Sea Level Change in Hong Kong and ENSO DW Zheng 1,2, XL Ding 1, YQ Chen 1, C Huang 2 1 Department of Land Surveying and Geo-Informatics Hong Kong Polytechnic University, Hong Kong, China 2 Center for Astrogeodynamics Research Shanghai Astronomical Observatory, Shanghai, China

Introduction u This research is to study Frequency 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 …) u 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 u Tide gauge records Over forty five years of data provided by two stations: ­North Point (  ) ­Quarry Bay (  present) Very good continuity and availability (daily availability: 98%)

North Point

Tide Gauge Records and Leveling Measurements in HK (cont …) u Leveling measurements Carried out over same periods of time Varying frequency: from once a few years to 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 u 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 …) u Estimated variations and long-term rate of tendency PeriodAmplitudePhase(1954.0) 18.6-year 2.04  0.39 cm0.41  0.64yr Annual  0.38 cm0.46  0.01yr Semi-annual 5.40  0.38 cm-0.14  0.01 yr

Constant =  1.60 cm Liner rate = 0.19  0.04 cm/yr d 57 =  1.32 cm d 86 = 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 u Conventional inverted barometer correction DSL =  g where  - density of seawater; g - gravitational acceleration;  P - atmospheric pressure variation (  P=P  P mean ) P - local mean atmospheric pressure P mean - global mean atmospheric pressure

Effect of Atmospheric Pressure Variations on Sea Level Change (cont…) u Estimation of inverted barometer correction

Data of global and local sea surface pressures (Pressure leading for three months for the correlation)

u 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 2.07       0.63 Annual       0.01 Semiannual 5.95  0.40  0.15   0.39  0.15   0.40  0.14  0.01 Constant  0.80 cm  0.57 cm  0.58 cm Rate 0.22  0.02cm/y 0.22  0.02cm/yr 0.20  0.02 cm/yr C p =  0.20 (The estimated phases are related to ) 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 C p = 0.95 which is very close to the theoretical value (0.99)

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

Seven-point moving average of residuals of monthly sea level data (Jan  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 Oceans measured by TOPEX during El Niño and La Niña (Dong and Huang, 1999) HK

u Correlation analysis In 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 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 level changes; Phase of SOI is 2 months earlier. ­Negative correlation between SST and the sea level changes; Phase of SST is 1 month earlier. ­Portions of interannaul frequency band in the squared coherency spectra are significant. ­El Niño and La Niña events in the eastern Pacific also influence the sea level changes in the South China Sea (or South Asia). The tropical atmospheric circulation variations may be the main reason.

Sea Level, LOD, AAM and the Next El Niño u The atmosphere, the oceans and the solid earth rotation interact with each other u ENSO are processes of the interactions u The angular momentum conserves

Sea Level, LOD, AAM and the Next El Niño (cont…) u HK interannual sea level changes are highly correlated with the interannual LOD and AAM u The last La Niña has finished and the next El Niño is brewing up

Assessment of Future Sea Level Changes in HK u Obvious rising tendencies in both relative and absolute sea level (RSL and ASL) variations u Extrapolations show 10 and 30 cm further rise by mid of this century

Conclusions and Discussions u 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.

u 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…)