# Presentation: 164741

## Barrier Layer Formation during Westerly Wind Bursts and Implications for ENSO

- Meghan Cronin
*NOAA Pacific Marine Environmental Laboratory*
- Cronin, M. F., and M. J. McPhaden, 2002: Barrier layer formation during westerly wind bursts.* J. Geophys. Res*., 107, 8020, doi:10.1029/2001JC001171.
*Data sources: TAO project office (PMEL), R. Lukas (UH), D. Tang (NTU), K. Kutsuwada (Tokai U), R. Weisberg (USF), Delcroix et al. (2000), Xie and Arkin (1997), Reynolds and Smith (1994).*

**Notes:**

For those expecting to hear about EPIC (the eastern Pacific Climate Processes Experiment), sorry, I’m here to talk to you about another COARE analysis.

Title is... The basis of this talk is a paper I did with Mike McPhaden that came out in JGR last December. 

Data I will be showing come from a variety of sources. The primary data set are temperature and salinity measurements from enhanced TAO moorings maintained by PMEL. Salinity at two of these moorings were provided by R. Lukas. I’ll acknowledge these other data sources as I show them. 

## Barrier Layer Thickness (BLT) -- Thickness of layer between base of mixed layer and base of isothermal layer near top of thermocline.

- From: *Lukas and Lindstrom (1991)*

***Effects of Barrier Layer:**** *
* **Reduced entrainment (“Barrier to entrainment mixing”)*
* **Increased trapping of momentum and heat*

**BL**

**No BL**

**Notes:**
What is a barrier layer.  A barrier layer is the layer between the base of a shallow mixed layer and the base of the isothermal layer near the top of the thermocline.  

Thus the CTD cast on the left has no BL, while the cast on the right shows a bl that is ~45 m thick. These are the casts made famous by Lukas and Lindstrom in 1991. 

The layer is called a Barrier Layer because it acts as a barrier to entrainment of cold thermocline water into the surface. Therefore the BL can have an important impact on the surface heat budget and SST. 
Additionally, barrier layer causes the surface heating and wind forcing to be trapped in a mixed layer that is shallower than expected based on thermocline depth. As a consequence, wind-forced jets can be faster and shallower than would be expected based upon temperature. 

I’ll discuss how these might influence ENSO and climate in few moments. Meanwhile...

## From: Ando and McPhaden (1997)

**Meghan Cronin:**
- Thickest BL on eastern edge of warm/fresh pool.
- BLT migrates zonally with ENSO.

**Notes:**
I want to give you an idea of the large-scale structure and variability of barrier layers in the equatorial Pacific in relation to the SST and SSS fronts. 

I’ve scanned in part of a figure from Ando and McPhaden.   Each of these panels is a time /longitude plot with time extending up from  1979 to 1994. 
Each + marks a bin averaged CTD estimated of MLT (SST), MLS (SSS), and BLT. 
The shade in left panel marks SST > 28C and the EN of 1982, 1987 and 1991 are clearly seen. 

In the middle panel, SSS greater than 35psu are shaded. Note that the there is a strong salinity front in the eastern portion of the warm pool. The Fresh Pool extends eastward  during El Nino. 

Right panel shows the BLT, with shading indicating thickness > 20 m .  Note: while BLT is thicker in western eq Pacific, there is substantial interannual variability. 

## Effects of Barrier Layer on Climate

- From: *Lukas and Lindstrom (1991)*

***Effects of Barrier Layer on Climate***
* ****Warmer SST **** (Lukas and Lindstrom 1991):*
*      **BL preconditions WARM pool for El Nino?*
*** ******Coupled hydrological cycle?**** (COARE)*
*      **Rain causes BL causes Warm SST causes rain?*
* ****Increased trapping of momentum  ****(Maes et al. 2002): *
***     ****Extends fetch of WWB and onset of El Nino*

**BL**

**No BL**

**Notes:**
Is this interannual BL variability caused by the zonal displacement of the warm/fresh pool? Or does this BL variability somehow influence the ENSO cycle?  What might be the effects of a barrier layer on climate ? 

