A heat budget for the Northern California Shelf during CODE 2

Abstract

Moored current, water temperature, and meteorological observations made during the summer of 1982 as part of the Coastal Ocean Dynamics Experiment (CODE 2) are used to study both the mean and fluctuating heat budget for the northern California shelf. The volume considered extends from the sea surface to the bottom, from the coast to near the shelf break (13 to 23 km offshore), and 56 km alongshore. The largest terms in the mean heat budget over a 105‐day period spanning the summer season are a surface heat flux of 1.6 × 1011 W into the ocean and an offshore heat flux of 2.4 × 1011 W due primarily to an offshore flow of relatively warm water in the upper 30 m. Cooling due to an alongshore heat flux divergence is 0.6 × 1011 W. Fluctuations with time scales from days to weeks are large relative to the means. The correlation between observed changes in the heat content of the volume and the changes predicted by estimating the heat fluxes into and out of the volume is 0.82. The primary balance in the fluctuating heat budget is heating and cooling of the volume in response to on/offshore fluxes of heat in the upper 30 m. A simple two‐dimensional Ekman model reproduces the major features of the observed offshore heat flux fluctuations, correlation 0.82, indicating that this offshore heat flux is consistent with wind‐driven upwelling. However, at alongshore scales less than 56 km there is considerable alongshore variability in the cross‐shelf heat flux, particularly over the outer shelf. This spatial variability is presumably due to eddy motions. Standard deviations of the alongshore heat flux are about 50% of the cross‐shelf heat flux. Two modes of alongshore heat flux variability are observed. The dominant mode is characterized by large equatorward heat fluxes over the outer shelf during the early part of the upwelling season. This mode is associated with strong vertical shears in alongshore velocity in the vicinity of the upwelling front which results in equatorward flow of the warm surface water relative to the colder deep water. The second mode of alongshore heat flux variability is concentrated over the inner half of the shelf and is related to the wind such that poleward heat fluxes occur in the northern portion of the volume during wind relaxations (periods of weak winds). The presence of an anomalous lens of warm water over the shelf during CODE 2 is also examined.

Cross-shelf circulation and momentum and heat balances over the inner continental shelf near Martha's Vineyard, Massachusetts

“…The substantial cooling of the inner shelf during summer observed here is similar to the summertime coastal upwelling on the West Coast of the United States in that the dominant terms in the heat balance on time scales of months are the surface heat flux Q s and the cross-shelf advective heat flux (e.g., Lentz, 1987). Nevertheless, there are important differences between the persistent upwelling observed here at the Node, and the summertime upwelling on the West Coast that is driven by the along-shelf wind stress.…”

“…The derivation closely follows those of Lentz (1987), Lentz and Chapman (1989), Dever and Lentz (1994), and Austin (1999), but is extended to include the effects of surface gravity waves. The additional temperature balance terms due to the waves are equivalent to those in the tracer transport equation found by McWilliams et al (2004).…”

“…On shorter time scales (days to weeks), advective transport of heat is important both over the inner shelf and at mid-shelf. Cross-shelf and along-shelf advective heat flux divergences are important in the synoptic heat budget in the Middle Atlantic Bight (Austin, 1999), in the South Atlantic Bight , and off northern California (Lentz, 1987;Lentz and Chapman, 1989;Dever and Lentz, 1994).…”

“…On the West Coast of North America, cross-shelf advection of heat is an important cooling mechanism for the shelf waters on long time scales in summer, due to the prevailing upwelling-favorable wind stress (Halliwell and Allen, 1987;Strub et al, 1987) and the resulting persistent coastal upwelling circulation (e.g., Lentz, 1987;Lentz and Chapman, 1989;Bryden et al, 1980;Dever and Lentz, 1994). In contrast, the wind stress in the Middle Atlantic Bight does not have a strong mean and the winds are generally weak in summer (Saunders, 1977;Lentz, 2007b), so the upwelling circulation at mid-shelf is weak in the Middle Atlantic Bight compared to on the West Coast.…”

Mechanisms for nutrient delivery to the inner shelf: Observations from the Santa Barbara Channel

“…However, the cross-isobath velocity profiles measured at the shallow moorings are not representative of the net onshore transport of nutrients on the scale of a kelp forest. This is due, in part, to overlapping surface and bottom Ekman layers, which negate cross-shelf transport in the inner shelf (Austin and Lentz 2002), but it is also due to the fact that subinertial cross-isobath currents are typically weak, and small errors in the determination of these currents will create large errors in the direct assessment of the net onshore transport of nutrients (e.g., Lentz 1987;Rudnick and Davis 1988). Hence, a direct assessment of the flux of DIN to the kelp forest as the product of the advective velocity and DIN concentration was not appropriate.…”