Long Term Trends in Global Sea Level-Analysis and Predictions
|School||Ocean University of China|
|Keywords||Sea level change Long term trend Steric East China Sea Western Pacific warm pool IPCC SERE A2 scenario Prediction|
Based on the data of tide-gauge, satellite altimeter, GRACE satellite, temperature and salinity and results of CCSM3 model, the long-term sea level trends and associated dynamics of global ocean and warm pool in the Western Pacific are analyzed in this paper, sea level changes of the global ocean and East China Sea in the 21st century under IPCC SERE A2 scenario are predicted with the CCSM3 and POP models.The reconstruction data of tide gauge and satellite altimeter (Church et al,2004) shows the rising rate of the global sea level during 20th century is 1.80mm/yr with a significant acceleration of 0.0092mm/yr2;if the acceleration maintains, the global mean sea level will rise about 279mm at the end of 21st century relative to the year 2000.The CCSM3 model shows, not including the water mass exchange between air/land and sea, only the steric change can cause a global sea level rise of about 4.0cm during the 20th century, with an acceleration of 0.0048mm/yr2.The global mean sea level rising rate observed by tide gauges is 1.5mm/yr during 1948-2008 and the rising rate is up to 3.0mm/yr based on satellite altimeter data during 1993-2008.Steric change contributes about 25% to the observed global mean sea level during last half of the 20th century; since the satellite altimeter observations in 1993,contribution of steric change increases to 33%-50%.Contribution of steric change can also be obtained by subtracting the GRACE mass estimation from the satellite altimeter sea surface height. During 2003-2008,the satellite altimeter observation shows a global sea level trend of 2.9mm/yr, and the GRACE satellite data gives a sea level rise rate of 1.2mm/yr causued by the water mass changes, obtaining a steric sea level change of 1.7mm/yr. More accurate estimate of long-tem setric sea level change needs longer records.In the warm pool of the Western Pacific, large inter-annual and decadal sea level variability associated with ENSO may obscure the analysis of long-term sea level change. Both observations and climate models show the long-term sea level trend of the warm pool is almost the same to the global mean. The annual sea level of the warm pool changes in reverse to the ENSO index, and the long term sea level trend is significantly affected by the ENSO cycles.During 1993-2008,there is a strong sea level rising trend with a rate of 7.8mm/yr, which is much larger than the trend of global mean in the same period; while during 1970-1992 the sea level in the area decreases. During 1950-2008,the rising rate is about 2.05 mm/yr, equivalents to the trend of global mean. CCSM3 model results also shows that the sea level in the warm pool has strong interannual variability, with amplitude up to 20cm and linear trends of the mean total steric, thermosteric and halosteric sea level in the warm pool during 1900-1999 are 0.32mm/yr,0.26mm/yr and 0.06mm/yr, respectively; thermosteric effect is dominant, causing a sea level trend comparable to those of the global averaged.Under the SERE A2 scenario, simulations of CCSM3 model show that comparing with the beginning of the century, the dynamic sea level changes at the end of 21st century have strong regional characteristics:large increase of sea level occurs in the Arctic Ocean which is related with the greater surface warming, increasing freshwater input and sea ice melting; significant reduction of sea level occurs in the high-latitude Southern Ocean and larger rise of sea level is found in the Atlantic Ocean than the Pacific Ocean.CCSM3 simulation also shows that in the 21st century sea level will rise in acceleration; the global sea level will rise 30cm during the whole century just considering the contribution of the thermosteric and halosteric change, not including the water mass inputs (land ice melting, etc.).The thermosteric sea leve change in the North Atlantic basin is more than 0.4m, significantly larger than the Pacific. Halosteric change averaged in the global extent is almost equal to 0,but has significant influence on regional steric sea level changes.The thermosteric and halosteric anomalies that contribute to sea level change are very different between ocean basins.The halosteric signal is larger than thermosteric change in Arctic Ocean, and they both cause the positive seal level rise. Significant warming of the North Atlantic Ocean can extend to 2500m deep,while the salinity change partially counteracts sea level rise due to this warming. The steric changes in the North Pacific are largely confined to the upper 500m.In the Southrn Ocean, temperature and salinity change can extend to the whole water column due to the intense vertical convection.POP prediction shows that at the end of 21st century, sea level in the East China Sea will rise about 12-20cm.Sea-level rise in the Bohai Sea is relatively larger, up to 17cm;the rise amplitude in east coast(16cm) of the East China Sea is larger than that in the west coast(13cm).Sea level will rise about 20cm near Ryukyu Islands.On the whole, the sea level rise in the East China Sea in 21st century is mainly from the ocean mass redistribution due to ocean dynamic change, contributing 65% of the total sea-level rise, and the rest of 35% change is from the steric change. If added the impact of land-based glaciers (Mitrovica et al.,2001;IPCC,2007),the sea level in the East China Sea will rise 22-40cm in 21st century.