Large-Scale Mid-Depth North Atlantic Circulation Identified from ARGO Float Trajectory Data Using the Optimal Decomposition Method

Although the North Atlantic is a subject of intensive theoretical and practical studies, knowledge on large-scale mid-depth circulation and especially its variability in the whole area is fragmentary. In modern scientific literature there are several attempts to reconstruct the large-scale mid-depth circulation pattern for the whole North Atlantic using climatic data array (Schmitz and McCartney, 1993; Reid, 1994; Lozier et al., 1995) but most oceanographers simply assumed that “this large-scale pattern of mid-depth flows does not vary too widely” (Reid, 1994).

During the last ten years considerable progress has been achieved in studying pathways and transports of intermediate circulation through analysis of subsurface float (SOFAR, RAFOS, ALACE, PALACE, SOLO) trajectories directly or with hydrological observations (Lavender et al., 2000; Zhang et al., 2001; Bower et al., 2002; Lavender et al., 2004). All these studies are focused only on local regions of the North Atlantic Ocean such as the Labrador and Irminger Seas.

The ultimate goal of the present paper is to show the existence of strong mid-depth circulation (at 1000 m) and temperature (at 950 m) in the North Atlantic from the ARGO float data collected between November 2003 and January 2005 using the optimal spectral decomposition (OSD) method developed for the analysis of sparse and noisy data (Eremeev et al., 1992; Ivanov et al., 2001; Chu et al., 2003a, b).

Basin and sub-basin scale mid-depth circulation at 1000 m and temperature at 950 m are reconstructed from ARGO float observations in the North Atlantic between November 2003 and January 2005. A new bi-modal circulation regime is identified from the analysis of circulation and temperature patterns. Each mode presents a pair of cyclonic–anticyclonic gyres but differs one from the other by location of the cyclonic gyre. The cyclonic gyre stretches from the southeast to the northwest for the first mode and stretches from the southwest to the northeast for the second mode. The first mode increases the westward transport of warm and saline water from the Mediterranean Sea. The second mode decreases the across Atlantic transport of this water. In comparison to Reid (1994) and other researchers who identified the second mode as the only existing quasi-steady state system, the present study provides the evidence that the second mode is an intermediate state of the North Atlantic circulation and demonstrates the transition between the modes.