Improving sea surface temperature in a regional ocean model through refined sea surface temperature assimilation

Infrared (IR) and passive microwave (PMW) satellite sea surface temperature (SST) retrievals are valuable to assimilate into high-resolution regional ocean forecast models. Still, there are issues related to these SSTs that need to be addressed to achieve improved ocean forecasts. Firstly, satellite SST products tend to be biased. Assimilating SSTs from different providers can thus cause the ocean model to receive inconsistent information. Secondly, while PMW SSTs are valuable for constraining models during cloudy conditions, the spatial resolution of these retrievals is rather coarse. Assimilating PMW SSTs into high-resolution ocean models will spatially smooth the modeled SST and consequently remove finer SST structures. In this study, we implement a bias correction scheme that corrects satellite SSTs before assimilation. We also introduce a special observation operator, called the supermod operator, into the Regional Ocean Modeling System (ROMS) four-dimensional variational data assimilation algorithm. This supermod operator handles the resolution mismatch between the coarse observations and the finer model. We test the bias correction scheme and the supermod operator using a setup of ROMS covering the shelf seas and shelf break off Norway. The results show that the validation statistics in the modeled SST improve if we apply the bias correction scheme. We also find improvements in the validation statistics when we assimilate PMW SSTs in conjunction with the IR SSTs. However, our supermod operator must be activated to avoid smoothing the modeled SST structures on spatial scales smaller than twice the PMW SST footprint. Both the bias correction scheme and the supermod operator are easy to apply, and the supermod operator can easily be adapted for other observation variables

The weakening AMOC under extreme climate change

Changes in the Atlantic Meridional Overturning Circulation (AMOC) in the quadrupled CO2 experiments conducted under the sixth Coupled Model Intercomparison Project (CMIP6) are examined. Increased CO2 triggers extensive Arctic warming, causing widespread melting of sea ice. The resulting freshwater spreads southward, frst from the Labrador Sea and then the Nordic Seas, and proceeds along the eastern coast of North America. The freshwater enters the subpolar gyre north of the separated Gulf Stream, the North Atlantic Current. This decreases the density gradient across the current and the current weakens in response, reducing the infow to the deepwater production regions. The AMOC cell weakens in tandem, frst near the North Atlantic Current and then spreading to higher and lower latitudes. This contrasts with the common perception that freshwater caps the convection regions, stifing deepwater production; rather, it is the infow to the subpolar gyre that is suppressed. Changes in surface temperature have a much weaker effect, and there are no consistent changes in local or remote wind forcing among the models. Thus an increase in freshwater discharge, primarily from the Labrador Sea, is the precursor to AMOC weakening in these simulations

Slowdown of the Atlantic meridional overturning circulation in a coupled simulation

The Atlantic meridional overturning circulation (AMOC) is projected to weaken under global warming. This study investigates this weakening overturning and addresses the question regarding which aspects lead to such a weakening. A selection of suggestions on mechanisms which can be responsible for the AMOC weakening are presented, and the aim is to answer if the weakening is consistent with one or several of these suggestions. The study makes use of outputs from a coupled climate model participating in the Coupled Model Intercomparison Project Phase 6 (CMIP6), and the model of use is NorESM2-LM. The chosen model simulation experiences a quadrupling of the atmospheric CO2 concentration, which results in a rapid and intense global warming and a slowdown of the AMOC. The results suggest that the weakening of the AMOC is consistent with a reduction in the meridional density gradient. A reduction in the meridional density gradient is associated with a reduction of the eastward shear through the thermal wind relation, thus slowing down the eastward flowing branch of the North Atlantic Current (NAC). The weakening of this branch is communicated to the currents it is feeding and being fed by, such that the entire system of surface currents flowing northward slows down. Increased freshwater into the northern North Atlantic is the key to reducing the meridional density gradient. However, the large-scale gradient that stretches from the tropics to the northernmost regions (the subpolar gyre and the Nordic Seas) does not weaken during the very first years of AMOC weakening. Instead, a weakening of a smaller-scale gradient across the eastward flowing NAC is found. Moreover, the results suggest that changes in the local winds in the northern North Atlantic may contribute to the weakening of the AMOC by directly weakening the currents in the subpolar gyre region. This possibility has not been fully tested, and further investigation is required