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In revisions at the Journal of Physical Oceanography Improvements to the Equatorial Pacific Cold Tongue Region in an OGCM Possible Implications for the NCEP GODAS / CFS Kristopher B. Karnauskas, Raghu Murtugudde, Antonio J. Busalacchi

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In revisions at the Journal of Physical Oceanography

Improvements to the Equatorial Pacific Cold Tongue Region in an OGCM

Possible Implications for the NCEP GODAS / CFS

Kristopher B. Karnauskas, Raghu Murtugudde, Antonio J. Busalacchi

University of Maryland College Park, Md., USA

Photo of Isla Isabela NOAA R/V Ka’imimoana 2 May 2005

NOAA NCEP/EMC 6/20/06


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In revisions at the Journal of Physical Oceanography

Improvements to the Equatorial Pacific Cold Tongue Region in an OGCM

Possible Implications for the NCEP GODAS / CFS

“The currents of the sea are rapid and sweep across the [Galapagos] archipelago, and the gales of wind are extraordinarily rare…”

“Considering that these islands are placed directly under the equator, the climate is far from being excessively hot; this seems chiefly caused by the singularly low temperatures of the surrounding water…”

- British Naturalist Charles Darwin in 1859 and 1839


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Introduction: The Pacific cold tongue is…

TMI SST Annual Mean (1998-2005)

  • A zonal band of cold SST along the equator in the east-central Pacific.

  • The result of coastal upwelling and Ekman divergence, a true coupled air- sea interaction process.

  • Linked to tropical cloud and precipitation patterns, ocean biological productivity and carbon cycling.


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Introduction: The Pacific cold tongue is…

Mean CMAP Precipitation

  • Linked to tropical cloud and precipitation patterns, ocean biological productivity and carbon cycling.


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Introduction: The Pacific cold tongue is…

Mean SeaWiFS Surface Chlorophyll-a

  • Linked to tropical cloud and precipitation patterns, ocean biological productivity and carbon cycling.


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Introduction: The Pacific cold tongue is…

Mean Ocean-Atmosphere Carbon Flux (Takahashi et al., 1999)

  • Linked to tropical cloud and precipitation patterns, ocean biological productivity and carbon cycling.

  • Linked to tropical cloud and precipitation patterns, ocean biological productivity and carbon cycling.

  • Where ENSO events produce large SST anomalies.


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Introduction: The Pacific cold tongue is…

TMI SST: December 1997

  • Linked to tropical cloud and precipitation patterns, ocean biological productivity and carbon cycling.

  • Linked to tropical cloud and precipitation patterns, ocean biological productivity and carbon cycling.

  • Where ENSO events produce large SST anomalies.


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Introduction: Tropical cold bias

(Vecchi et al., 2005)

  • Problem: most OGCMs produce a cold tongue that is too cold and extends too far west…

  • Problem: most OGCMs produce a cold tongue that is too cold and extends too far west…

  • Presents obstacles for coupled GCMs to produce realistic tropical cloud and precipitation patterns… one potential culprit in the “double ITCZ” problem.

  • A relevant theme in ongoing and upcoming research programs, e.g. Pacific Upwelling and Mixing Physics (PUMP).

  • Modeling studies aimed at diagnosing the cold bias have focused on the surface energy budget (Kiehl, 1997), atmospheric feedbacks (Gordon et al., 2000; Sun et al., 2003), biological attenuation of shortwave radiation (Murtugudde et al., 2002, Marzeion et al., 2005) and otherwise coupled air-sea interactions (Luo et al., 2004).

  • These studies represent progress, but the cold bias remains a serious problem. There also remain problems with the EUC and SEC.


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Introduction: Current ocean modeling

Image courtesy Dave Behringer and Yan Xue, http://www.nws.noaa.gov/ost/climate/STIP/GODAS.htm

The GODAS, a part of the CFS, is based on the MOM3 OGCM.

Currently 1° x 1° with 1/3° meridional within 10° of the equator.

1982-2004


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Introduction: Current ocean modeling

The GODAS, a part of the CFS, is based on the MOM3 OGCM.

Currently 1° x 1° with 1/3° meridional within 10° of the equator.

1982-2004


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Introduction: Current ocean modeling

The GODAS, a part of the CFS, is based on the MOM3 OGCM.

Currently 1° x 1° with 1/3° meridional within 10° of the equator.


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Introduction: Current ocean modeling

  • What has not been considered explicitly are the effects of horizontal resolution and the inclusion of the Galapagos Islands in the models.

