On the changing nature of the regional connection between the North Atlantic Oscillation and sea surface temperature

Abstract

Evidence is presented that the correlation between the North Atlantic Oscillation (NAO), in terms of the NAO index, and the North Atlantic sea surface temperature (SST) is not stationary. This is inferred from both reanalysis data from the U.S. National Centers for Environmental Prediction/National Center for Atmospheric Research (NCEP/NCAR) and the Kaplan sea surface temperature data set. Two phases of winterly North Atlantic atmosphere‐ocean covariability are identified by means of linear regression and correlation analysis. During the recent decades since the late 1960s/early 1970s and during the first 3 decades of the twentieth century, the North Atlantic SST is strongly correlated to the regional atmospheric circulation in the North Atlantic sector, i.e., the North Atlantic Oscillation. During these periods the NAO index, defined as the difference of normalized sea level pressures on the Azores and Iceland, is characterized by pronounced decadal variability and by mainly positive values. In contrast, the NAO index is only weakly correlated to the North Atlantic SST from the 1930s to the early 1960s, when the NAO index is characterized by weak decadal variability. Remote influences, in particular from the tropical Pacific region, become important, especially for the SST in the western tropical North Atlantic.

The North Atlantic thermohaline circulation simulated by the GISS climate model during 1970–99

“…He suggested that year-toyear SST fluctuations arise from heat fluxes at the atmosphere-ocean interface; on longer timescales, the changes in the ocean circulation play a primary role in SST variability. The conclusions of Bjerknes (1964) were confirmed by many later studies (Cayan, 1992a(Cayan, , 1992bDeser and Blackmon, 1993;Kushnir, 1994;Halliwell, 1998;Walter and Graf, 2002;Wu and Gordon, 2002).…”

“…Simulated regional surface air temperature and precipitation patterns are in agreement with large-scale patterns typical for the positive NAO trend (Hurrell, 1995;Thompson and Wallace, 2001). Previously reported analyses of the North Atlantic SST (Cayan, 1992a(Cayan, , 1992bDeser and Blackmon, 1993;Kushnir, 1994;Dickson et al, 1996;Grötzner et al, 1998;Delworth and Greatbatch, 2000;Curry and McCartney, 2001;Walter and Graf, 2002;Brauch and Gerdes, 2005) have shown that on interannual timescales the ocean surface over the North Atlantic is strongly influenced by the NAO, especially in the winter. In fact, the ocean surface temperature and salinity anomalies develop as a time-integrated response to persistent NAO air-sea heat and freshwater flux anomalies.…”

“…This strong cooling in the North Atlantic surface ocean tends to cancel out at least part of the effect of the reduced surface densities from previously described warming. It has been noted in earlier studies that, on interannual timescales, surface ocean temperature and salinity anomalies develop directly in response to NAO airsea heat and moisture flux anomalies (Bjerknes, 1964;Cayan, 1992aCayan, , 1992bDeser and Blackmon, 1993;Kushnir, 1994;Walter and Graf, 2002). On longer timescales of several decades, surface ocean temperature and salinity anomalies are primarily driven by changes in meridional ocean transports of heat and salt (Halliwell, 1998;Wu and Gordon, 2002).…”

On Multidecadal and Quasi-Decadal North Atlantic Variability

“…The covariability of the tripole SST pattern and the NAO has been shown to vary on multidecadal time scales by Walter and Graf (2002). Their correlation weakens from the 1930s to the early 1960s, and increases during the last decades of the century.…”

“…Their correlation weakens from the 1930s to the early 1960s, and increases during the last decades of the century. As noted by Walter and Graf (2002), these periods correspond respectively to a warm and a cold phase of the multidecadal variability. During the latter, the NAO was characterized by enhanced variability on the quasidecadal time scale, matching the amplitude modulation of the quasi-decadal SST oscillation in Fig.…”