The focus of Friday map discussion for 26 Mar'04 was, not surprisingly, on the apparent South Atlantic hurricane off the coast of southern Brazil. This storm appears to be the first-ever reported hurricane in the South Atlantic Ocean during the modern satellite era (last 30 years). Prior to the start of reliable GOES imagery (mid 1970s) the absence of South Atlantic hurricanes in the historical record can not be equated to evidence of their absence. The lack of South Atlantic Ocean hurricanes is commonly attributed to a combination of three factors: a) SSTs are usually < 27 C, b) mean 850-200 hPa wind shears are usually > 10 m/s, and c) African easterly waves are virtually unknown in the tropical South Atlantic. As to prior possible tropical systems in the South Atlantic, I am aware of only two events. The first was a possible TD/TS that formed in April 1991 out of an area of convection that may initially have been associated with an African disturbance. The second was the very recent (19 Jan'04) convective flare-up that may have briefly achieved TD status off the coast of Brazil. My personal view is that while it is quite probable that other "tropical-like" systems may have gone undetected even in the modern satellite era, it is unlikely that what appears to be a genuine Cat 1 storm could have escaped detection.
The indirect evidence that the current South Atlantic storm achieved hurricane status (Cat 1) is pretty persuasive and includes spectacular visible imagery that showed a clear eye signature, SSM/I and TRMM imagery that revealed the presence of a quasi- closed eye wall ring of deep convective clouds with an implied eye radius of 20-25 km, and AMSU (advanced microwave sounding unit) observations from the University of Wisconsin which indicated the likely existence of a warm-core storm with a central pressure down to 974 hPa on Friday afternoon. Although these remotely sensed measurements make a good case for the existence of a Cat 1 hurricane off the coast of southern Brazil, to my knowledge there are as yet no direct measurements of sustained 65 kt surface winds in the storm (the highest measured surface winds in the QuickScat imagery that I have seen are in the 50 kt range). One possible way to reconcile the 50 kt QuickScat surface wind measurements with satellite-derived indicators of a Cat 1 hurricane in the absence of any independent surface wind measurements from mobile ships is to assume that a slightly stable boundary layer has confined the implied higher hurricane-force winds to just above the surface.
As part of our discussion of the South Atlantic hurricane (hereafter "NoName" #1) we looked at the following fields: 1. Loops of past 15-day SH 500 hPa Z/Z'. 2. Loops of past 15-day SH 1000-500 hPa thickness and thickness anomalies. 3. Loops of past 15-day SH/tropical theta on the dynamic tropopause from Mike Dickinson. 4. Loops of past 15-day SH/tropical 850 hPa Z/theta-e from Mike Dickinson. 5. Satellite loops/stills from multiple sources. 6. Mean/anomaly fields for multiple variables derived from the CDC Web site. 7. Surface obs/soundings from Brazilian sources. An extended discussion established the following points: 1. Sea surface temperatures in the storm formation area ranged from near normal (24-25 C) to slightly below normal (~-0.5 C). 2. Estimated deep-layer (850-200 hPa) shears in the storm formation area were generally < 10 m/s. 3. A quasi-stationary weak cold-core upper-level trough was situated east of southern Brazil since 12 Mar'04. 4. The cold-core upper-level trough was cutoff for much of the last 15 days. 5. The upper-level cutoff cyclone east of southern Brazil was isolated from the westerlies by a persistent ridge that lay poleward. 6. The upper-level cutoff cyclone east of southern Brazil was reinvigorated several times in the last 15 days by the northern (equatorward) ends of upstream troughs that weakened and thinned as they crossed the Andes and southern Argentina. 7. Weak surface baroclinic cyclone formation occurred over central Argentina to the east of the Andes in conjunction with weak trough passage across the Andes. 8. Modest cyclone intensification was observed when these weak surface cyclones reached the east coast of South America and moved offshore. 9. A persistent area of cyclonic flow was found at the surface east of southern Brazil beneath the persistent upper-level cutoff cyclone. 10. "Noname" #1 formed as a tropical transition from the last of the baroclinic systems to cross the east coast of South America over 24-25 C SSTs. 11. Based upon the fields available and in the absence of mesoscale observations, nothing struck the discussants as unique about the formation of "NoName" #1. 12. According to the GFS, 850 hPa theta-e values were only in the 325-335 K range in the narrow band of higher moisture that fed the storm. 13. Based upon the WV loops, the tropical transition appeared to occur beneath a narrow ribbon of Amazonian moisture that turned westward and then northwestward from the main column of moisture that swept poleward along the eastern flank of the cutoff cyclone. Otherwise the environment over and to the west of the storm was quite dry in mid and upper levels.