Because BL reduce entrainment of cool thermocline water into the mixed layer, they can cause SST to become warmer. Lukas and Lindstrom in 1991 speculated that BLs might cause the warm pool to get very warm, and thus precondition the warm pool for El Nino onset. 

To the degree that SST affects convection and rainfall, a positive feedback can exist in which : Rainfall -> BL  formation -> SST warming  -> more Rainfall.  Understanding whether the hydrological cycle was coupled in this way in the WP region was one of the main scientific questions motivating the COARE. 

The trapping effect can also have climatic implications. In a recent and very interesting coupled model study, Maes and co-
author showed that BL caused not only warmer SSTs, but also caused faster/shallower eastward surface currents that displaced the edge of the warm pool further east.  When they turned off the trapping effect, the WP was confined to west of the dateline and no El Nino formed. 
Of  course to understand these coupled processes, we must understand “What causes BLs to form?” This is the subject of my talk. 

## Formation Mechanisms...Are barrier layers formed by rain? 

- COARE authors (Tomczak 1995, Wijesekera and Gregg 1996, Soloviev and Lukas 1996, etc.)
-     *Rain formed barrier layers are thin and dissipate rapidly.*
- Zhang and McPhaden (2000)
-     *MJO rainfall is associated with wind stirring and barrier layer thinning.*

**Notes:**
How do barrier layers form?  Are barrier layers formed by rain? 

During COARE there were numerous surveys of rain formed barrier layers. These studies consistently found that rain formed barrier layers tend to be thin and dissipate rapidly. 

As shown by Zhang and McPhaden (2000), the problem is that rainfall tends to occur during the convective phase of the intra-seasonal MJO. Zhang and McPhaden used data from 11 moorings from March 1991-March 1994 to obtain a composite MJO time series.  MJO convective phase tended to be associated with net surface cooling, increased latent heat flux, precipitation, wind stress, westerly wind, SST cooling, mixed layer deepening and BL thinning. 

Bottom line – according to the Zhang and McPhaden analysis, MJO WWB cause barriers to dissipate rather than form !   

## or...Are barrier layers formed through subduction and advection? 

- Lukas and Lindstrom (1991)
*      “**Subduction Hypothesis”. *
- Vialard and Delecluse (1998)
*      **Supporting model results *
- Roemmich et al. (1994)
*      “**Fresh jets” associated with salinity front.*

**Notes:**
If BL are not formed by rainfall, then how are they formed? 
The answer is: through subduction or advection.

## Lukas and Lindstrom (1991) “Subduction Hypothesis”

- Evap

- Precip

- warm
- salty

- warm
- fresh

- warm
- salty

*Climatological barrier layer near dateline caused by subduction of salty warm SEC water beneath fresh warm pool water. *

**Notes:**
According to the Lukas and Lindstrom “Subduction Hypothesis”, trade wind convergence near the dateline causes a zonal convergence near the dateline, which causes the warm salty SEC to subduct beneath the fresh warm pool water, thus forming a climatological barrier layer near the dateline. 

## From: Vialard and Delecluse (1998)

**Notes:**
Using an OGCM run with realistic forcing from 1984-1993, Vialard and Delecluse showed that BL formed through zonal subduction similar to the Lukas Lindstrom hypothesis. 

The model produced a salinity front between the fresh warm pool and the salty SEC. Compositing periods in which the barrier layer was > 10 m, and centering the sections to the SSS front to remove ENSO variability, they arrived at this cartoon. 

The basic elements from the LL schematic are shown here. This is the thermocline, top of thermocline, mixed layer, ...

One difference though is that the surface currents in the freshpool are large and eastward. Also, the salty SEC flow is in the upper portion of the thermocline, not in the mixed layer. 

Although this composite was based on barrier layer thickness, it looks similar to what I would expect for a WWB composite. 

