Variability and frontogenesis in the large‐scale oceanic frontal zones

Abstract

Sea surface temperature (SST) from weekly global advanced very high resolution radiometer data for the period of 1982-1992 and estimates of the surface forcing due to wind stress and net heat flux are used to investigate a global monthly climatology of large-scale oceanic frontal zones (OFZs), the variability of SST gradient in several frontal zones, and the meridional frontogenesis in the North Pacific. Subpolar frontal zones are identifiable in the SST gradient field throughout the whole year while subtropical frontal zones appear as seasonal phenomena in the northern hemisphere. The magnitude and position of subtropical fronts were found to vary synchronously in both hemispheres, with maximum intensity in November to March. During this period, subtropical fronts in the northern hemisphere become clearly identifiable, separate features, while in the southern hemisphere their southern boundaries tend to merge with the subpolar front. In the northern hemisphere the northward shift of the subtropical frontal zones during late boreal spring results in a merger of the subpolar and subtropical zones by August. In the southern hemisphere, subtropical fronts separate from the subpolar front during this period and become clearly distinguishable, though less intense. Singular spectrum analysis of the 11-year time series of SST gradient in several OFZs revealed a seasonal signal as well as interannual variations that, in the equatorial zone, are related to E1 Nifio events. The amplitude of the annual cycle is comparable in the subtropical and subpolar North Pacific, but the subtropics leads the subpolar zone by about 4 months. Combined forcing due to wind stress and net surface heat flux was found to be sufficient to describe most of the observed frontogenesis variability in North Pacific midlatitudes on seasonal timescales, but not on shorter timescales. In the subtropical zone, the Ekman convergence alone was not sufficient to provide the observed frontogenesis/frontolysis on seasonal time scales. In fact, when the maximum frontogenesis associated with Ekman convergence occurs in late spring/early summer, this is counteracted by the frontolytic effect of Ekman pumping and the dominating frontolytic effect of surface heat forcing. In summer, Ekman convergence is more important in the subpolar area where it is enhanced by the smaller frontogenetic effect of both Ekman pumping and surface heat forcing. Surface forcing due to wind stress and heat flux considered in this study does not explain the high-amplitude intraseasonal frontogenesis variations, especially in the subpolar region. 1Permanently at P.P. Shirshov Institute of Oceanology, Russian derstanding the ocean's thermodynamic balance has been the subject of several investigations since the mid-1970s [e.g., Roden, 1972, 1975; Lynn, 1986]. Reliable satellite measurements of sea surface temperature (SST) in the early 1980s provided a new basis for frontal investigations (albeit, with the limitations imposed by using SST as a tracer of OFZ), and a number of regional stu...

The Azores Confluence Zone

“…Volkov and Fu (2011) considered 20-years of satellite altimetry and reanalysis data (winds) to demonstrate that the primary contribution to the frontogenesis at the Azores Current is provided by the meridional Ekman-induced current convergence that always acts to tilt the near-surface isopycnalsthereby enhancing the front. In fact, this result also agrees with previous studies of subtropical frontal zones in the North Pacific (Kazmin and Rienecker, 1996;Qiu and Chen, 2010). Volkov and Fu (2011) have also discussed the possibility of the associated high EKE of the AC to be modulated by Ekman convergence.…”

Fronts in the Southern Indian Ocean as inferred from satellite sea surface temperature data

“…It appears obviously that the fronts in the western part of the basin (20°-60°E) are marked by higher SST gradients than those situated in the central and eastern parts (Fig. 11c) where SST gradients are more diffuse and basic fronts are hardly seen in accordance to the general eastwards decrease of SST gradients generally observed in the Indian Basin (Kazmin and Rienecker, 1996). The general observed eastward decrease of frontal SST gradients can be ascribed to the decrease of overall SST latitudinal gradients across the whole investigated region (around 1.2°C/°lat and 0.8 °C/°lat at 40°E and 140°E respectively, Fig 11).…”

“…In order to retain the maximal spatial resolution of the data, the analysis was based not on the monthly averaged data, as it was made in the study of Kazmin and Rienecker (1996), but on the weekly averaged data for the weeks in the middle of each month of the three years.…”

“…Satellite SST maps have already been used to study dynamics of the PF (Moore et al, 1997;Moore et al, 1999), while Kazmin and Rienecker (1996) used satellite SST data to obtain a global picture and reveal seasonal variability of the main large-scale oceanic frontal zones. This paper represents the first investigation of the structure and space-time variability of the complete frontal system of the Southern Indian Ocean (30°-60°S, 20°-150°E) based on satellite weekly SST data set for the period of 1997-1999, with an emphasis on poorly known or unknown aspects of location or behavior of different fronts.…”

Causes of the large warm bias in the Angola–Benguela Frontal Zone in the Norwegian Earth System Model

“…This result indicates that the ocean heat flux also plays a certain role for frontogenesis/frontolysis of the ABFZ in addition to the oceanic current. A deeper understanding of these features requires a diagnostic study of the frontogenesis function (e.g., Kazmin and Rienecker 1996;Jacobs et al 2014;Giordani and Caniaux 2014), but is beyond this study.…”