Upper-Ocean Circulation and Tropical Atlantic Interannual Modes

Abstract

The impact of tropical Atlantic Ocean variability modes in the variability of the upper-ocean circulation has been investigated. For this purpose, we use three oceanic reanalyses, an interannual forced-ocean simulation, and satellite data for the period 1982–2018. We have explored the changes in the main surface and subsurface ocean currents during the emergence of Atlantic meridional mode (AMM), Atlantic zonal mode (AZM), and AMM–AZM connection. The developing phase of the AMM is associated with a boreal spring intensification of North Equatorial Countercurrent (NECC) and a reinforced summer Eastern Equatorial Undercurrent (EEUC) and north South Equatorial Current (nSEC). During the decaying phase, the reduction of the wind forcing and zonal sea surface height gradient produces a weakening of surface circulation. For the connected AMM–AZM, in addition to the intensified NECC, EEUC, and nSEC in spring, an anomalous north-equatorial wind curl excites an oceanic Rossby wave (RW) that is boundary-reflected into an equatorial Kelvin wave (KW). The KW reverses the thermocline slope, weakening the nSEC and EUC in boreal summer and autumn, respectively. During the developing spring phase of the AZM, the nSEC is considerably reduced with no consistent impact at subsurface levels. During the autumn decaying phase, the upwelling RW-reflected mechanism is activated, modifying the zonal pressure gradient that intensifies the nSEC. The NECC is reduced in boreal spring–summer. Our results reveal a robust alteration of the upper-ocean circulation during AMM, AZM, and AMM–AZM, highlighting the decisive role of ocean waves in connecting the tropical and equatorial ocean transport.

Modulation of Equatorial Currents and Tropical Instability Waves During the 2021 Atlantic Niño

“…The downwelling equatorial Kelvin wave that initiated the Atlantic Niño caused a strong reduction of near-surface westward flow close to the equator during May 2021. This is in agreement with a recent study by Martín-Rey et al (2023), who found that the nSEC is weakened during the onset of an Atlantic Niño event. However, while they showed that subsurface currents change little, we observe strengthening and deepening of the Equatorial Undercurrent (EUC) during the onset of the Atlantic Niño from May to June.…”

“…This is in agreement with a recent study by Martín‐Rey et al. (2023), who found that the nSEC is weakened during the onset of an Atlantic Niño event. However, while they showed that subsurface currents change little, we observe strengthening and deepening of the Equatorial Undercurrent (EUC) during the onset of the Atlantic Niño from May to June.…”

Impact of the North Atlantic Oscillation on the Decadal Variability of the Upper Subtropical‐Tropical Atlantic Ocean

“…The tropical Atlantic Ocean plays an important role in the global climate system as it is a gateway for the northward branch of the Atlantic Meridional Overturning Circulation (AMOC) that transports warm water from the South to the North Atlantic (Martín-Rey et al, 2023;Schott et al, 2004;Tuchen et al, 2022b). Additionally, the subtropical and tropical Atlantic are connected via shallow meridional overturning cells, so-called subtropical cells (STCs) (Liu et al, 1994;McCreary & Lu, 1994;Schott et al, 2004).…”