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George R. Halliwell Jr.

Abstract

North Atlantic winter surface atmospheric circulation anomalies that vary over decadal and longer periods are characterized by examining the life history of individual anomaly features present from 1950 to 1992. Individual features observed on surface pressure anomaly (PA) maps propagated to the east-northeast during the early to mid-1950s and to the south from 1964 to 1984. Standing PA fluctuations were observed at other times. The nonstationary statistical properties of this atmospheric variability were not apparent in earlier studies because the statistical analysis techniques used assumed stationarity or assumed the atmosphere was dominated by standing patterns of variability. Observed winter sea surface temperature anomaly (SSTA) patterns that vary over decadal and longer periods were driven in part by these surface atmospheric anomalies through the associated anomalous surface turbulent heat flux patterns. The ocean tends to be anomalously cold (warm) where the surface wind speed is anomalously large (small). Local atmospheric forcing of winter SSTA remains important out to longer periods than previously realized. Although SSTA appears to respond passively to this atmospheric forcing, a complete understanding of the ocean–atmosphere variability documented here will require an understanding of processes responsible for driving the atmospheric circulation anomalies.

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George R. Halliwell Jr.

Abstract

The North Atlantic winter sea surface temperature anomaly (T sa ) response to anomalous surface atmospheric circulation anomalies that vary over decadal and short-term inter-decadal periods is simulated for 1950 through 1992. Anomalous ocean variability is driven by adding monthly COADS-derived anomalous fields of vector wind stress and wind speed to the climatological annual cycle forcing. A simple model is derived relating winter T sa to the integrated atmospheric forcing present earlier in time that is responsible for its existence. The basin- scale structure of forced winter T sa patterns depends on the structure of the atmospheric forcing along with regional differences in the dominant mixed layer processes that generate T sa . For example, when the atmospheric subtropical high and subpolar low pressure systems are simultaneously strong, enhanced flow around, and baroclinic adjustments within, the subtropical gyre results in anomalously warm water in the Gulf Stream region off the U.S. East Coast. At the same time, however, the open ocean generally cools because the westerlies and trades are anomalously strong. By analyzing T sa variability not driven by the atmosphere, an anomalously cold decade is identified characterized by rapid onset and termination that both occur within one year. The onset during 1968 coincides with the appearance of the great salinity anomaly, while the termination during 1977 coincides with an abrupt Northern Hemisphere climate shift that is particularly evident in the Pacific.

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George R. Halliwell Jr.
and
Peter Cornillon

Abstract

We describe properties of large-scale fluctuations in the wind field represented by two analyses (FNOC and NOAA ATOLL), and in the SST field represented by maps derived from 5-day composite AVHRR images, within an 11° longitude by 10° latitude domain during FASINEX (January through June 1986). FNOC and ATOLL wind and wind stress time series were highly correlated with each other (<0.8) over the entire domain, and they were also highly correlated with time series measured at the FASINEX site ≥0.89. At periods of 2–13 days, clockwise →τ energy exceeded anticlockwise energy by an order of magnitude, while the energy was nearly partitioned equally at periods of 13–40 days. Mean ATOLL winds were everywhere more eastward than mean FNOC winds, probably in part because the ATOLL (FNOC) analyses are intended to represent 850 mb (19.5 m) winds. The 850 mb winds are expected to be more eastward due to the thermal wind balance with the mean southward air temperature gradient. The SST maps represented large-scale variability with reasonable accuracy, although a mean positive SST bias of about 0.54.6°C resulted from the image compositing procedure. This bias did not seriously affect the ability to detect either large-scale SST features or frontal zones in the 5-day maps. A single frontal zone up to 3° of latitude wide, which crossed the domain in a predominantly zonal direction, was the dominant large-scale frontal feature in the SST maps. Perturbations in the strength of this frontal zone shifted westward at several kilometers per day.

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George R. Halliwell Jr.
and
Peter Cornillon

Abstract

We analyzed the influence of wind-deriven horizontal heat advection on the large-scale [O(1000) km wavelength] variability of both the upper-ocean mixed-layer heat content and the subtropical frontal zone (SFZ) within an 11° by 10° domain in the western North Atlantic Ocean during FASINEX (January through June 1986). By estimating heat advection due to both Ekman transport and interior geostrophic (Sverdrup minus Ekman) transport from a slab mixed layer heat balance equation using satellite-derived sea surface temperature (Ts ) and wind analysis maps, it was found that these processes could not account for the observed variability in either beat content or the SFZ. The annual cycle of surface vertical heat flux had the dominant influence on the heat content. Even when the average heat balance was analyzed during a 4-month time interval when the net influence of the annual cycle was nearly zero (mid-January to mid-May 1986), westward-propagating Ts spatial anomaly features with peak-to-peak scales of several hundred kilometers apparently had the dominant influence on heat content. The influence of Ekman transport appeared to become marginally detectable only when terms in the heat equation were zonally averaged across the entire analysis domain, apparently reducing the influence of the propagating anomaly features. Ekman transport did act to maintain the SFZ during the 4-month interval, and thus may have been ultimately responsible for its existence, but the large-amplitude variability in heat content and the SFZ driven by other processes made this impossible to prove conclusively in the FASINEX region.

