Upper–Lower Layer Coupling of Recurrent Circulation Patterns in the Gulf of Mexico

Abstract

Abstract The study of the relationship between the upper and lower layers in the Gulf of Mexico (GoM) has experienced a lot of progress in recent years. Nevertheless, an examination of their coupling for the entire GoM in a statistically consistent manner is still needed. Layer thickness data from a GoM 21-yr free-running simulation are used to examine the coupling between the upper (<250 m) and lower (>1000 m) layers, focusing on the dominant modes of variability through a Hilbert empirical orthogonal function (HEOF) analysis. The results show that the three leading modes are associated with the coupling between both layers during the life cycle of the Loop Current (LC) and the LC eddy (LCE) separation process, consistent with previous observational studies. These modes are cyclical, with periodicities in agreement with the mean LCE separation period, indicating recurrence of the circulation patterns. The fourth mode of the upper layer is associated with the translation of LCEs and their dissipation in the northwestern GoM, while in the lower layer it captures variability related to the strengthening of the circulation along the Sigsbee Gyre western branch. This mode does not show cyclicity, suggesting persistence of the associated circulation patterns with a dominant time scale of 14 months. Evidence and corroboration of recently observed lower-layer circulation features are provided. The application of the HEOF technique used here can complement the three-dimensional oceanic assimilation methods by projecting surface information to depth in a statistically consistent manner. Significance Statement The purpose of this study is to better understand the relationship between the upper and lower ocean layers in the Gulf of Mexico. While much is known about their variability separately, an examination of the coupling through the whole region and in a long time period is still needed. We use a free numerical simulation of the circulation in the Gulf to accomplish this goal. The dominant circulation patterns in the lower layer are tied to the upper ones and are governed by the same temporal scales. Our findings point to a way to better understand the response of deep circulation to upper circulation, and may contribute to a better prediction of ocean dynamics.