The western North Pacific subtropical high (WNPSH) in boreal summer shows a remarkable enhancement after the early 1980s. Whereas the sea surface temperature (SST) in the North Indian Ocean (NIO) and the equatorial eastern Pacific had been noted to have remarkable local or remote effects on enhancing the WNPSH, the influence of the Atlantic SST, so far, is hardly explored. This article reports a new finding: enhanced relationship between the tropical Atlantic (TA)-SST and the WNPSH after the early 1980s. Regression study suggests that the warm TA-SST produced a zonally overturning circulation anomaly, with descending over the equatorial central Pacific and ascending over the tropical Atlantic/eastern Pacific. The anomalous descending over the equatorial central Pacific likely induced low-level anticyclonic anomaly to the west and therefore enhanced the WNPSH. One implication of this new finding is for predictability. The well-known "spring predictability barrier" (i.e., the influence of El Niño-Southern Oscillation (ENSO) falls dramatically during boreal spring) does not apply to the TA-SST/WNPSH relationship. The TA-SST shows consistently high correlation starting from boreal spring when the ENSO influence continues declining. The TA-SST extends the predictability of the WNPSH in boreal summer approximately one season earlier to boreal spring.
This paper reports a new finding and related mechanism: the forcing effect of the tropical Atlantic (TA) sea surface temperature (SST) on the atmosphere–ocean coupling in the western North Pacific (WNP) and northern Indian Ocean (NIO). Since the early 1980s, the TA SST has increased and, notably, exhibited an enhanced interannual statistical relationship with the WNP subtropical high and NIO SST in boreal summer. Empirical diagnostics reveal the following spatial pattern linking the TA SST and the atmosphere–ocean coupling in the Pacific and Indian Ocean: 1) a cyclonic (anticyclonic) circulation pair straddling the equator over the eastern Pacific, 2) an anticyclonic (cyclonic) circulation pair straddling the equator in the WNP and Indian Ocean, 3) overturning circulation with ascending (descending) and descending (ascending) anomalies over the TA and tropical western Pacific, respectively, and 4) positive (negative) SST anomaly in the TA and NIO. The characteristics of this pattern are consistent with those of a WNP–NIO coupling pattern identified in a previous study. Empirical diagnostics and numerical simulations indicate that the TA SST serves as a forcing to induce low-level divergence and streamfunction anomalies in the Indian Ocean and the western Pacific. The latter in turn induces anomalous heat storage in the NIO and enhances the WNP–NIO coupling system, which is an intrinsic pattern engendered by the atmosphere–ocean interaction in the region. Without the remote influence of the TA SST forcing, the WNP–NIO coupling pattern and its impacts on the summer monsoon and TC variability in South Asia, East Asia, and the WNP would be considerably less significant than observed.
The western North Pacific (WNP) is a region where tropical cyclones (TCs) occur most frequently. However, no TCs occurred in August, climatologically the month of most frequent TC genesis, in 2014. Such absence of TC activity in August was approximately 3σ below the climatological mean (5.1) and occurred for the first time during 1945–2014. This study investigates the large‐scale factors responsible for the absence. While July–August 2014 was a period of weak El Niño condition, rainfall in the eastern North Pacific was unusually high, exceeding the 95th percentile for the period 1979–2014. We demonstrate that the unusually positive sea surface temperature anomaly in the eastern North Pacific (ENP) resulted in such high rainfall amount. Diabatic heating associated with the unusual rainfall resulted in a Walker circulation anomaly in the Pacific, which in turn led to anomalous downward motion in the West Pacific and suppressed TC genesis in the WNP. Moreover, an eastward propagating intraseasonal oscillation (ISO) was identified in the Indo‐Pacific in August. The dry‐phase ISO induced subsidence anomaly in the WNP. The combination of the dry ISO phase in the WNP and a west‐east overturning circulation anomaly triggered by the warm sea surface temperature in the ENP led to extremely dry and warm conditions and the absence of TCs in the WNP. Both empirical diagnostics and numerical simulations confirm this result.
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