Report of RMS Discussion Meeting 21 June 2000: “Some aspects of the general circulation of the atmosphere” ‐ Presidential Address

Abstract

Abstract:The extent to which the North Atlantic Oscillation (NAO) is in¯uenced by changes in the ocean state is an issue that has attracted much recent attention. Although there have been counter claims, the weight of evidence clearly suggests that forcing by the ocean of year-to-year changes in the NAO is a weak in¯uence by comparison with atmospheric internal variability. The NAO is thus very different in character to the Southern Oscillation (SO), and its predictabilityÐat least on seasonal-to-interannual timescalesÐis almost certainly much lower. Although weak, the in¯uence of the ocean on the NAO is not negligible. In a previous study we found that wintertime North Atlantic climate, including the NAO, was signi®cantly in¯uenced by a tripole pattern of North Atlantic SST anomalies. Here we report the results of experiments to further elucidate the nature of this in¯uence. We show that the tripole pattern induces a signi®cant response both in the tropical Atlantic and at mid-to-high latitudes. The low latitude response is forced by the low latitude SST anomalies, but the high latitude response is in¯uenced by the extratropical SST anomalies as well as those in the tropics. Furthermore, we ®nd evidence of nonlinear interaction between the in¯uence of the tropical and extratropical SST anomalies. Lastly, we investigate the feedback from the atmosphere onto the SST tripole. We ®nd that the expected negative feedback is signi®cantly modi®ed at low latitudes by the dynamical response of the atmosphere. *

The role of Atlantic Ocean-atmosphere coupling in affecting North Atlantic oscillation variability

“…In support of the mechanisms proposed in sections 3.2 and 3.3, many AGCM studies suggest a positive feedback between the SST tripole as a whole and the NAO [Venzke et al, 1999;Rodwell et al, 1999;Watanabe and Kimoto, 2000b;Cassou and Terray, 2001a;Sutton et al, 2001;Terray and Cassou, submitted;Peng et al, 2002], although not all [Robertson, 2001]. Analysis of the observational record should thus at least provide a hint for such a feedback.…”

“…It is worth emphasizing that aquaplanet GCMs typically lack such interactions [e.g., Ting, 1991], and even GCMs with zonal asymmetries do not systematically produce equivalent barotropic structures in response to middle latitude SST anomalies [Kushnir and Held, 1996]. Accordingly, predicted atmospheric responses to an anomalous meridional SST gradient along 45ºN can vary between AGCMs, although a strengthening of the westerlies in response to a strengthening of the meridional SST gradient along 45ºN appears to be robust [e.g., Watanabe and Kimoto, 200b;Sutton et al, 2001;Peng et al, 2002]. This is consistent with a positive feedback on the NAO from the extra-tropical part of the SST tripole, but model studies suggest the feedback amplitude is weak compared to intrinsic variability.…”

“…Second, the interaction of the propagating Rossby wavetrain with middle latitude transient eddies ultimately shape the net response of the Northern Hemisphere to the tropical forcing [Hoskins and Sardeshmukh, 1987;Held et al, 1989;Hoerling and Ting, 1994]. An illustration of this in the context of NAO/tropical Atlantic interactions can be found in, for instance, Watanabe and Kimoto [1999], Sutton et al [2001] and Terray and Cassou [submitted].…”

Inter-hemispheric Decadal Variations in SST, Surface Wind, Heat Flux and Cloud Cover over the Atlantic Ocean.

“…On decadal time scales, observational studies suggest that SST variability may be correlated over the whole Atlantic basin from the South Atlantic to Greenland (Xie and Tanimoto 1998;Rajagopalan et al 1998;Tourre et al 1999;Tanimoto and Xie 1999). During boreal winterspring seasons, as part of this so-called panAtlantic decadal oscillation (PADO), the tropical dipole, with a strong cross-equatorial SST gradient that forces the ITCZ to move in the meridional direction, is apparently involved in a two-way tropical-extratropical interaction with the NAO (North Atlantic Oscillation) as model studies suggest (Xie and Tanimoto 1998;Okumura et al 2001;Sutton et al 2001). When a northward cross-equatorial SST gradient occurs, sea level pressure (SLP) anomaly field shows a southward gradient, whose centers of action are displaced poleward of maximum tropical SSTAs and found at 30 (Tanimoto and Xie 1999).…”

Relationship between monthly rainfall in northwest Iberian Peninsula and North Atlantic sea surface temperature

“…It has been suggested that analysis of North Atlantic climate variability must take into account the role of the whole Tropics, including both Pacific and Indian Ocean sectors Hurrell et al, 2004). On the other hand, the SSTA in March and April that correlates with rainfall in May, suggest a strengthening of the westerlies in response to a strengthening of the meridional SST gradients that has been seen in wintertime (Sutton et al, 2001).…”