Fig. 4: Model-derived North Atlantic surface MLT budget anomalies during 2023 relative to the 1981–2010 climatological mean.
From: Drivers of the extreme North Atlantic marine heatwave during 2023
a, Anomalous surface warming over the North Atlantic (equator to 60° N) during May-August decomposed into each surface heat flux term (and the net surface heat flux), the net vertical mixing and entrainment terms and the advection plus other minor terms. The total anomalous MLT tendency (red bars) is primarily a balance between net surface heat flux anomalies (orange bars) and cooling anomalies at the base of the mixed layer due to vertical mixing and entrainment (green bars). The shortwave radiation anomalies are decomposed into anomalies due to MLD variations alone (MLD′), anomalies due to surface heat flux variations alone (QSW′) and anomalies due to the radiative flux through the base of the mixed layer (QSW,H′). b–g, June and July anomalous mixed layer warming (b,e) versus that due to anomalies in net surface heat fluxes (c,f) and total ocean advection, mixing and entrainment (d,g). Units are °C per month throughout. The net surface heat flux anomalies are calculated for the mixed layer, subtracting out shortwave radiation penetration at the base of the mixed layer (Methods). The model simulation shown here is that forced by the ERA5 atmospheric reanalysis fields. Similar results are obtained from the JRA55-forced ocean model simulation. Note that this analysis calculates the MLT budget terms centred on each month, while the warming discernible in Fig. 1c–f is from one month to the next (that is, between monthly averages, centred at the start of each month). Also note that this analysis does not include the effects of interannual variability in shipping emissions. See Methods and Extended Data Fig. 11 for further details.
