Interdecadal Sea Level Variations in the Pacific: Distinctions Between the Tropics and Extratropics

Abstract

Long tide gauge records from Fremantle and San Diego are used to examine interdecadal sea level fluctuations and their relationship to Pacific climate variability. The sea level difference between the tide gauges and the Southern Oscillation Index (SOI) provide a consistent depiction of trade wind variations along the equator over the past century. The sea level difference and SOI exhibit weak interdecadal variability prior to the late 1970s, followed by an extended 40‐year period of high levels at San Diego and low at Fremantle, and low SOI levels signifying weakened equatorial trade winds. By referencing the tide gauge records to global mean sea level, we infer that Pacific trade winds exhibited weak departures from mean conditions on interdecadal time scales prior to the late 1970s and that a prolonged El Niño‐like lull dominated the tropics at the end of the century. A recent shift suggests that the trades are reverting back to the El Niño‐like state. These tropical interdecadal variations have counterparts in the extratropical North Pacific after the 1970s, as captured by the Pacific Decadal Oscillation, but the early twentieth century quiescent phase in the tropical Pacific is not reflected in the extratropics, which exhibits more energetic oscillatory behavior over the same time span.

Forcing Factors Affecting Sea Level Changes at the Coast

“…These patterns in SSH trends are similar to those in sea surface temperatures, these two periods being times of decreasing and increasing PDO index, respectively. Adapted from Merrifield and Thompson (2018) Dynamical processes along continental shelves have been investigated most intensively along the Atlantic coast of North America, where there is interesting dynamics related to the Gulf Stream, and high-quality tide gauge and meteorological data sets are available (e.g., Thompson and Mitchum 2014;Frederiske et al 2017). The latter found a strong correlation between coastal sea level and decadal steric variability in the subpolar gyre which is probably caused by variability in the Labrador Sea that is propagated southward.…”

Impacts of Basin-Scale Climate Modes on Coastal Sea Level: a Review

“…At decadal time scales, remote tropical forcing—rather than local forcing—is the dominant mechanism. Decadal variations in thermocline depth and sea level of the eastern Pacific as far north as southern California are remotely driven by variability in trade winds of the tropical Pacific (Clarke and Lebedev 1999; Thompson et al 2014; Merrifield and Thompson 2018). Further north, recent decadal sea level trends at the coast are more closely related to remote equatorial wind forcing with a substantially smaller contribution from local longshore winds (Thompson et al 2014), even though local forcing is important for interannual SLAs in the higher latitude region; local wind stress curl may also be an important driver (Bromirski et al 2011), but it is ineffective in accounting for coastal sea level variance in statistical regressions (Chelton and Davis 1982; Thompson et al 2014).…”

“…Interannual and decadal fluctuations of the easterly trade, which are linked to climate modes and inter-basin interactions (see Sect. 3.3.2), cause the west–east seesaw of decadal SLAs (e.g., Carton et al 2005; Bindoff and Willebrand 2007; Kohl et al 2007; Lombard et al 2009; Timmermann et al 2010; McGregor et al 2012; England et al 2014; Han et al 2014a; Merrifield and Thompson 2018). Local land subsidence, however, may also be an important factor in the amplification of relative SLR (e.g., Ballu et al 2011; Becker et al 2012).…”

“…Since local longshore wind and IB effect are important for driving coastal SLAs in extratropical oceans, it is important to make the distinction between extratropical climate indices (e.g., PDO) and tropical-centered indices (e.g., IPO), even though they are highly correlated. For instance, the PDO index can capture the PDO phase transitions in the North Pacific, whereas the IPO index cannot always capture them (Thompson et al 2014; Merrifield and Thompson 2018). This is because the PDO is an empirical mode and not a physical mode, which reflects a superposition of multiple processes including teleconnection to the tropics, variability of Aleutian Low due to stochastic weather noise, and oceanic processes in the North Pacific (Schneider and Cornuelle 2005; Newman et al 2016).…”

Sea Level, Tidal, and River Flow Trends in the Lower Columbia River Estuary, 1853–present

“…Similarly, El Niño conditions produce interannual variability in sea level and a spatially variable sea level anomaly that decreases from the equator northward (e.g., Hamlington et al, 2015). Shifts in Pacific Basin trade winds depressed sea level rise rates in the eastern Pacific between roughly 1980-1990(e.g., Bromirski et al, 2011Merrifield, 2011) but may have recently reversed and produced elevated rise rates (e.g., Hamlington et al, 2016;Merrifield &Thompson, 2018). Additionally, vertical land motion caused by glacial isostatic adjustment and tectonics cause significant (order 1 mm/year) variations in RSL rise, often over length scales as small as 10 km (Burgette et al, 2009;National Research Council, 2012).…”