## From: Vialard and Delecluse (1998)

***Ocean Response to Westerly Wind Burst (WWB)***
*** ******Net surface heat loss and SST cooling ***
*** ******Rainfall (freshening) ***
*** ******Eastward surface jet “Yoshida Jet”  ***
*** ******Ekman convergence onto equator  ***
*** ******Thermocline downwelling ***
*** ******Kelvin waves, Rossby waves, ... ***
*** ******Pressure driven subsurface countercurrent***
*** ******BL formation or dissipation?***

**Notes:**
The ocean response to WWB includes...

So does a WWB cause bl formation ? Or dissipation?  

## Roemmich et al. (1994)                 “Fresh Equatorial Jets”dS/dx     d/dx(Pz)     d/dt (uz)dS/dx tilts into dS/dz     Barrier Layer

***Problem:****  Shear instability *
***Problem:****  Wind forcing?*

- Fresh

- Salty

- (Low P)

- (High P)

**Notes:**
Finally, I want to mention the very intriguing barrier layer formation mechanism put forth by Roemmich et al.  Through “Fresh eq jets” .

Roemmich notes first that from the hydrostatic relation, a zonal salinity gradient will give rise to a zonal gradient in the vertical pressure derivative.... Which we see by switching the order of derivatives, will on the equator give rise to time rate of change in the shear.  This shear acting on the zonal salinity gradient will tend to tilt the zonal gradient into the vertical and thus generate salinity stratification. (i.e. barrier layer). 

When I read this paper, I really thought this was cool. 

There’s a couple of problems though. First, as the shear intensifies, eventually, shear instability will form that will destroy the barrier layer.  Second, in the real world there is wind forcing that is likely to swamp this out.     

## Barrier Layer is stratified by salinity

- What processes form z ~ Sz?

-  1. Horizontal Advection      	4. Stretching/squashing
-  2. Vertical Advection    	 	5. Surface flux and mixing
-  3. Tilting                              	(6. Penetrative radiation)

**Notes:**
However, it made me think about tilting and equations.  How do you describe barrier layer formation mathematically? 

Barrier layers form when salinity stratification forms within an isothermal layer. Therefore, we need to consider the vertical derivative of the heat and salt budgets. 

Term 1  on RHS is horizontal advection of the stratification into the region.
Term 2 is vertical advection
Term 3 is the tilting term whereby a horizontal gradient is tilted into a vertical stratification
Term 4 is vertical stretching of the layer
Term 5 is curvature in the flux profile. Note that at the surface the salt flux is S0 * E-P, and the heat flux is the net surface heat flux. 
Term 6 is the penetration radiation profile which can cause the barrier layer to be warmer than the surface mixed layer. 
Often terms in one budget are correlated with terms in the other – Rainfall is correlated with net surface heat loss, advection of salintiy is correlated with heat advection, .... Therefore when considering processes forming salinity stratification, we must consider how these occur without forming corresponding temperature stratification. 

## Barrier Layer is stratified by salinity

- What processes form z ~ Sz?

-  **1. Horizontal Advection**      	4. Stretching/squashing
-  2. Vertical Advection    	 	5. Surface flux and mixing
-  3. Tilting                              	(6. Penetrative radiation)

## Barrier Layer is stratified by salinity

- What processes form z ~ Sz?

-  1. Horizontal Advection      	4. Stretching/squashing
-  **2. Vertical Advection**    	 	5. Surface flux and mixing
-  3. Tilting                              	(6. Penetrative radiation)

## Barrier Layer is stratified by salinity

- What processes form z ~ Sz?

-  1. Horizontal Advection      	4. Stretching/squashing
-  2. Vertical Advection    	 	5. Surface flux and mixing
-  **3. Tilting**                              	(6. Penetrative radiation)

## Barrier Layer is stratified by salinity

- What processes form z ~ Sz?

-  1. Horizontal Advection      	**4. Stretching/squashing**
-  2. Vertical Advection    	 	5. Surface flux and mixing
-  3. Tilting                              	(6. Penetrative radiation)

## Barrier Layer is stratified by salinity

- What processes form z ~ Sz?

