A Significant In-Situ Diurnal Warming Event in the Labrador Sea Unobserved by Satellite Observations

Diurnal warming (DW) at the ocean surface occurs when there is sufficient solar heating in the absence of vertical mixing. Observations of a DW event of 1.5 °C confined to the upper 2 m in the Labrador Sea at ~55°N were conducted with an upwardly-rising microstructure profiler. DW has been well described using satellite and in-situ observations, but there are very few reports at northerly latitudes. Contemporaneous satellite observations indicate a region that is largely obscured by clouds thereby preventing spaceborne detection of this DW event. Combining our in-situ observations with the ERA5 reanalysis product, we derive a set of conditions for potential DW in the Labrador Sea: shortwave radiation above 600 W/m²; total cloud cover less than 30%; and 10-m windspeed lower than 4 m/s. Based on this analysis, DW events in the Labrador Sea have the potential to occur more frequently than satellites can observe. A first look at microstructure profiler data from other regions of the North Atlantic indicates that such conditions can also be derived for these regions.