The role of non-local moisture sources in the development of a subtropical cyclone over the Southwestern South Atlantic – A numerical case study

Abstract:

Subtropical cyclones over the Southwestern South Atlantic Ocean develop in a region where the sea surface temperature is below the required threshold for tropical development. However, there is a large moisture source region situated to the northeast of the most cyclogenetic area near the South American Coast. The South Atlantic Subtropical High flow advects water vapour originated from ocean evaporation in this region directly to the cyclogenetic area, suggesting that this horizontal moisture convergence balances the weaker fluxes below the cyclone (local source) and provide the necessary fuel to the convective activity. In order to investigate this hypothesis, the numerical model WRF version 3 is used to carry out sensitivity experiments where a subtropical cyclone is simulated first without the non-local moisture source (the moisture advected by the Subtropical High) and then without the local moisture source (evaporation from the underlying ocean). The case study is a subtropical cyclone developed in March 1999 near the South American Coast, associated to a mid-tropospheric cut-off low and strong low level moisture convergence. The model correctly simulates the hybrid structure of the cyclone and the synoptic environment where it is inserted. In the absence of non-local moisture source, the cut-off in 500 hPa fails to develop, due to changes in the horizontal temperature advection field. This prevents the strong atmospheric instability caused by the cold low aloft, and then there is no cyclogenesis. Without the local source of moisture, the cyclone is formed, but it is much weaker than in the control simulation, and lacks the low tropospheric warm core (a distinctive feature of the subtropical cyclones). In both cases, the tropospheric warming by cloud formation is strongly reduced, and the surface low does not deepen. The horizontal temperature advection is also decreased, and this impacts the incipient cyclogenesis. These results show that this subtropical cyclone depended on both local and non-local moistures sources acting together to destabilize the environment and provide the atmospheric latent heating. It is likely true for most of the hybrid cyclones in this region.