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George R. Halliwell Jr.
and
Dennis A. Mayer

Abstract

Frequency response properties of North Atlantic (5°–57°N) sea surface temperature anomaly (Tsa ) variability with periods of several months to 20 years are characterized using the Comprehensive Ocean-Atmosphere Data Set (COADS). Significant direct forcing of Tsa variability by the anomalous wind field (primarily through the resulting anomalous surface turbulent heat flux) is observed in the westerly wind and trade wind belts. To characterize properties of the large-scale climatic Tsa response to this forcing over the entire frequency band resolved, it is necessary to consider the dual role of anomalous surface heat flux as both the dominant local forcing mechanism and the dominant damping mechanism, the latter through a negative linear feedback (Newtonian relaxation). At frequencies where wind forcing is important, good agreement exists between the frequency response function estimated from data and the same function theoretically predicted by a simple stochastic forcing model where the locally forced response is damped by a negative linear feedback with a decay time scale of 3 mo. To make this comparison, the total anomalous surface heat flux represented by the standard bulk formula was decomposed into two components, one primarily representing the local wind forcing and the other primarily representing negative feedback damping. In the westerlies, wind forcing is effective over periods from several months to 8 yr, primarily 2–4 yr, and is ineffective at periods of 8–20 yr. These fluctuations are primarily forced in the western part of the basin then propagate to the east and northeast across the Atlantic at a characteristic speed of 6 km day−1. When time series of winter-only Tsa are analyzed, however, wind forcing of winter to winter Tsa variability remains significant at decadal and longer periods. In the trades, wind forcing is effective over periods from 8 mo to 13.3 yr, primarily 2-3 yr and 7–13.3 yr, and significant seasonal differences are not observed.

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George R. Halliwell Jr.
and
Christopher N. K. Mooers

Abstract

Weekly Gulf Stream paths within 1000 km downstream of Cape Hatteras were obtained for 1975–78 from the Navy's weekly EOFA charts based on satellite IR imagery. They displayed two dominant meander modes: first, a standing meander energetic over periods between 4 months and at least 4 years; and second. down-stream-propagating meanders that were most energetic at periods of several weeks. The long-period standing meander was confined between nodes located at the separation point near Cape Hatteras (i.e., where the Stream's mean path turns seaward) and at a point about 600 km farther downstream. The rms amplitude was 36 km at the antinode. The amplitude of propagating meanders increased rapidly in the first 200 km downstream of the separation point, where the capture of warm-core eddies was common. Farther down-stream, the predominant meanders had a wavelength averaging 330 km, a period averaging 1.5 month, a phase speed averaging 8 cm s−1, a downstream group speed averaging 17 cm s−1, and downstream exponential spatial growth rate averaging 3.2 × 10−3 km−1. They were energetic over a broad wavenumber-frequency band (periods of 1–6 months and wavelengths of 200 to more than 800 km) due to variable wavelengths, propagation speeds, and inter-meander space and time scales. The energetic wavenumber band was broadest near 4 cpy; it narrowed and shifted to larger wavenumbers with increasing frequency. The amplitude and frequency of occurrence of propagating meanders had large variability over time scales of a few months and longer.

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George R. Halliwell Jr.
and
J. S. Allen

