globalObserving decadal variability in the oceans

with altimetry and in situ data

Dean Roemmich1, Josh Willis2, and John Gilson1

1Scripps Institution of Oceanography, UCSD, 2Jet Propulsion Laboratory

OSTST Hobart, March 2007

Thanks to many contributors!

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Objective: Use global datasets (altimetry and in situ) to synthesize regional studies of decadal variability.

Outline• Global patterns of decadal variability, 1993 – 2006.• S. Hemisphere subtropical/subpolar variability.• The tropical Pacific and Indian Ocean.• N. Hemisphere subtropical/subpolar variability.• Decadal variability in heat transport. • Global summary.

Steric height, 0/2000 db 2006 annual average from Argo

Datasets and data products:• ALT: AVISO gridded (reference) altimetric height; 1993 – Aug 2006 (Ducet

et al, 2000).• Argo: 150,000 Argo profiles from 2004-2006 gridded 1o x 1o x 1 month

(similar to Coriolis product, which also uses non-Argo data). • HRX: High Resolution XBT transects.• WOCE and CLIVAR hydrographic sections.• Willis-A: 0-750 m, 1993-2006 gridded temperature; first guess based on

ALT (Willis et al, 2004; similar to ARMOR product, Guinehut et al, 2004)• Willis-B: 0-750 m, 1993-2006 gridded temperature; in situ data only.• WOA01: World Ocean Atlas 2001 (combined data types, Conkright et al.,

2002).• WGHC: WOCE gridded hydrographic climatology (station data only,

WOCE-weighted, Gouretski and Koltermann, 2004).• NCEP/NCAR reanalysis wind stress.• WHOI FSI data (~210 out of 2800 presently active floats) excluded from

Argo gridded datasets (including Willis); probably recoverable.

Altimetric SL rise 1993-2003

In-situ SL rise (ARMOR) 1993-2003

Difference ALT – INSITU 1993-2003

Left, 10-year trends from Morrow et al, 2007

Steric height (Willis-B): 2003/5 minus 1993/55o x 3o smthed, 0/750 db

ALT(AVISO): 2003/5 minus 1993/55o x 3o smoothed

- Southern subtropical gyres strengthened. - Southern Ocean warmed. - Western tropical Pacific/eastern Indian warmed. - NPC, NAC strengthened. - Northern subpolar gyres weakened. - Globally, ALT(3 cm), SH(1.3 cm + deep contribution)

Two views of decadal change from ALT and steric height:

Zonally-averaged 10-year difference 2003/5 minus 1993/5

Top: Altimetric height (blue) Thermosteric height (red, Willis-A) Thermosteric height (green, Willis-B)

Bottom: Colors: Temperature diff. (Willis-A)Contours: Mean Temperature (Argo)

ALT

Steric

Zonal average of altimetric height, steric height, and temperature

More work is needed on instrument biases to understand mostly-XBT to mostly-Argo transition.

ALT

Steric

Zonally-integrated 10-year difference 2003/5 minus 1993/5

Top: Altimetric height (blue) Thermosteric height (red, Willis-A) Thermosteric height (green, Willis-B)

Bottom: Colors: Temperature diff. (Willis-A)Contours: Mean Temperature (Argo)

The dominance of the Southern Hemisphere Oceans stands out in the zonally integrated quantities. The largest decadal change occurred at 40oS.

Zonal integral of altimetric height, steric height, and temperatureZo

nal i

nteg

ral o

f hei

ght a

nom

aly

x 10

4 m2

T(zonal avg): Argo (2004/6) minus WGHC

T(zonal avg): Argo (2004/6) minus WOA01

T(zonal avg): Willis-A 2003/5 minus 1993/5

Zonally averaged temperature change for different climatologies

Similarity in pattern suggests decadal and multi-decadal changes are not greatly different.

Zonally averaged temperature difference by ocean basin, Argo (2004/06) minus WOA01

XY-avg of ΔT

Average temperature anomaly is 0.03oC (8 x 1022 J globally)Strongest warm anomalies at 40oS, 60oNCool anomalies (upward displacement) at 20oS, 20oN, 40oN.

XY-avg of ΔS

• Atlantic (0-200 m) has freshwater deficit in Argo data relative to WOA01 (.05 psu)• Pacific (0-200 m) has surplus (.01 psu). • Freshening at intermediate levels in Southern Hemisphere in all oceans.• Net freshening 0-2000 m.

Zonally averaged salinity difference by ocean basin, Argo (2004/06) minus WOA01

Southern subtropical gyres and SAF

Colors: ALT 10-yr changeContours: Argo DH 1000/2000

Colors: Argo – WGHC DH 0/2000Contours: Argo DH 1000/2000

Colors: ALT 10-yr changeContours: SLP 10-yr change

Cai (2006) “an increasing trend of the midlatitude positive curl with isopleths shifting southward, indicating an intensifying, southward-shifting subtropical gyre circulation.”

Qiu and Chen (2006) compared ALT (upper panel) with SSH trend from a simple wind-driven model, emphasizing the importance of propagating response.

