On the nature of decadal anomalies in North Atlantic sea surface temperature

Abstract

North Atlantic sea surface temperature data from the Comprehensive Ocean‐Atmosphere Data Set were used to investigate the behavior of temperature anomalies on multiple‐year timescales during the period 1948–1992. Monthly anomaly time series for each 2° square from the equator to 70°N were low‐pass filtered at 4 years and normalized by the local standard deviation. Attention is focused on the extreme events, the upper and lower deciles, of the anomaly time series. A 45‐year sequence of January maps shows the already familiar phenomena of generally cold conditions prior to 1951, a long warm interval from 1951 through 1967, and again a cold period from 1968 through 1977. The years 1978 through 1982 were largely devoid of persistent strong anomalies, but moderate cold conditions returned during 1983–1986. Warm conditions dominated the North Atlantic from 1987 onward. Within these thermal epochs, however, a total of five cold anomaly features and nine warm anomaly features have been identified. These features have individual lifetimes of 3 to 10 years. A typical size is 20° of latitude or longitude, but they range from barely detectable to spanning the width of the basin, the latter especially in lower latitudes. Most of the anomalies move long distances along certain preferred paths. These paths generally follow the routes of the subarctic and subtropical gyres. Anomalies originating off North America along the boundary between the gyres move northeastward toward the Norwegian Sea along the approximate route of the North Atlantic Current. Midlatitude anomalies originating at the eastern boundary tend to spread both northward and southward along the coast. The speed of these movements (1–3 km d−1) is generally less than the expected speed of the near‐surface ocean circulation. Simple ideas about the effects of beta dynamics and air‐sea heat exchanges are briefly considered but do not provide a satisfactory explanation for the movements of the anomalies. The long timescale of these extreme events and the continuity of their movements suggest a useful degree of predictability of sea surface temperature based on persistence and propagation of features.

Midwestern streamflow, precipitation, and atmospheric vorticity influenced by pacific sea-surface temperatures and total solar-irradiance variations

“…The average speed of this transport could be approximately three years, the time lag that Favorite and McLain (1973) observed in the movement of a warm-temperature anomaly traveling the same course. Temperature and salinity anomalies have also been tracked over a period of several years in the Atlantic Ocean (Hansen and Bezdek, 1996). A comparison between the SST in the area 130°-140°E and 20°-30°N (Area 1) with the SST in Area 2 is significant (R = 0.43) from 1970 to 2000 when the SST for Area 1 is lagged three years (Figure 4(E)).…”

North Atlantic climate variability: phenomena, impacts and mechanisms

“…A large freshening event (dubbed the Great Salinity Anomaly; GSA) occurred in the subpolar gyre in the early 1970s (Dickson et al, 1988;Levitus, 1989). Observations suggest it may have been linked to coherent changes in the overlying atmospheric circulation and an extreme manifestation of quasi-decadal fluctuations (Kushnir, 1994;Hansen and Bezdek, 1996;Reverdin et al, 1997;Belkin et al, 1998).…”

“…Gu and Philander (1997) hypothesize that exchange between extratropical and tropical water masses through thermocline ventilation can induce decadal changes in the depth of the equatorial thermocline in the Pacific, which in turn can modify equatorial wave speeds and give rise to decadal SST variability. Hansen and Bezdek (1996) presented observational evidence of anomalous SST variability on decadal scales extending into the tropical Atlantic along the preferred path of the subtropical gyre. Malanotte-Rizzoli et al (2000) discuss and model water-mass pathways between the subtropics and the tropics in the Atlantic.…”

“…They argue that such behaviour is likely to be a consequence of advection of heat by anomalous ocean currents and possible feedback on the overlying atmosphere. Spreading of SST anomalies along the path of the Gulf Stream and North Atlantic Current (see Figure 4, modified from Sutton and Allen, 1997-see also the observations of Hansen and Bezdek, 1996 and the modelling study of Visbeck et al, 1998 andKrahmann et al, 2001). Winter SST anomalies born in the western subtropical gyre appear to propagate eastwards with a transit time of a decade or so.…”

The Turbulent Oscillator: A Mechanism of Low-Frequency Variability of the Wind-Driven Ocean Gyres

“…Although significant evidence of the decadal and longer variability of the ocean is found in the midlatitude sea surface temperature (e.g., Kushnir 1994;Hansen and Bezdek 1996), hydrography (Qiu and Joyce 1992), and altimetry (Qiu 2003;Qiu and Chen 2005) data, the underlying dynamical mechanisms of this variability remain to be established. Present theoretical understanding is based on the null hypothesis: that the ocean integrates high-frequency atmospheric forcing and responds in terms of the red variability spectrum (Hasselman 1976).…”