Using the NCEP reanalysis variables and TC dataset for the period 1960−2008, the present study examines the different impacts of ENSO Modoki and canonical ENSO events on TC frequency over WNP during boreal summer and fall. Different from the Niño-3.4 index, which may mix up eastern Pacific warming and central Pacific warming, EMI and Niño-3 index can distinguish two types of Pacific warming events. The results show that TC frequency in the WNP basin is significantly positively correlated with EMI during JJA. On the contrary, the relationship of TC frequency and Niño-3 index becomes weakened owing to mutual cancellation between enhanced and reduced TC frequency in the different domains. Less TCs occur in the northern part of WNP, while more TCs form in the southeastern part of WNP during the canonical El Niño years. These differences can be attributed to the different large-scale circulation anomalies between the two types of ENSO events. An anomalous cyclonic circulation in response to a broad heating source dominates over the WNP during the El Niño Modoki years. In contrast, in response to the a zonally elongated and meridional dipole pattern of heating over the WNP during the canonical El Niño years (heating source being located in the equatorial central Pacific and heating sink just north of it), an anomalous anticyclonic circulation persists in the subtropics, while an cyclonic shear associated with the equatorial westerly anomalies is dominant in the southeast part of the WNP.

  A baroclinic numerical model is used to validate the relationship between wind anomalies and heating forcing. A realistic summer (JJA) mean state serves as the basic state. The heating source/sink, whose profiles are similar to those of the observed OLR anomalies, is prescribed to force the atmosphere in the presence of the same realistic mean state. The simulated results are qualitatively consistent with the observed, suggesting that the different distributions of heating related to two types of ENSO events can play essential roles in the distinct modulations on the TC frequency over the WNP basin. In the end, it is worthwhile noting that, with a probable increasing tendency of El Niño Modoki occurrence in terms of observed data and scenario simulation under climate change projections, the present results urge that the modulation of ENSO Modoki on TC activity should be taken into account in TC activity prediction in the context of global warming.

Chen, G. H., and C. Y. Tam. 2010: Different impacts of two kinds of Pacific Ocean warming on tropical cyclone frequency over the western North Pacific. Geophys. Res. Lett., 37, L01803, DOI: 10.1029/2009GL041708.

Figure 1. Composites of SSTA (shading; units: ℃), 850-hPa wind anomalies (vectors, see scale arrow at the upper right; units of scale arrow: m s^-1) and OLR anomalies (dash/solid contours indicating negative/positive values; contour interval 3W m^-2; zero contours omitted) during JJA for (a) El Niño Modoki and (b) canonical El Niño events. OLR anomalies are computed based on data starting from 1980 and with a T15 truncation applied for clarity.