Low-level cloud variability over the equatorial cold tongue

An examination of interannual low-level cloud variability within and on the northern edge of the South American stratocumulus deck indicates atmospheric advection over varying SST has a prominent role on the southern edge of the equatorial cold tongue. Analysis of International Satellite Cloud Climatology Project (ISCCP) and NCEP reanalysis data shows that, over the equatorial region, lower-tropospheric stability (LTS) is the dominant factor associated with low-level cloud interannual anomalies. However, an area of pronounced cloud variability located on the southern side of the equatorial cold tongue also exhibits particularly strong negative correlation between cloud amount and atmospheric advection over varying SST. In this region, low-level cloud fraction and cloud optical thickness tend to increase with anomalous cold advection, and the combination of advection plus LTS explains significantly more cloud variability than LTS alone. Although higher clouds can obscure the satellite's view of low-level clouds, that issue does not affect these results.

Covariability between temperature advection and low-level cloud properties is also seen in the Extended Edited Cloud Report Archive (EECRA), a collection of visual synoptic cloud reports made by observers on ships. Cumuliform clouds occur more frequently with anomalously cool advection, and stratiform clouds and clear sky conditions occur more frequently with strong warm advection. In the latter case, advection of warm air over cooler water serves to stabilize the surface layer, thus inhibiting the upward mixing of moisture, but in the former case, cool or weak warm advection encourages air-sea fluxes and upward convective transport of moisture. This proposed mechanism describing how temperature advection affects cloud properties through changes to surface stability and air-sea fluxes is supported by soundings and measurements of air-sea temperature difference, relative humidity, and wind speed.

Reanalysis wind and SST fields show that anomalously cold advection south of the equator occurs when the equatorial cold tongue is weak whereas anomalously warm advection occurs when the equatorial cold tongue is strong and well defined. Low-level cloud variability also projects weakly onto ENSO, with warm ENSO events having more cumuliform clouds in EECRA and greater cloud amount in ISCCP.

Examination of output from the NCAR CCSM3.0, GFDL CM2.0, and GFDL CM2.1 indicates that these three coupled atmosphere-ocean GCCMs do not simulate the observed relationship between low-level cloud amount and advection anywhere south of the equator in the eastern Pacific. This suggests that an important regional negative cloud feedback may be missing from the models in which colder equatorial SST favors less cloud fraction and cloud optical thickness due to stabilization of the near-surface layer as relatively warm air is advected over colder water.