North Atlantic Ocean control on surface heat flux on multidecadal timescales

Abstract

Nearly 50 years ago Bjerknes suggested that the character of large-scale air-sea interaction over the mid-latitude North Atlantic Ocean differs with timescales: the atmosphere was thought to drive directly most short-term--interannual--sea surface temperature (SST) variability, and the ocean to contribute significantly to long-term--multidecadal--SST and potentially atmospheric variability. Although the conjecture for short timescales is well accepted, understanding Atlantic multidecadal variability (AMV) of SST remains a challenge as a result of limited ocean observations. AMV is nonetheless of major socio-economic importance because it is linked to important climate phenomena such as Atlantic hurricane activity and Sahel rainfall, and it hinders the detection of anthropogenic signals in the North Atlantic sector. Direct evidence of the oceanic influence of AMV can only be provided by surface heat fluxes, the language of ocean-atmosphere communication. Here we provide observational evidence that in the mid-latitude North Atlantic and on timescales longer than 10 years, surface turbulent heat fluxes are indeed driven by the ocean and may force the atmosphere, whereas on shorter timescales the converse is true, thereby confirming the Bjerknes conjecture. This result, although strongest in boreal winter, is found in all seasons. Our findings suggest that the predictability of mid-latitude North Atlantic air-sea interaction could extend beyond the ocean to the climate of surrounding continents.

Oceanic control of multidecadal variability in an idealized coupled GCM

“…12, bottom row panels), the correlation is positive almost all over the small basin. At those timescales, ocean-atmosphere heat fluxes are mainly driven by the atmosphere, consistent with the stochastic forcing of the ocean in the Frankignoul and Hasselmann (1977)'s paradigm, and the results of Gulev et al (2013). However, a negative correlation is observed along the western boundary of the subpolar gyre, and spreads over a wider region as the resolution increases.…”

“…Using observational data, Gulev et al (2013) have confirmed the Bjerknes (1964) assumption for the North Atlantic sector: Q ′ is driven by ocean dynamics on long-term (multidecadal timescales), but by the atmospheric dynamics on short-term (interannual to decadal timescales). To investigate this issue in our model, we compute now the short-term signal by taking the deviation from the longterm signal, i.e.…”

“…the 10-year-smoothed temperature and heat fluxes anomalies. The 10-year smoothing window appears to be an ideal time filtering to clearly separate the oceanic and atmospheric role in the heat fluxes variability (Gulev et al 2013). On short-term (Fig.…”

A delayed oscillator model for the quasi-periodic multidecadal variability of the NAO

“…Figure 7a shows the upward surface heat flux anomalies in association with the NAT. The heat flux anomaly has a similar pattern to the NAT, reflecting that the atmosphere acts to damp the SST anomalies over multidecadal timescales (Gulev et al 2013). As a result, the low-level air temperature is affected and shows anomalous cooling and warming in the subpolar and subtropical basins, respectively (Fig.…”

Observational Advances in Estimates of Oceanic Heating

“…The dominant role of heat transport convergence in driving long-term OHC changes in the North Atlantic was also deduced through comprehensive analyses of ORA models [26,73]. These multi-decadal OHC changes exert a strong influence on surface temperature patterns such as the Atlantic Multi-decadal Oscillation [11], which subsequently drive turbulent heat fluxes at the air-sea interface and associated atmospheric responses [24].…”