Winter North Atlantic low cloud anomalies associated with the Northern Hemisphere Annular Mode

Ship observations of low cloud anomalies over the Atlantic Ocean from Jan. 1956 through Mar. 1995 are linearly related to the Northern Hemisphere annular mode (NAM) and to the corresponding anomalies of surface pressure, surface wind and sea surface temperature (SST). Positive NAM anomalies are associated with coherent cloud anomaly patterns: a band of increased bad weather stratiform cloud along the storm track between 40 degreesN and 50 degreesN, positive anomalies of precipitating stratiform and convective clouds together with negative anomalies of fair weather stratiform clouds north of this band, and negative anomalies of cumulonimbus with weak increases in fair weather stratiform clouds south of this band. Near 10 degreesN, positive NAM anomalies are associated with increased trade winds and increases in both fair weather and bad weather stratiform clouds, resulting in an increase of total low cloud amount. Low cloud anomalies are also related to anomalies in SST and surface wind divergence.