the role of the southern ocean in sea level change · this project is going to focus on the role of...

The role of the Southern Ocean in sea level changeDr Xuebin Zhang | Senior Research Scientist

3 May 2018

CSHOR Sea-level Project Team

• CSIRO: Dr. Xuebin Zhang (project leader) and Dr.Kewei Lyu (postdoc)

•UNSW: Prof. John Church, Prof. Trevor McDougulland Dr. Yuehua Li (postdoc)

•UTAS: Prof. Matt King, Dr. Steven Phipps and Dr.Christopher Watson

•QNLM: Prof. Xianyao Chen

CSHOR sea level project | Xuebin Zhang2 |

Ocean and climate processes affecting global and regional sea level


IPCC AR5Fig. 13.1

Warming (cooling) of the ocean (thermal expansion/contraction)

Change in mass of glaciers and ice sheets (Barystatic)

Changes in liquid water storage on land (Barystatic)

“regional sea level change and coastal impacts” is one of five Grand Challenges identified by the WCRP

21st century GMSLR projections


0.98 m

0.44 m

0.74 m

0.53 m

0.61 m

0.28 m

0.55 m

“The collapse of marine-based sectors of the Antarctic Ice Sheet, if initiated, could cause GMSL to rise substantially higher (estimated at several tenths of a metre ) than the likely range during the 21st century.” (IPCC AR5)

Medium confidence in likely ranges. Very likely that the 21st century mean rate of GMSLR will exceed that of 1971-2010 under all RCPs.

Motivation for CSHOR sea-level project

The Southern Ocean is a key area for improving understanding and projections of ocean heat content and sea level change because:

It is one of the key areas where heat enters the ocean, resulting in heat storage in the upper ocean and in the abyssal layers, and contributing to sea-level rise through thermal expansion (Purkeyand Johnson, 2010; Roemmich et al. 2015; Wijffels et al. 2016).

Southern Ocean interacts with cryosphere in complicated ways, in particular a warming ocean is critical to the dynamic response of the Antarctic Ice Sheet (Church et al. 2013; DeConto and Pollard 2016).


Achievements and highlights from FY 17-18

• Novel analysis of global mean sea level budget and identifying source of acceleration

• Ocean heat uptake & redistribution based on CMIP5 climate models

• Apparent multi-decadal oscillation in sea levels

• Selecting and tuning up models for perturbation experiments

• Uncertainties of Antarctic Ice Sheet mass flux time series

CSHOR sea level project | Xuebin Zhang

Latest global sea level budget analysis leads to identifying the dominant processe responsible for increasing SLR rate

7 |

Missing SLR acceleration?

• GMSL has been rising at a faster rate during the

satellite altimetry period (since 1993) than previous

decades, and is expected to accelerate further over

coming century.

• However, the accelerations observed over century

and longer periods have not been clearly detected in

altimeter data over about two decades. In fact, the

rate of GMSL rise decreases from 3.2 mm/yr in the

first decade to 2.8 mm/yr in the second decade,

which was suggested to be relate to natural

variability (e.g., Cazenave et al. 2014).

• This lack of observed acceleration of GMSL contrasts

with a simultaneously increased contribution from

the Greenland Ice Sheet and a less certain increase

from Antarctica Ice Sheet.


A novel perspective by Watson et al. (2015)


• Unlike others, Watson et al. re-examined historical altimetry data, compared them with tide gauge observations, and identified mission-specific biases

• ‘Bias drift’ results vary systematically as a function of VLM applied.

• Results were in contrast to other groups, and in the absence of a reprocessed record, evaluation of the impact on the GMSL record was warranted.

Sea level budget closure

• To understand the increasing rate of sea level rise by examining the sea level budget and tracing down the main drivers.

• This question was investigated at similar times by two groups: (Chen et al. NCC, 2017 & Dieng et al. GRL, 2017)


=?

Church et al. IPCC Ch13, 2013

Instantaneous rate budget closure by using Ensemble Empirical Mode Decomposition (EEMD) analysis

(GPS VLM, Watson et al. 2015)

=? Chen et al. NCC, 2017

• The steric component contributes about ~55% of the total GMSL rate in 1993, reducing to ~30% by 2014, despite itself not changing much over the period.

• Increasing contributions come from the mass components, the largest, and statistically significant increase comes from the Greenland Ice Sheet, which is less than ~5% of the GMSL rate during 1993 but more than ~25% during 2014.

11 | CSHOR sea level project | Xuebin Zhang

12 |

ARGO estimates of heat uptake for the upper 2000 m since 2006, which were dominated by mid-latitude southern

hemisphere (Roemmich 2015, Nature CC)


Since 1970’s, the ocean absorbs ~93% of heat uptake of our climate system (IPCC AR5 Ch. 3).

