CGD's Dr. Stephen Yeager

Abstract

The interannual variability in upper-ocean (0-400 m) temperature and governing mechanisms for the period 1968-97 are quantified from a global ocean hindcast simulation driven by atmospheric reanalysis and satellite data products. The unconstrained simulation exhibits considerable skill in replicating the observed interannual variability in vertically integrated heat content estimated from hydrographic data and monthly satellite sea surface temperature and sea surface height data. Globally, the most significant interannual variability modes arise from El Niño-Southern Oscillation and the Indian Ocean zonal mode, with substantial extension beyond the Tropics into the midlatitudes. In the well-stratified Tropics and subtropics, net annual heat storage variability is driven predominately by the convergence of the advective heat transport, mostly reflecting velocity anomalies times the mean temperature field. Vertical velocity variability is caused by remote wind forcing, and subsurface temperature anomalies are governed mostly by isopycnal displacements (heave). The dynamics at mid- to high latitudes are qualitatively different and vary regionally. Interannual temperature variability is more coherent with depth because of deep winter mixing and variations in western boundary currents and the Antarctic Circumpolar Current that span the upper thermocline. Net annual heat storage variability is forced by a mixture of local air-sea heat fluxes and the convergence of the advective heat transport, the latter resulting from both velocity and temperature anomalies. Also, density-compensated temperature changes on isopycnal surfaces (spice) are quantitatively significant.

Figure caption: Spatial maps of the first empirical orthogonal function (EOF) mode for the model (top panel) and Reynolds-Smith (R/S) (middle panel) monthly SST anomalies (1982-1997) after removal of the average seasonal cycle. The spatial pattern correlation is 0.96. The contour interval is 0.2deg C, and the variances associated with each EOF are given as percentages of the respective total variances. The time series of the Reynolds-Smith (red) and model (black) first mode principal components are shown in the bottom panel.

Support: NOAA Office of Global Programs ACCP Grant NA86GP0290, NSF Grant OCE96-33681, and the WHOI Ocean and Climate Change Institute.

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Yeager, S. G., and W. G. Large, 2007: Observational Evidence of Winter Spice Injection, J. Phys. Oceanogr., accepted.

Abstract

Temperature and salinity (T/S) profiles from the global array of Argo floats support the existence of spice formation regions in the subtropics of each ocean basin where large, destabilizing vertical salinity gradients coincide with weak stratification in winter. In these characteristic regions, convective boundary layer mixing generates a strongly density-compensated (SDC) layer at the base of the well-mixed layer. The degree of density compensation of the T/S gradients of an upper ocean water column is quantified using a bulk vertical Turner angle (T_uB) between the surface and upper pycnocline. The winter generation of the SDC layer in spice formation zones is clearly seen in Argo data as a large amplitude seasonal cycle of T_uB in regions of the subtropical oceans characterized by high mean T_uB. In formation regions, Argo floats provide ample evidence of large, abrupt spice injection (T/S increase on subducted isopycnals due to vertical mixing) associated with the winter increase in T_uB. A simple conceptual model of the spice injection mechanism is presented which is based on known behavior of convective boundary layers and supported by numerical model results. It suggests that penetrative convective mixing of a partially density-compensated water column will enhance the Turner angle within a transition layer between the mixed layer and the upper pycnocline, generating seasonal T/S increase on density surfaces below the mixed layer. Observations are consistent with this hypothesis.

In OGCMs, regions showing a high mean and seasonal amplitude of T_uB are also the sources of significant interannual spice variability in the permanent pycnocline. Decadal changes in the North Pacific of a model hindcast simulation show qualitative resemblance to the observed multi-year time series from the Hawaii Ocean Time-series (HOT) ALOHA station. Modelled pycnocline variations near Hawaii can be linked to high T_uB seasonality and winter spice injection within a formation region upstream of ALOHA, suggesting that spice injection may explain the origins of observed large, interannual variations on isopycnals in the ocean interior.

Figure caption: Instantantaneous T_ub (bulk 200m vertical Turner angle) measured by Argo profilers, plotted as a composite of all available float data collected between 1999-2006 from months July, August, and September (top panel). Seasonal mean T_ub computed from the POP ocean model hindcast (bottom panel).

Support: NSF and NASA contract X06AD58G.