Geographical Variability of the First Baroclinic Rossby Radius of Deformation

Global 1° × 1° climatologies of the first baroclinic gravity-wave phase speed c¹ and the Rossby radius of deformation λ1 are computed from climatological average temperature and salinity profiles. These new atlases are compared with previously published 5° × 5° coarse resolution maps of λ₁ for the Northern Hemisphere and the South Atlantic and with a 1° × 1° fine-resolution map of c₁ for the tropical Pacific. It is concluded that the methods used in these earlier estimates yield values that are biased systematically low by 5%–15% owing to seemingly minor computational errors. Geographical variations in the new high-resolution maps of c₁ and λ₁ are discussed in terms of a WKB approximation that elucidates the effects of earth rotation, stratification, and water depth on these quantities. It is shown that the effects of temporal variations of the stratification can be neglected in the estimation of c₁ and λ₁ at any particular location in the World Ocean. This is rationalized from consideration of the WKB approximation

The importance of interocean exchange south of Africa in a numerical model

A fine resolution numerical model of the Southern Ocean (the Fine Resolution Antarctic Model (FRAM)) has been used to investigate the way in which heat is supplied to the South Atlantic. The heat budget in the model is compared with other estimates and is found to be broadly realistic. The temperature structure in the Atlantic, and therefore the meridional heat transport, depend heavily on the input of heat from the Indian Ocean via the Agulhas Retroflection region. FRAM is compared with three models which do not exhibit a significant input of heat from the Indian Ocean. These models also have a lower equatorward heat transport in the South Atlantic. Horizontal resolution affects the amount of Agulhas transfer with coarser resolution leading to lower heat transport in the Atlantic, a result which has implications for ocean models used in climate simulations

Surface cloud forcing in the East Pacific stratus deck/cold tongue/ITCZ complex

Author Posting. © American Meteorological Society 2006. This article is posted here by permission of American Meteorological Society for personal use, not for redistribution. The definitive version was published in Journal of Climate 19 (2006): 392–409, doi:10.1175/JCLI3620.1.Data from the Eastern Pacific Investigation of Climate Studies (EPIC) mooring array are used to evaluate the annual cycle of surface cloud forcing in the far eastern Pacific stratus cloud deck/cold tongue/intertropical convergence zone complex. Data include downwelling surface solar and longwave radiation from 10 EPIC-enhanced Tropical Atmosphere Ocean (TAO) moorings from 8°S, 95°W to 12°N, 95°W, and the Woods Hole Improved Meteorology (IMET) mooring in the stratus cloud deck region at 20°S, 85°W. Surface cloud forcing is defined as the observed downwelling radiation at the surface minus the clear-sky value. Solar cloud forcing and longwave cloud forcing are anticorrelated at all latitudes from 12°N to 20°S: clouds tended to reduce the downward solar radiation and to a lesser extent increase the downward longwave radiation at the surface. The relative amount of solar radiation reduction and longwave increase depends upon cloud type and varies with latitude. A statistical relationship between solar and longwave surface cloud forcing is developed for rainy and dry periods and for the full record length in six latitudinal regions: northeast tropical warm pool, ITCZ, frontal zone, cold tongue, southern, and stratus deck regions. The buoy cloud forcing observations and empirical relations are compared with the International Satellite Cloud Climatology Project (ISCCP) radiative flux data (FD) dataset and are used as benchmarks to evaluate surface cloud forcing in the NCEP Reanalysis 2 (NCEP2) and 40-yr ECMWF Re-Analysis (ERA-40). ERA-40 and NCEP2 cloud forcing (both solar and longwave) showed large discrepancies with observations, being too large in the ITCZ and equatorial regions and too weak under the stratus deck at 20°S and north to the equator during the cool season from July to December. In particular the NCEP2 cloud forcing at the equator was nearly identical to the ITCZ region and thus had significantly larger solar cloud forcing and smaller longwave cloud forcing than observed. The net result of the solar and longwave cloud forcing deviations is that there is too little radiative warming in the ITCZ and southward to 8°S during the warm season and too much radiative warming under the stratus deck at 20°S and northward to the equator during the cold season.This research was supported by grants from the NOAA Office of Global Programs, Pan American Climate Studies

Heat and Energy Balances in the Upper Ocean at 50°N, 140°W during November 1980 (STREX)

Subsurface temperature data and surface meteorological data are analyzed from thermistor chain moorings deployed near 50°N, 140°W during the Storm Transfer and Response Experiment (STREX). The upper-ocean heat and potential energy (PE) contents to 90 m are examined for an 18-day period and their changes compared to the sources and sinks of heat and turbulent kinetic energy (TKE). Heat and TKE do not balance in the vertical dimension alone. The heat content change, for example, averages −200 W m⁻² while the net cooling at the surface, estimated from bulk formulas for latent and sensible heat fluxes and radiation measurements, averaged only −86 W m⁻². Advection of heat and PE, in either the vertical or horizontal, play major roles in the budgets of this area. We describe a method for using the large-scale wind stress and SST data around the site to compute the advection in the Ekman layer and close the heat (to 23%) and TKE (to 24%) budgets. Though the heat and PE contents exhibit long-term trends, there are two marked events associated with storms on 15 and 27 November 1980 that account for much of the overall cooling and PE change. The advection estimates mimic the episodic character of the heat and PE contents and are clearly important on the short, storm time scale. The relative contributions of horizontal and vertical advection are quite different for the two storms, showing that the upper-ocean response very much depends on the proximity and orientation of the storm as it moves past the observational site. The TKE budget is complex, and some terms can only be estimated by uncertain parameterizations so that the relative importance of surface production, shear production, and advection is unclear. Still, the fact emerges that mixed layer deepening is dominated by wind-forcing even during the season of significant cooling

Spatial Fluctuations North of the Hawaiian Ridge

A closely spaced hydrographic section from Oabu, Hawaii to 28°N, 152°W and then north along 152°W shows strong eddy or current features with dynamic height signatures of about 30 dyn cm across 150 km and associated geostrophic surface velocities of approximately 60 cm s⁻¹. Two such features are found between Hawaii and the Subtropical Front, which is located at 32°N. Similar features have been observed on a number of other hydrographic and XBT sections perpendicular to the Hawaiian Ridge. It is hypothesized that the features are semipermanent, are due to the presence of the Ridge, and are related to the North Hawaiian Ridge Current of Mysak and Magaard

Air-sea interaction over ocean fronts and eddies

Air-sea interaction at ocean fronts and eddies exhibits positive correlation between sea surface temperature (SST), wind speed, and heat fluxes out of the ocean, indicating that the ocean is forcing the atmosphere. This contrasts with larger scale climate modes where the negative correlations suggest that the atmosphere is driving the system. This paper examines the physical processes that lie behind the interaction of sharp SST gradients and the overlying marine atmospheric boundary layer and deeper atmosphere, using high resolution satellite data, field data and numerical models. The importance of different physical mechanisms of atmospheric response to SST gradients, such as the effect of surface stability variations on momentum transfer, pressure gradients, secondary circulations and cloud cover will be assessed. The atmospheric response is known to create small-scale wind stress curl and divergence anomalies, and a discussion of the feedback of these features onto the ocean will also be presented. These processes will be compared and contrasted for different regions such as the Equatorial Front in the Eastern Pacific, and oceanic fronts in mid-latitudes such as the Gulf Stream, Kuroshio, and Agulhas Return Current. © 2008 Elsevier B.V. All rights reserved