Short timescale events

Next I will shed light on the formation mechanisms of the high frequency daily events of large barrier layers in this ITCZ region. As stated, in the central tropical Atlantic, the barrier layers change their position over the year, according to the migration of the ITCZ. So to examine the growth and decay mechanisms of the BLT, vertical sections are taken over different locations in the ITCZ-box depending on where the localized BLT signal exists.

Barrier layer growth

The largest BLT peaks in this region are found every year during December-February.

Figure 4.31 presents the BLT from one of the peak events (red circles in Figure 4.29) during 30-12-2004. The figure shows the progression of the growth and decay of this BLT from 29-12-2004 to 01-01-2005. It is clearly noticeable that the large BLT at 20.72^◦W, 2.51^◦N occurs due to the ITCZ rainfall passing over it. The E-P-R magnitude is largely negative in that area, meaning large precipitation (Figure 4.31 row 1, row 5).

The SSS is seen to decrease at that location, with a freshwater lens forming on top of the saline water (Figure 4.31 row 4). The MLD and ILD are deep in the background in that region, but as soon as the rainfall takes place, the MLD shoals due to salinity changes, while the ILD still remains unaffected (Figure 4.31 row 2, row 3). This gives rise to the thick BLT on 30-12-2004 and 31-12-2004.

The event of barrier layer growth from 28-12-2004 to 30-12-2004 is illustrated in Figures 4.32. The 60 m thick barrier layer present on 30-12-2004 extending from 1-4^◦N in this section along 20.72^◦W, forms since 28-12-2004 due to a drastic shallowing of the MLD (see also dashed curves in Figure 4.32a,g,h). The shallowing of the mixed layer causes the BLT to increase and is associated with a positive salinity vertical gradient

4.5 Barrier layer in the ITCZ region 75

Figure 4.31: Snapshots from 29-12-2004 to 01-01-2005 showing the growth and decay of the maximum BLT on 30-12-2004 (black box delineates the ITCZ region): (row 1) BLT (meters), with vectors showing surface currents, (row 2) ILD (meters), (row 3) MLD (meters), (row 4) SSS (psu), with vectors showing surface currents and (row 5) E-P-R (×10⁻⁷ m/s).

tendency, meaning an increase in the salinity vertical gradient from 2 days before until the day of the peak BLT (Figure 4.32a). A similar, but weaker, pattern is seen in the temperature gradient tendency (not shown).

Looking at the individual contributions of salinity and temperature for the density stratification (Figure 4.32i,j,k), it can be noticed that once again the MLD is mainly controlled by salinity, as the salinity stratification almost entirely match that of density, and almost no temperature stratification exists in the upper 30 m. It can be also seen that the isohalines mostly move together or compress at the surface during those 3 days (Figure 4.32g,h), due to fresher water on the surface. This is the general case in 100% of the peak BLT cases.

Analyzing now the terms of the balance equations, it is clear that co-located with

Figure 4.32: Vertical section at 20.72^◦W of the salinity gradient balance terms averaged from 28-12-2004 to 30-12-2004 (units are×10−7

psu/m.s). Black (green) solid lines correspond to the ILD, black (green) dashed lines to the MLD for 30-12-2004 (28-12-2004). Salinity vertical gradient (psu/m) with isohalines (psu) superimposed for (g,h) both days. Corresponding (i) density, (j) salinity and (k) temperature stratification (×10−4

/s²) on 30-12-2004.

4.5 Barrier layer in the ITCZ region 77 this change in MLD, there is a positive salinity gradient tendency, meaning that the salinity gradient increased from the 28-12-2004 to the 30-12-2004 at those particular depths. This can almost entirely be explained by the turbulent mixing term which contains the surface forcing (Eq. 4.1 and Eq. 4.3 in section 4.2). At the air-sea interface, turbulent salinity flux depends on precipitation and evaporation. The other terms of the salinity balance do not contribute much; horizontal advection and tilting negligibly contribute to the positive tendency. Vertical advection and stretching are also small and oppose the affect of turbulent mixing, horizontal advection and tilting.