  • The Galapagos Islands are directly on the equator, in the midst of the cold tongue… and in a very critical place in terms of surface fluxes and subsurface currents.

  • Current operational ocean modeling at NOAA NCEP does not include the Galapagos Islands. GODAS horizontal resolution is arguably sufficient to represent them.


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Introduction: Current ocean modeling

Galapagos topography in the GODAS / CFS

Thanks to Dave Behringer (NCEP/EMC) for kindly providing this figure.


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Introduction: Current ocean modeling

  • What has not been considered explicitly are the effects of horizontal resolution and the inclusion of the Galapagos Islands in the models.

  • The Galapagos Islands are directly on the equator, in the midst of the cold tongue… and in a very critical place in terms of surface fluxes and subsurface currents.

  • Current operational ocean modeling at NOAA NCEP does not include the Galapagos Islands. GODAS horizontal resolution is arguably sufficient to resolve them.

  • Effects of the Galapagos Islands in an OGCM (MOM3) were looked at by Eden & Timmerman (2004) but there are some concerns about how that was done. Also, ET-04 focused on very local-scale effects (TIWs and island upwelling). However, ET-04 provides useful comparison with our study.



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Experimental philosophy

Kraus-Turner (1967) type mixed layer model

Price et al. (1986) dynamical instability model

Chen et al. (1994) HYBRID...

  • Gent & Cane (1989) OGCM coupled to an atmospheric mixed layer (just surface fluxes; Murtugudde et al., 1996) and hybrid vertical mixing scheme (Chen et al., 1994a). This model has been extensively tested and used successfully in the tropical Pacific (e.g. Kessler et al., 1998; Chen et al., 1994a,b and many others).

  • Nicely captures the 3 major physical processes of vertical entrainment-mixing:

  • Mixed layer entrainment-detrainment (related to atmospheric forcing)

  • Shear flow instability (Richardson-dependent)

  • Free convection in thermocline (instant adjustment)


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Experimental philosophy

  • Gent & Cane (1989) OGCM coupled to an atmospheric mixed layer (just surface fluxes; Murtugudde et al., 1996) and hybrid vertical mixing scheme (Chen et al., 1994a). This model has been extensively tested and used successfully in the tropical Pacific (e.g. Kessler et al., 1998; Chen et al., 1994a,b and many others).

  • Some modifications: higher-order Shapiro filter (8th), shorter time-step (30 min.), and grid-stretching in the zonal direction as opposed to just y-stretching…


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Experimental philosophy

  • A total of 4 one-year climatology simulations… after being spun up onto each grid independently (60 years plus “spin-overs”).

  • Coarse Uniform zonal resolution, no Galapagos

  • Coarse+G Uniform zonal resolution, with Galapagos

  • FineHigher (stretched) zonal resolution, no Galapagos

  • Fine+GHigher (stretched) zonal resolution, with Galapagos

  • Forcing was climatological ECMWF winds, Xie & Arkin precipitation, ISCCP clouds, and ERBE shortwave.

  • Validation data used were Reynolds, TMI and TAO.

XRES: uniform 3/4, YRES: 1/3° stretching to 1°

XRES: 1/4° stretching to 1°, YRES: 1/4° stretching to 1°





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Results: Comparing annual mean SST

Coarse

Coarse+G

Fine

Fine+G

TMI

Contour Interval: 1°C

Shaded: SST < 23°C

Bold contour: 26°C


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Results: Comparing annual mean SST

Coarse

Coarse+G

Fine

Fine+G

TMI

Contour Interval: 1°C

Shaded: SST < 23°C

Bold contour: 26°C


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Results: Comparing annual mean SST

Coarse

Coarse+G

Fine

Fine+G

TMI

Contour Interval: 1°C

Shaded: SST < 23°C

Bold contour: 26°C


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Results: Comparing annual mean SST

Coarse

Coarse+G

Fine

Fine+G

TMI

Contour Interval: 1°C

Shaded: SST < 23°C

Bold contour: 26°C


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Results: SST seasonal cycle

A cold tongue index

(1998-2005 Annual Mean TMI SST)


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Results: SST seasonal cycle


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Results: Seasonal SST fields

Mar-Apr-May

Sep-Oct-Nov

Coarse

Coarse+G

Fine

Fine+G

TMI

Contour Interval: 1°C

Shaded: SST < 24°C

Bold contour: 27°C

Contour Interval: 1°C

Shaded: SST < 22°C

Bold contour: 26°C


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Results: Seasonal SST fields