The available evidence suggests that the precursor disturbance to "NoName" #1 was an ordinary, but small scale, system that formed through ordinary baroclinic processes within the envelope of a larger-scale upper-level trough (points 3-12 above). Similar developments in the NH occur in conjunction with polar low formation beneath high-latitude cutoff cyclones, small-scale cyclone formation beneath cutoff cyclones over the Mediterranean Sea, and small-scale cyclone formation beneath cutoff cyclones aloft at the southwestern ends of long PV tails over the western Atlantic. At issue is what determines what much smaller subset of baroclinic developments of this type will undergo a tropical transition to a warm-core disturbance as happened, for example, with Hurricane Diana in 1986 and Hurricane Michael in 2000. The baroclinic development of these two storms and their subsequent tropical transition is documented observationally and numerically in papers by Bosart and Bartlo (1991) and Davis and Bosart (2001, 2002, 2003), all in the MWR. The scientific insights gained from these papers and their possible operational application are summarized in Davis and Bosart (2004; to appear in BAMS later this year). Critical factors in tropical transition appear to be the development of a region of weak deep-layer shear near the cyclone center in conjunction with PV destruction and ridging aloft in response to diabatic heating associated with deep convection over relatively warm SSTs (points 1-2 above).
A critical issue with regard to the formation of "NoName" #1 is whether there was something very atypical about the larger-scale environment within which it formed and/or whether the development was essentially part of a chaotic stochastic process. Based upon our *very* preliminary analysis and discussion, nothing appears to be out of the ordinary about the baroclinic formation process for those of us conditioned to "NH thinking". The real issue is what made the storm undergo tropical transition into a warm-core Cat 1 hurricane. The SST analysis is no help because contrary to initial expectations the SSTs were normal to slightly below normal (24-25 C) in the region where the storm underwent tropical transition (note that these SSTs were still 1-2 C below optimal values for TC formation).
Null cases need to be examined critically to see if there are any obvious environmental differences between the far more common baroclinic events that never proceed to a tropical transition from the (one) event so far that won the TC jackpot. For example, the question can be asked as to whether the persistent (two week) upper-level cyclone east of Brazil was an extremely rare event at this time of the year. That this upper- level cyclone was enveloped by an equally persistent ridge on all sides ensured that fresh surges of cool air from the southern oceans could not reach southern Brazil as often is commonly observed to happen in early autumn. The end result of this process is that slow-moving weak baroclinic systems were favored just to the east of South America in a region of persistent cyclonic flow but one also devoid of strong (and deep) shear because of the mini Rex-like block created by the ridge lying poleward of the cutoff cyclone. The possible role of larger-scale deformation processes in concentrating moisture and thermal gradients in this situation should be explored further (point 13 above).
It is my view that the "NoName" #1 event represents a challenging decadal scale research opportunity. Unless mesoscale obs in the vicinity of the storm materialize unexpectedly out of thin air, we are never going to know the wind, temperature, and moisture fields offshore. At issue is whether after having thoroughly documented the evolution of the larger-scale flow structure and its impact on baroclinic storm formation it will be possible to conduct insightful mesoscale simulations of the tropical transition of "NoName" #1 given the observed larger-scale flow configuration. Progress may occur through the result of numerical simulations of "NoName" #1 and a careful comparisons of these results to warm-core storm development in the NH as typified by polar lows, Mediterranean cyclones and certain Atlantic hurricanes. The difficulty of the task should not be underestimated, however. As noted in separate email messages, Bob Hart's phase space diagrams for "NoName" #1 were not especially revealing and suggested that most of the global models (e.g., GFS, NOGAPS, ECMWF) had difficulty resolving the initial conditions properly and developing a warm-core storm.