Contrasting the flavors of El Niño-Southern Oscillation using sea surface salinity observations

Abstract

The recent detection of a central Pacific type of El Niño has added a new dimension to the El Niño‐Southern Oscillation climatic puzzle. Sea surface salinity (SSS) observations collected during 1977–2008 in the tropical Pacific are used to contrast the three eastern Pacific (EP) (1982–1983, 1991–1992, 1997–1998) and seven central Pacific (CP) (1977–1978, 1986–1988, 1990–1991, 1992–1995, 2002–2003, 2004–2005, 2006–2007) types of El Niño events, as well as the six EP (1985–1986, 1988–1989, 1995–1996, 1999–2001, 2005–2006, 2007–2008) and two CP (1983–1984, 1998–1999) types of La Niña events. The EP El Niño events result in large (∼30° longitude) eastward displacements of the eastern edge of the low‐salinity warm pool waters in the equatorial band, a resulting well‐marked SSS freshening (∼−1) near the dateline, and a SSS increase (∼+1) below the mean position of the South Pacific Convergence Zone (SPCZ). The CP El Niño events are characterized by smaller (50%) eastward displacements of the eastern edge, a ∼15° longitude westward shift of the equatorial SSS freshening, and a comparatively reduced (∼50%) SSS increase in the SPCZ. A qualitative analysis indicates that changes in zonal currents and precipitation can account for the observed contrasted signature in SSS. Eastward current anomalies appear over most of the equatorial band during EP El Niño events. In contrast, there is a tendency for zonal current convergence slightly west of the dateline during CP El Niño events, consistent with the confinement of the warm/fresh pool in the western central equatorial basin, the related quasi‐inexistent northeastward migration of the SPCZ, and associated heavy precipitation regime.

ENSO indices from sea surface salinity observed by Aquarius and Argo

“…Following the definition of Trenberth (1997) and Trenberth and Stepaniak (2001), we calculate the Niño-3, Niño-4, and Niño-3.4 indices as regional averages of SST at (5°S-5°N, 150°-90°W), (5°S-5°N, 160°E-150°W), and (5°S-5°N, 170°-120°W), respectively. The correspondence between the SSS front and ENSO has been reported by previous studies (e.g., Delcroix 1998;Picaut et al 2001;Maes et al 2004;Bosc et al 2009;Singh et al 2011;Qu et al 2014). Here, we note that the correlation between the Niño-S34.8 and Niño-4 indices reached as high as 0.88 during the Argo period (January 2005-September 2013), with the latter leading by 1 month (Fig.…”

“…Based on a gridded SSS product derived from observations of Voluntary Observing Ships, TAO/TRITON moorings, and Argo profilers, Singh et al (2011) recently introduced an index called the SSS ENSO index, defined as the difference between the normalized SSS anomalies averaged in the regions (25°-10°S, 160°E-160°W) and (2°S-2°N, 150°E-170°W). For comparison, the SSS ENSO index calculated from the Argo data is shown in Fig.…”

Interannual sea surface salinity variations observed in the tropical North Pacific Ocean

“…Figure 1a shows the standard deviation (STD) of SSS (represented by 10 dbar salinity), which is low-passed with a 13 month running mean filter to underscore interannual signals. The strongest SSS variability appears in the far western equatorial Pacific, with STD > 0.2 (psu) and extending southeastward to the SPCZ, which has been investigated by previous studies [e.g., Delcroix and Henin, 1991;Delcroix et al, 2011;Singh et al, 2011]. Another relative maximum of variability occurs in the tropical North Pacific (TNP) between 7 N-15 N with STD > 0.15 near the dateline, and extends westward.…”

“…In the tropical Pacific Ocean, large seasonal SSS variations often occur in the intertropical convergence zone (ITCZ) and South Pacific convergence zone (SPCZ) regions and mainly controlled by rainfall changes [e.g., Delcroix and Henin, 1991;Delcroix et al, 1996;Hénin et al, 1998]. At interannual time scale, SSS variations are of larger amplitudes and more complicated patterns in response to El Niño-Southern Oscillation (ENSO)-related freshwater forcing and ocean current advection [Maes, 2000;Delcroix and McPhaden, 2002;Delcroix et al, 2011;Singh et al, 2011]. Modeling studies suggested that interannual variations of SSS and its associated barrier layer [Lukas and Lindstrom, 1991] are intrinsically connected with the dynamics of ENSO [e.g., Vialard and Delecluse, 1998;Maes et al, 2005;Yim et al, 2008] and playing a critical role in the evolution of specific events [e.g., Zheng and Zhang, 2012].…”

Formation and variability of the South Pacific Sea Surface Salinity maximum in recent decades

“…as they were shown to have a weak regional impact on observed SSS [Singh et al, 2011]. In contrast, the east-box MLS does not exhibit interannual changes, even during 1998-1999.…”