Mar-Apr-May

Sep-Oct-Nov

Coarse

Coarse+G

Fine

Fine+G

TMI

Contour Interval: 1°C

Shaded: SST < 24°C

Bold contour: 27°C

Contour Interval: 1°C

Shaded: SST < 22°C

Bold contour: 26°C


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Results: Seasonal SST fields

Mar-Apr-May

Sep-Oct-Nov

Coarse

Coarse+G

Fine

Fine+G

TMI

Contour Interval: 1°C

Shaded: SST < 24°C

Bold contour: 27°C

Contour Interval: 1°C

Shaded: SST < 22°C

Bold contour: 26°C


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Results: Seasonal SST fields

Mar-Apr-May

Sep-Oct-Nov

Coarse

Coarse+G

Fine

Fine+G

TMI

Contour Interval: 1°C

Shaded: SST < 24°C

Bold contour: 27°C

Contour Interval: 1°C

Shaded: SST < 22°C

Bold contour: 26°C


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Results: SST seasonal cycle

2° x 2° box indices

(1998-2005 Annual Mean TMI SST)


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Results: SST seasonal cycle

Coarse Resolution


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Results: SST seasonal cycle

High Resolution


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Results

The EUC


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Results: Zonal Currents and Temperature

GODAS

TAO Clim.

Adapted from Large, et al. (2001)

Coarse

Fine

Coarse+G

Fine+G

(Sep-Oct-Nov)


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Results: Zonal currents at 100m

* This is our model “off the shelf.” No tuning.


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Results: Zonal currents in the RA6 & GODAS

Fine

Fine+G

Mean Zonal Currents

RMSE w.r.t. TAO

(Adapted from Behringer and Xue, 2004)

(Annual Mean)


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Results: Termination of the EUC

Fine

Fine+G


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Results: Termination of the EUC

Fine

Fine+G


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Results: (Non-)Termination of the EUC

(Eden & Timmerman, 2004)


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Results: The EUC from all angles

0m 10m 20m 30m 40m 50m 60m 70m 80m 90m 100m 110m 120m 130m 140m 150m 160m 170m 180m 190m 200m

Fine

Fine+G


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Results: The EUC from all angles

0m 10m 20m 30m 40m 50m 60m 70m 80m 90m 100m 110m 120m 130m 140m 150m 160m 170m 180m 190m 200m

Fine

Fine+G


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Results: The EUC from all angles

0m 10m 20m 30m 40m 50m 60m 70m 80m 90m 100m 110m 120m 130m 140m 150m 160m 170m 180m 190m 200m

Fine

Fine+G


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Results: The EUC from all angles

0m 10m 20m 30m 40m 50m 60m 70m 80m 90m 100m 110m 120m 130m 140m 150m 160m 170m 180m 190m 200m

Fine

Fine+G


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Results: The EUC from all angles

0m 10m 20m 30m 40m 50m 60m 70m 80m 90m 100m 110m 120m 130m 140m 150m 160m 170m 180m 190m 200m

Fine

Fine+G


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Results: The EUC from all angles

0m 10m 20m 30m 40m 50m 60m 70m 80m 90m 100m 110m 120m 130m 140m 150m 160m 170m 180m 190m 200m

Fine

Fine+G


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Results: The EUC from all angles

0m 10m 20m 30m 40m 50m 60m 70m 80m 90m 100m 110m 120m 130m 140m 150m 160m 170m 180m 190m 200m

Fine

Fine+G


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Results: The EUC from all angles

0m 10m 20m 30m 40m 50m 60m 70m 80m 90m 100m 110m 120m 130m 140m 150m 160m 170m 180m 190m 200m

Fine

Fine+G


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Results: The EUC from all angles

0m 10m 20m 30m 40m 50m 60m 70m 80m 90m 100m 110m 120m 130m 140m 150m 160m 170m 180m 190m 200m

Fine

Fine+G


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Results: The EUC from all angles

0m 10m 20m 30m 40m 50m 60m 70m 80m 90m 100m 110m 120m 130m 140m 150m 160m 170m 180m 190m 200m

Fine

Fine+G


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Results: The EUC from all angles

0m 10m 20m 30m 40m 50m 60m 70m 80m 90m 100m 110m 120m 130m 140m 150m 160m 170m 180m 190m 200m

Fine

Fine+G


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Results: The EUC from all angles

0m 10m 20m 30m 40m 50m 60m 70m 80m 90m 100m 110m 120m 130m 140m 150m 160m 170m 180m 190m 200m

