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%)

North Point

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)

Interannaul Sea Level Changes and ENSO Events Interannual sea level changes in HK

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…)

(a) and (c) : estimated cross-correlation(b) and (d): squared coherence

(a)

(b)

(c)

(d)

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.

Discussions:– Data sets (sea level and relative meteorology) in

HK are of good quality– Future research has been planned– The research aims to contribute to sea level

studies of the APSG

Conclusions and Discussions (cont…)