Interdecadal change in the genesis activity of the first tropical cyclone of each year over the western North Pacific

This study analyzed time series of the genesis latitude, longitude, and date of the first tropical cyclone (TC) each year over the 38 years between 1979 and 2016. Statistical change-point analysis applied to these three variables showed that a shift in climate regime occurred around 1998. More specifically, recent TCs have shown a strong tendency to occur more northwest in the western North Pacific (WNP), and day of TC genesis tend to be delayed. Also, we compared differences between the periods 1998 to 2016 (post-1998) and 1979–1997 (pre-1998) in terms of outgoing longwave radiation (OLR), total cloud cover, precipitable water, precipitation, vertical wind shear, 850 hPa relative vorticity, and sea surface temperature (SST). Our results showed that a favorable environment for TC genesis was formed near the South China Sea (SCS) and the Philippines and an unfavorable environment for TC genesis was formed in the southeastern part of the WNP. Analysis of stream flow showed that an anomalous cyclonic circulation at 850 hPa was formed in the SCS and an anomalous large anticyclonic circulation was formed in the North Pacific. From these circulations, a ridge extended to the east sea of the Philippines, and consequently, anomalous trade winds were strengthened in the equatorial Pacific. Such anomalous atmospheric circulation seems to be associated with the cold Pacific Decadal Oscillation (PDO) phase. At 200 hPa, the anomalous anticyclonic circulation was strengthened in the SCS, and an anomalous cyclonic circulation formed in the east sea of the Philippines, which strengthened anomalous westerlies in the equatorial Pacific. Furthermore, this circulation pattern is found to be related with a strengthening of Walker circulation. Therefore, during the post-1998, when trade winds were strengthened by the development of Walker circulation, the cold PDO phase was strengthened, the location of TC genesis moved toward the northwestern WNP, and TC genesis day tended to be delayed.

Impact of the Anomalous Latent Heat Flux Over the Kuroshio Extension on Western North American Rainfall in Spring: Interannual Variation and Mechanism

The Kuroshio and its extension (KE) significantly influences regional climate through meridional heat transport from the tropical ocean. In this study, the observational and reanalysis datasets are used to investigate the impact of the latent heat flux (LHF) over the KE region on downstream rainfall and the underlying mechanism. The result shows a “seesaw” structure in rainfall anomaly, dominating the Western Canada and the southwestern North America with a correlation coefficient of 0.77 between the two modes. In strong LHF years, strengthened LHF favors to enhance precipitation in the Western Canada and reduce that in the southwestern North America. This is primarily associated with an anomalous cyclonic circulation over the KE region, which enhances southwesterly precipitation and latent heating in the middle troposphere. The heating excites an anomalous cyclonic circulation to its west and an anticyclonic circulation to its east, helping to reinforce the existing anomalous cyclonic circulation in turn and form a positive feedback. The conditions associated with La Niña events favor to above processes. To the upper troposphere, the deepened anomalous cyclonic circulation due to enhanced eddy activities and atmospheric baroclinic instability over the KE strengthens subtropical westerly jet stream and thereby extends eastward on the 200 hPa level. Correspondingly, an elongated zonally lower level cyclonic circulation anomaly across the North Pacific leads to a moisture convergence in the Western Canada, which is mainly resulted from the anomalous positive vorticity advection over the left side of the exit region of the jet stream. The opposite circumstance occurs in weak LHF years, presenting an opposed anomalous circulation and rainfall pattern.

Impacts of ENSO and IOD on Snow Depth over the Tibetan Plateau: Roles of Convections over the Western North Pacific and Indian Ocean

<p>This is a consensus that snow over the Tibetan Plateau (TP) modulates the regional climate significantly. Possible causes for the interannual variability of snow over the TP, however, are under debate, especially regarding the independent roles of El Niño-Southern (ENSO) and Indian Ocean dipole (IOD). Based on in-situ observational data analyses and model simulations, our study shows that impacts of ENSO and IOD on snow depth (SD) over the TP are different during early winter. In particular, ENSO mostly affects SD over the eastern TP, while IOD affects SD over the central-western TP. Both above-normal snowfall and cold temperature anomaly contribute to deeper-than-normal SD, with the former playing a more important role. Diabatic cooling of the suppressed convection over the western North Pacific that related to the positive phase of ENSO could excite an anomalous cyclonic circulation and strong cold temperature anomalies over the eastern TP. There is an enhanced moisture transported over the eastern TP from the tropics due to the anomalous cyclonic circulation; along with strong cold temperature anomalies, resulting in above-normal snowfall in the eastern TP. On the other hand, anomalous convection over the western Indian Ocean related to the positive IOD could generate a wave-train propagating northeastward and induce an anomalous cyclonic circulation over the central-western TP. The associated anomalous circulation transports extra moisture from the tropics to the central-western TP, providing conditions favorable for more snowfall over the central-western TP. Opposite conditions tend to occur during negative phases of ENSO and IOD.</p>

