Ocean–atmosphere interactions: Bottom up in the tropics

news & views of the most important factors influencing pest risk, and improve our ability to provide realistic scenarios for the future. Scenarios of future pest and disease distributions under climate change support prioritization in agricultural research programmes. For example, if it is likely that a particular disease will become more important in a region, crop-breeding programmes can respond by incorporating better resistance to that disease in locally adapted varieties. The prevalence of crop pests is a function of many factors, so identifying particular drivers of change is challenging 9 (Fig. 1). If we wish to ask whether crop pests have altered distributions because of climate change, this is complicated by the many other factors that have changed simultaneously, even when long-term data are available10. It is reasonable to expect that higher temperatures will often reduce limitations on pest overwintering and increase the number of pest generations per year (Fig. 1A). However, other factors simultaneously influence pest risk at any location, including host genotypic and phenotypic resistance, where phenotypic resistance may respond to weather variables such as temperature (Fig. 1B). When multiple observers and multiple levels of sampling effort are involved, this adds another layer of uncertainty when comparing the actual distribution of pests and reported distributions (Fig. 1C). Bebber and colleagues4 provide a stimulating analysis of changes in pest distributions, along with a new set of hypotheses to engage scientists working with pests. Future ‘big data’ analyses may address the geographic distribution of pest genomes and microbial metagenomes associated with plants and soil, including analysis of the geographic spread of genes important in crop damage. Cell phone availability may facilitate analysis of global digital images of crop damage. Better data archiving systems and more data sharing are needed to support future synthetic analyses. For addressing large-extent questions, we also need advances in methods to evaluate more directly the factors that lead, not only to pest risk, but also to reporting of observations, to support understanding of what variables may be good proxies for sampling effort.  ❐ Karen A. Garrett is in the Department of Plant Pathology, Kansas State University, Manhattan, Kansas 66506, USA. e-mail: References 1. Coakley, S. M., Scherm, H. & Chakraborty, S. Ann. Rev. Phytopathol. 37, 399–426 (1999). 2. Jeger, M. J. & Pautasso, M. New Phytol. 177, 8–11 (2008). 3. Shaw, M. W. & Osborne, T. M. Plant Pathol. 60, 31–43 (2011). 4. Bebber, D. P., Ramotowski, M. A. T. & Gurr, S. J. Nature Clim. Change 3, 985–988 (2013). 5. Mayer-Schönberger, V. & Cukier, K. Big Data: A Revolution That Will Transform How We Live, Work, and Think (Houghton Mifflin Harcourt, 2013). 6. Sutherst, R. W. J. Biogeogr. 30, 805–816 (2003). 7. Venette, R. C. et al. BioScience 60, 349–362 (2010). 8. Pasiecznik, N. M. et al. EPPO Bull. 35, 1–7 (2005). 9. Garrett, K. A. et al. Plant Pathol. 60, 15–30 (2011). 10. Hannukkala, A. O., Kaukoranta, T., Lehtinen, A. & Rahkonen, A. Plant Pathol. 56, 167–176 (2007). OCEAN–ATMOSPHERE INTERACTIONS Bottom up in the tropics A study reveals that recent warming in the Indian Ocean and in the Pacific ‘warm pool’ has caused a cooling near the top of the tropical troposphere above, leading to less water vapour entering the stratosphere. Qiang Fu W ater vapour in the stratosphere is a greenhouse gas. It is constrained from entering the stratosphere in the tropics by the thermal boundary between the stratosphere and troposphere1 — the tropical tropopause, the coldest point in the lower atmosphere. Cold-point temperatures at the tropical tropopause (Fig. 1a) have important implications for both stratospheric chemistry 2 and global climate change3. The importance of the spatial distribution of temperature (Fig. 1b) is well recognized, as the temperature minimum is relevant to cloud formation and subsequent dehydration through atmospheric circulation4. In the boreal winter, for example, the lowest cold-point temperatures over the warm pool in the tropical western Pacific govern the amount of water vapour that enters the stratosphere5. It is thus critically important to understand how the zonal (longitudinal) structure of the tropical cold-point temperature would respond to global warming. Now, reporting in the Journal of Geophysical Research, Garfinkel and co-workers6 find that the warming in the tropical upper troposphere over the past 30 years has been strongest over the Indo-Pacific warm pool, where cooling near the tropopause has been strongest. They suggest that warming in the Indian Ocean and the Pacific warm pool has led to zonal asymmetry in atmospheric temperature trends, and that such trends may continue in the future. Temperatures near the tropical tropopause are determined by a complex combination of stratospheric (top-down) and tropospheric (bottom-up) processes7. The zonal structures at 100 hPa (Fig.1b) closely resemble the mean pattern of the equatorial planetary waves — large-scale perturbations of the atmospheric dynamical structure. These are driven by massive convection over the Indo-Pacific warm pool8, with the lowest temperatures and largest cirrus cloud fractions over the western Pacific and Maritime Continent (which includes the islands of Indonesia, NATURE CLIMATE CHANGE | VOL 3 | NOVEMBER 2013 | www.nature.com/natureclimatechange © 2013 Macmillan Publishers Limited. All rights reserved New Guinea and Malaysia, and the surrounding shallow seas)9. The signature of the equatorial planetary waves is also evident in the temperature variability over intraseasonal to interannual timescales9. The responses of temperature structures at 100 and 250 hPa are reversed in sign because the maximum amplitude of equatorial planetary waves with opposite phases occurs at these two levels. The temperatures and cloud fraction near the tropical tropopause are also strongly modulated by extra-tropical stratospheric waves. These drive the Brewer–Dobson circulation (BDC) — a large-scale latitudinal circulation in the stratosphere with air rising across the tropical tropopause, moving polewards and sinking towards the extra-tropical troposphere — which is particularly evident in their seasonal cycles9–11 (Fig.1c). In contrast to the equatorial planetary waves, the extra-tropical stratospheric waves are associated with zonally symmetric temperature anomalies in the lower stratosphere. 957 news & views a 101 30 Cold-point temperature 180 200 5 220 240 260 Temperature (K) 30° N 280 300 202 199 196 15° N 3 19 Lattitude 10 0° 196 199 202 205 120° W 15° S 30° S 0 60° E 120° E 180° 0.24 0.16 193 Pressure (hPa) 15 Troposphere Altitude (km) 20 Cold-point tropopause 102 103 b 25 Stratosphere 0.08 0 –1 0 20 ms 60° W Longitude 12 194 10 195 8 196 6 197 4 198 199 2 2007 2008 2009 2010 Year (...truncated)