Gustavo Goni

Effects of a Warm Oceanic Feature on Hurricane Opal

On 4 October 1995, Hurricane Opal deepened from 965 to 916 hPa in the Gulf of Mexico over a 14-h period upon encountering a warm core ring (WCR) in the ocean shed by the Loop Current during an upper-level atmospheric trough interaction. Based on historical hydrographic measurements placed within the context of a two-layer model and surface height anomalies (SHA) from the radar altimeter on the TOPEX mission, upperlayer thickness fields indicated the presence of two warm core rings during September and October 1995. As Hurricane Opal passed directly over one of these WCRs, the 1-min surface winds increased from 35 to more than 60 m s21, and the radius of maximum wind decreased from 40 to 25 km. Pre-Opal SHAs in the WCR exceeded 30 cm where the estimated depth of the 208C isotherm was located between 175 and 200 m. Subsequent to Opal’s passage, this depth decreased approximately 50 m, which suggests upwelling underneath the storm track due to Ekman divergence. The maximum heat loss of approximately 24 Kcal cm22 relative to depth of the 268C isotherm was a factor of 6 times the threshold value required to sustain a hurricane. Since most of this loss occurred over a period of 14 h, the heat content loss of 24 Kcal cm22 equates to approximately 20 kW m22. Previous observational findings suggest that about 10%‐15% of upper-ocean cooling is due to surface heat fluxes. Estimated surface heat fluxes based upon heat content changes range from 2000 to 3000 W m 22 in accord with numerically simulated surface heat fluxes during Opal’s encounter with the WCR. Composited AVHRR-derived SSTs indicated a2 8‐38C cooling associated with vertical mixing in the along-track direction of Opal except over the WCR where AVHRR-derived and buoy-derived SSTs decreased only by about 0.58‐18C. Thus, the WCR’s effect was to provide a regime of positive feedback to the hurricane rather than negative feedback induced by cooler waters due to upwelling and vertical mixing as observed over the Bay of Campeche and north of the WCR.

Application of Oceanic Heat Content Estimation to Operational Forecasting of Recent Atlantic Category 5 Hurricanes

Research investigating the importance of the subsurface ocean structure on tropical cyclone intensity change has been ongoing for several decades. While the emergence of altimetry-derived sea height observations from satellites dates back to the 1980s, it was difficult and uncertain as to how to utilize these measurements in operations as a result of the limited coverage. As the in situ measurement coverage expanded, it became possible to estimate the upper oceanic heat content (OHC) over most ocean regions. Beginning in 2002, daily OHC analyses have been generated at the National Hurricane Center (NHC). These analyses are used qualitatively for the official NHC intensity forecast, and quantitatively to adjust the Statistical Hurricane Intensity Prediction Scheme (SHIPS) forecasts. The primary purpose of this paper is to describe how upper-ocean structure information was transitioned from research to operations, and how it is being used to generate NHC’s hurricane intensity forecasts. Examples of the utility of this information for recent category 5 hurricanes (Isabel, Ivan, Emily, Katrina, Rita, and Wilma from the 2003–05 hurricane seasons) are also presented. Results show that for a large sample of Atlantic storms, the OHC variations have a small but positive impact on the intensity forecasts. However, for intense storms, the effect of the OHC is much more significant, suggestive of its importance on rapid intensification. The OHC input improved the average intensity errors of the SHIPS forecasts by up to 5% for all cases from the category 5 storms, and up to 20% for individual storms, with the maximum improvement for the 72–96-h forecasts. The qualitative use of the OHC information on the NHC intensity forecasts is also described. These results show that knowledge of the upper-ocean thermal structure is fundamental to accurately forecasting intensity changes of tropical cyclones, and that this knowledge is making its way into operations. The statistical results obtained here indicate that the OHC only becomes important when it has values much larger than that required to support a tropical cyclone. This result suggests that the OHC is providing a measure of the upper ocean’s influence on the storm and improving the forecast.

