Position and structure of the Subtropical/Azores Front region from combined Lagrangian and remote sensing (IR/altimeter/SeaWiFS) measurements

Abstract

The position and structure of the North Atlantic Subtropical Front is studied using Lagrangian flow tracks and remote sensing (AVHRR imagery: TOPEX/POSEIDON altimetry: SeaWiFS) in a broad region (∼31° to ∼36°N) of marked gradient of dynamic height (Azores Current) that extends from the Mid-Atlantic Ridge (MAR), near ∼40°W, to the Eastern Boundary (∼10°W). Drogued Argos buoy and ALACE tracks are superposed on infrared satellite images in the Subtropical Front region. Cold (cyclonic) structures, called ‘Storms’, and warm (anticyclonic) structures of 100–300 km in size can be found on the south side of the Subtropical Front outcrop, which has a temperature contrast of about 1°C that can be followed for ∼2500 km near 35°N. Warmer water adjacent to the outcrop is flowing eastward (Azores Current) but some warm water is returned westward about 300 km to the south (southern Counterflow). Estimates of horizontal diffusion in a Storm (D=2.2×102 m2 s−1) and in the Subtropical Front region near 200 m depth (Dx=1.3×104 m2 s−1, Dy=2.6×103 m2 s−1) are made from the Lagrangian tracks. Altimeter and in situ measurements show that Storms track westwards. Storms are separated by about 510 km and move westward at 2.7 km d−1. Remote sensing reveals that some initial structures start evolving as far east as 23°W but are more organized near 29°W and therefore Storms are about 1 year old when they reach the MAR (having travelled a distance of 1000 km). Structure and seasonality in SeaWiFS data in the region is examined.

Ocean structure and climate (Eastern North Atlantic): in situ measurement and remote sensing (altimeter)

“…Kelly & Watts, 1994;Pingree et al, 2002). The data processing methods and sla tracking techniques are given in Pingree & Sinha (1998) and Pingree et al, (1999a) and Fourier analysis of sla in time has been used to determine the periods of dominant structure at di¡erent latitudes and longitudes. ESAs ERS-1 and ERS-2 data, and CNES/NASA TOPEX/POSEIDON data for CLS 'MSLA' products were also used, even if not displayed, and for these data processing procedures the reader is referred to AVISO (1996), Le Traon et al (1995 and Le Traon et al (1998).…”

“…This sla was called anomaly R, R standing for Meddy (Richardson et al, 2000) or wave (Rossby) or Reference sla but it could have been T for Meddies (T0, T1, T2) and 'Taylor Wave'. R was followed in the manner of Pingree et al (1999a) which insists on continuity of structure so that some integral part of a feature is being followed. This method of tracking (as opposed to travel curves at the ¢xed latitudes presented here) allows the anomaly to change latitude and can £ag merging with other structures or splitting of structure.…”

A Review of Abrupt Climate Change Events in the Northeastern Atlantic Ocean (Iberian Margin): Latitudinal, Longitudinal, and Vertical Gradients

“…1a), also a branch of the AzC, 148 becomes a prominent feature off western Iberia, the thermal subtropical front (at ~17°C) is 149 shifted northward and reaches the SW-Iberian margin [Pingree et al, 1999]. Along with this shift 150…”

Can the Subtropical North Atlantic permanent thermocline be observed from space?

“…Overall, these results for temperature show that we can interpret maps of sla (see ¢gure 16 of Pingree et al, 1999, Can the Subtropical North Atlantic permanent thermocline be observed from space? R. Pingree et al 719 Journal of the Marine Biological Association of the United Kingdom (2002) Figure 9.…”

“…When this has been done we will then be in a better position to say what the altimeter is detecting in this region, e.g. Rossby Waves, variations in current strength, current meanders, fronts or waves and eddies (Cromwell et al, 1996;Cipollini et al, 1997;Pingree et al, 1999;Cipollini et al, 2000). Although it is shown that the altimeter data can represent the displacement of the permanent thermocline, we make it quite clear that this is an indirect result through correlation with near surface geostrophic velocity and not surface temperature.…”