Seasonal and interannual variability of sea temperature and surface heat fluxes in the northwest Atlantic.

• Main Author: Umoh, Joseph U.
• Other Authors: Ph.D.
• Format: Text
• Language: English
• Published: Dalhousie University 2014
• Subjects: Physical Oceanography; Canada; North Atlantic; Northwest Atlantic
• Online Access: http://hdl.handle.net/10222/55341

The Northwest Atlantic is a region of extremely strong sea surface temperature (SST) variability. The seasonal cycle in SST of about 16$\sp\circ$C, and the anomalies about this seasonal cycle that occasionally exceed 5$\sp\circ$C, are the largest in the North Atlantic. The purpose of this study is to explain the seasonal and interannual variability of water temperature in this region. Analysis of hydrographic data from the Scotian Shelf and Slope shows that the seasonal temperature signal is confined to the top 75 m of the water column. As a first step in the development of a model to explain the seasonal temperature variability, the heat budget is examined. The most important term in the long-term mean heat budget is horizontal advection, with a contribution of about $-$40W m$\sp{-2}$, and is almost exactly balanced by the combined effect of the net surface heat flux ($Q=25$W m$\sp{-2}$), horizontal mixing (11W m$\sp{-2}$) and vertical diffusion (6W m$\sp{-2}$). On the seasonal time scale, about 85% of the local rate of heat storage ($\partial H$/$\partial t$) can be accounted for by Q. Horizontal advection and Q together explain about 99% of $\partial H$/$\partial t$. Motivated by the results of the heat budget, the seasonal cycle is modelled by a modified 1-D heat diffusion equation: $\partial T$/$\partial t$ = $\partial$/$\partial z(K\sb{v}\partial T$/$\partial z)$ + $\Gamma(z,t)$, where $\Gamma(z,t)$ is dominated by horizontal advection. Considerable attention is paid to the estimation of $K\sb{v}$, an extremely important parameter in the model. Three methods of estimating $K\sb{v}(z,t)$, on a monthly time scale, are presented. In two of the methods, $K\sb{v}(z,t)$ is assumed to vary with density stratification, and hence with depth and time, as $K\sb{v}(z,t)=K\sb{\rm o}(1+\alpha N\sp{p})\sp{-1}$. An important contribution of this work is providing an effective way of determining the parameters p, $\alpha$ and $K\sb{\rm o}$. The attraction of this approach is that it does not allow negative diffusivities. $T(z,t)$ predicted using the seasonal varying $K\sb{v}(z,t)$ compares much more favorably with observations than $T(z,t)$ calculated using the best constant $K\sb{v}$. This emphasizes the importance of allowing $K\sb{v}(z,t)$ to vary with depth and time. The modified 1-D heat diffusion model is also used to study the origin of the cold intermediate layer (CIL), and it is shown that both local heating and horizontal advection of cold water are needed to maintain the permanent CIL on the Scotian Shelf. Focusing now on the monthly anomalies (i.e. variations about the seasonal cycle), over 90% of $Q\sp\prime$ may be accounted for by the latent and sensible heat flux. Overall, $Q\sp\prime$ has a smaller spatial scale and shorter time scale than SST anomalies (SSTAs). Previous studies have hypothesized that SSTAs are the result of stochastic forcing by $Q\sp\prime$. For the first time, this study has quantified through numerical modelling, the contribution of $Q\sp\prime$ to the evolution of the SSTAs. The results show that $Q\sp\prime$ is not the primary cause of the interannual variability of SSTAs in the Northwest Atlantic, and suggest that the primary cause lies in the ocean. Empirical modelling indicates that, on the whole, the SSTAs originate from variations in the top 50 m of the water column. It is speculated that fluctuations in the transport of water from the Gulf of St. Lawrence and the inshore Labrador Current are the dominant cause of the interannual variability of SSTA in the Northwest Atlantic. Thesis (Ph.D.)--Dalhousie University (Canada), 1992.