Decadal Variation of Rainfall Seasonality in the North American Monsoon Region and Its Potential Causes

This study shows that the North American monsoon system’s (NAMS) strength, onset, and retreat over northwestern Mexico exhibit multidecadal variations during the period 1948–2009. Two dry regimes, associated with late onsets, early retreats, and weaker rainfall rates, occurred in 1948–70 and 1991–2005, whereas a strong regime, associated with early onsets, late retreats, and stronger rainfall rates, occurred in 1971–90. A recovery of the monsoon strength was observed after 2005. This multidecadal variation is linked to the sea surface temperature anomalies’ (SSTAs) variability, which is a combination of the Atlantic multidecadal oscillation (AMO) and the warming SST trends. These SST modes appear to cause an anomalous cyclonic circulation and enhanced rainfall over the southeastern United States and the Gulf of Mexico, which in turn increases the atmospheric stability over the monsoon region. However, these SST modes cannot fully explain the circulation and rainfall anomalies observed during the early-retreat monsoons. An expansion of the North Atlantic surface high (NASH) in recent decades also contributes to the anomalous circulation associated with the early retreats of the NAMS. A northwestward expansion of the NASH further enhances the anomalous cyclonic circulation and rainfall over the southeastern United States and the Gulf of Mexico. Its associated northwestward shift of the subtropical jets over the western United States enhances subsidence over the NAMS region. The combined effects of the AMO, the warming trends, and the NASH expansion on atmospheric circulation contribute to a stronger and more persistent earlier retreat during the recent dry regime (1991–2005), while the earlier dry regime (1948–70) appears to be only influenced by the positive phase of the AMO.

Interdecadal Variability of Summer Rainfall in Taiwan Associated with Tropical Cyclones and Monsoon

This study investigates interdecadal variability of summer (June–August) rainfall in Taiwan for the 1950–2008 period. Summer rainfall in Taiwan is partitioned into two components: tropical cyclone (TC) rainfall caused by TC passage and seasonal monsoon rainfall associated with monsoon southwesterly flows. The joint interdecadal mode of TC rainfall and seasonal monsoon rainfall is extracted by empirical orthogonal function (EOF) analysis. The first interdecadal mode features an increasing trend plus a near-20-yr oscillation. The spatial patterns of this mode are uniform in sign over the entirety of Taiwan with positive anomalies for TC rainfall and negative anomalies for seasonal monsoon rainfall. These results reveal that TC rainfall and seasonal monsoon rainfall tend to vary inversely in interdecadal variability, with a positive trend in TC rainfall and a negative trend in seasonal monsoon rainfall. Large-scale regulating processes associated with this interdecadal rainfall mode are interpreted from the correlation patterns. Significant warm sea surface temperature (SST) anomalies exist in the tropical central and eastern Pacific and the Indian Ocean. At the low levels, an anomalous large-scale divergent center occurs in the Australian regions, which in turn evokes an anomalous cyclonic circulation in the subtropical North Pacific. Taiwan is on its western edge and affected by anomalous northeasterly flows, in company with weakening in the prevailing southwesterly flows and moisture transport from the South China Sea into Taiwan. As such, negative seasonal monsoon rainfall anomalies occur in Taiwan with a decreasing trend. The subtropical anomalous cyclonic circulation also weakens vertical wind shear over the major TC genesis region, that is, the Philippine Sea. Warm SST anomalies in this region and accompanying anomalous ascending motion provide additional favorable conditions for TC genesis. More TCs are thus formed in the Philippine Sea. The appearance of an anomalous cyclonic circulation in the subtropical North Pacific reflects a weakening of the Pacific subtopical high, which tends to retreat eastward and provides southeasterly or southerly flows on its western boundary to guide TCs formed in the Philippine Sea northwestward toward Taiwan. TC frequency and TC rainfall thus increase in Taiwan with an increasing trend.