-  1. Horizontal Advection      	4. Stretching/squashing
-  2. Vertical Advection    	 	**5. Surface flux and mixing**
-  3. Tilting                              	(6. Penetrative radiation)

## Barrier Layer is stratified by salinity

- What processes form z ~ Sz?

-  1. Horizontal Advection      	4. Stretching/squashing
-  2. Vertical Advection    	 	5. Surface flux and mixing
-  3. Tilting                              	**(6. Penetrative radiation)**

## BL  Formation Mechanisms

**Notes:**
This cartoon illustrates the formation processes.  In this cartoon, the solid line is the salinity gradient, and dashed line is the top of the thermocline. Black is before, and grey is after. Stippling represents the barrier layer between a shallow halocline and the top of the thermocline.

Advection of BL from one region to another,

Stretching within the barrier layer can cause bl to thicken, but not form.

Tilting. Note that the shear can be either caused by a surface intensified eastward current, or a subsurface intensified westward current. If however the salinity gradient was reversed for this given shear, then the tilting would result in heavy water overlying lighter water and result in convective mixing. 

Finally, as illustrated here, bl can form through rainfall in the absence of strong mixing.   

## From: Vialard and Delecluse (1998)

***Ocean Response to Westerly Wind Burst (WWB)***
*** ******Net surface heat loss and SST cooling    -- mixing, Q******x***
*** ******Rainfall (freshening)  -- (E-P)******x***
*** ******Eastward surface jet “Yoshida Jet”   -- U******z***
*** ******Ekman convergence onto equator  -- V******z***
*** ******Thermocline downwelling   --  W******z***
*** ******Kelvin waves, Rossby waves, ... -- ∂(U, V, W), ***
*** ******Pressure driven subsurface countercurrent  -- U******z***
*** ******BL formation or dissipation?***

**Notes:**
The ocean response to WWB includes all the ingredients for barrier layer formation. In particular, the zonal gradient in the net surface heat flux will tend to cause cooling in the west and warming in the east (reduce the large scale SST gradient). While rainfall and evaporation zonal gradient will tend to enhance the large-scale SSS gradient.  
So it seemed the question of whether WWB lead to BL formation or dissipation needed revisiting. 

## BLT: TAO moorings enhanced with salinity

**BLT:** *TAO moorings enhanced with salinity*
*154E***, 156E, **160.5E***, 165E*
**Courtesy: R. Lukas*
*BLT = MLD - MLD**T*
**SST:*** Reynolds and Smith*
**SSS:*** Delcroix et al.*

**Notes:**
In this analysis I use daily averaged temperature and salinity data from moorings at 154E, 156E, 161E and 165E to compute barrier layer thickness. These are shown as pixel time series in the middle and left panels in relation to Reynolds and Smith SST and the Delcroix et al. monthly gridded SSS.

In many ways this is similar to the Ando and McPhaden figure I showed earlier, although it is based on a different data set. Really these analyses are complementary. The CTD analysis traded temporal resolution for spatial coverage. With the moored estimates of BLT, we trade  spatial coverage for temporal resolution. With the moored data, we can resolve WWB variability. 

Like Ando McPhaden, we see large barrier layers forming on the western edge of the salinity front prior to the eastward displacement of the warm fresh pool. 

## Rain: Xie and Arkin

**Rain:** *Xie and Arkin*
**Zonal Wind:** *TAO*
**BLT:** *TAO moorings enhanced with salinity*

**Notes:**
The leftmost panel shows the time-longitude plot of the rainfall (shaded) and zonal winds. As found by Zhang and McPhaden and others, the rainfall tends to occur during periods of westerly winds. 

Now look at these pixel time series of the BLT. During several of these WWB, really huge bls formed. 

## Slide 22

**Notes:**
In particular, I will focus on two WWB that formed thick bl, and fortuitously occurred during intensive field surveys: the first is the November 1989 WWB (described in detail by McPhaden et al 1992) and the second is the October 1992 WWB during COARE. 

