Sea Level Change in Hong Kong and ENSO
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Transcript of Sea Level Change in Hong Kong and ENSO
Sea Level Change in Hong Kong and ENSO
DW Zheng1,2, XL Ding1, YQ Chen1, C Huang 2
1Department of Land Surveying and Geo-Informatics Hong Kong Polytechnic University, Hong Kong, China
2 Center for Astrogeodynamics ResearchShanghai Astronomical Observatory, Shanghai, China
Introduction
This research is to studyFrequency features and long-term tendency in HK sea level change
Effects of atmospheric pressure on HK sea level change
Relationships between interannual sea level change and El Niño and La Niño events
Future sea level change in HK
Introduction (cont …)
Data sets adopted : Tide gauge records and leveling measurements of the last forty five years
Global and local atmospheric pressure data from NCEP, USA
Southern Oscillation Index (SOI) and Sea Surface Temperature (SST) data in tropical Pacific from NCEP, USA
Tide Gauge Records and Leveling Measurements in HK Tide gauge records
Over forty five years of data provided by two stations: North Point (1954.0 1986.0) Quarry Bay (1986.0 present)
Very good continuity and availability (daily availability: 98%)
Tide Gauge Records and Leveling Measurements in HK (cont …)
Leveling measurementsCarried out over same periods of time
Varying frequency: from once a few years to 2 - 3 times a year
Estimated ground subsidence at tide gauge stations: 4.2 (0.3) mm/yr and 4.8(0.2) mm/yr, respectively
Frequency Features of Sea Level Change in HK Frequency features
Stable seasonal and long-term (18.6-year) tidal variations in the monthly mean sea level data
Interannual variations change with time
Frequency Features of Sea Level Change in HK (cont …)
Estimated variations and long-term rate of tendency
Period Amplitude Phase(1954.0)
18.6-year 2.040.39cm 0.410.64yr
Annual 10.850.38cm 0.460.01yr
Semi-annual 5.400.38cm -0.140.01yr
tdd
kkk
Ptkcbta
tSL
8657
3
1)/2sin(
Constant = 132.34 1.60 cm
Liner rate = 0.19 0.04 cm/yr
d57 = 13.87 1.32 cm
d86 = 1.02 1.06 cm/yr
Contributions from interannual fluctuations to sea level change are apparent
Effect of Atmospheric Pressure Variations on Sea Level Change
Conventional inverted barometer correction
DSL = g
where - density of seawater;
g - gravitational acceleration; P - atmospheric pressure variation (P=PPmean)
P - local mean atmospheric pressure
Pmean - global mean atmospheric pressure
Effect of Atmospheric Pressure Variations on Sea Level Change (cont…)
Estimation of inverted barometer correction
3
1)/2sin(
ktpkkkt PCPtcbtaSL
Data of global and local sea surface pressures
(Pressure leading for three months for the correlation)
Estimated effect of pressure variation on sea level
Solution with IB Solution without IB With solved IB Theoretic IB Period Amplitude Phase Amplitude Phase Amplitude Phase (year) (cm) (year) (cm) (year) (cm) (year) 18.6yr 2.070.40 4.200.58 2.070.39 4.200.85 1.940.40 4.260.63 Annual 15.221.23 0.380.01 15.500.39 0.380.01 10.720.40 0.460.01 Semiannual 5.950.40 0.150.0 5.970.39 0.150.01 5.510.40 0.140.01 Constant 135.510.80 cm 135.640.57 cm 132.840.58 cm Rate 0.220.02cm/y 0.220.02cm/yr 0.200.02 cm/yr Cp=-0..950.20
(The estimated phases are related to 1958.0)
• Atmospheric pressure variations mainly influence the annual sea level change and its amplitude is depressed by 30 %
• Constant term is 3 cm larger• Linear rate is slightly higher• Estimated Cp= 0.95 which is very close to the theoretical value (0.99)
Seven-point moving average of residuals of monthly sea level data (Jan. 1954 Mar. 2001)
Interannual sea level changes in Hong Kong are related to El Niño and La Niña events
Sea level is dropping during El Niño events
Sea level is rising during La Niña events
The amplitudes of changes due to ENSO are up to 10 cm
Interannaul Sea Level Changes and ENSO Events (cont…)
Sea level changes in the Pacific and Indian Oceansmeasured by TOPEX during 1997-98 El Niño and 1998-99 La Niña
(DongandHuang,1999)
HK
HK
Correlation analysisIn time domain - cross-correlation of interannual sea level changes with SOI and SST
In frequency domain - coherence spectrum of sea level with SOI and SST
2211
12 )()(
22112122 )(/)()()( SfSSfRfRf
Interannaul Sea Level Changes and ENSO Events (cont…)
Results of analysis: Positive correlation between SOI and the sea levelchanges;PhaseofSOIis2monthsearlier.
Negative correlation between SST and the sea levelchanges;PhaseofSSTis1monthearlier.
Portions of interannaul frequency band in the squaredcoherencyspectraaresignificant.
ElNiñoandLaNiñaevents in theeasternPacificalsoinfluencethesealevelchangesintheSouthChinaSea(or SouthAsia). The tropical atmospheric circulation variationsmaybethemainreason.
Sea Level, LOD, AAM and the Next El Niño The atmosphere, the oceans and the solid
earth rotation interact with each other ENSO are processes of the interactions The angular momentum conserves
0 oceanatmosphereearthsolid
MMM
Sea Level, LOD, AAM and the Next El Niño (cont…)
HK interannual sea level changes are highly correlated with the interannual LOD and AAM
The last La Niña has finished and the next El Niño is brewing up
Assessment of Future Sea Level Changes in HK
Obvious rising tendencies in both relative and absolute sea level (RSL and ASL) variations
Extrapolations show 10 and 30 cm further rise by mid of this century
Estimations of future sea level in Hong KongEpoch
absolute sea level relative sea level
2010's 2 cm (18 cm, 22 cm) 7 cm (23 cm, 27 cm)
2030's 6 cm (22 cm, 27 cm) 20 cm (36 cm, 40 cm)
2050's 10 cm (27 cm, 31 cm) 32 cm (48 cm, 53 cm)
Conclusions and Discussions
Conclusions:– Sea level rise and ground settlement in HK: 1.9 and
4.5 mm/yr– Atmospheric pressure variations depress the
amplitude of annual sea level changes by 30 %– Up to 10 cm interannual variations in sea level of
HK are related to ENSO events– The next El Niño is brewing up– Future sea level rise could impose a serious problem
in HK. By mid of this century, the absolute and relative MSL will rise by 10 and 30 cm, with possible maximum values of 30 and 50 cm.