Southern hemisphere winter cold-air mesocyclones: climatic environments and associations with teleconnections

Abstract

International audienceCold-air mesocyclones remain a forecasting challenge in the southern hemisphere middle and higher latitudes, where conventional observations are lacking. One way to improve mesocyclone predictability is to determine their larger-scale circulation environments and associations with teleconnection patterns. To help realize this objective, reanalysis datasets on atmospheric and upper-ocean synoptic variables important in mesocyclone development are composited and compared to previously published mesocyclone spatial inventories. These analyses demonstrate a consistent association between higher frequencies of mesocyclones, greater sea ice extent and large positive differences in the SST minus low-altitude air temperature fields, coinciding with enhanced westerly low-level winds having a southerly component. Composites in the 1979 2001 period also were formed for opposite phases of El Niño Southern Oscillation (ENSO), the Southern Annular Mode (SAM) and the Trans-Polar Index (TPI). Regions likely to be favorable for mesocyclone development relative to climatology were identified. The largest (smallest) variations in meso-cyclogenesis occur in the South Pacific (South Indian Ocean, south of Australia), and are dominated by ENSO. The SAM and TPI are of secondary importance, yet still influential, and exhibit strong regional-scale variations

Atmospheric and upper ocean environments of Southern Ocean polar mesocyclones in the transition season months and associations with teleconnections

“…The results presented in Table 2 confirm those for the winter season [ Claud et al , 2009]. Mesocyclones occur preferentially in association with either low temperatures at 500 hPa or proximity to large gradients of T500.…”

“…In Claud et al 's [2009] study for the SH winter‐season months, mesocyclone frequencies over the open ocean were found to increase when and where the difference between the SST and the temperature at 925 hPa (SST‐T925) increases, in conjunction with south to southwesterly low‐level (925 hPa) winds ( V 925). These associations suggested strong upward fluxes of heat and moisture, particularly west and southwest of midtropospheric troughs and frontal cyclones.…”

“…For example, in most years, the sea ice continues to expand equatorward during September. Accordingly, we included September in our recent study of mesocyclone‐circulation teleconnection associations for the winter season [ Claud et al , 2009]. The month of May was not considered because there is no mesocyclone inventory available for this month.…”

“…In addition to the ENSO and SAM, zonally asymmetric teleconnections of essentially extratropical origin show important associations with temperature, meridional wind and precipitation for key sectors of the Southern Ocean, the first two variables also being connected to regional anomalies of the Antarctic sea ice [ Harangozo , 1997]. In particular, the Trans‐Polar Index (TPI), typically defined as the Sea Level Pressure (SLP) anomaly difference between Hobart and Stanley [ Pittock , 1980, 1984; Rogers and van Loon , 1982; Carleton , 1989, 2003], depicts the eccentricity of wave number one, and has been shown to influence sea ice and mesocyclone distributions in the winter season [ Claud et al , 2009]. In its positive (negative) phase, TPI is associated with an enhanced ridge (trough) in the Australian region and an enhanced trough (ridge) in the South American Sector.…”

Mesoscale Cyclone Activity over the Ice-Free Southern Ocean: 1999–2008

“…While there are numerous hemispheric factors that can influence the interannual variability of cyclone activity over the Southern Ocean (e.g., Simmonds and King 2004), the southern annular mode (SAM), El Niñ oSouthern Oscillation (ENSO), and the wavenumber-1 pattern in the SH pressure field are thought to be particularly influential with respect to mesoscale cyclone activity (e.g., Claud et al 2009). To expose the degree of influence these three modes of variability have over the seasonal frequency of mesoscale cyclone activity, system density fields were produced for each individual season of each year and a time-synchronous (i.e., zero lag) correlation was calculated between each grid point of these fields and three indices that are commonly used to represent the amplitude and phase of the SAM, ENSO, and wavenumber-1 pattern, respectively.…”