# Presentation: 851526

## Arctic sea level changefrom ERS-2 altimetry

- Andy Ridout and Seymour LaxonCentre for Polar Observation & Modelling, University College London, UK

**Arctic sea level change****from ERS-2 altimetry**

- 2007 ENVISAT SymposiumMontreux, Switzerland, 23–27 April 2007

## Multi-Satellite Data Set

- All GDR/OPR data converted to RADS
- Measurements corrected where needed and/or recommended
- Geophysical corrections (tides, MSS, wet, iono, SSB, IB) added/replaced to ensure consistency
- Delft orbits added on ERS and Envisat
- Arctic data analysis
- Only ERS-1, ERS-2 and Envisat reach to 82oN
- Data need to be retracked over ice covered regions
- Develop new instrument corrections (e.g. SSB)
- Fewer reliable models available

## Multi-Satellite Data Set

|  | ERS-1 | ERS-2 | GFO | TOPEX | Jason | Envisat |
| --- | --- | --- | --- | --- | --- | --- |
| Range | USO and SPTR correction |  | USO correction | Internal calibration |  | USO; bias
40.92 cm |
| Orbit | DGM-E04 | DGM-E04 | PGS7777 | JGM-3 | EIGEN-CG03C | EIGEN-Grace02S |
| Iono | NIC07 / JPL GIM |  |  | Smoothed dual-frequency |  |  |
| Wet tropo | Microwave radiometers corrected for driftsArctic: NCEP reanalysis model |  |  |  |  |  |
| Dry tropo | ECMWFArctic: NCEP reanalysis model |  |  |  |  |  |
| Tides | FES2004 ocean, load, long-period tides; pole tide; solid earth tide |  |  |  |  |  |
| Inv baro | MOG2D: dynamic model |  |  |  |  |  |
| SSB | BM3 | Hybrid | BM3 | CSR A, B | Non-Par | Non-Par |
| MSS | CLS01 Mean Sea Surface ModelArctic: GGM02C Geoid |  |  |  |  |  |

## GSLR Database
Edit data for quality, using limits on SLA, SWH, SWH, etc.
Combine data per “cycle” per 1ox1o cell
Keep ascending and descending separate
Store average latitude, longitude, SLA, dry tropo, wet tropo, etc., per cell
This significantly reduces the size of the database, facilitating research

- Edit data for quality, using limits on SLA, SWH, SWH, etc.
- Combine data per “cycle” per 1ox1o cell
- Keep ascending and descending separate
- Store average latitude, longitude, SLA, dry tropo, wet tropo, etc., per cell
- This significantly reduces the size of the database, facilitating research

- Means of:
-  lat, lon
-  SLA
-  dry tropo
-  wet tropo
-  iono
-  inv baro
-  ....

-  TX, J1: 9.92 days
-  GFO: 8.54 days
-  E1/E2/N1: 7.75 days

- Sea Level Change Database

## Creating Time Series

- Determine global average per “cycle”
- Use all averaged points *i*
- Weight by the original number of points in cell *i*
- Weight by latitude
- Remove occasional outlier cycle from database

- 3-monthboxcar

## Merging Time Series

- Single trend and annual cycle
- Estimate offset per satellite

## Merged Time Series

- Global sea level trend: 3.14 mm/year
- Time series match extremely well

## Merged Time Series

- Common annual signal (4.5 mm amplitude) removed
- All time series within 8 mm from each other

## Estimating Local Sea Level Trends

- 5-Parameter fit
*a* = offset for ascending
*d* = offset for descending
- Absorbs geographically correlated errors
- Setup normal equations
- All “cycles”, each cell

| a |  |  |  |  |  |
| --- | --- | --- | --- | --- | --- |
| d |  |  |  |  |  |
| b |  |  |  |  |  |
| c |  |  |  |  |  |
| s |  |  |  |  |  |

| a |  |  |  |  |  |
| --- | --- | --- | --- | --- | --- |
| d |  |  |  |  |  |
| b |  |  |  |  |  |
| c |  |  |  |  |  |
| s |  |  |  |  |  |

| a |  |  |  |  |  |
| --- | --- | --- | --- | --- | --- |
| d |  |  |  |  |  |
| b |  |  |  |  |  |
| c |  |  |  |  |  |
| s |  |  |  |  |  |