Fine

Fine+G


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Results: The EUC from all angles

0m 10m 20m 30m 40m 50m 60m 70m 80m 90m 100m 110m 120m 130m 140m 150m 160m 170m 180m 190m 200m

Fine

Fine+G


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Results: The EUC from all angles

0m 10m 20m 30m 40m 50m 60m 70m 80m 90m 100m 110m 120m 130m 140m 150m 160m 170m 180m 190m 200m

Fine

Fine+G


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Results: The EUC from all angles

0m 10m 20m 30m 40m 50m 60m 70m 80m 90m 100m 110m 120m 130m 140m 150m 160m 170m 180m 190m 200m

Fine

Fine+G


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Results: The EUC from all angles

0m 10m 20m 30m 40m 50m 60m 70m 80m 90m 100m 110m 120m 130m 140m 150m 160m 170m 180m 190m 200m

Fine

Fine+G


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Results: The EUC from all angles

0m 10m 20m 30m 40m 50m 60m 70m 80m 90m 100m 110m 120m 130m 140m 150m 160m 170m 180m 190m 200m

Fine

Fine+G


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Results: The EUC from all angles

0m 10m 20m 30m 40m 50m 60m 70m 80m 90m 100m 110m 120m 130m 140m 150m 160m 170m 180m 190m 200m

Fine

Fine+G


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Results: The EUC from all angles

0m 10m 20m 30m 40m 50m 60m 70m 80m 90m 100m 110m 120m 130m 140m 150m 160m 170m 180m 190m 200m

Fine

Fine+G


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Results: The EUC from all angles

0m 10m 20m 30m 40m 50m 60m 70m 80m 90m 100m 110m 120m 130m 140m 150m 160m 170m 180m 190m 200m

Fine

Fine+G


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Results: The EUC from all angles

0m 10m 20m 30m 40m 50m 60m 70m 80m 90m 100m 110m 120m 130m 140m 150m 160m 170m 180m 190m 200m

Fine

Fine+G


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Results: The EUC from all angles

Zonal Velocity at 93°W

Depth [10 m]

Fine

Depth [10 m]

Fine+G


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Results: The EUC from all angles

Zonal Velocity at 91°W

Depth [10 m]

Fine

Depth [10 m]

Fine+G


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Results: The EUC from all angles

Zonal Velocity at 89°W

Depth [10 m]

Fine

Depth [10 m]

Fine+G


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Results: Response of SST and AO NHF

Contours: ΔNHF

every 10 w m-2

Fine+G – Fine

Shading: ΔSST

NHF = Qsw – [ εσBSST4 + LV ρairCMUA( qs–qA) + CPρairCHUA(SST–θA) ]

(Sep-Oct-Nov)


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Results: SST response

Fine+G – Fine

(Vecchi et al., 2005)

(Sep-Oct-Nov)


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Results: SST response

Fine+G – Fine

(Sep-Oct-Nov)


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Results:Local SST response

(Eden and Timmerman, 2004)

Fine+G – Fine

(Sep-Oct-Nov)


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Results:Why results differ from ET-04?

  • Similar: Strong upwelling/colder SST right up against the west side of the Galapagos Islands. Very local.

  • Different:Termination of EUC = broader tropical SST response.

Why?

  • Galapagos in ET-04 too small and entirely south of the equator…

  • The EUC in ET-04 is right on the equator, not ½° or so south…

  • Vertical mixing schemes?

  • ET-04 imposed surface heat flux with Haney-type formulation?

  • ET-04 domain was 10°N to 10°S?

  • ET-04 results are actually the average of 10 years of output (the 90’s) …different from a true “climatology” run?



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Results: the EUC? Temperature vs. Depth

Sharp gradient

24°C outcrops ~130°W

More diffuse gradient

24°C outcrops ~115°W

Diffuse gradient

24°C outcrops ~115°W

All averaged 2°N to 2°S

(Sep-Oct-Nov)


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Results: the EUC? Temperature vs. Depth

Temperature at 110°W (°C)

All averaged 2°N to 2°S

(Sep-Oct-Nov)


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Results: the EUC? Temperature vs. Depth

dT/dz at 110W (°C per 10m)

~MLD

All averaged 2°N to 2°S

(Sep-Oct-Nov)


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Results: the EUC? Dynamical adjustment

τx

Warm

Cold

x

Sv

Thermocline

z

SEC

Zonal Circulation

Why might these improvements result from the termination of the EUC?

