The diurnal warm layer and its consequences for the upper ocean: from EUREC4A-OA observations and the global coupled ICON-ESM

<p><span lang="EN-US">The uppermost 0-20m depth of the ocean within the mixed layer (ML) were investigated on </span><span lang="EN-US">diurnal</span><span lang="EN-US"> scales using data collected during the EUREC</span>^{<span lang="EN-US">4</span>}<span lang="EN-US">A campaign in the western tropical Atlantic. The results are compared against data from the global coupled Earth System model ICON. In both datasets is the diurnal impulse generator the penetrating shortwave solar radiation, heating the first meters of the ocean. During day </span><span lang="EN-US">on top of the ML a stably stratified near-surface layer, called the diurnal warm layer (DWL), can be formed. </span><span lang="EN-US">Depending on the wind conditions or the amount of incoming solar radiation</span><span lang="EN-US"> the depth of such a DWL can reach from several centimeters to tens of meters</span><span lang="EN-US">. Associated to the stable stratification (and the wind) shear is produced which propagates down with time.</span><span lang="EN-US"> At that point, the model and the observations start to differ.</span> <span lang="EN-US">Using high-resolution current measurements of ADCP&#8217;s mounted on saildrones the detailed structure of the descending shear layer is observed.</span> <span lang="EN-US">The cycle of shear instability leads the diurnal mixing cycle, typically by 2&#8211;3 h, consistent with the time needed for instabilities to grow and break (observed by microstructure measurements).</span> <span lang="EN-US">In the morning, the turbulence decays and the upper ocean restratifies. At this point, wind accelerates the near-surface flow to form a new unstable shear layer, and the cycle begins again. </span><span lang="EN-US">Since the study area is located around 15</span><span lang="EN-US">&#176;</span><span lang="EN-US">N, the excited layers are affected by the </span><span lang="EN-US">Coriolis force, which causes </span><span lang="EN-US">the </span><span lang="EN-US">descending</span><span lang="EN-US"> shear layer</span><span lang="EN-US"> to</span><span lang="EN-US"> rotat</span><span lang="EN-US">e</span><span lang="EN-US"> around the </span><span lang="EN-US">inertial</span> <span lang="EN-US">frequency</span><span lang="EN-US"> of 1.8 days.</span> <span lang="EN-US">Compared to the global coupled earth system model, these processes cannot be represented in such detail here. This leads to lower shear (and also mixing) at the different time and depth. Different model configurations show that even small differences in the upper 20m of the ocean, such as those observed, can lead to quite large changes in the model, e.g., a different strength of the ocean current system down to 1000m depth.</span></p>