The persistence of summer sea surface temperature anomalies (SSTAs) shows strong seasonal dependence, which might prove useful as reference for seasonal to interannual climate predictions.

For the North Pacific, previous studies on SSTA persistence and large-scale air–sea interaction tended to focus on the cold season. However, there is an evident contradiction for the persistence of summer SSTAs (Fig. 1).

Fig. 1 Lag correlation of SSTAs as a function of the start month (ordinate) and lag month (abscissa): leading principal component of the North Pacific SSTA (left); SSTA over (32°N, 159°W) (right). Contour interval: 0.1; the 0.4 and 0.6 contours are thickened. (Image by IOCAS)

Associate Professor ZHAO Xia and WANG Jing from the Institute of Oceanology of the Chinese Academy of Sciences (IOCAS), and Assistant Professor YANG Guang from the First Institute of Oceanography, State Oceanic Administration, investigated the persistence of summer SSTAs in the North Pacific. They also examined its mechanism and interdecadal variability.

Their findings, published in Advances in Atmospheric Sciences, showed that summer SSTAs can persist for a long time (approximately 8–14 months) around the Kuroshio Extension (KE) region.

"This long persistence may be strongly related to atmospheric forcing, because the mixed layer is too shallow in the summer to be influenced by the anomalies at depths in the ocean," said ZHAO Xia. "The longwave radiation flux has a dominant influence. The effect of shortwave radiation flux anomalies is not significant. This result is different from that of the previous studies."

Researchers further indicated that the persistence of summer SSTAs displays pronounced interdecadal variability around the KE region, which appears very weak during 1950–1982, but becomes stronger during 1983–2016.