Causes of the El Niño and La Niña Amplitude Asymmetry in the Equatorial Eastern Pacific

Abstract

The amplitude asymmetry between El Niñ o and La Niñ a is investigated by diagnosing the mixed-layer heat budget during the ENSO developing phase by using the three ocean assimilation products: Simple Ocean Data Assimilation (SODA) 2.0.2, SODA 1.4.2, and the Global Ocean Data Assimilation System (GODAS). It is found that the nonlinear zonal and meridional ocean temperature advections are essential to cause the asymmetry in the far eastern Pacific, whereas the vertical nonlinear advection has the opposite effect. The zonal current anomaly is dominated by the geostrophic current in association with the thermocline depth variation. The meridional current anomaly is primarily attributed to the Ekman current driven by wind stress forcing. The resulting induced anomalous horizontal currents lead to warm nonlinear advection during both El Niñ o and La Niñ a episodes and thus strengthen (weaken) the El Niñ o (La Niñ a) amplitude. The convergence (divergence) of the anomalous geostrophic mixed-layer currents during El Niñ o (La Niñ a) results in anomalous downwelling (upwelling) in the far eastern equatorial Pacific, which leads to a cold nonlinear vertical advection in both warm and cold episodes.

Strong and moderate nonlinear El Niño regimes

“…5), the nonlinear advection (NDH) does not contribute with amplified warming, but plays a substantial role in weakening the cooling during the decay of the strong EN (low end of T in Fig. 5c; Su et al 2010). On the other hand, the radiative feedback associated with clouds becomes negative for strong EN and reduces the net warming (see Sect.…”

“…Thus, although it has been previously parametrized in simple models of the recharge-discharge family (Jin et al 2003;Timmermann et al 2003;An and Jin 2004), we believe that the neglect of wave dynamics could qualitatively affect the results (e.g. Su et al 2010).…”

“…Although the ocean analyses provide comprehensive datasets that are heavily used almost as equivalent to observations, it has to be kept in mind that these can be strongly influenced by the numerical models used in data sparse regions, such as the far eastern equatorial Pacific, and for weakly constrained variables such as vertical motions associated with wave dynamics (Su et al 2010). …”

“…A major limitation to observationally study the strong EN is that only two of these events have been observed comprehensively, and even so only the 1997-1998 EN had subsurface temperature and surface winds fully observed by the TAO/TRITON array, while ocean analysis based on data assimilation suffer from model dependence (Su et al 2010). From an empirical perspective, it would then be very difficult to propose generalizations purely based on observations.…”

Symmetry of the Atlantic Niño mode

“…Meinen and McPhaden [2000] reported that positive anomalies in warm water volume (WWV) are associated with larger SST anomalies than negative WWV anomalies of comparable size. Su et al [2010] found that nonlinear zonal and meridional temperature advection by ocean currents was crucial for El Niño and La Niña amplitude asymmetry. Kang and Kug [2002] showed that wind stress anomalies related to La Niña are shifted westward compared to those related to El Niño.…”