## Nov 1989 WWB at 0°, 165°E

*Large U**z** ~1ms**-1**/100m*

*Large V**z** !*

*U**x **+ V**y** causes deep *
*thermocline*

*Rain ~ 30% of *
*freshening*

*Easterlies*
*shift to *
*westerlies*

***BLT~100m !***

**Meghan Cronin:**
- 1. Easterlies shift to westerlies.
- 2. Typical WWB response seen:
- a. eastward accel. (Yoshida Jet)
- b. Deepening thermocline (dashed) and surface cooling
- 3. But Mixed layer remains shallow due to rapid freshing, caused by
- a. Rainfall (this accounts for 30%)
- b. surface intensified freshwater advection from west (and north).
- 4.  What causes the shear above thermocline?

**Notes:**
This figure shows time series from the 0 165 E TAO buoy for the Nove 1989 WWB. We see all the typical WWB responses: 
Eastward Yoshida Jet 
 strong shear too in the meridional
 SST cooling, and deepening thermocline due to Ekman convergence and zonal convergence....
 However, while the top of the thermocline deepens (as indicated by the dashed), the mixed layer base (solid) remained shallow due to this rapid freshening. 
Rainfall might account for ~30% of the freshening. The remaining must come from advection from the west or the north. 

## Are these Roemmich’s Fresh Jets?     If dS/dx ~ .2 psu / 100 km.... 

*Might be responsible for 50% of shear.*
*Once shear has formed, a feedback will form between wind trapping and S(x) tilting into S(z). *

**Notes:**
However, if there was a zonal salinity gradient, could this have caused the observed zonal shear via the Roemmich et al. mechanism? 

Crunching through the numbers, we see that this might account for 50% of shear. In otherwords, this might have started the shear and tilting process. Once a stratification begins, wind trapping can cause the shear to intensify and further tilt the salinity gradient. 

## From: McPhaden et al. (1992)

- Before WWB

- After WWB

**Notes:**
Note too that it appears that the tilting also happened in the meridional direction. 

These meridional – depth sections of T, S, and density are from before the WWB and after the WWB. 

## Summary of Case Study 1

- Extremely thick (~100 m) and long-lived barrier layer formed in response to Nov 1989 WWB.
- Barrier layer formed by tilting both zonal and meridional salinity gradients into vertical.
- Feedback between formation of Sz and Uz.

## Oct 92 WWB (COARE)

**Oct 92 WWB (COARE)**

**Notes:**
2nd case study is from the COARE period. This second case study is important not only because of the interest in the COARE period, but also because we have BLT measurements from 4 sites and therefore can look at the zonal structure of the BL formation process. 

Panels show...

During the October 1992 WWB, a 65 m thick bl formed at 161E and 165E. However no bl formed at 156E and 154E. 

Can we see tilting during this formation period? 

## Zonal-depth section for 27 Oct – 1 Nov, 1992 156°E: tilting produces mixing. 161°E: tilting produces thick barrier layer. 

**T**** and S**

**U**** and S**
*ADCP data courtesy:    Kutsuwada, TAO, Weisberg *

**Notes:**
This figure shows the average zonal –depth section of Temperature and salinity (top). 
And zonal flow and salinity (bottom) for the 5-day period between 27 Oct and 1 Nov. 
Blues are westward flow. Reds are eastward flow. 

The section is salty in the lower left (deep-east) corner. Temperature above the thermocline iswarmer to the east. The center of the warm pool is to our east here. 
At 154E and 156E the zonal T gradient and zonal S gradient tend to be density compensating. 

Consequently, tilting at these sites produces mixing.   In contrast, the shear at 165E produces thick BL.

## Summary

- Long-lived, thick barrier layers can form in response to WWBs through tilting of zonal and meridional salinity gradients into vertical stratification.
- Salinity gradients can cause shear. Feedback between mometum trapping (Uz   Sz) and tilting (Sz   Uz ) can produce thick barrier layers.

## Summary continued...

- Mechanism requires a salinity gradient in absence of temperature gradient. WWB Q0 and E-P fields work in correct sense, but also need large-scale salinity front.
- During ENSO cool event, salinity front is in western Pacific. This is why Nov 1989 WWB produced thick BL at 165°E.