The Gulf Stream region is a primary location for midlatitude storm cyclogenesis and growth. However, the influence of sea surface temperature (SST) on storms in the region is still under question, particularly after a storm has developed. Using the Weather Research and Forecasting (WRF) model, a storm that intensified as it transited northward across the Gulf Stream is simulated multiple times using different SST boundary conditions. These experiments test the storm response to changes in both the absolute value of the SST and the meridional SST gradient. Across the different simulations, the storm strength increases monotonically with the magnitude of the SST perturbations, even when the perturbations weaken the SST gradient. The storm response to the SST perturbations is driven by the latent heat release in the storm warm conveyor belt (WCB). During the late stages of development, the surface fluxes under the storm warm sector regulate the supply of heat and moisture to the WCB. This allows the surface fluxes to govern late-stage intensification and control the storm SST sensitivity. The storm warm front also responds to the SST perturbations; however, the response is independent of that of the storm central pressure. These modeling results suggest that the SST beneath the storm can have just as important a role as the SST gradients in local forcing of the storm.

[1] Midlatitude cyclone tracks over the Southern Ocean are constructed for the 1999-2006 period using two surface data sets: European Centre for Medium-range Weather Forecasts (ECMWF) sea-level pressure analyses on one hand, and on the other hand modified analyses in which high-wavenumber pressure variability derived from scatterometer swaths has been injected with a wavelet-based method. A comparison of track statistics reveals the differences between the two data sets. The fluxes of momentum and sensible and latent heat associated with these midlatitude cyclones are calculated and sorted by life span. Three aspects of these cyclone flux statistics are investigated. (1) The momentum flux into the ocean is stronger inside cyclones than over the rest of the Southern Ocean, while the ocean loses more sensible and latent heat outside of the cyclones. (2) The momentum flux into the ocean and the loss of sensible and latent heat by the ocean are larger when calculated from the scatterometer-modified analyses than when calculated from the original ECMWF analyses. (3) Mesoscale cyclones (short-lived cyclones) contribute a significant amount of the fluxes between the atmosphere and the Southern Ocean, although over slightly different geographic areas from longer-lived cyclones.

Abstract. A spectral decomposition of QuikSCAT surface wind vectors reveals different levels of variance and different values of the spectral slope in various regions of the world ocean for the 12 months investigated. The traditional considerations on the factors affecting the shape of the spectra are reviewed and compared to the results. In particular, the influence of large-scale synoptic systems is shown by comparing the steeper and more energetic spectra of the midlatitudes to the shallower spectra of the tropics. Similarly, the signature of convection is investigated by comparing spectra in the tropical convectively active and dry zones of the Pacific Ocean. Spectra of vorticity and divergence are calculated, along with spectral vorticity-to-divergence ratios. Their spatial and temporal variations are discussed. It is hypothesized that when convection is enhanced in the tropics, the spectral analysis captures the mesoscale/synoptic structures in which convection is embedded and that the spectra exhibit some of the characteristics of their midlatitude counterparts (i.e., steeper and more energetic).

Storm-track analysis is applied to the meridional winds at 10 m and 850 hPa for the winters of 1999–2006. The analysis is focused on the North Atlantic and North Pacific Ocean basins and the Southern Ocean spanning the region south of the Indian Ocean. The spatial patterns that emerge from the analysis of the 850-hPa winds are the typical free-tropospheric storm tracks. The spatial patterns that emerge from the analysis of the surface winds differ from the free-tropospheric storm tracks. The spatial differences between the surface and free-tropospheric storm tracks can be explained by the influence of the spatial variability in the instability of the atmospheric boundary layer. Strongly unstable boundary layers allow greater downward mixing of free-tropospheric momentum (momentum mixing), and this may be the cause of the stronger surface storm tracks in regions with greater instability in the time mean. Principal component analysis suggests that the basin-scale variability that is reflected in the storm-track signature is the same for the free-tropospheric and surface winds. Separating the data based on the boundary layer stability shows that the surface storm track has a local maximum in the region of maximum instability, even when there is no local maximum in the free-tropospheric storm track above the region. The spatial patterns of the surface storm tracks suggest a positive feedback for storm development as follows: 1) an existing storm generates strong free-tropospheric wind variability, 2) the momentum mixing of the unstable boundary layers acts to increase the ocean–atmosphere energy fluxes, and 3) the fluxes precondition the lower atmosphere for subsequent storm development.

[1] A wavelet-based method is described for incorporating swaths of surface pressure derived from scatterometer measurements into surface pressure analyses obtained from the European Centre for Medium-range Weather Forecasts (ECMWF). The resulting modified pressure fields are used to identify low-pressure centers over the Southern Ocean and to build statistics of midlatitude cyclones during 7 years of the SeaWinds-on-QuikSCAT operational period (July 1999 to June 2006. The impact of the scatterometer-derived pressure swaths is assessed with a statistical analysis of cyclone characteristics (central pressure, radius, depth) performed in parallel on the ECMWF and on the modified pressure fields. More low-pressure centers (5-10% depending on the season) are identified with the modified pressure fields, in particular incipient lows captured earlier than ECMWF and more short-lived mesoscale cyclones (with a life span less than 4 days). The cyclones identified with the modified pressure fields are characterized by lower central pressure and tighter isobars on average. A parallel spectral analysis reveals $1% additional energy at scales less than 2000 km in the modified pressure fields.Citation: Patoux, J., X. Yuan, and C. Li (2009), Satellite-based midlatitude cyclone statistics over the Southern Ocean: 1. Scatterometer-derived pressure fields and storm tracking,