07.22.11: Behavior of density surfaces around WHOTS – comparison of WHOTS observations and HYCOM model¶
Starting from the assumption that nitrate contours within the nutricline follow closely density contours, I study the behavior of isopycnal surfaces in the 1-day averaged WHOTS observations and in a similar manner in the HYCOM model. The time series of potential density from WHOTS and HYCOM are plotted in the lower panel of Figs. 1 and 2, respectively. One isopycnal surface, σ = 24.4 kg/m3 is also plotted with a black contour. The upper panel of each figure shows the sea surface heigh (SSH) or sea surface height anomaly (SSHA) at the location of WHOTS. In Figs. 3 and 4 are then plotted either SSH or SSHA together with the depth of σ = 24.4 kg/m3 at the WHOTS location. We see that most of the variability in the depth of the isopycnal surface is associated with SSH variability. The correspondence is not perfect because 1) the flow is not zero below that isopycnal surface and 2) the stratification changes with the passage of different masses. This is itself not a new result at all.
There is an important difference between the model and the observations. The anomalies in depth of the isopycnal surface that is on average within the nutricline are smaller and biased toward negative values in the HYCOM model compared to the observations.
Figure 1: (upper) Sea surface height anomaly (SSHA) from AVISO at WHOTS location. (lower) Potential density during WHOTS (NOTE: The last two years of data have not been total reprocessed). All WHOTS quantities are 1-day averaged.
Figure 2: Same as in Fig. 1 but from the HYCOM model. Notice that the size of the time window is the same as in Fig. 1.
Figure 3: Depth of the isopycnal surface σ = 24.4 kg/m3 from WHOTS and sea surface height anomaly (SSHA) from AVISO at WHOTS location.
Figure 4: Same as Fig. 3 but from the HYCOM model.
In Spring of 2010 in HYCOM, there is a strong event of shallow isopycnal surface. Figs. 5 and 6 show that this event is associated with a density front that has migrated from the north and is associated with submesoscale processes (not only horizontal stirring but also surface mixed layer instability) that trigger submesoscale structure in the density field but not in the SSH field (explaining why the time series of the depth of the isopycnal surface has a higher frequency component than that of SSH in Fig. 5). According to the model this is the only event where the isopycnal surfaces that are on average at the depths of the nutricline can reach the depths of the euphotic zone. During this event, large vertical component of velocity may input some nutrients on the cold side of the front.
Figure 5: (upper) Depth of the isopycnal surface σ = 24.4 kg/m3 and SSHA at WHOTS location from the HYCOM model. (lower) Depth of the isopycnal surface σ = 24.4 kg/m3 and SSH around the WHOTS location and for Mar. 24, 2010.
Figure 6: Vertical section of potential density and Rossby number along the longitude of the WHOTS location in HYCOM on Mar. 24, 2010.
Although there is one strong event in Spring of 2009 in WHOTS, it is difficult to tell if similar events occur also in Spring of other years as they have the same amplitude as any other event. However, Fig. 7 does indicate that there is a seasonal cycle to the high-frequency noise (from 1-day to 30-day period) with large noise in Spring and isolated noisy event in late Summer. This could suggest that the break-up into submesoscale structure during Spring seen in the model might also happen in the ocean. NOTA: When loopking at the squared difference over the full depth, I realized that the increase in difference follows the sinking of the seasonal thermocline. Thus, the seasonal increase may be due to the passage of high stratification which makes the difference between the 1-day and 3-day signals larger than usually.
Figure 7: (a) 1-day and 30-day runnning mean σ at 120 m depth. (b) Difference squared between the two signals of panel (a).
In OFES, do we have the same density structure? Is nutrient utilized before it reaches the latitude of the WHOTS mooring? See Figs. 4 and 5 in this note.
Does the ARGO climatology show also the meridional migration of large-scale density fronts? Yes.
Did nutrient events in ARGO data occur near the large-scale density front (which may be obtained from the ARGO climatology)?
Plotted thanks to the script full_WHOTS_sig_analysis_1.m in RESEARCH/PROJECTS/MARINE_BIOLOGY/SUBMESOSCALE_PROCESSES/WHOTS/analysis on the main disk.