SURFACE CLIMATE VARIATIONS OVER THE NORTH-ATLANTIC OCEAN DURING WINTER - 1900-1989

The low-frequency variability of the surface climate over the North Atlantic during winter is described, using 90 years of weather observations from the Comprehensive Ocean-Atmosphere Data Set. Results are based on empirical orthogonal function analysis of four components of the climate system: sea surface temperature (SST), air temperature, wind, and sea level pressure. An important mode of variability of the wintertime surface climate over the North Atlantic during this century is characterized by a dipole pattern in SSTs and surface air temperatures, with anomalies of one sip east of Newfoundland, and anomalies of the opposite polarity off the southeast coast of the United States. Wind fluctuations occur locally over the regions of large surface temperature anomalies, with stronger-than-normal winds overlying cooler-than-normal SSTs. This mode exhibits variability on quasi-decadal and biennial time scales. The decadal fluctuations are irregular in length, averaging approximately 9 years before 1945 and approximately 12 years afterward. There does not appear to be any difference between the wind-SST relationships on the different time scales. The decadal fluctuations in SSTs east of Newfoundland are closely linked to decadal variations in sea ice in the Labrador Sea, with periods of greater than normal sea ice extent preceding by approximately 2 years periods of colder-than-normal SSTs east of Newfoundland. Another dominant mode of variability is associated with the global surface warming trend during the 1920s and 1930s. The patterns of SST and air temperature change between 1900-29 and 1939-68 indicate that the warming was concentrated along the Gulf Stream east of Cape Hatteras. Warming also occurred over the Greenland Sea and the eastern subtropical Atlantic. The warming trend was accompanied by a decrease in the strength of the basin-scale atmospheric circulation (negative phase of the North Atlantic Oscillation). In marked contrast to the dipole pattern, the wind changes occurred downstream of the largest SST anomalies; hence, the gradual surface warming along the Gulf Stream may have been a result of altered ocean currents rather than local wind forcing.