Interannual Variability in Low Stratiform Cloud Amount over the Summertime North Pacific in Terms of Cloud Types

Using long-term (1958–2008) ship-based cloud observations and reanalysis data, interannual variability in the low stratiform cloud (LSC) amount of stratocumulus (Sc), stratus (St), and sky-obscuring fog (FOG) is examined over the summertime North Pacific. The correlation between the LSC amount and the estimated inversion strength is positive but relatively weak, compared with the well-known linear relationship for their seasonal variabilities. This reflects the regional contrast: the correlations are stronger in the southeastern North Pacific (SE NP) and weaker in the northwestern North Pacific (NW NP). Regarding the LSC types, variations in Sc amount are large over the SE NP and correlated with the inferred capping inversion strength. Variations in FOG amount are large over the NW NP and correlated with the inferred surface-based inversion strength. The compensating variations between the Sc and FOG amounts result in an apparent small variation in the total LSC amount in this region. Variations in St amount are small over the whole North Pacific. The increase in the Sc amount over the SE NP is linked to the local cold sea surface temperature (SST) anomalies with a positive feedback, whereas the increase in the FOG amount over the NW NP is related to warm moist advection across the SST front caused by the anticyclonic sea level pressure anomalies over the north-central North Pacific. The former is associated with an El Niño in the preceding winter and the latter with a wavelike teleconnection pattern along the summertime Asian jet.