Paulo Polito and Olga Sato Seminar - "Rossby waves simulated by GCMs" / "Subtropical mode water in the South Atlantic"
Climate Change Research Centre seminar room, level 4 Mathews Building, Kensington Campus, Sydney
"Can the OFES and CESM models reproduce long Rossby waves?"
by Paulo Polito
General circulation models solve primitive equations that have both the physics and the space-time resolution necessary to provide a realistic reproduction of long, first mode baroclinic Rossby waves. In the present context these waves are a key ingredient in the set-up of the subtropical gyre, where they carry energy westward without an efficient eastward counterpart. This asymmetry reinforces western boundary currents and is a relevant part of the wind-driven circulation. In a stratified ocean, internal Rossby waves move the isopycnals O[10-100 m] in the vertical, which in turn is reflected at the surface as O[1-10 cm] vertical anomalies detected by satellite altimetry. Sea surface height anomaly estimates from two ocean general circulation models (OFES and CESM) were used to determine whether climate numerical models reproduce the surface signature of first mode baroclinic Rossby waves in a manner similar to the observations from satellite altimetry in the South Atlantic basin. A series of filters was applied to isolate the westward propagating signals corresponding to Rossby waves in the altimetric record and the same set of filters was applied to the model's output. The results suggest that in low latitudes the physical processes are well captured by both models. However, as latitude increases, the added complexity of the CESM model with a coupled ocean--atmosphere that includes freshwater input and surface heat fluxes makes significant difference in the Rossby waves forecast.
"Investigating the formation and maintenance of subtropical mode water in the South Atlantic"
by Olga Sato
Similar to other basins, the western boundary region of the South Atlantic subropical gyre exhibits a subtropical mode water (SASTMW), just northeast of the Brazil-Malvinas confluence. It manifests as a sustained layer of homogeneous volume of water where the net cumulative heat balance through the surface over time is negative. The distinct presence of SASTMW is verified either when it is trapped between the seasonal and the main thermocline during most of the year or even more evidently when it outcrops at surface during the austral winter and early spring months. The outcropping is a result of vigorous exchange of heat between the ocean and the atmosphere when both boundary layers experience convective mixing due to the air-sea interactions. The amount of water formed depends on the strength and duration of this thermodynamical forcing. In this seminar I will present the results of SASTMW studies using Argo profiles and a 14-year long time series, from 2002 to 2015, of global monthly climatology of gridded T and S profiles based on in situ measurements to i) detect the SASTMW, ii) map its extent, and iii) estimate the volume that is formed each year and of how much is preserved in the subsurface. This continuous and tridimensional description of the ocean allowed us to determine the SASTMW volume as well to investigate its persistence and interannual variability. The results show that the SASTMW is found near the edge but still within the domain of the subtropical gyre. The detected SASTMW shows a strong seasonal cycle during all stages of its existence, however there is a 2-months phase lag between its outcropped and submerged phases. The time series of the SASTMW volume shows a clear interannual variability mostly due to changes that were introduced during the formation periods.
Brief Biography: Paulo Polito and Olga Sato are professors in physical oceanography from the Institute of Oceanography at the University of São Paulo, Brazil. Prior to that, Paulo and Olga obtained a PhD in Physical Oceanography from the University of Rhode Island, US. They both did postdoc at the Jet Propulsion Laboratory NASA/Caltech (US), and in the Brazilian National Institute for Space Research. Paulo research interests are large scale ocean dynamics, Rossby waves, satellite oceanography and air-sea interaction. Olga is interested in remote sensing, air-sea fluxes and mode water formation in the South Atlantic Ocean.