Dynamics of the Brazil-Malvinas Confluence based on inverted echo sounders and altimetry

We use data from Geosat altimeter and from 10 inverted echo sounder (IES) moorings deployed in the SW Atlantic Ocean off the Argentine continental shelf to investigate several aspects of the dynamics of the upper layer in the Brazil-Malvinas Confluence region. We use the altimeter data to estimate the sea height anomalies at each IES location and use the IES data to compute the upper layer thickness, taken in this work to go to the depth of the 8°C isotherm. We first discuss the sea height and upper layer thickness variations caused by the passage of the Brazil Current, Malvinas Current, and warm anticyclonic and cold cyclonic eddies. We introduce a two-layer model in which we decompose the sea height into its baroclinic and barotropic contributions. We then propose a method to monitor the thickness of the upper layer and the barotropic and baroclinic transports as a function of the sea height anomalies and the statistics of the upper layer thickness and reduced gravity for the region. We compute the reduced gravity values from the slope of a linear fit between the sea height anomalies and the upper layer thicknesses. We estimate the reduced gravity values for this region to range from 0.005 to 0.011 m s−2. We also estimate the mean barotropic sea height difference using two methods: conservation of mass and conservation of potential vorticity. Finally, we compute the time series for the baroclinic and barotropic transports during the Geosat Exact Repeal Mission time period. Our results suggest that the mean baroclinic transport in the upper layer decreases from 12 Sv at around 35°S to 7 Sv at 37°S. Our results also indicate that there is a significant barotropic contribution to the upper layer transport in the confluence region.

Ocean thermal structure monitoring could aid in the intensity forecast of tropical cyclones

Accurate prediction of the track and intensity of tropical cyclones is highly important for planning the evacuation of densely populated coastal areas and for impact assessment. Though forecasts of Atlantic hurricane tracks have improved greatly during recent years, large errors in intensity forecasts still remain. Dynamical and statistical models are currently being used, with a different range of success, to predict the location of tropical cyclone intensity changes. Statistical prediction models attempt to quantify the relationship between tropical cyclone intensification and variables that can be estimated or observed in real time.

Agulhas ring dynamics from TOPEX/POSEIDON satellite altimeter data

The transfer of warm water from the Indian Ocean into the South Atlantic subtropical gyre takes place in the form of rings and filaments formed when the Agulhas Current retroflects south of Africa between IS and 25E. A survey of the rings formed from September 1992 until December 1995 in the Retroflection region was carried out using TOPEX/POSEIDON altimeter data. A two-layer model was used to estimate the upper layer thickness from the altimeter-derived sea-surface height anomaly data. An objective analysis scheme was used to construct a map of upper layer thickness every ten days. Seventeen rings and their trajectories were identified using these maps. The shedding of rings from the Agulhas Current was neither continuous nor periodic, and for long periods there is no formation of rings. Several rings remained in the region for more than a year and, at any given time, 2 to 6 rings coexisted in the region east of the Walvis Ridge. The results showed that the number of rings translating simultaneously in this region is larger during the first half of each year. The upper layer transport of the Agulhas Current in the Retroflection region was computed and a close association between high variations in transport and ring shedding was found. Rings translated WNW at translation speeds ranging from 5 to 16 km day-l following formation. The values of available potential energy computed for the rings place them among the most energetic rings observed in the world oceans, with values of up to 70 X lOIs J. Transport computations indicate that each ring contributes in the average approximately 1 Sv of Agulhas Current waters to the Benguela Current.

A census of North Brazil Current Rings observed from TOPEX/POSEIDON altimetry: 1992–1998

Six years of TOPEX/POSEIDON altimeter data are used to investigate the formation of rings and eddies shed by the North Brazil Current. Upper layer thickness maps were used to identify 34 of these features formed in the North Brazil Current retroflection region, an average of more than 5 rings and eddies per year. The ensemble of ring trajectories closely parallels the 500 m isobath, and one out of six rings penetrate into the Caribbean Sea through the southern Lesser Antilles. The rest of the rings and eddies follow a northern trajectory past Barbados once they reach 58W. Their estimated mean translation speed is 14 km/day and their mean length scale is approximately 100 km. Our results suggest that the formation rate of NBC rings and eddies is nearly twice that previously thought, and that they may account for more than 1/3 of the interhemispheric transport within the Atlantic meridional overturning cell.

