relatively short satellite time series. Additionally, the authors highlight it will be important to revisit the results of the study with truly eddy-rich models, like the ones employed in nextGEMS.
References:
Beech, N., Rackow, T., Semmler, T., Danilov, S., Wang, Q., & Jung, T. (2022). Long-term evolution of ocean eddy activity in a warming world. Nature Climate Change, 12, 910-917. https://doi.org/10.1038/s41558-022-01478-3
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