Adjustment of tide gauge measurements in KAWA located in Kawaihae, Hawaii
Description
Tide gauge is the equipment that used to measure the sea-level rise. Although the measurements of high-quality satellite altimeters have covered near-global ocean areas, the measurement periods are still too short and, for this reason, the estimates of long-term sea-level changes are still dependent on the sparsely distributed network of tide gauge stations. Thus, it is essential to conduct the quality-control of the tide gauge data. When studying the relative sea-level rise in the Hawaii Island (Figure 1) using the monthly mean values of sea level provided by the Permanent Service for Mean Sea Level (PSMSL) (http://www.psmsl.org/), we found the rate of relative sea-level rise derived from KAWA (6.6±0.6 mm/year during 1988-2016) was much higher than that of HIHA (2.3±0.3 mm/year during 1975-2016) (Figure 2b). Figure 2a shows that the relative sea-level rise change (RSLC) difference (KAWA-HIHA) in the common period of 1988-2016 presents an obvious discontinuity (jump) in October 2006. The trends of the RSLC differences are almost the same before and after the jump, 0.6±0.5 mm/year VS 0.7±0.9 mm/year. The sharp change should be attributed to the sudden equipment or land movement as the sea-level trend is impossible to have such an offset in one month. Figure 1 shows the distribution of earthquakes with magnitudes larger than 5.0 (M ≥ 5.0) since 1951 in the Hawaii area. It indicates that most seismic activities occurred on the Island of Hawaii, among which a pair of earthquakes with magnitudes of 6.7 and 6.1, respectively, occurred on October 15, 2006, near KAWA. We surmise that the sudden jump of RSLC difference resulted from the abrupt land or equipment movement at KAWA caused by the earthquakes. Because the offset had a detrimental effect on the velocity estimation of RSLC at KAWA, an adjustment of 86.3521 mm was applied to the RSLC data at KAWA after October 2006. After the correction, the correlation of RSLC between KAWA and HIHA increased from 0.7864 to 0.9520; the KAWA shows an RSLC rate of 2.2±0.5 mm/year during 1988-2016 (Figure 2b), and the RSLC difference shows a trend of 0.5±0.2 mm/year (Figure 2a). This project was funded by the Hawaii Department of Transportation, HWY-06-16, entitled "Statewide Highway Shoreline Protection Program Study Update."