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Observations Supporting Decadal Predictions. Roger Lukas University of Hawaii. Climate Research Committee Forum on Decadal Predictability. 12/2/2008. Observations Supporting Decadal Predictions.

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observations supporting decadal predictions

Observations Supporting Decadal Predictions

Roger LukasUniversity of Hawaii

Climate Research Committee Forum on Decadal Predictability

12/2/2008

observations supporting decadal predictions1
Observations Supporting Decadal Predictions
  • Focus here is on decadal prediction initialization, but we must also consider prediction validation and model improvement
  • Assimilation formalisms provide initial state estimates from observationsand a framework for assessing model errors relative to observational errors
oke et al

Altimeter

Altimeter

T/S

XBT

XBT

SST

SST

Moorings

Moorings

Oke et al.

Assimilated and with-held observations

Assimilated observations

  • Observing System Experiments (OSEs)
    • Assimilate real observations
    • Systematically with-hold observation types

Evaluation/

Validation

Forecast

or

BGF

Analysis

or

Forecast

T/S

GODAE Final Symposium, 12 – 15 November 2008, Nice, France

overview
Overview
  • Focus on ocean state initialization
        • Identified source of decadal predictability in some models
        • Oceanic impacts (fisheries, coastal inundation, shipping)
  • Status of ocean components of climate observing system
  • Identify some gaps and weaknesses
  • Discuss strategy for enhanced observational impacts on decadal time scales
  • A few requirements
ocean observing systems have advanced in the last decade
Ocean observing systems have advanced in the last decade
  • In situ surface meteorology + scatterometer + SST
  • Global sea level through altimetry and in situ gauges
  • Tropical moored arrays
    • TAO, Triton, Pirata implemented in Pacific and Atlantic
    • Under development in Indian Ocean
  • Argo float array
  • RAPID/MOCHA 26°N AMOC
  • OceanSITES (time-series stations)
    • NSF: HOT + BATS
    • NOAA: Atlantic and Pacific ORS moorings
slide7

Initial Global Ocean Observing System for Climate Status against the GCOS Implementation Plan and JCOMM targets

Total in situ networks

60%

February 2008

87%

100%

62%

81%

100%

43%

79%

24%

48%

Milestones

Drifters 2005

Argo 2007

sea level
Sea Level
  • Tide gauges + altimeters
        • ENSO
        • Integral constraints on heat content
        • global rise rate
        • Marginally eddy-resolving
  • Understanding decadal sea level variations is problematic – spatial variability is large
      • Salinity contributions not well constrained
      • Deep water mass variations may be important
      • Eddies may be important feedbacks on WBCs (~50% of decadal fluctuations), but can’t be initialized
slide11

Decadal change of N. Atl. MOC at 26N estimated by an ECCO-GODAE product (Wunsch & Heimbach 2006)

  • Complex vertical structure:
  • Weakening northward transport above 1000 m
  • Strengthening southward transport of NADW
  • Strengthening northward transport of abyssal water
  • No significant decrease of northward heat transport (upper-ocean warming enhances vertical temperature gradient to offset weakening of upper MOC).
  • Opposite trends of MOC strength at 26N & 50N.
slide12

Argo floats March 2008

Note dots are larger than mesoscale eddies

increasing demand on float power duration and reliability
Increasing demand on float power; duration and reliability
  • Biogeochemical sensors (e.g. oxygen, fluorometer, …)
  • Sampling upper few meters requires additional sensors
  • Deeper profiling
  • Ice detection
more argo floats needed
More Argo floats needed

Smith et al. (2007)

“… improvements in DePreSys relative to NoAssim on decadal time scales result mainly from initializing H.”

“Furthermore, a substantial increase in the number of subsurface ocean observations through the Argo program should substantially improve our ability to initialize the ocean in future …”

Signal/noise requires more profiles in space/time to reduce aliasing noise

slide18

AMOC

  • Great progress with AMOC (RAPID/MOCHA array)
  • Need more measurements to partition effects of AMOC constituent variations

Church (2007, Science)

slide19

Energetic high frequency variations

LF ~mass balance

Array concept works

Kanzow et al. (2007)

slide21

A complete AMOC observing system would include:

• The Nordic Sea overflows

• Production and export of dense waters from the Labrador Sea

• The time varying strength of the AMOC in the subpolar North Atlantic following vertical entrainment and mixing processes

• The time varying strength of the AMOC in the subtropical North Atlantic (e.g., RAPID).

• The time varying strength of the AMOC in the subtropical South Atlantic.

US CLIVAR report 2008-1

some obvious observational weaknesses not prioritized
Some Obvious Observational Weaknesses(NOT prioritized)
  • AMOC meridional structure, deep convection regions
  • Boundary currents
        • US (NSF/OOI and NOAA/IOOS)
        • South Atlantic; W. Pacific
        • LLWBCs
  • Surface salinity (and global rainfall)
  • Deep thermohaline structure (>2000 m)
  • High latitude time-series generally
        • NSF/OOI plans: Station PAPA, Irminger Sea, SP (55S/90W)
strategy for observations supporting decadal predictions
Strategy for Observations supporting Decadal Predictions
  • We can’t get many more new realizations so need to consider observational strategy
  • Paleo can help extend temporal coverage, but need observations to calibrate proxies
  • Limited DoF in time may be overcome to some extent by DoF in space
  • Need multivariate time-series for validation

DoF = degrees of freedom

requirements for observations supporting decadal predictions
Requirements for Observations supporting Decadal Predictions
  • Need to improve surface forcing estimates going forward, and reanalyses
    • Consistent, accurate instrument calibrations are crucial
  • Need more integral constraints
    • e.g. surface salinity is an integral constraint and errors don’t directly feedback onto atmosphere (Aquarius satellite mission + in situ)
    • e.g. regional tomography array for deep convection regions
  • Careful observing system experiments e.g. Oke et al. and Lee et al. (Nov 2008, GODAE Final Symposium)
oke et al oses and osses
Oke et al.: OSEs and OSSEs

Simulated

observations

Assimilated “observations”

  • Observing System Experiments (OSEs)
    • Assimilate real observations
    • Systematically with-hold observation types

Evaluation/

Validation

Forecast

or

BGF

Analysis

or

Forecast

  • Observing System Simulation Experiments (OSSEs)
    • Assimilate pretend “observations” … from a model
    • Systematically include different observation types … including future observation types

GODAE Final Symposium, 12 – 15 November 2008, Nice, France

some conclusions
Some Conclusions
  • We are not oversampling the ocean
  • Prioritizing gaps relative to decadal prediction requires better understanding and models
  • Shouldn’t neglect decadal signals that could add prediction skill
        • Decadal variations arising from tropics?
        • Decadal variations of midlatitude Pacific?
  • Harder to sustain/improve existing observing systems than to start new ones
        • research funding is entrepreneurial
        • transition from research to operations
  • Must build multi-decadal time-series for the future
contributors
Contributors
  • Bob Weller
  • Bill Johns
  • Bo Qiu
  • Detlef Stammer
  • Bruce Cornuelle
  • Niklas Schneider
  • Axel Timmermann