For a given τx and ocean geometry, you have an equilibrium equatorial dynamical balance.

Change the geometry in such a way that disrupts one component of the balance, and you must have a new, adjusted dynamical balance.


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Results: the EUC? Dynamical adjustment

τx

η

SEC

Warm

Cold

x

Sv

Thermocline

EUC

z

P

Zonal Circulation

Why might these improvements result from the termination of the EUC?

For a given τx and ocean geometry, you have an equilibrium equatorial dynamical balance.

Change the geometry in such a way that disrupts one component of the balance, and you must have a new, adjusted dynamical balance.


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Results: the EUC? Dynamical adjustment

η

SEC

Warm

Cold

x

Sv

Thermocline

EUC

z

P

Zonal Circulation

Why might these improvements result from the termination of the EUC?

For a given τx and ocean geometry, you have an equilibrium equatorial dynamical balance.

Change the geometry in such a way that disrupts one component of the balance, and you must have a new, adjusted dynamical balance.


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Results: the EUC? Dynamical adjustment

Warm

Cool

x

Sv

z

P

Zonal Circulation

Why might these improvements result from the termination of the EUC?

η

SEC

Cool

Thermocline

Galapagos

EUC

For a given τx and ocean geometry, you have an equilibrium equatorial dynamical balance.

Change the geometry in such a way that disrupts one component of the balance, and you must have a new, adjusted dynamical balance.


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Results: the EUC? Dynamical adjustment

nz

η = h(1) b(1) + Σ h(k) b(k)

k=2

g

g

Zonal Circulation

Why might these improvements result from the termination of the EUC?

b(k) = α g (T – Tref)

(Sep-Oct-Nov)


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Results: the EUC? Dynamical adjustment

Realistic Indonesian throughflow?

Zonal Circulation

Why might these improvements result from the termination of the EUC?

(with 1σ shown)

(Sep-Oct-Nov)


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Results: the EUC? Dynamical adjustment

Zonal Circulation

  • Why might these improvements result from the termination of the EUC?

1. Basin-wide adjustment of the zonal pressure gradient beginning with the EUC

2. Slower SEC so reduced cold SST advection along the surface

3. Reduced SEC-EUC shear so MUCH less vertical entrainment-mixing (EMX)

4. All of these promote a deeper thermocline and warmer SST


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Results: the EUC? Dynamical adjustment

South

North

Meridional Circulation

Why might these improvements result from the termination of the EUC?


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Results: the EUC? Dynamical adjustment

South

North

Meridional Circulation

Why might these improvements result from the termination of the EUC?

(Sep-Oct-Nov)


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Results: the EUC? Dynamical adjustment

or, schematically…

South

North

Meridional Circulation

Why might these improvements result from the termination of the EUC?

* reduced speeds in meridional overturning cells

(Sep-Oct-Nov)


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Results: the EUC? Entrainment Mixing

Why might these improvements result from the termination of the EUC?

Fine+G – Fine

Shading: ΔEMX (Wm-2)

(Sep-Oct-Nov)



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Summary & implications the EUC?

  • The bottom line: Given sufficient zonal resolution (to get the EUC right), the Galapagos Islands terminate the EUC and lead to improved modeling of SST in the Pacific cold tongue region, and for good reasons… adjusted equatorial dynamics and less entrainment-mixing.

Isla Isabela, Galapagos Archipelago From NOAA R/V Ka’imimoana 2 May 2005

Galapagos in Fine+G


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Summary & implications the EUC?

TMI

Fine+G

Overall SST Improvement

Coarse

(Sep-Oct-Nov)

Reynolds

(Sep-Oct-Nov)


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Results: the EUC? SST seasonal cycle


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Results: the EUC? SST response

Fine+G – Fine


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Summary & implications the EUC?

  • The cold bias problem has for some time been a mainstay of OGCMs.

  • These SST improvements could lead to substantial improvements in tropical cloud and precipitation patterns, ocean biological productivity, and carbon cycling in coupled models.

  • The GODAS cold bias could be improved by…

  • (a) increasing the zonal resolution- just enough to get the EUC right

  • (b) adding the Galapagos Islands. This should be given consideration in the next iteration of the NCEP GODAS.

  • Future work should address the impact of the Galapagos Islands on interannual variability (e.g. ENSO events) and prediction of seasonal and interannual variability in the tropical Pacific (e.g. NINO forecasts).


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Thank You. the EUC?

University of Maryland

MODIS (Terra) true color March 12, 2002 Courtesy NASA GSFC


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