Roemmich et al (2007)Comparison of Argo, ALT, WOCE, and CLIVAR repeat hydrography.Gyre acceleration was seen in all data, including float trajectories at 1000 m and temperature to 2000 m.

South Pacific

SSH and 0/2000 db SH

1000/2000 db SH

South Pacific – Salinity

Fresh anomalies in intermediate waters of the Southern Hemisphere seen in WOCE relative to earlier data (Wong et al, 1999)continue to freshen in Argo relative to WOCE.

Argo minus WOA01 salinity on σθ = 26.9

Argo minus WOCE salinity on σθ = 26.9

SURVOSTRAL HRX lineSR3 repeat hydrography line

Morrow et al, 2007

South of Australia

Both the High Resolution XBT transect and the SR3 repeat hydrography line showed a southward shift of the STF and SAF. In the latter, the warming was seen to 2000 m depth (Morrow, et al, 2007).

HRX lineSR3 line

1992/6

2000/4

Δ

Tropical Pacific/Indian

A multi-decadal decrease in the Pacific tradewinds (McPhaden and Zhang, 2002) began to reverse in the most recent decade (McPhaden and Zhang, 2004).

The recent 10-year signal in the ocean is a warming and downward displacement of the thermocline in the western tropical Pacific, cooling in the east.

NCEP Anomaly in wind stress: 2000/2006 minus 1990/2000

Zonal wind stress averaged over 10oN-10oS, 150oE-80oW, NCEP

.01 N/m2

Indian Pacific

Top: Blue – ALT 10-yr diff, Red – Willis_B 10-yr diff

Colors: Along equator Willis_B 10-yr diff, Contours: Argo mean T

12-month running mean

60-month running mean

Tropical Pacific surface salinity(Delcroix et al., 2007)

On decadal time-scale, SSS in the designated regions vary in phase(+) or out of phase (-) with the PDO.This is consistent with precipitation variability in the western, but not eastern region.

Trend : -0.23 psu / 30 years

A multi-decadal trend is seen in SSS in the western warm pool region, consistent with precipitation.

+

++-

North Pacific

ALT, 2003/05 – 1993/95

Willis-B, SH 0/750 db:2003/05 – 1993/95

Colors: Argo – WOA01 DH 0/2000 db

Contours: Argo DH 0/2000 db

HRX line

Cummins and Freeland (2007)

Douglass et al, 2006

North PacificThe NPC increase is seen in ALT, broadscale and HR XBT, and Argo data.

Zonal velocity at ~150oW from HR XBT

North Atlantic

ALT, 2003/05 – 1993/95

Willis-B, SH 0/750 db:2003/05 – 1993/95

Colors: Argo – WOA01 upper 200 m salinity anomaly

Contours: Argo DH 0/2000 db

Pattern is similar to North Pacific.

Subpolar gyre decreasing in strength (Hakkinen and Rhines, 2004)

High salinity, reversing the previous decadal freshening (Hatun et al, 2005, Peterson et al, 2006) is attributed to increased input of subtropical waters.

North Atlantic

The first EOF dominates the 10-year change in ALT(Hakkinen and Rhines, 2004, also Fu, 2004, Greiner et al 2006)

Decadal increases in surface salinity are seen in time-series in the northern Atlantic (Hatun et al, 2005), and attributed to increased input of subtropical waters.

Decadal variability in heat transport

Hydrographic transects in 1957, 1981, 1992, 1998, 2004.

SW Pacific heat budget, Roemmich et al, 2005

N. Atlantic MOC and heat transport, Bryden et al, 2005

N Pacific heat budget, Douglass et al, 2007

A high priority is to observe the interannual to decadal variability in meridional ocean heat transport (Douglass et al, 2007) and overturning circulations (Bryden et al, 2005), as well as air/sea feedback in western boundary current regions (e.g. Kelly, 2004).However, a comprehensive WBC observing system is not yet in place, and historical datasets are subject to serious aliasing issues (e.g. Wunsch and Heimbach, 2006).

Summary (1) – Decadal ocean variability

• Decadal anomalies in the wind-driven circulation are largely responsible for spatial variability of ocean heat gain.

• Patterns of variability span more than one ocean basin at middle and high latitude in both hemispheres.

• Decadal changes extend to depths of at least 2000 m.

• In comparison of Argo salinity to WOA01, salinity increased in the upper 200 and decreased below. The Pacific became fresher and the Atlantic saltier (net transfer of freshwater).

• The Southern Hemisphere dominates global decadal variability in heat and freshwater content.

Summary (2) – Ocean observations

• Datasets exist for decadal variability in ocean temperature, 0 – 750 m, but attention is needed to biases in XBT data.

• Southern Ocean observations are very sparse in pre-Argo times.

• Pre-Argo salinity observations are not adequate for description of global/decadal variability.

• Development of boundary current and heat transport observations is needed, as well as abyssal observations.

• Climate research applications place high demands on ocean data quality, requiring .

Winter SSS (colors) and σθ (contours) from Argo

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Phys. Oceanogr., 36, 2012 – 2024.