- Spread normal equations
- Gaussian σ = 1o
- Smoothes results
- Allows results in rarified areas

## ERS-1/2, Envisat (Sep 1992 – Mar 2007)
Very consistent features
All latitudes show sea level rise, except around 45oN
Sea level drop confined to N-Indian Ocean, Kuroshio, Gulfstream

- Very consistent features
- All latitudes show sea level rise, except around 45oN
- Sea level drop confined to N-Indian Ocean, Kuroshio, Gulfstream

- Local Sea Level Change

- E1/E2/N1

- TX/J1

## Local Sea Level Change

- ERS-1/2, TOPEX, GFO, Jason-1 and Envisat(Apr 1992 – Mar 2007)
- Very consistent features
- All latitudes show sea level rise, except around 45oN
- Sea level drop confined to N-Indian Ocean, Kuroshio, Gulfstream

## Annual cycle: Amplitude and phase

## Arctic OPR data coverage

- Winter

- Autumn

## Creating an Arctic Ocean data set

- Specular echoes
- Originate from leads and thin new ice
- Separated from diffuse and complex echoes
- Retracking (Peacock, Laxon, Ridout)

- Specular

- Complex

- Diffuse

**Notes:**
Radar altimeter echo types observed over sea ice. Specular returns (i) originate from leads and thin new ice, complex echoes (ii) originate from the surface of ice floes. Complex echoes occur when a heteregeneous mixture of ice and water occurs within the altimeter footprint and are discarded since an unambiguous elevation cannot be retrieved. This is illustrated on the right with ERS altimeter footprints are overlaid on AVHRR imagery. Diffuse echoes are only observed where consolidated ice cover fills the altimeter footprint. Specular echoes occur over visible and sub-pixel lead. Data gaps indicate the location of complex echoes. (Laxon et al., 2001) 

## Arctic retracked data coverage

- Winter

- Autumn

## SSB for Arctic data set

- Building an SSB model
- Retracker does not provide wave height
- Use backscatter as proxy for wave height
- Determine SSB as difference between sea heights from OPR and retracker
- Reasonably good fit for SWH < 6 m

## Arctic retracked sea level

- Winter

- Autumn

## Arctic OPR sea level

- Winter

- Autumn

## Arctic Sea level change

- Good comparison between OPR and retracked in permanently open region
- Unique results in ice covered regions
- Sea level *drop* of ~4 mm/year
- Only small part associated with GIA

- OPR

- retracked

- ICE5G

## Arctic sea level: Annual signal

- High amplitude signals in some coastal regions
- High in September (minimum ice cover)
- Consistent with freshening ocean

- Amp

- Phase

## Melting of sea ice

- sea ice

- saline water

- ~10%

- ~2%

- fresh water

- Impact of melting sea ice on sea level
- To first order: sea level does not change when ice melts
- However: fresh water has lower density than ocean
- Fresh water raises sea level
- Consistent with annual cycle visible in altimeter data
- Trend more difficult to explain

## Mass trend from GRACE

- Morison *et al., *2007
- Trend in mass equivalent to -2.43 cm/yr
- Attributed to loss in (heavier) saline water

## Conclusions

- Global multi-satellite sea level change
- Rate over last 15 years is about 3.1 mm/yr
- Clear regional differences in rates
- Arctic sea level change from ERS-2
- Specular data retracked
- Much better coverage than OPR data
- Highest sea level in September is consistent with maximum fresh water
- Arctic sea level drops by 4 mm/yr
- Small part due to GIA
- Trend more difficult to explain, but consistent with GRACE observations