THE COARE EVENT:
I'M NOT HAPPY WITH THE TAO 5-DAY WINDS AND REYNOLDS SST. THESE
FIELDS ARE TOO SMOOTH.
Notice that high rainfall and low zonal winds is not sufficient to
produce a thick BL since otherwise would have developed one at 156E
~Oct 15-20 1992. Instead, the zonal current and shear seem to be
ARE THESE 2 FIGURES SIMPLE ENOUGH?
THE ZONAL PERSPECTIVE:
The zonal extent of the fp and wp vary interannually. BL tends to be thick ~165E when the warm pool and fresh pool are limited to the western Pacific. During El Nino, Ando and McPhaden show that thick bl extend across the Pacific and are slightly less thick in the western Pacific. This variability has been discussed by Vialard and Delecluse, Henin et al., Ando and McPhaden and others.
Note, the SSS in above figure is from DELCROIX ET AL. (2000). Check out similar figure using only moored SSS. Notice have stronger gradients.
Although we know that the BL will reduce entrainment cooling, I challenge you to find a correlation between the SST and BL. SST is controlled by many factors, one of which is mixing.
What's new here is that this is from an independent data set of previously published longterm BL observations (Ando and McPhaden). BLT are computed from moored data. Zhang and McPhaden used this data set to compute BLT for Sep 1992 - Dec 1993. Their method is slightly different, althoug the BLT produced are roughly similar to our 0, 165E values for that period. Our time series is longer and also done at other zonal sites.
Although we have a more limited spatial (and temporal) range than the CTD analysis (Ando and McPhaden), we have better time resolution. Like the Zhang and McPhaden analysis, we can resolve local variability due to westerly wind bursts. These burst are intrinsically related to the termination of the trades and the horizontal convergence of surface flow that is responsible for the SSS front. While the Zhang and McPhaden analysis focused on local balances, we are interested in the spatial structure and the role of advection.
While this is true, cross correlation between zonal wind and BLT, and dBLT/dt are not significant. During the extended period beyond COARE, there are WWB that also resulted in very thick BLs. For example, See WWBs in late 1989, late 1992, and Spring 1994.
THE BIG EVENT at 0, 165E in NOV89
The source of this fresh water is from the north and the west.
However, this explaination does not take into consideration zonal convergence of the flow. Some of the water may have also originated further to the west. The gridded monthly SSS from shipdata (Delcroix et al. 2000) show water fresher than 34.5 west of 155E. For this water to reach 0 165E in 13 days would require a ~ 1 m/s eastward jet. This is observed. The fact that the zonal current reversed direction also indicates that this is a region of zonal convergence and thus frontogenesis. ... Which we need, since if the dSdt during Nov 22-24 was entire caused by zonal advection, the dSdx would need to be between 0.14 psu/100 km and 0.8 psu / 100 km. Rain is occurring, so in fact, the required gradient is probably less than this. However this is a big front!
Note however that while the surface eastward jet is trapped above the mixed layer, the subsurface westward jet is below the top of the thermocline and therefore is not at the level required by the zonal subduction barrier layer formation mechanism. Rather, this looks like a surface phenomena.
McPhaden et al. Figure 9 shows 34.4 psu water at ~3-4N. With a 30 cm/s southward current, that freshwater would reach the equator in ~13 days. Northward subsurface flow also has the effect of tilting zonal gradients into the vertical. However during the big freshening Nov 20-22, the meridional current was northward! which would be bringing Salty water to the equator rather than freshwater. Later on, southward advection at the surface and northward advection at depth probably played a role in maintaining the bl.
THE MERIDIONAL PERSPECTIVE
Now, how to get that freshwater to the equator? Switch the winds to westerlies and the Ekman convergence will bring this water to the equator. ... in theory. In practice, the Ekman convergence can sometimes be displaced meridionally or wobble around the equator. We won't worry about that, except that this can sometimes cause a non-symmetric v around the equator and also a v on the equator.
THE BIG EVENT AT 0 161 AFTER THE OCT92 WWB
At the start (16-20 Oct) we see westward flow at the surface advecting
warm salty water to the west. MLD ~MLDT and both are shallow.
21-25 Oct: The WWB begins to cause eastward flow in the west, thus a dudx convergence. Downwelling at 156E and mixing causes the MLD & MLDT to deepen.
26-30 Oct: dudx & d(rain)/dx ?? cause the dSdx to intensify.
31 Oct - 4 Nov: dUdz & dVdz between 157.5E and 165E now tilts the dS/dx & dS/dy? front into the vertical, generating a ~30 m thick BL.
dVdz might be most important for first WWB after period of easterlies.
Meghan F. Cronin
Pacific Marine Environmental Laboratory
7600 Sand Point Way NE
Seattle, WA 98115 USA
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