This material is based upon work supported by the National Center for Atmospheric Research, which is a major facility sponsored by the National Science Foundation under Cooperative Agreement No. 1852977.

CESM Insights Into Drivers of Regional Sea

Level Rise in Recent Decades

(Fasullo and Nerem 2018 PNAS; Hamlington et al. 2019 GRL)

June 18, 2019

John Fasullo CAS/CGD Project Scientist

Contributions from R.S. Nerem (U Colorado) B. Hamlington (JPL)

Altimeter-Era Sea Level Trends (AVISO)

Trends over the altimeter era (1993-2018) exhibit strong spatial structure.

What drives these patterns? How will they evolve?

2019 CESM Workshop: Fasullo: Sea Level Drivers

Global

~3 mm/yr

Altimeter-Era Sea Level Trends vs Modes (AVISO)

Early in the altimeter record, trends

were shown to be strongly influenced

by ENSO.

A) Trends in the first half are

influenced by El Niño in its early

years and La Niña later in the era.

∴ Trend is La Niña-like

B) Trends in the second half are

influenced strongly by La Niña in

early years and strong El Niño

events late in the era.

∴ Trend is El Niño-like

2019 CESM Workshop: Fasullo: Sea Level Drivers

Trends: Global mean removed

2019 CESM Workshop: Fasullo: Sea Level Drivers

Altimeter-Era Sea Level Modes (AVISO)

Later, an influence of the PDO similar to

that of ENSO (A) was also identified (B).

A transition in the PDO from strong

positive values (1990s) to negative values

(2000-13) – are plausible explanations of

the trend pattern, which resembles the

inverse of the PDO regression.

But are the full altimeter-era trends in

fact dominated by the noise of internal

variability?

Altimeter-Era (1993-2018) Large Ensemble Forced Responses

CESM (A, 40mem) and ESM2M (B,

30 mem) forced responses

(ensemble mean trends) for the

altimeter era show strong spatial

structure; resembling the PDO in

many respects (zonal contrast

across Pacific).

The magnitude of the FRs is sizeable;

doubling rates of rise in some

regions; eliminating rise in others.

Some model dependence exists in the

FR, particularly in the deep tropical

Pacific Ocean (10N/S, Walker cell)

but many common features exist:

western subtropical and Atlantic

Ocean maxima; eastern Pacific,

Southern Ocean minima.

1993-2018 Ensemble Mean Trends (SSH mm/yr; sfc Winds m/s/yr)

CESM

ESM2M

mm/yr

Global Mean SSH Removed

Altimeter-Era (1993-2018) Large Ensemble Forced Responses

CESM (A, 40mem) and ESM2M (B,

30 mem) forced responses

(ensemble mean trends) for the

altimeter era show strong spatial

structure; resembling the PDO in

many respects (zonal contrast

across Pacific).

The magnitude of the FRs is sizeable;

doubling rates of rise in some

regions; eliminating rise in others.

Some model dependence exists in the

FR, particularly in the deep tropical

Pacific Ocean (10N/S, Walker cell)

but many common features exist:

western subtropical and Atlantic

Ocean maxima; eastern Pacific,

Southern Ocean minima.

CESM

ESM2M

mm/yr

1993-2018 Ensemble Mean Trends (SSH mm/yr; sfc Winds m/s/yr)

Global Mean SSH Removed

Emergence of Forced Responses in CESM

2019 CESM Workshop: Fasullo: Sea Level Drivers

Patterns of the forced response

evolve over time / can be correlated

with trends of individual members to

assess emergence.

Key findings:

1) FRs evolve as a f(time)

2) FR has emerged for the

altimeter era: correlations for 1993-

2018 for every LE member exceed

the 95% threshold of those from the

1800 yr control run (r=0.25)

3) A similar FR persists and

strengthens in coming decades.

Distribution of Ensemble Member Pattern

Correlations with the Forced Response

Single Forcing Runs show:

Regional trends during the altimeter era include

important contributions from ozone, aerosols, O3.

correlation

Altimeter-Era (2020-2045) Forced Responses

In coming decades, western

subtropical Pacific maxima extend

further eastward and patterns in

other basins intensify.