Heat uptake in the Southern Ocean


Ocean temperature trend (oC/decade)

Argo (2005-2014)

D&W(1950-2008)

CMIP5(1950-2005)

CMIP5(1996-2090)


Southern Hemisphere ocean temperature projection: CMIP5 ensemble mean

Major warmingoccurs around thegyre boundary


Temperature change due to

isopycnalvertical

movement

SpicinessOcean temperature

change under RCP8.5

heaving

Decomposing ocean temperature changes


Is there a 60-year oscillation in global mean sea level? (Chambers et al. 2012)

16 |

Identifying multi-decadal oscillations in individual sea-level components


Meyssignac et al. 2017: Evaluating Model Simulations of Twentieth-Century Sea-Level Rise.Part II: Regional Sea-Level Changes.

The “apparent” 60 year Oscillation in Forced Sea-level Change


• “Apparent” multi-decadal signal in forced sea level –possible contribution from natural variability?

• Complex Spatial structure (noise?)

• Some similarity to observed patterns – Need to understand patterns and to compare quantitatively with observations

• Considering regional natural variability requires additional analysis

Dynamic sea level fingerprint from CMIP3/5 climate models and downscaled with a 0.1o eddy-resolving OGCM


CMIP3/5 - 1.0o

Downscaled – 0.1o

• How to explain these regional patterns? Dirven by winds, heat flux or freshwater forcing?

• Meso-scale eddies are basically missing in climate models, but they play significant roles in heat transport and exchange (Griffies et al. 2015), the ocean kinetic energy budget (Ferrari and Wunch 2008).

• Through numerical perturbation experiments, separate impacts of various forcings (e.g., wind vs melting; natural vs anthropogenic) on sea level and ocean dynamics in the Southern Ocean .

A hierarchy of models


•Global models:OFAM3 (1/10o)ACCESS-ocean (1o, 0.25o)

• Regional models based on MITgcm (poleward of 25oS, 1o, 1/3o, 1/6o, 1/12o) (Collaboration with Drs Matt Mazloff and Bruce Cornuelle of Scripps Institution of Oceanography)

• How robust are the uncertainties?

• Williams et al. 2014 reported serial correlations in GRACE (gravity field) time series which increased rate uncertainties 4-6x

• Uncertainties in the Input-Output estimates are being considered (examining noise properties of climate model outputs)

Recent Antarctic ice mass balance estimates


Regional uncertainties in ice mass flux• To date, simple statistical models

have been used to compute the uncertainty in trends of Antarctic surface mass balance (net precipitation)

• Wouters et al., Nature Geo, (2013) examine auto-regressive models for the entire ice sheet

• We are examining basin-scale noise properties to determine appropriate noise models and trend uncertainties for improved basin-scale (i.e., regional) mass flux estimates

Wouters et al 2013


Trend and acceleration uncertainty for Antarctica. Uncertainty (95% range, blue area) in mass trend and acceleration estimates due to stochastic ice sheet variability as function of observation length for the AIS.

Outlook for FY 18-19

• Perturbation experiments based on a suite of OGCMs (CSIRO)

• Ocean heat uptake & redistribution based on CMIP5/6, and above perturbation experiments (CSIRO)

• Historical ocean heat content estimate using a better vertical interpolation scheme (UNSW)

• Uncertainties of Antarctic Ice Sheet mass flux time series (UTAS)

• Ice sheet modelling (UTAS)

Ice sheet modelling based on PISM


IPCC WGI Ch 3

The Earth’s energy balance is central to understanding climate change

Oceans absorbed >90% of the energy storage. But there are still major uncertainties in upper ocean heat content changes27 |

Domingues et al. (2008) improved estimates of Global Ocean Heat Content (3-year running mean)

28

Levitus et al.

Ishii et al.

Domingues et al.

But, vertical interpolation biases could be a significant fraction of the change. Can we provide the next step change in improved estimates of upper ocean heat content change from 1970 to present?


Ocean heat content estimates prior to CTD and Argo observations need to interpolate vertically between the sparse observations.

Need to improve estimates of changes in ocean heat content

The simple technique used by Domingues et al. (2008) results in significant biases.

An improved technique using piecewise cubic Hermite interpolating polynomial developed by Barker and McDougall have smaller biases. Potential biases in upper ocean heat content

estimates


Tests based on an artificial snapshot of temperature field, which provides the testing bed for vertical interpolation schemes.

This project is going to focus on the role of the Southern Ocean in sea-level change, through examining thermal expansion (heat uptake and redistribution) and mass input from Antarctic Ice Sheet, with the aim to provide more reliable future projections of global and regional sea levels for better adaptation and mitigation planning.

This project will use various in-situ and satellite observations, CMIP5/6 climate models, stand-alone or coupled ice-sheet models, eddy-resolving ocean models, and perturbation/sensitivity experiments.

The project is on track and is building momentum to deliver more novel findings.

Summary


CSHORXuebin ZhangSenior Research Scientist

t +61 3 6232 5043e [email protected] https://cshor.csiro.au

Thank you

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