Marine Heatwave Imprints on Salinity in the Northwest Atlantic

Climate modes synergistically influence marine heatwaves in the North Sea

Climate modes synergistically influence marine heatwaves in the North Sea

Abstract. Global shelf seas have experienced unprecedented marine heatwaves (MHWs) in recent decades. Although MHWs have been extensively studied at the global scale, their regional variability and underlying mechanisms remain poorly understood, particularly in shelf seas influenced by multiple climate modes. Here, we examine MHW variability in the Northeastern Atlantic shelf using a correlation-based k-means clustering approach. Two distinct subregions with contrasting seasonal patterns are identified. In winter, the southern North Sea experiences increased MHW frequency, intensity, and duration. This enhancement is linked to a positive East Atlantic Pattern, which intensifies westerly winds and enhances warm Atlantic inflow through both atmospheric and oceanic pathways. In contrast, the northern North Sea shows enhanced MHW frequency and duration in summer, while MHW intensity weakens. This summer response is modulated by Atlantic Multidecadal Variability, with its positive phase strengthening Pacific–Atlantic connections via Rossby wave propagation, altering cloud cover and surface radiative forcing. A shallow mixed layer, enhanced stratification, and circulation-induced upwelling favor frequent and persistent but less intense summer MHWs. This north–south contrast demonstrates that different combinations of atmospheric and oceanic processes shape MHW variability across the shelf, providing a diagnostic and mechanistic framework for understanding regional MHW variability and its potential predictability.

Marine Heatwaves and NAO-Related Ocean–Atmosphere Variability in the North Atlantic

Increasing greenhouse gas concentrations are placing severe pressure on the Earth system, particularly on the ocean, which plays a vital role in carbon and heat uptake, and overall climate regulation. Consequently, the ocean is experiencing an accelerated warming, leading to an increase in the occurrence of extreme seawater temperature events, called Marine Heatwaves (MHWs). According to the most common definition, an MHW event is identified when local temperatures exceed the 90th percentile threshold of the climatology for at least five consecutive days. In this study, the definition was modified by calculating both the mean and the 90th percentile of SST over the entire available historical period (1982–2022), rather than using a fixed 30-year baseline. While MHWs can develop as a function of multiple drivers (including subsurface heat re-emergence, anomalously warm water masses, ocean heat advection, reduced vertical mixing, and mixed-layer stratification associated with surface heat gain), this study focuses on synoptic-scale atmospheric conditions associated with MHW occurrence and characteristics in the North Atlantic basin, from 1982 to 2022, with the objectives of identifying spatial-temporal trends of MHWs, examining the atmospheric conditions associated with their occurrence and exploring their relationship with prevalent climate variability modes. The results show positive trends in MHW frequency, duration, and intensity, albeit characterised by significant zonal and meridional variability, with noticeable differences between composite patterns of frequency and maximum intensity, according to the prevailing North Atlantic Oscillation (NAO) mode. The annual NAO appears to modulate the spatial distribution of MHWs, with its positive phase favouring MHWs in mid-latitude regions, while the negative phase impacts subpolar and tropical regions. Furthermore, concerning case-specific events, the stationarity of high-pressure systems, with weak pressure gradients, reduced wind speeds and increased solar radiation appears to be associated with the occurrence of the analysed events, while atmospheric instability appears to signal their decline, likely linked to enhanced wind-induced ocean mixing.

Unexpected Ecological Vulnerability in Tropical Pacific Heatwave Refuge

The equatorial western‐central Pacific (WCP), characterized by low marine heatwave (MHW) occurrence and weak MHW intensity, is generally considered a refuge from MHWs. However, by integrating satellite observations with in situ measurements from BGC‐Argo floats and TAO/TRITON buoys, this study reveals that the WCP exhibits the strongest phytoplankton response to MHWs in the global ocean, with near‐surface chlorophyll concentration (CHL) declining by up to 50% per unit MHW intensity. This extreme response primarily arises from a concurrent reduction in both horizontal and vertical nutrient supply during MHWs. Eastward current anomaly inhibits zonal advection of nutrient‐rich water from the east, while enhanced salinity stratification and shoaled mixed layer hamper vertical nutrient supply. Contrary to the near‐surface, subsurface phytoplankton increase substantially during MHWs, mainly resulting from the improvement in nutrient and light conditions at depths below the shoaled mixed layer and near the base of the deepened euphotic layer. As the WCP hosts the world's most productive tuna fisheries and intense subsurface MHWs, our findings reveal a previously unrecognized ecological vulnerability with important implications for fishery management and carbon cycle in this vital marine system.

Record-Breaking Marine Heatwave Event in the Yellow Sea During Summer 2024 and Its Underlying Mechanisms

Marine heatwaves (MHWs) are persistent extreme warm events in the ocean that pose substantial threats to marine ecosystems, fisheries, aquaculture, and offshore energy infrastructure. In 2024, the Yellow Sea experienced the most intense MHW on record in terms of cumulative intensity, with sea surface temperature (SST) anomalies exceeding 5 °C and an exceptional duration of 118 days. Using the ERA5 atmospheric reanalysis and GLORYS12V1 ocean reanalysis, this study systematically investigates the characteristics, driving mechanisms, and extremity of this event. Mixed-layer heat budget analysis indicates that enhanced shortwave radiation was the primary contributor to the warming, which is closely linked to the westward-extending and northward-shifting subtropical high. During MHW decay, sea surface cooling is dominated by enhanced latent heat flux, closely linked to typhoon and cold air activities. Further analysis links the positive SST anomalies to the North Atlantic and the Barents Sea warming, which triggered a Eurasian teleconnection wave train. These results highlight the importance of cross-basin climate connectivity in driving regional maritime temperature extremes.

Marine heatwaves in the Northeast Pacific intensify landfalling atmospheric rivers on the west coast of North America

Extreme precipitation along the west coast of North America is often associated with atmospheric rivers (ARs), fueled by evaporation from the ocean. When ARs interact with marine heatwaves (MHWs), they can form compound extreme events with amplified hydrological impacts. Here, we quantify how MHWs influence the intensity and precipitation of landfalling ARs through thermodynamic air–sea interaction processes. We use high-resolution regional coupled ocean–atmosphere ensemble simulations to isolate the influence of large-scale MHW-related sea surface temperature (SST) anomalies while constraining the synoptic-scale atmospheric circulation. Focusing on well-documented AR events during the 2013–16 Northeast Pacific MHW, we show that anomalously warm SSTs enhance evaporation and lower-tropospheric moisture availability, leading to a robust increase in integrated vapor transport and intensified landfalling ARs. The enhanced moisture transport results in earlier onset and substantially increased coastal precipitation, particularly over drought-vulnerable regions of California. Moisture-budget diagnostics demonstrate that this amplification arises from a direct thermodynamic response to SST anomalies, rather than indirect modulation through changes in large-scale atmospheric circulation. Insights gained from this case study identify a thermodynamic pathway linking MHWs and ARs, highlighting the role of persistent oceanic thermal anomalies in shaping compound hydrological extremes under continued climate warming.