Thus, in addition to acceleration in the

global mean rate of sea level rise,

many regions that have exhibited

elevated rates of rise in the altimeter

era should expect those elevated

rates to persist rather than reverse.

Antarctica, west coast of South

America should expected past lower

rates of rise to persist (also due to

gravitational fingerprints).

2020-2045 SSH Trends (mm/yr)

CESM

ESM2M

Regional Trends from Satellite Altimeter RecordHamlington et al. 2019 GRL

Rate, Natural Variability Removed: 1993-2005 Rate, Natural Variability Removed: 2006-2018

Correlation: -0.31

Correlation with 1993-2018 rate pattern: 0.70 and 0.75

Rate, Natural Variability Retained: 1993-2005 Rate, Natural Variability Removed: 2006-2018

Correlation between early and late era trends are negative…

When modes are removed, correlation with each other

and background trend is strong.

Regional Trends from Satellite Altimeter RecordHamlington et al. 2019 GRL

With internal modes removed, altimeter era trends remain highly correlated with raw trends.

Hamlington et al. 2019 GRL; Figure 4B

Conclusions

2019 CESM Workshop: Fasullo: Sea Level Drivers

Large Ensembles are a useful tool for evaluating the emergence of the patterns of forced

response over discrete intervals (e.g. satellite records).

Regional sea level trends in every member of the CESM and ESM2M large ensembles

correlate with their respective forced responses to a degree that exceeds the 95%

confidence limit (estimated from their piControl runs). On average the FR explains

about 30% of the spatial variance of trends in the altimeter era.

The forced response is transient. It evolves over time and often is not well-characterized

by the 1850-2100 trend.

Emergence has now been corroborated in recent observational studies. It allows for an

estimation of the anticipated trends over the next several decades. How best to do so

remains a research topic.

Manuscripts:

Fasullo and Nerem, 2018: Altimeter-Era Emergence of the Patterns of Forced Sea Level Rise in Climate Models and

Implications for the Future, Proc. Nat. Acad. Sci.

Hamlington, Fasullo, and Nerem, 2019: Uncovering the Pattern of Forced Sea Level Rise in the Satellite Altimeter

Record, Geo. Res. Lett.

Conclusions

2019 CESM Workshop: Fasullo: Sea Level Drivers

Large Ensembles are a useful tool for evaluating the emergence of the patterns of forced

response over discrete intervals (e.g. satellite records).

Regional sea level trends in every member of the CESM and ESM2M large ensembles

correlate with their respective forced responses to a degree that exceeds the 95%

confidence limit (estimated from their piControl runs). On average the FR explains

about 30% of the spatial variance of trends in the altimeter era.

The forced response is transient. It evolves over time and often is not well-characterized

by the 1850-2100 trend (particularly true in fields with slow adjustment – e.g. ocean).

SLR emergence has now been corroborated in recent observational studies. This also

allows for an estimation of the anticipated trends over the next several decades. How

best to do so remains an active research question.

Manuscripts:

Fasullo and Nerem, 2018: Altimeter-Era Emergence of the Patterns of Forced Sea Level Rise in Climate Models and

Implications for the Future, Proc. Nat. Acad. Sci.

Hamlington, Fasullo, and Nerem, 2019: Uncovering the Pattern of Forced Sea Level Rise in the Satellite Altimeter

Record, Geo. Res. Lett.

2019 CESM Workshop: Fasullo: Sea Level Drivers

END

Altimeter-Era (1993-2020) Ocean Heat Content Trends: CESM/ESM2M

Ocean heat content is a primary driver

of SSH trend patterns. Regions of

greater warming exhibit greater

rates of rise.

In addition, the coefficient of

expansion is greater for warm water

and thus SSH rise is greater in

warm oceans for a given warming.

1993-2020 OHC Trends (108 J/yr)

CESM

ESM2M

108 J/yr

Drivers of Altimeter SSH Trends Inferred from Single-Forcing LENS

2019 CESM Workshop: Fasullo: Sea Level Drivers

Key Findings:• O3, aerosols, GHG important

• SH dipole driven by GHG, O3

• Atlantic dominated by aerosols

• GHG influence in North Atlantic

• Pacific trends from all 3.