Low-Frequency Sea Level Variability and the Inverted Barometer Effect

Abstract

For a dynamical interpretation of sea level records, estimates are needed of the isostatic, or so-called inverted barometer, signals (?ib) associated with the ocean response to atmospheric loading. Seasonal and longer-period ?ib signals are evaluated over the global ocean for the period 1958?2000 using monthly sea level pressure fields from two different atmospheric reanalyses. Variability and linear trends in ?ib agree well for the two reanalyses in most regions but less so over the Southern Ocean, where uncertainties in ?ib seem to be largest. The standard deviation of ?ib ranges from <1 cm in equatorial regions to >7 cm in the regions of the Aleutian and Iceland lows and parts of the Southern and Arctic Oceans. When compared to a global tide gauge dataset, both seasonal and interannual ?ib signals are found to contribute importantly to the sea level variance in many mid- and high-latitude records, with seasonal signals important as well in tropical records from India and Southeast Asia. For these records, subtracting ?ib from the data can lead to changes in variance of 40% or more. Over the period of study, linear trends in ?ib are mostly negative at low and midlatitudes and can cause negative biases in tide gauge estimates of global mean sea level rise that are comparable in magnitude to the effects of postglacial rebound. In agreement with previous findings, ?ib signals are found to introduce anomalous behavior in local records (e.g., substantially weaker upward trends in the Mediterranean), and their removal can also reduce formal trend uncertainties. Accounting for ?ib effects can be even more important when analyzing relatively short (decadal) records, such as those available from satellite altimetry.