Air - Sea Interactions

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The Atmosphere and the Ocean form a coupled system, exchanging heat, momentum and water at the air-sea interface. In the long term, the convergence / divergence of oceanic heat transport provide source / sinks of heat for the atmosphere, and partly shape the mean climate of the Earth. Understanding and analysing to which extent the Atmosphere and the Ocean actually feel each other is the subject of large scale air - sea interactions.

The following gives a brief overview of the various aspects of air - sea interactions (using observations, theory and numerical models) studied at EAPS, with on - line references.

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Although less widely acknowledged in the public than El Nino and the Southern Oscillation, Atlantic air - sea interactions are the subject of intense research, to which the EAPS Department contributes significantly. Two strong climatic signals exist in the Atlantic:

• The North Atlantic Oscillation (NAO)
  

  
  A modulation in strength and position of the surface westerlies, with large repercussion in precipitation and surface temperature over Northern Europe (Fig. 1). A particular attention has been given to this phenomenon because it explains a significant fraction of the positive trend seen in hemispheric averaged temperature over the last 30 yrs.
  
  
  
  Figure 1: (left) Positive NAO phase (right) Negative NAO phase. In each situation, the shift in the path of the jetstream / storm track system induces a large - scale precipitation / temperature anomaly over Europe, Greenland and the east coast of the US. The transition from one phase of the NAO to the other is only weakly predictable, and occurs on monthly as well as pluri - annual timescales.
  
  
  The physics governing the NAO is highly complex, and is certainly driven, to first order, by intrinsic atmospheric processes (interaction between the jetstream, the storms and the planetary waves). A theory for understanding the fundamental nature of atmospheric modes of variability, like the NAO, has recently been proposed by Goodman and Marshall (2000), using the concept of neutral vectors. It has also been suggested that the Atlantic ocean might be involved in the seasonal (see Czaja and Frankignoul, 2000 ; (see Fig. 2) and decadal ( Goodman and Marshall, 1999 ; Marshall et al., 2000; Czaja and Marshall, 2000 ; see Fig. 3) evolution of the NAO. Indeed, positive and negative NAO phases are associated with large changes in surface heat and momentum fluxes at the air - sea interface, to which the Ocean circulation is sensitive.
  
  
  
  Figure 2: Observed patterns of sea surface temperature (SST, color) and height anomalies at 500 mb (contoured every 5m, dashed for negative - low pressure) having the strongest covariance when SST leads the height field by 4 months. The height field pattern is reminiscent of the NAO signature at 500 mb, while the SST pattern has centers of action in the midlatitudes (north of 20N) and the tropics. As discussed in Czaja and Frankignoul (2000), this large scale SST pattern can be interpreted as inducing a negative NAO phase. The strongest forcing comes from the SST anomalies north of 20N, but there is also a significant forcing of the NAO from the Tropical Atlantic.
  
  
  
  
  Figure 3: Theoretical power spectra of cross - Gulf Stream SST anomalies, from various stochastic models. The reference case (dashed curve) where no role for the ocean circulation in impacting SST is allowed shows no spectral structures on timescales longer than about 10 yrs (flat spectrum). However, as soon as the ocean circulation is active (other curves), one sees the appearance of a broad band peak near 10 - 20 yrs. This structure is even more pronounced when coupling between the Atmosphere and Ocean is allowed (thin and thick continuous curves). From Czaja and Marshall (2000).
  
  
  Links: NAO thematic website of D. Stephenson , Annular mode website .
  
  
  

• Tropical Atlantic Variability (TAV)
  

  
  Fluctuations in the strength of the trade and cross equatorial winds in the Tropical Atlantic, associated with changes in interhemispheric SST gradient (Fig. 4). These have a large impact on precipitation over Northeast Brazil, Sahel, and the Guinea coastal region.
  
  
  
  
  Figure 4: see legend above.
  
  Understanding the cause of the fluctuations shown in Fig. 4 is made difficult by the various processes potentially impacting Tropical Atlantic variability (i) the influence of the NAO (ii) the remote forcing from ENSO (iii) local air - sea interactions over the tropical Atlantic sector ( Van den Vaart and Marshall, 2000 ).
  
  
  In addition to off-equatorial SST anomalies, tropical Atlantic variability has also a component along the equator, which may be related to modulation in the strength of the oceanic subtropical cell ( Inui and Manalotte-Rizzoli, 2000 , Fig. 5).
  
  
  
  
  
  Figure 5: (left) model simulations of SST anomalies along the equator, showing the alternance of warm and cold episodes, each persisting for several years. (right) the mean subtropical cell simulated by the model (latitude - depth plot of the circulation, every Sverdrup). The fluctuations in this cell might contribute to the SST variability seen on the left. From Inui and Manalotte - Rizzoli (2000).
  
  
  Both the NAO and TAV phenomena are associated with large-scale anomalies in sea surface temperature (SST) and most of the research being done at EAPS is to understand:
  
• if the atmosphere is sensitive to the latter
  
• if the ocean circulation can impact these SST patterns, thereby potentially leaving its imprint on climate.
  
  
  
  
  People involved in air-sea interactions
  
  Professors:
  John Marshall
  Paola Manalotte - Rizzoli
  Kerry Emmanuel
  
  
  Post-docs:
  Tomoko Inui
  Arnaud Czaja
  Paul Van Den Vaart
  Fabio D'Andrea
  
  Graduate students:
  Jason Goodman