Figure 1.
SST (contoured every ) used in experiment (a) CNTL (b) SMTH and (c) COOL. Panel (d) gives the difference CNTL-SMTH (contoured every , positive continuous, negative dashed). The colour curves in panels (a), (b) and (c) are the average horizontal trajectories for the respective experiments, as discussed in Section 2.2 (magenta: , green: , blue: ). The diamonds on these curves indicate the average locations when parcels leave the boundary layer. The colour curves in (d) indicate the CNTL (continuous lines) and SMTH (dashed lines) mean trajectories, reproduced from panel (a) and (b), respectively.
Figure 2.
Snapshots at h of (a) vertical velocity (w, in m / s) at and (b) equivalent potential temperature (, in K) at m. The corresponding surface temperature (=SST used in CNTL over the ocean) is shown in black with a contour interval of , starting from . The ‘white corners’ indicate the limit of the nested 12 km domain in that particular portion of the Northwest Atlantic.
Figure 3.
Back trajectories from the core of ascending motion at h at mid-levels (the ‘release volume’ in black) in (a) the CNTL, (b) the SMTH and (c) the COOL experiments. The corresponding SST is shown in black with a contour interval of . Note that only trajectories originating from low levels over the ocean at h are shown. The colour coding refers to different sets of trajectories, depending on the height at h: (magenta), (green) and (blue).
Table 1.
Number of back-trajectories feeding the ‘release volume’ from low levels for the different experiments discussed in the text (rows labelled ‘all’). Contributions from different layers are also indicated based on the height of the parcels at h.
Figure 4.
Evolution of the parcels’ height (in m, y-axis) through time (in h, x-axis) for the (a) CNTL and (b) SMTH experiments. The height of the boundary layer is indicated as a dashed line. The colour coding is the same as in Fig. 3.
Figure 5.
Evolution of SST (x-axis, in deg) and (y-axis, in K) along the mean trajectories (CNTL, continuous lines; SMTH, dashed lines) from h until the time their height exceeds that of the boundary layer. Values are given every hour (crosses) and the initial (final) location is indicated by a diamond (star). The same colour coding as in Fig. 3 is used.
Figure 6.
Snapshots of vertical velocity at h and a height of 4.5 km, decomposed into low-pass (panels a,b,c) and high-pass components (panels d,e,f) – see main text for details of the spatial filtering. The CNTL experiment is shown at 12 km resolution in panels (a,d) and at 40 km resolution in panels (c,f). The SMTH experiment at 12 km is shown in panels (b,e). The associated SST distributions are shown in black contours (same contouring as in Fig. 1).
Figure 7.
Distribution of potential vorticity (in PVU, ) at time h and height km. (a) CNTL at 12 km (b) SMTH at 12 km (c) CNTL at 40 km. The associated SST distributions are shown in black contours (same contouring as in Fig. 1).
Figure 8.
Fraction of the time in winter (in %) when the condition in eq. (3) is met for a surface (a) equal to an equilibrium value (b) equal to the actual at 950 hPa – see Section 3. The wintertime mean SST is displayed in black contours with a contour interval of . All data from ERA interim in boreal winter. Note the factor of four difference between the colour bars in (a) and (b).
Figure 9.
Number of back-trajectories (circles) with heights km at h and reaching low levels over the ocean at h (from Table 1 for all experiments): red for CNTL, blue for COOL and green for SMTH. The arrows indicate a measure of oceanic forcing when comparing CNTL/SMTH or CNTL/COOL (see main text). Note that the number of trajectories was normalized such that the number for CNTL at 12 km resolution is unity in order to facilitate the comparison between experiments.