Annual and Interannual Changes in the North Atlantic STMW Layer Properties

Abstract

Subtropical mode waters (STWMs) are water masses formed in winter by convective mixing on the equatorward side of western boundary currents in the subtropical gyres. After the return of the seasonal stratification in spring, it is found at the stratification minimum between the seasonal and main pycnoclines. By characterizing STMW primarily at the density gradient minimum, previous studies were limited in their ability to describe STMW properties over large temporal and spatial scales. Rather than using a density-based characterization, the North Atlantic STMW layer was identified here by its much smaller temperature gradient relative to the more stratified seasonal and main thermocline, its temperature, and its large thickness. By using this temperaturebased characterization, this study was able to develop a climatology using the large number of XBTs deployed between 1968 and 1988 and contained in the World Ocean Atlas 1994 historical hydrographic database and to use this climatology to examine STMW properties on large spatial and long temporal scales. Three different characterizations were used to assess the degree of convective renewal of the STMW layer during the 1968-88 winters. Two characterizations were based on comparing the winter mixed layer properties to the STMW layer properties in the previous fall, while the third characterization involved comparing the temperature gradient through the STMW layer in the spring to the STMW layer temperature gradient in the previous fall. Based on these characterizations, there was considerable spatial and temporal variability in the renewal of the STMW layer'

The interannual variability of oceanic CO2 parameters in the northeast Atlantic subtropical gyre at the ESTOC site

“…Interannual anomalies affecting an open ocean site such as ESTOC occur synchronously over large areas, which may, for instance, extend over a substantial fraction of the North Atlantic subtropical gyre. The relationship between the North Atlantic circulation and variability in the same, with modes of climate variation, is largely acknowledged [e.g., Cullen et al , 2001; Alfutis and Cornillon , 2001]. Decadal variations in the position of the Azores Front have been shown, with a shift from a more northern position in the 1980s to a more southern position in 1990s [ Siedler et al , 2005].…”

“…Very low but, nevertheless, statistically significant anomalies (below 0.3 with p < 0.05) are to be observed between the instantaneous effect of both modes of climate variability and the seawater properties at ESTOC. Following Alfutis and Cornillon [2001], the NAO and EA indices, and hydrographic and carbon dioxide anomalies were divided over three periods: December through to March, April through to July, and August through to November. In winter (defined here as December through to March: DJFM.)…”

Interannual variability of the oceanic CO2 sink in the subtropical gyre of the North Atlantic Ocean over the last 2 decades

“…In deeper subtropical mode waters (STMW), the mean rate of change of nDIC, over the 1988−2001 time period, was significantly higher than surface waters, increasing at a rate of +2.2 ± 0.26 μ moles kg −1 yr −1 [ Bates et al , 2002] (Table 3). Here STMW is defined as those waters with temperatures ranging from 17.8° to 18.4°C, by a salinity of ∼36.5 ± 0.05, and by a minimum in the vertical gradient of isopycnic potential vorticity [ Klein and Hogg , 1996; Jenkins , 1998; Hanawa and Talley , 2001; Alfutis and Cornillon , 2001]. The rate of increase of nDIC in STMW from 1988 to 2003 is similar at +2.1 ± 0.28 μ moles kg −1 yr −1 to earlier estimates [ Bates et al , 2002].…”

The Fate of North Atlantic Subtropical Mode Water in the FLAME Model

“…Examples of additional filtering are the exclusion of waters east of the Mid-Atlantic Ridge (Worthington 1976) or 358W (Kwon and Riser 2004) to remove Madeira Mode Water (Siedler et al 1987) and the imposition of a limit on the stratification based on the vertical temperature gradient (e.g., Klein and Hogg 1996;Alfultis and Cornillon 2001;Kwon and Riser 2004) or potential vorticity q (e.g., Talley and Raymer 1982;Klein and Hogg 1996;Forget et al 2011).…”