Objective Detection of Oceanic Eddies and the Agulhas Leakage

Mesoscale oceanic eddies are routinely detected from instantaneous velocities derived from satellite altimetry data. While simple to implement, this approach often gives spurious results and hides true material transport. Here it is shown how geodesic transport theory, a recently developed technique from nonlinear dynamical systems, uncovers eddies objectively. Applying this theory to altimetry-derived velocities in the South Atlantic reveals, for the first time, Agulhas rings that preserve their material coherence for several months, while ring candidates yielded by other approaches tend to disperse or leak within weeks. These findings suggest that available velocity-based estimates for the Agulhas leakage, as well as for its impact on ocean circulation and climate, need revision.

Three Agulhas rings observed during the Benguela Current Experiment

A field program to study the circulation of the Benguela Current and its extension into the southeastern Atlantic Ocean has completed the survey and instrument deployment phase. We report here new observations of three Agulhas rings north and west of Cape Town, South Africa. Three mesoscale anticyclonic rings initially identified by means of TOPEX/POSEIDON altimetry were surveyed with expendable bathythermographs, conductivity-temperature-depth-oxygen profiles, direct current measurements from a lowered acoustic Doppler current profiler, a hull-mounted acoustic Doppler current profiler, and satellite-tracked surface drifters. Characteristics of the rings are presented, and their origins are discussed. Two are typical Agulhas rings surveyed at different times after their generation; the third Agulhas ring has an anomalous water mass structure whose most likely origin is the Subtropical Front.

Interannual variations in the Atlantic meridional overturning circulation and its relationship with the net northward heat transport in the South Atlantic

[1] Variability of the Atlantic Meridional Overturning Circulation (AMOC) and its effect on the net northward heat transport (NHT) in the South Atlantic are examined using a trans-basin expendable bathythermograph (XBT) high-density line at 35S (AX18). The time-mean AMOC is 17.9 ± 2.2 Sv during 2002–2007. Although the geostrophic transport dominates the time-mean AMOC, both geostrophic and Ekman transports are important in explaining the AMOC variability. The contributions of geostrophic and Ekman transports to the AMOC show annual cycles, but they are out ofphase,resultinginweakseasonalvariabilityoftheAMOC. The NHT variability is significantly correlated with the AMOC, where a 1 Sv increase in the AMOC would yield a 0.05 ± 0.01 PW increase in the NHT. Partition of transport into the western and eastern boundaries and interior suggests that, to quantify changes in the AMOC and NHT, it is critical to monitor all three regions. Citation: Dong, S., S. Garzoli, M.Baringer,C.Meinen,andG.Goni(2009),Interannualvariations intheAtlanticmeridionaloverturningcirculationanditsrelationship with the net northward heat transport in the South Atlantic, Geophys. Res. Lett., 36, L20606, doi:10.1029/2009GL039356.

Investigation of the Brazil Current front variability from altimeter data

The southwestern Atlantic Ocean is characterized by the confluence of the Brazil and Malvinas Currents, which form very strong surface and subsurface fronts that can be detected from hydrographic and remote sensing observations. Three data sets, consisting of TOPEX/Poseidon-derived sea height anomalies, the climatologically derived depth of the 10°C isotherm, and reduced gravity, are used in conjunction with a two-layer dynamical ocean scheme to monitor the Brazil Current front and to investigate its variability during a 6 year period (1993–1998). Results reveal that the fronts exhibit motions that are larger zonally than meridionally, showing strong interannual variability with annual mean amplitudes that range from 1° to 6°. The annual and semiannual components account for more than 75% of the variability of the frontal oscillations. In the annual cycle the frontal motions appear to be related closely to fluctuations in the baroclinic transport of the Brazil Current and are only influenced by the Malvinas Current when the Brazil Current transport is very small.