Decadal and long-term sea level variability in the tropical Indo-Pacific Ocean

Abstract

International audienceIn this study, we analysed decadal and long-term steric sea level variations over 1966-2007 period in the Indo-Pacific sector, using an ocean general circulation model forced by reanalysis winds. The simulated steric sea level compares favourably with sea level from satellite altimetry and tide gauges at interannual and decadal timescales. The amplitude of decadal sea level variability (up to ~5 cm standard deviation) is typically nearly half of the interannual variations (up to ~10 cm) and two to three times larger than long-term sea level variations (up to 2 cm). Zonal wind stress varies at decadal timescales in the western Pacific and in the southern Indian Ocean, with coherent signals in ERA-40 (from which the model forcing is derived), NCEP, twentieth century and WASWind products. Contrary to the variability at interannual timescale, for which there is a tendency of El Niño and Indian Ocean Dipole events to co-occur, decadal wind stress variations are relatively independent in the two basins. In the Pacific, those wind stress variations drive Ekman pumping on either side of the equator, and induce low frequency sea level variations in the western Pacific through planetary wave propagation. The equatorial signal from the western Pacific travels southward to the west Australian coast through equatorial and coastal wave guides. In the Indian Ocean, decadal zonal wind stress variations induce sea level fluctuations in the eastern equatorial Indian Ocean and the Bay of Bengal, through equatorial and coastal wave-guides. Wind stress curl in the southern Indian Ocean drives decadal variability in the south-western Indian Ocean through planetary waves. Decadal sea level variations in the south-western Indian Ocean, in the eastern equatorial Indian Ocean and in the Bay of Bengal are weakly correlated to variability in the Pacific Ocean. Even though the wind variability is coherent among various wind products at decadal timescales, they show a large contrast in long-term wind stress changes, suggesting that long-term sea level changes from forced ocean models need to be interpreted with caution

Nonlinear subsidence at Fremantle, a long‐recording tide gauge in the Southern Hemisphere

“…It thus fits Douglas's [1991] criterion of >60 years and Houston and Dean's [2013] criterion of >75 years for the reliable estimation of MSL trends and accelerations, respectively, though interannual and decadal variability of MSL remains problematic along the Western Australian coast [e.g., Amin, 1993;Nidheesh et al, 2013]. Nevertheless, the length of the FREM record means that it has been used in many global and regional sea level assessments [e.g., Holgate and Woodworth, 2004;Fenoglio-Marc and Tel, 2010;Bouin and W€ oppelmann, 2010;Houston and Dean, 2012;Burgette et al, 2013, as well as the references cited earlier].…”

“…This is more challenging than using geodetic-only techniques because of interannual and decadal sea level variability due to a strong El Nino Southern Oscillation signal along the Western Australian coast [e.g., Amin, 1993;Nidheesh et al, 2013], and additional uncertainty of radar altimetry close to the coasts [e.g., Deng et al, 2002].…”

The Sea Level Response to External Forcings in Historical Simulations of CMIP5 Climate Models*

“…Wind stress as a driver of sea level change Many studies have been conducted on the underlying mechanism that connects the changes in external forcing to the sea level, using a range of models, and focusing on different regions. For instance, in the Indian and Pacific Oceans it was shown that DSL change is mainly driven by wind stress, using either a simplified dynamical ocean model (Timmermann et al 2010) or an ocean general circulation model (Nidheesh et al 2013). In addition, Piecuch andPonte (2011, 2012) pointed out that buoyancy forcing is important to explain interannual variability in sea level in both the tropical Atlantic and the tropical Pacific regions.…”

Spatial Patterns of Sea Level Variability Associated with Natural Internal Climate Modes

“…Previous studies suggest that over the past few decades change in wind forcing is the main cause for decadal sea level variability in the Pacific, including the intensified SLR in the western tropical Pacific since the early 1990s (e.g., Qiu 2002;Carton et al 2005;Bindoff et al 2007;Köhl et al 2007;Köhl and Stammer 2008;Roemmich et al 2007;Timmermann et al 2010;Merrifield and Maltrud 2011;McGregor et al 2012;Nidheesh et al 2013;Qiu and Chen 2012;Han et al 2014a). Given that a significant portion of the basinscale coherent sea level pattern is associated with the PDO and NPGO, one can conclude that it is the surface wind changes associated with the PDO and NPGO that are the major cause for the basin-wide sea level patterns.…”

“…2,3,6), model experiments that use the extracted winds as surface forcing to quantify the roles played by the PDO and NPGO have not yet been performed. Moreover, the regression method may not cleanly isolate the wind signals associated with the climate modes (Palanisamy et al 2015), and reanalysis winds that were used in the regression analyses have significant uncertainties (e.g., Wittenberg 2004), with apparent discrepancies or even opposite signs in their multidecadal trends since the 1960s (Nidheesh et al 2013). Yet, reanalysis winds have been widely applied to investigate decadal climate variability due to their longer data records than satellite winds, and decadal trends of sea level over tropical oceans are very sensitive to the wind trends (McGregor et al 2012).…”