This study is focused on a single phenomenon, the subtropical midsummer drought (which is technically not a drought but a minimum in the climatological annual cycle of precipitation during the rainy season in the inter-American seas region). The investigator will approach the problem at three different spatial scales. At the smallest scale, local atmospheric boundary layer and convective cloud processes that ultimately deliver rainfall will be studied to discern why precipitation diminshes. At the regional scale, features such as ocean-land contrasts, upstream effects and feedbacks, and orographic influences on flow and precipitation will be studied. On the largest scale, the PI will examine the role of large-scale general circulation features on the background state of the global atmosphere. Much of the larger-scale work pivots on the hypothesis that the North Atlantic subtropical high is a driving force for the ""drought"". However, the cloud-scale work is where the process-scale investigations will occur, and the regional scale links these effects together. The research will be pursued on three parallel cross-cutting tracks: observational analysis, dynamical diagnosis and modeling. The PI plans to support a separate PhD student, or perhaps a pair of consecutive MS students, to focus on each track. A better understanding of regional climate should result. There also could be positive impacts on weather and climate prediction across subtropical North America and the southern United States including forecasting of Atlantic and eastern Pacific hurricanes as a result of this research.
This project was motivated by the puzzling observation that summer rainfall in the northwest Atlantic, including South Florida and on through to Mexico and the eastern Pacific, has a notch: rainfall in July-August is lower than June and September. What is different about the heart of midsummer, as compared to the summer season more broadly? How does this feature of the mean or basic climatology of the region affect life there? How can we understand its basic mechanism of action in terms of rain-bearing cloud processes? Given that global climate models have a similar feature -- surprisingly, for such a subtle aspect of seasonality -- might it be ultimately driven by a robust planetary-scale process? Might we learn to predict its year-to-year variability, by discovering its connection to such larger, planetary-scale climate subsystems? Do other geographically analogous regions (the western edges of subtropical ocean basins) have a similar signal? The team divided up these questions and pursued them at the length of 3 PhD dissertations. For the Northern Hemisphere, we found that the midsummer rain notch was driven by the South Asian monsoon, halfway across the world, through a chain of causality that is surprisingly robust, for how many links it involves (too complex to detail here; Figure 1 summarizes them schematically). Intrigueingly, at similar latitudes in South America, a notch is also seen in the rainfall climatology, there in February (Figure 2). Brazilian climatologists were little aware of this feature, but the second-most-famous Brazilian song ever - "The Waters of March" - has some aspects that appear to relate to this poignant last pulse of rainy weather in March that marks the end of their wet green summer season. Since the Australian and South African monsoons are milder and less dominant than the South Asian monsoon in northern summer, it is not as easy to pin down the causes, but we have indications that the same basic mechanism -- involving the brief mid-summer farthest-poleward excursion of the boundary between westerlies and easteriles -- is at play in the Southern hemisphere too. At subtropical latitudes, the oceanic summer climate is dry while the land climate is wet, so that wind from the east, off the oceans, brings a reduction of rainfall, counter-intuitively. The clouds react to the drier airmass they find themselves in by raining both less often, and less in amount, creating the distinctive signature in the rainfall trace. The year to year variability of this midsummer rain notch is thus tied up with the year to year fluctiations of the monsoons, giving a further impetus to increase our understanding of monsoon meteorology, in hopes of better prediction.