Figure 4.33 illustrates an event of the peak BLT in September, which was also seen to be one of the months with peak BLT in this region. The snapshots present the growth and decay of barrier layers present on 05-09-2011, one of the many large BLT peaks circled in red in Figure 4.29. In this event, we see that the intense localized precipitation is responsible for the MLD to shoal over a nearly constant ILD, giving rise to 50 m BLT on 05-09-2011. The emergence of the freshwater signal at that spot is seen in the SSS field of 05-09-2011 which is accompanied by a drastic localized shoaling of MLD.

The 50 m thick barrier layer present on 05-09-2011 extending from 5-9^◦N in the section along 25.56^◦W, forms since 03-09-2011 due to dominantly a shallowing of the MLD during those 3 days (see dashed curves in Figure 4.34a,g,h). In September the salinity gradient is high at the surface due to vertical compression of isohalines and shallowing of the MLD due to fresh water at the surface. At the MLD on 05-09-2011, the density stratification is completely explained by the salinity stratification (Figure 4.34i,j,k). The term dominantly contributing to the formation of this BLT is the turbulent mixing of this surface forcing of freshwater (Figure 4.34a,f).

The same behavior of the isohalines is seen for all the events marked in red thus giving a 100% occurrence over the entire year, mostly with largest BLT magnitudes in December, September and April-June. The compression of isohalines at the surface due to turbulent mixing of freshwater on the surface is the main reason for the shoaling of the MLD and the formation of these barrier layers under the ITCZ. On examining all the events with peak BLT, along with the shoaling of MLD there was generally no deepening of ILD. Only rarely a negligible deepening of ILD occurred.

Barrier layer decay

In order to study the decay of the above described barrier layers, the averages of the salinity and temperature gradient balance terms are now taken between the day of the peak BLT and a few days after the peak, when we see the barrier layer getting thinner.

Along with the formation, Figure 4.31 and Figure 4.33 show the decay of the large barrier layers on 30-12-2004 and 05-09-2011, respectively. We see that the decay of these barrier layers on both days is due to dominantly the mixed layer deepening soon after the episode of the intense ITCZ rainfall is past.

Figure 4.33: Same as Figure 4.31 for an event with maximum BLT on 05-09-2011 In the shown case of 05-09-2011 (Figure 4.35), the mixed layer deepens during the course of the averaged 3 days (see depth change between dashed black and dashed green curves). The pattern of salinity gradient tendency is now the opposite of the one seen before when analysing the growth of BLT in the region. The deepening of the MLD is associated with a negative salinity vertical gradient tendency (blue in Figure 4.35a) above the resulting MLD (green curve) and to a positive tendency below it.

This reveals a downward shift of the salinity gradient (see change from Figure 4.35g to Figure 4.35h) and an increase in its magnitude due to moving apart of isohalines at the surface. In fact this is happening in the majority of the cases identified (95/123 times, 77%).

The dominant terms contributing to the tendency are turbulent mixing, tilting and horizontal advection. Stretching is negligible, while vertical advection counteracts the above processes. In the minority of cases, MLD deepens in 24/123 cases (19.31%) by the compression of isohalines. Apart from the deepening of MLD, a minor shoaling of ILD contributes to the thinning of the barrier layer in 48/123 times (39.02%). This

4.5 Barrier layer in the ITCZ region 79

Figure 4.34: Same as Figure 4.32, but vertical section at 25.56^◦W of the salinity gradient balance terms averaged from 03-09-2011 to 05-09-2011. Black (green) solid lines correspond to the ILD, black (green) dashed lines to the MLD for 05-09-2011 (03-09-2011). Corresponding (i) density, (j) salinity and (k) temperature stratification on 05-09-2011.

Figure 4.35: Vertical section at 25.56^◦W of the salinity gradient balance terms averaged from 05-09-2011 to 07-09-2011 (units are×10⁻⁷ psu/m.s). Salinity vertical gradient (psu/m) with isohalines (psu) superimposed for (g,h) both days. Black (green) solid lines correspond to the ILD, black (green) dashed lines to the MLD for 07-09-2011 (05-09-2011).

happens mostly when the isotherms are compressed vertically due to turbulent mixing.

Thus we see in the ITCZ region that the BLT forms and decays generally because of a change in MLD. Turbulent mixing of the freshwater on the surface plays a major role in the formation, while turbulent mixing along with a tilting of the salinity front into vertical, locally destroys the BLT. In most of the cases there is no ILD change in the formation or decay of the BLT here.