Seasonal dynamics in the Azores-Gibraltar Strait region: A climatologically-based study

摘要

Annual and seasonal mean circulations in the Azores-Gibraltar Strait region (North-Eastern Atlantic) are described based on climatological data. An inverse box model is applied to obtain absolute water mass transports consistent with the conservation of volume, salt and heat and the equations of the thermal wind. The large-scale gyre circulation (Azores Current, Azores Counter Current, Canary Current and Portugal Current) is well-represented in climatological data. The Azores Current annual mean transport was estimated to be 6.5 ± 0.8 Sv (1 Sv ＝ 10~6 m~3/s) eastward, exhibiting a seasonal signal with minimum transport in the spring (5.3 ± 0.8 Sv) and maximum transport in autumn (7.3 ± 0.8 Sv). The Azores Current transport is twice that of the Azores Counter Current in spring and autumn and is four-times higher in summer and winter. The southward Portugal and Canary Currents show similar seasonal cycles with maximum transports in spring (3.5 ± 0.6 and 6.6 ± 0.4 Sv, respectively). The overturning circulation within the area has an annual mean magnitude of 2.2 ± 0.1 Sv and two seasonal extremes; the highest in summer (2.6 ± 0.1 Sv) and the lowest in winter (1.7 ± 0.1 Sv). Of the annual mean, about two thirds (1.4 Sv) of the overturning circulation results from water mass transformation west of the Strait of Gibraltar: the downwelling and recirculation of upper Central Water (0.6 Sv) in the intermediate layer, the entrainment of Central Water (0.6 Sv) into the Mediterranean Outflow and the contribution of Antarctic Intermediate Water (0.2 Sv) to the Mediterranean Outflow. The remaining 0.8 Sv relates to the overturning in the Mediterranean Sea through the two-layer exchange at the Gibraltar Strait. Accordingly, the density level dividing the upper-inflowing and lower-outflowing limbs of the overturning circulation was found to be σ_1, ＝ 31.65 kg m~(-3) (σ_1, potential density referred to 1000 db), which is above the isopycnal that typically separates Central and Mediterranean Water (σ_1 ＝ 31.8 kg m~(-3)). In terms of water masses, we describe quantitatively the water mass composition of the main currents. Focusing on the spread of Mediterranean Water, we found that when the northward Mediterranean Water branch weakens in spring and autumn, the westward Mediterranean Water vein strengthens, and vice versa. The maximum net transports of Mediterranean Water across the northern and western sections of the box were estimated at -1.9 ± 0.6 Sv (summer) and -0.8 ± 0.2 Sv (spring), respectively. Within the error bar (0.2 Sv), we found no significant net volume transport of Mediterranean Water across the southern section.