Variability of the subtropical mode water in the Southwest Pacific

Abstract

The variability of Subtropical Mode Water (STMW) in the Southwest Pacific is investigated using a 28 year‐long time series (1986–2014) of high‐resolution expendable bathythermograph data north of New Zealand (PX06) and a shorter time series, the Roemmich‐Gilson monthly Argo optimal interpolation for the 2004–2014 period. The variability in STMW inventories is compared to the variability in air‐sea heat fluxes, mixed layer depths and transport of the East Auckland Current (EAUC) to assess both the atmospheric and oceanic roles influencing the formation and decay of STMW. The STMW north of New Zealand has a short lifespan with little persistence of the water mass from 1 year to the next one. Deeper mixed layers and negative anomalies in surface heat fluxes are correlated with increased formation of STMW. The heat content of the STMW layer is anticorrelated with inventories, particularly during the El Niño years. This suggests that large volumes of STMW are coincident with cooler conditions in the prior winter and less oceanic heat storage. There is significant seasonal and interannual variability in STMW inventories, however there are no trends in STMW properties, including its core layer temperature over the last decade. The variability of the winter EAUC transport is highly correlated with the STMW inventories and thermocline depth in the following spring, suggesting ocean dynamics deepen the thermocline and precondition for deeper mixed layers.

Variability of the South Pacific Western Subtropical Mode Water and Its Relationship With ENSO During the Argo Period

“…The outcropping of the 20°C isotherm marks the opening of a ventilation window, where some portion of the SPWSTMW is exposed to the atmosphere. However, this outcropping process is not shown by Fernandez et al's (2017) results (e.g., their fig. 4b).…”

“…The STMW has been extensively studied throughout the global ocean over the past few decades (Fernandez et al, 2017; Hanawa & Talley, 2001; Holbrook & Maharaj, 2008; Oka & Qiu, 2012; Roemmich & Cornuelle, 1992; Speer & Forget, 2013). However, compared with its Northern Hemisphere counterpart, fewer studies have focused on the STMW in the Southern Hemisphere oceans.…”

“…The main reason is that southern subtropical gyres are less sampled than the northern ones, and thus, STMW has been, so far, detected in small amount. Recently, as more observational data products, such as the Digital Atlas of Southwest Pacific upper Ocean Temperatures (DASPOT) (Holbrook & Bindoff, 2000a, 2000b) and the Argo‐gridded data set (Roemmich & Gilson, 2009), became available, the STMW in the south Pacific Ocean has received increasing attention (Fernandez et al, 2017; Holbrook & Maharaj, 2008; Sato & Suga, 2009). A good example of this is the STMW in the southwestern Pacific (SPWSTMW) lying to the north of the Tasman Front.…”

“…Zonally, the SPWSTMW extends from the east coast of Australia to about 170°W, with a temperature range of 14–20°C and a density range of 25.25–26.5 kg/m 3 (Holbrook & Maharaj, 2008; Roemmich & Cornuelle, 1992). Studies of the SPWSTMW have recently focused on its role as a heat reservoir in memorizing the wintertime atmospheric conditions in its formation region (Fernandez et al, 2017; Holbrook & Maharaj, 2008; Wang et al, 2015).…”

“…The ENSO is one of the most prominent modes of large‐scale variability in the global ocean (Alexander et al, 2002; Freund et al, 2019; McPhaden et al, 2006; Taschetto et al, 2014; Taschetto & England, 2009; Yu et al, 2017). The ENSO‐induced changes in ocean circulation and air‐sea fluxes over the South Pacific are believed to directly contribute to the SPWSTMW variability (Fernandez et al, 2017; Holbrook & Maharaj, 2008; Roemmich & Cornuelle, 1992; Wang et al, 2015). For example, Holbrook and Maharaj (2008) claimed that the anomalous cooling of the southwest Pacific Ocean during El Niño periods can enhance winter time convection, which leads to more SPWSTMW formed in the region.…”