# Presentation: 033955

## Air-sea heat fluxes in the stratocumulus deck / cold tongue / ITCZ complex of the eastern tropical Pacific 

- Meghan F. Cronin (NOAA PMEL)
- Chris Fairall (NOAA ETL)
- Michael J. McPhaden (NOAA PMEL)
- Robert Weller (WHOI)

## Eastern Pacific Investigation of Climate (EPIC) experiment

- This study uses ship and buoy data along 95W and at 20S, 85W to evaluate air-sea heat fluxes in the stratus deck / cold tongue / ITCZ complex.

**Notes:**
This figure shows winds, SST and rain in the eastern tropical Pacific.
During boreal fall, the cold season, the equatorial cold tongue is well formed and extends far west. The ITCZ is at its northernmost extent, stratus deck extends from the cold waters off Chile across the equator. 

During the warm season in the boreal spring, the cold tongue and stratus deck recede, and a double ITCZ sometimes forms.

The challenge for modelers is that if you don’t get the stratus correct, you won’t get the cold tongue right. And if you don’t get the cold tongue right, you won’t get the ITCZ, and vice versa. If you don’t get one right, the others won’t be correct.

Thus we have the E P I C (EPIC) experiment. As part of EPIC the easternmost TAO line was enhanced with 3 buoys and sensors, and an IMET mooring was deployed at 20S, 85W.

## Buoy and ship time series 

**Buoy and ship time series**** **

**Notes:**
Enhancements along 95W began in Dec 1999 and ended in late 2003. 
Enhancements at 20S began in Oct 2000 and are continuing. 

This slide shows time series from buoy and ships in the stratus deck region at 20S, 85W (left), in the cold tongue region (middle) and in the ITCZ (right column).
Shown here are  solar radiation and clearsky radiation in dashes; wind speed; specific humidity from rel humidity, sea-air temperature difference, and SST. Also measured but not shown are LWR, BP, rain, subsurface temperature, sea and subsurface salinity and 10 m currents. 

## Slide 4

**Notes:**
Our goal is to use these buoys as reference sites for NWP. As a first step we need to compare the buoy measurements to shipboard measurements shown here by circles. 

I’ve highlighted a few of these comparisons. Initial comparisons are quite good, although care must be taken in making proper temporal and spatial averaging. This is all I will say about this portion of the study.

## Latent and sensible heat flux calculations from buoy data

- Used Fairall et al. (2003) v3.0a bulk flux algorithm.
- Used hourly-averaged data (fill gaps with telemetered daily-averaged data).
- Applied warm layer and cool skin corrections to extrapolate 1 m SST to surface (Tskin).
- Used winds relative to surface currents.

## Q0 = Qsw - Qlw - Qlat - Qsen

**Notes:**
Each panel shows monthly timeseries at 20 S and along 95W for the 4 terms contributing to the net surface heat flux:

Qnet = net shortwave radiation (LL panel) - net longwave radation (LR panel) - latent heat loss (UL panel) - sensible heat loss (UR panel)

Gaps. 

Easier to see pattern whens when bin into each month. (avg Jan, avg Feb,...)

## Q0 = Qsw - Qlw - Qlat - Qsen

-  **Q****0**** ~ Qsw - Qlat**
** ****Qlat and Qsen max over frontal region**
** ****Qlat near zero over cold tongue and is weakest during warm season. **

**Notes:**
As before, Shortwave (LL), net longwave (LR), sensible (UR) and latent heat loss (UL). 

Not necessarily mean annual cycle since based on 3-4 years of gappy data. 
Nevertheless a few patterns can be seen here:

1) terms on the right (sensible heat flux & longwave) have much smaller dynamic range than the terms on the left. Variability in net surface heat flux is controlled primarily by solar radiation and latent heat loss. I will return to this point later. 

2)  Max latent and sensible heat loss are co-located. 
3) QE is near zero along equator. 

## PBL stabilized over cold tongue and destabilized over frontal region

-  **Q****0**** ~ Qsw - Qlat**
-  **Qlat and Qsen max over frontal region**
** ****Qlat near zero over cold tongue and is weakest during warm season. **

**Notes:**
The patterns of Latent heat loss  (LL), Sensible heat loss (LR), can be understood by looking at the corresponding zonal (UR) and meridional (UL) wind fields. 

1)  Max latent and sensible heat loss are co-located with maximum meridional winds. Consistent with destabilized PBL bringing dry air and strong winds to surface. 
2) Minimum latent heat loss on equator is due to weak winds there and is consistent with stabilized PBL over cold tongue. 
3) However, the weakest latent heat loss on equator is during the warm season not the cold season. So largescale variations in winds also play role.

## Cloud Forcing is reduction or enhancement in surface radiation caused by clouds

**ITCZ**
**cold tongue**
**stratus deck**

-  **Q****0**** ~ Qsw - Qlat**
** ****Qlat and Qsen max over frontal region**
** ****Qlat near zero over cold tongue and is weakest during warm season. **
** ****Qsw and Qlw modulated by clouds.**

**Notes:**
Now lets look at the patterns of net shortwave and longwave radiation.
These fields are strongly modulated by clouds.
Solar Cloud forcing (shown in the upper left panel) is defined as the reduction in shortwave radiation by the clouds. 
Longwave cloud forcing (upper right panel) is the enhancement in surface longwave radiation by the clouds. 

For example, clouds associated with theITCZ cause reduction of  ~120    W/m2 in the surface solar radiation. 

Low clouds tend to radiate at higher temperature therefore the stratus deck has high ir cloud forcing during latter half of year in southern hemisphere.

## Q0’ ~ Qsw - Qlat ~? d(HC)/dtHeat Content ~  Cp hh ~ 30m

**ITCZ**
**cold tongue**
**stratus deck**

-  **Q****0**** ~ Qsw - Qlat**** **
** ****Qlat and Qsen max over frontal region**
** ****Qlat near zero over cold tongue and is weakest during warm season. **
** ****Qsw and Qlw modulated by clouds.**
** ****Ocean processes also important**

## How do NWP latent heat fluxes compare to buoy? 

**ITCZ**
**cold tongue**
**stratus deck**

** ****NCEP2 latent heat flux is much stronger than buoy (cold bias) **
** ****ECMWF is more realistic**

## What are SST biases associated with NWP latent heat flux “bias”? 

**ITCZ**
**cold tongue**
**stratus deck**

** ****SST biases are HUGE when integrated over a year. **
** ****ECMWF has a warm bias in stratus deck region and cold bias in cold tongue / ITCZ complex. **
** ****NCEP2 has a cold bias everywhere. **

**Notes:**
An error of 40 W/m2 in latent heat loss is equivalent to 0.05 mm/hr error in evaporation, or equivalently .5 m/year. 

## Conclusions

**Stabilized PBL causes low latent heat loss over cold tongue; and destabilized PBL causes maximum latent heat loss over frontal region.**
**Solar radiation, latent heat loss and ocean processes control SST variability in the east Pacific stratus deck / cold tongue / ITCZ  complex.**
**ECMWF latent heat loss appears to be more realistic than NCEP2.**
**ECMWF has a warm bias (due to latent heat flux) in stratus deck region and cold bias in cold tongue / ITCZ complex.  NCEP2 has a cold bias everywhere.**
**SST biases associated with latent heat biases are HUGE when integrated over a year. **