Assessment of seasonal-to-decadal variability and trends of regional sea level in the North Pacific Ocean using satellite altimetry and tide gauges

  • 藍 文浩

Student thesis: Doctoral Thesis

Abstract

A considerable progress is observed in understanding a global mean sea level rise but regional sea level variations that deviate from a global average rate remain poorly understood In this study we present a comprehensive analysis of sea level data around Taiwan from 1993 to 2015 and an analysis of seasonal-to-decadal variability and trends of regional sea level in the North Pacific Ocean using satellite altimetry and tide gauges Results show that datum shifts and vertical land motions in gauge records have significant impacts on sea level trends with respective average contributions of 7 3 and 8 0 mm/yr whereas ocean tides and inverted barometer effects which represent 9% and 14% of the observed trend respectively have relatively minor impacts Thus datum shifts and vertical land motion effects must be removed in the tide gauge records for accurate sea level estimates The estimated land motions show that the southwestern plain in Taiwan has large subsidence rates For example the Boziliao Dongshi and Wengang tide gauge stations exhibit a rate of 24–31 mm/yr as a result of groundwater pumping The absolute sea level trends which are derived from the tide gauges or satellite altimetry around Taiwan agree well with each other and both are estimated to be 2 2 mm/yr for 1993–2015 This estimate is significantly lower than the global average sea level rise trend of 3 2 mm/yr from satellite altimeters We suggest that a recent hiatus in sea level rise in this region exhibits good agreement with the interannual and decadal variabilities associated with the El Ni?o-Southern Oscillation and Pacific Decadal Oscillation The results of sea level budget show that steric sea level and ocean mass components contribute to the total absolute sea level in Northeast Taiwan at a similar rate (approximately –4 9 mm/yr to –2 2 mm/yr and 1 9 mm/yr respectively) but contribute approximately 62%–74% (8 3 mm/yr to 9 9 mm/yr) and 14% (1 8 mm/yr) in Southeast Taiwan correspondingly In the western ocean of Taiwan the temperature and salinity data are lacking; thus the estimated steric sea level is inaccurate Interannual-to-decadal variability and trends of sea level in the North Pacific Ocean are analyzed using tide gauge and satellite altimeter data covering 1993–2016 The absolute sea level trends derived from satellite altimeter data in the Northwest and Northeast Pacific Ocean are estimated to be 3 3 ± 0 2 mm/yr and 2 3 ± 0 2 mm/yr and the similar rates of absolute sea level rise (of 3 7 mm/yr and 2 3 mm/yr) are observed from all coastal tide gauge records covering the same time span respectively Over the entire North Pacific Ocean the absolute sea level trends are 2 8–3 3 mm/yr from tide gauges and satellite altimetry which are similar to the global average trend of 3 2 mm/yr A similar average trend in the sea level of 2 9 ± 0 1 mm/yr is observed when considering the effects of the ENSO and PDO Moreover a uniform spatial distribution with a range of 0–5 mm/yr is detected The seasonal sea level cycles in the North Pacific Ocean are explored using tide gauges in 1950–2016 and satellite altimeter data in 1993–2016 through Ensemble Empirical Mode Decomposition method The seasonal cycle can explain 60%–93% of the sea level variability in the continental shelf of the Western Pacific Ocean while explaining less than 40% of the variance in the open ocean Significant annual amplitudes are found in the regions of the continental shelf of the Western Pacific Ocean Gulf of California eastern tropical Pacific and Kuroshio Extension with a range of 100–211 mm A semi-annual amplitude has a relatively minimal impact on the sea level variation in the North Pacific Ocean The inverted barometer effect produces ?116 mm to 88 mm of annual amplitudes in the Western Pacific coast regions especially from the north of South China Sea to the Sea of Japan whereas the annual amplitude in most area of study is lower The significant annual amplitudes of steric component are found in the Sea of Japan and Kuroshio Extension region wherein the largest value is 15 cm The annual amplitude has significantly decreased in the open ocean after removing the steric component from the observed sea level However the annual cycle of the residual sea level in the large areas of marginal seas remains strong Wind forcing is highly correlated with the residual seasonal sea level cycle in most areas of the marginal seas and the eastern tropical Pacific Therefore we suggest that wind forcing strongly influences the sea level changes in marginal seas of the North Pacific Ocean
Date of Award2018 Aug 31
Original languageEnglish
SupervisorChung-Yen Kuo (Supervisor)

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