CoAuthors

Peter Rhines (School of Oceanography, University of Washington, United States)

This work aims to (1) construct time-variable internal tide models accounting for the incoherent component and (2) monitor ocean heat change in the upper ocean over various time scales. For these goals, seasonal, annual and decadal variations of the mode-1 M2 internal tide are studied using 25 years of multisatellite altimeter data from 1992–2017. A series of global internal tide maps are constructed using subsetted altimeter data. A new mapping technique is employed to better suppress nontidal noise and retrieve westbound/eastbound internal tides. The resultant maps enable us to explore the temporal and spatial variability of internal tides (1- and 2-order derivatives). By dividing the data into two decades: 1992–2004 and 2005–2017, we construct two global internal tide fields that have almost identical spatial patterns. Interestingly, global statistical analysis reveals that there is a 6-degree phase difference between them (about 10 minutes in time), suggesting that the ocean stratification strengthened from the first to the second decade, which is mainly attributed to upper ocean warming. However, the phase difference is very complicated in space. In addition, four seasonal internal tide maps are constructed and all have similar spatial patterns. Systematic phase differences mainly occur in equatorial zones and marginal seas, consistent with the seasonal speed change of internal tides estimated from World Ocean Atlas 2013. We also construct global internal tides using yearly subsetted data. Due to limited data length and thus large uncertainties, annual variations are mainly studied in regions of strong internal tides. Significant phase changes are observed to be associated with numerous long-range internal tidal beams in different years.