North Paci®c Atmospheric and SST Anomalies in 1997: Links to ENSO? JAMES E. OVERLAND 1,* , NICHOLAS A. BOND 2 AND JENNIFER MILETTA ADAMS 2 1 NOAA, Paci®c Marine Environmental Laboratory, Seattle, WA 98115, USA 2 University of Washington, JISAO, Seattle, WA 98195, USA ABSTRACT In the summer of 1997, positive sea surface tempera- ture anomalies (SSTA) extended across the Gulf of Alaska (GOA) and into the eastern Bering Sea (EBS). The SSTA in the EBS are at least in part due to atmospheric causes. Anomalously high 925 mb tem- peratures and 700 mb geopotential heights and low 925 mb relative humidities, and hence decreased low cloud cover, occurred over the region during April to August. This resulted in enhanced warming of the GOA and EBS owing to increased insolation. The anomalous solar heating was particularly great in the EBS from mid-May to mid-July. The pattern of positive 700 mb height anomalies for April to August 1997 is similar to its counterpart formed by compos- iting the April to August anomalies that occurred during previous El Nin Ä os. The positive equatorial SSTA for 1997 was one of the strongest on record for summer months. The existence of an equatorial/high- latitude connection and the strength of the summer equatorial SSTA in 1997 suggest an El Nin Ä o/Southern Oscillation (ENSO) in¯uence in the GOA and EBS. The warming in the Bering Sea and North Paci®c during summer 1997 appears to be due in part to the con¯uence of three meteorological factors which favoured clear skies. There was not only an El Nin Äo in¯uence, but also a decadal trend toward higher 700 mb geopotential heights and a particularly strong blocking ridge weather pattern over the Gulf of Alaska in May. INTRODUCTION A major warming occurred in the eastern Bering Sea (EBS) during summer 1997, as noted in other papers in this issue (Napp and Hunt, 2001). The intent of this paper is to investigate whether the observed anomalies in the EBS occurred over a larger region and to investigate the relationship of the sea surface tempera- ture anomalies (SSTA) to concurrent meteorological anomalies. Our hypotheses are that the observed oceanographic anomalies in the EBS were in¯uenced, at least in part, by large-scale atmospheric processes, and that these processes have a connection to El Nin Äo/ Southern Oscillation (ENSO). Most studies of North Paci®c weather and climate variability concentrate on winter months (Hurrell, 1996; Mantua et al., 1997), but from the ®sheries standpoint, solar heating and wind mixing in the spring months April and May are crucial for primary productivity. Sustained temperature anomalies during the summer also in¯uence higher trophic levels. As we shall investigate, the spring and summer have their own modes of atmospheric variability. In winter, low- level air temperature anomalies at mid- and subarctic latitudes over the ocean are associated with tempera- ture advection. In spring and summer, direct heating from insolation contributes substantially to SSTA. The EBS is remote from tropical in¯uences by direct oceanographic connection; however, indirect connections exist through the atmosphere. Telecon- nections between tropical SSTA and the midlatitude atmosphere have been considered primarily during the winter because that is when the atmospheric structure is most suitable for poleward propagation of Rossby waves. Recent studies suggest that there can be a midlatitude response to tropical SSTA in all seasons (Livezey et al., 1997; Trenberth et al., 1998). Because the midlatitude background circulation differs between winter and summer, the nature of the midla- titude response to ENSO also varies seasonally. The spring/summer in¯uence of ENSO at midlatitudes is believed to be related to an equatorward extension of the midlatitude Paci®c jet stream and changes in mean ¯ow/eddy feedback (Higgins and Mo, 1997; Straus and Shukla, 1997). *Correspondence. e-mail: overland@pmel.noaa.gov Received 15 March 1999 Revised version accepted 25 June 1999 FISHERIES OCEANOGRAPHY Fish. Oceanogr. 10:1, 69±80, 2001 Ó 2001 Blackwell Science Ltd. 69