Cross-Equatorial Anti-Symmetry in the Seasonal Transport of the Western Boundary Current in the Atlantic Ocean

The western boundary current in the equatorial Atlantic Ocean is a main conduit for water-mass exchanges across the equator and thus a major pathway for the interhemispheric transports in the Atlantic Meridional Overturning Circulation (AMOC) system. In this study we quantify and examine the mean and seasonal variability of the equatorial western boundary current (EWBC) in the upper ocean layer using two data-assimilated products, the Estimating the Circulation and Climate of the Ocean (ECCO4r3) and the Simple Ocean Data Assimilation (SODA3). It is found that the EWBC between 10 degrees S and 10 degrees N exhibits two pronounced features in its seasonal variability: (1) the transport varies anti-symmetrically across the equator, that is, the northward EWBC strengthens to the north of the equator when it weakens to the south of the equator, and vice versa; and (2) the amplitude of seasonal variations is much greater in the northern hemisphere than in the south. We hypothesize that the cross-equatorial anti-symmetry in EWBC transport variability is attributable to the impingement of equatorial Rossby waves at the western boundary and the shape of the western boundary is the main cause for the amplified seasonal variability in the northern hemisphere. A simple 1 and 1/2-layer model is used to test and validate this hypothesis and to elucidate the role of wind forcing and topography plays in the seasonal variability in the EWBC transport. Plain Language Summary The equatorial western boundary current (EWBC) is the major pathway for transporting water from the South Atlantic to the North Atlantic in the upper ocean layer. Our analyses of two data-assimilated products show that the EWBC transport varies in anti-symmetric phase across the equator, which counters intuition that the EWBC is a continuous and coherent flow. We hypothesize that this anti-symmetry in EWBC's seasonal variability is attributed to the impingement of equatorial Rossby waves. Long Rossby waves are generated along the equatorial wave guide and propagate westward toward the western boundary. The direction of meridional velocity field induced by an equatorially symmetrical (i.e., odd-numbered meridional mode) Rossby wave is anti-symmetrical across the equator. Upon their impingement at the western boundary, such Rossby waves would result in an antisymmetrical response in the EWBC transport across the equator. A simple model is used to validate this hypothesis.

Automated summary

The study quantifies and examines the mean and seasonal variability of the equatorial western boundary current (EWBC) in the upper ocean layer using two data assimilation products. It reveals that the EWBC displays anti-symmetric variability across the equator and the amplitude of seasonal variations is greater in the northern hemisphere. The study attributes this anti-symmetry to the impingement of equatorial Rossby waves at the western boundary and the shape of the western boundary amplifies the seasonal variability in the northern hemisphere.