Abstract
A prominent weakening in equatorial Atlantic sea surface temperature (SST) variability, occurring around the year 2000, is investigated by means of observations, reanalysis products and the linear recharge oscillator (ReOsc) model. Compared to the time period 1982–1999, during 2000–2017 the May–June–July SST variability in the eastern equatorial Atlantic has decreased by more than 30%. Coupled air–sea feedbacks, namely the positive Bjerknes feedback and the negative net heat flux damping are important drivers for the equatorial Atlantic interannual SST variability. We find that the Bjerknes feedback weakened after 2000 while the net heat flux damping increased. The weakening of the Bjerknes feedback does not appear to be fully explainable by changes in the mean state of the tropical Atlantic. The increased net heat flux damping is related to an enhanced response of the latent heat flux to the SST anomalies (SSTa). Strengthened trade winds as well as warmer SSTs are suggested to increase the air–sea specific humidity difference and hence, enhancing the latent heat flux response to SSTa. A combined effect of those two processes is proposed to be responsible for the weakened SST variability in the eastern equatorial Atlantic. The ReOsc model supports the link between reduced SST variability, weaker Bjerknes feedback and stronger net heat flux damping.
Highlights
The equatorial Atlantic Ocean is characterized by interannual variations of sea surface temperature (SST), which can have significant impacts on the climate over the adjacent landmasses (Hirst and Hastenrath 1983; Folland et al 1986; Nobre and Shukla 1996)
Interannual SST variability in the eastern equatorial Atlantic featured a strong change in magnitude around the year 2000 (Figs. 1, 2a)
The Atlantic Niño mode is in part determined by ENSOlike dynamics (Servain et al 1982; Keenlyside and Latif 2007; Deppenmeier et al 2016; Lübbecke and McPhaden 2017), in particular by the Bjerknes feedback which can be decomposed into its three components: (1) the zonal wind response to eastern equatorial SST anomalies (SSTa), (2) the thermocline slope response to western equatorial wind anomalies and (3) the local response of SSTa to thermocline depth anomalies
Summary
The equatorial Atlantic Ocean is characterized by interannual variations of sea surface temperature (SST), which can have significant impacts on the climate over the adjacent landmasses (Hirst and Hastenrath 1983; Folland et al 1986; Nobre and Shukla 1996). While Tokinaga and Xie (2011) investigated trends in Atlantic cold tongue variability over the time period 1950–2009, several studies have discussed a shift in equatorial Pacific variability that occurred around the year 2000 and is clearly visible in many ENSO characteristics. A more recent study from Xu et al (2019), investigating the weakening of ENSO amplitude since the late 1990s, put it in the context of the transition from the Aleutian Low mode to the North Pacific Oscillation in the atmosphere that is responsible for a westward extension of negative sea level pressure anomalies This shift is proposed to have weakened the atmospheric responses to the zonal equatorial SSTa, and ENSO amplitude.
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