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Previous work has shown that anthropogenic aerosol (AA) forcing drives a strengthening in the Atlantic meridional overturning circulation (AMOC) in CMIP6 historical simulations over 1850–1985, but the mechanisms have not been fully understood. Across CMIP6 models, it is shown that there is a strong correlation between surface heat loss over the subpolar North Atlantic (SPNA) and the forced strengthening of the AMOC. Despite the link to AA forcing, the AMOC response is not strongly related to the contribution of anomalous downwelling surface shortwave radiation to SPNA heat loss. Rather, the spread in AMOC response is primarily due to the spread in turbulent heat loss. We hypothesize that turbulent heat loss is larger in models with strong AA forcing because the air advected over the ocean is colder and drier, in turn because of greater AA-forced cooling over the continents upwind, especially North America. The strengthening of the AMOC also feeds back on itself positively in two distinct ways: by raising the sea surface temperature and hence further increasing turbulent heat loss in the SPNA, and by increasing the sea surface density across the SPNA due to increased northward transport of saline water. A comparison of key indices suggests that the AMOC response in models with strong AA forcing is not likely to be consistent with observations.

Future changes in tropical cyclone properties are an important component of climate change impacts and risk for many tropical and midlatitude countries. In this study we assess the performance of a multimodel ensemble of climate models, at resolutions ranging from 250 to 25 km. We use a common experimental design including both atmosphere‐only and coupled simulations run over the period 1950–2050, with two tracking algorithms applied uniformly across the models. There are overall improvements in tropical cyclone frequency, spatial distribution, and intensity in models at 25 km resolution, with several of them able to represent very intense storms. Projected tropical cyclone activity by 2050 generally declines in the South Indian Ocean, while changes in other ocean basins are more uncertain and sensitive to both tracking algorithm and imposed forcings. Coupled models with smaller biases suggest a slight increase in average TC 10 m wind speeds by 2050., Key Points Biases in tropical cyclone distribution, frequency, and intensity are generally reduced in models at 25 km resolutionNorthern Hemisphere basins show mixed responses to future forcing, while Southern Indian Ocean activity projected to declineFuture changes in 10 m wind speed in coupled models are mixed, and models with lower bias suggest small increases

Leading up to and during the summer of 2015 sea surface temperatures (SSTs) in the eastern North Atlantic Subpolar Gyre reached anomalously low values while in the subtropical gyre just to the SSTs were anomalously warm. Recent observation and modelling studies have found evidence showing that these SST anomalies can be linked to the heat wave experienced over Europe that summer. The latest observation based data still shows anomalously cold temperatures, as well as the anomalously fresh conditions that went along the 2015 cold blob in the upper layers of the eastern North Atlantic Subpolar gyre. A second heat wave over Europe occurred in the summer of 2018 where the SSTs reached another minimum value. Therefore, being able to predict the development, enhancement and persistence of such an anomaly is essential for good seasonal and longer predictions. At present several modelling systems have had difficulties in simulating/maintaining the 2015 cold blob. In this work we apply a novel initialization technique using anomalous initialization from a forced ocean simulation to simulate the 2015 cold blob. Initial results show that the model is able to maintain the cold blob as well as have a strengthening of the cold blob, however, it has difficulties capturing the timing of this strengthening.

Labrador Slope Water is known to exist in the Slope Sea off the US eastern shelf as a relatively fresh and cool water mass deriving from the Labrador Current further north, and is present between the upper layer US shelf-derived water masses and the deeper Deep Western Boundary current waters, typically near 400-600m. This LSLW is investigated in the EN4 database and shown to penetrate as far south as Cape Hatteras (74-75°W), having previously only been described as far west as the Gulf of Maine (66°W). We then examine, using both EN4 and Line W observations, the changes of this water mass between 2005-2008, when the strength of Atlantic Meridional Overturning Circulation (AMOC) measured by the RAPID array at 26°N, was high, and 2009-2015, when the AMOC was low. We show that in the AMOC high period, there was a larger volume of the LSSW present on the northern side of the Gulf Stream system which resulted in an increased meridional slope of the isopycnals near these depths, commensurate with increased geostrophic transport, and also in a more southerly position, of the Gulf Stream after separation at Cape Hatteras. The LSLW could therefore play an important role in decadal timescale variations in the North Atlantic climate system through its impact on the Gulf Stream and AMOC.