An Extratropical Air–Sea Interaction over the North Pacific in Association with a Preceding El Niño Episode in Early Summer

Using data for the month of June from 1951 through 2000, this study examined the air–sea interactions over the North Pacific after El Niño matured during the preceding fall season. The principal findings of this work are the following: 1) a coherent region near the international date line (IDL) in the extratropical North Pacific revealed an area of significant negative correlations (SNCs) between the preceding November sea surface temperature (SST) in the Niño-3 region and the June SST in the North Pacific. Also, two indexes of the June Okhotsk high show a significant positive correlation with the November SST in the Niño-3 region during the 1963–2000 period. 2) The strong southeastward wave flux from the upstream area of the Okhotsk Sea over much of the North Pacific in the midlatitudes is associated with a strong preceding El Niño event, the development of the Okhotsk high, and a negative 500-hPa geopotential height/SST anomaly around the coherent region. The stationary wave propagation plays a major part in maintaining the low SSTs in the coherent region and suppressing the northward progress of the subtropical high. This process partially bridges the connection between the central equatorial Pacific warming (CEPW) and the East Asian summer monsoon. 3) A wave train–like anomaly in the SST (tilted northwest–southeast) was established and maintained in the North Pacific during the summer of 1998. This coincided with the direction of the atmospheric Rossby wave propagation as the strong southeastward wave flux was scattered over the midlatitude North Pacific. This event provides solid evidence that Rossby wave propagation plays an important role in forming an oceanic temperature wave train in the extratropical Pacific through the barotropic process.

Dynamical Processes Related to the Appearance of the Okhotsk High during Early Midsummer

The authors identified an upper-level pressure anomaly pattern corresponding to the interannual variability of the Okhotsk high in midsummer (late July and early August) as a predominant anomaly pattern in the Northern Hemisphere, by using objectively analyzed data. According to the results of empirical orthogonal function (EOF) analyses and composite analyses, a positive pressure anomaly appeared near the tropopause over eastern Siberia in years with strong Okhotsk highs. Examination of the heat budget in the lower troposphere revealed that a negative surface temperature anomaly observed in northern Japan was brought by the advection of the climatological temperature gradient from the anomalous wind associated with the upper-level anticyclonic anomaly. It was also demonstrated that the anomaly field over Siberia does not accompany predominant vorticity forcing or Rossby wave propagation from the west with a specific phase. However, positive kinetic energy conversion from the climatological basic field to the anomaly field is estimated. The energy conversion contributes to maintaining the anomaly pattern. By the numerical experiments using a linear barotropic model, it is suggested that the upper-level anomaly pattern related to the anomalous Okhotsk high appears through the interaction with the climatological basic field, even though the external forcings are homogeneously distributed.