Abstract

Along the west Coast of North America, the response of sea level to fluctuations in alongshore wind stress at large alongshore scales (> 1000 km) accounted for a substantial faction of the total sea level variance during summer 1973. Space-time contour plots of sea level and alongshore stress show that the response of sea level to poleward propagating wind stress events was generally stronger than the response to equatorward propagating events. Atmospheric forcing was most effective in two regions along the coast, with relatively strong forcing and response along northern California and Oregon, and somewhat weaker forcing and response along northern Baja California. The forced fluctuations in sea level propagated poleward away from these forcing regions, causing local sea level to be most correlated with alongshore wind stress earlier in time and at a distant equatorward 1ocation. Along the southern and central California coast, fluctuation in sea level were partly forced along northern Baja California, although some of the energy may have entered the domain from the south. Poleward of Crescent City, fluctuations in sea level were dominated by the response to alongshore stress in the northern forcing region, and were therefore poorly correlated with sea level to the south. Most of the sea level energy was contained in two frequency domain modes representing the northern and southern fluctuations in sea level. The southern mode had proportionally more energy than the northern mode at a frequency of 0.043 cpd, while the opposite was true for frequencies between 0.086 and 0.22 cpd. Sea level apparently responded more effectively in frequency bands where fluctuations in wind stress propagated poleward and acted over a longer alongshore distance. Along the British Columbia coast, local atmospheric forcing was relatively ineffective, and fluctuations in sea level were apparently dominated by free wave energy propagating poleward from the northern forcing region. Predictions of sea level response made from simple theory of wind-forced coastal-trapped waves were similar to the observed response, and accounted for up to 70% of the total variance along Oregon and Washington, poleward of the northern forcing region.

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George R. Halliwell Jr.
,
Peter Cornillon
, and
Deirdre A. Byrne

Abstract

Sea surface temperature (Ts ) maps of the region from 59.5° to 75.5°W, 22.5° to 33.5°N containing the western North Atlantic Subtropical Convergence Zone (STCZ) were derived from AVHRR/2 images. The 7- year mean annual cycle was removed and the maps were filtered in space and time to represent anomaly variability with wavelengths ≥ 220 km and periods ≥ 50 days. Warm and cold anomaly features were observed cast of 71°W between 26° and 32°N that propagated westward at 3–4 km day−1 and that occasionally exceeded ±1°C in amplitude. They are generally strong and persistent from fall to spring and are only marginally detectable during summer. During 1981–82, 1982–83, and 1985–86, individual features could be followed through the entire fall-spring interval. During 1983–84,1986–87,and 1987–88,they could typically be followed for 2–4 months, and during 1984–85, for only 1–2 months. The features were anisotropic during all fall-spring intervals except 1986–87, and they had characteristic wavelengths of ∼800 km in the minor axis direction and periods of ∼200 days. Local forcing by synoptic atmospheric variability alone could not amount for the existence of these features. Anomaly features propagated westward in a manner consistent with theoretical zonal dispersion properties of first-mode baroclinic Rossby waves, suggesting that the anomalies may be coupled to a field of wavelike eddies. Since the anomalies were confined to the zonal hand of large mean meridional Ts gradients associated with the STCZ, where meridional eddy currents are relatively effective at forcing anomalies these eddy currents could be largely responsible for their existence. In one case, however, the influence of eddies an vertical heat flux at the mixed layer base appeared to be important. The relatively strong and persistent 1985–86 anomaly features appeared during a several-day interval at the onset of relatively stormy fall weather and (presumably) rapid mixed-layer deepening.

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George R. Halliwell Jr.
,
Young Jae Ro
, and
Peter Cornillon

Abstract

Previous studies have documented the existence of a zonal band of strong, persistent, westward-propagating sea surface temperature (T s ) anomalies with zonal wavelengths of ≈800 km and periods of ≈200 days that are confined to the subtropical convergence zone (STCZ, roughly 26°–32°N). Two years of satellite-derived sea surface temperature (T s ) and sea surface elevation anomaly (η) maps of the Sargasso Sea (22.5°–33.5°N, 71.5°–59.5°W) are analyzed to determine how these anomalies are forced and why they an confined to the STCZ. A simple anomaly model forced by horizontal eddy currents and damped by a linear feedback mechanism explains many properties of the anomaly response. At wavelengths exceeding several hundred kilometers, forcing by horizontal eddy currents becomes less important relative to atmospheric forcing with increasing wavelength. The anomalies are confined to the STCZ partly because the large mean T s gradient there enables the horizontal eddy currents to be relatively effective at forcing anomalies. Also, the eddies that force these anomalies, wavelike features with wavelengths of ∼800 km and periods of ∼200 days, are themselves confined to the STCZ. These wavelike eddies were not detecting during earlier experiments such as MODE because the domains within which they were conducted were too small. Within the STCZ, zonal dispersion properties of the eddy field are consistent with baroclinic Rossby wave variability. To the north and south of the STCZ, however, zonal dispersion properties differ substantially from the properties observed within the STCZ. The eddy dispersion properties change abruptly across transition zones 1–2 degree wide centered at 32.5° and 25.5°N. A simple linearized reduced-gravity model is used to demonstrate that interaction between eddies and zonal mean currents can qualitatively account for the change is dispersion properties south of the STCZ, but not to the north within the Gulf Stream recirculation region.

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George R. Halliwell Jr.
,
Donald B. Olson
, and
Ge Peng

Abstract

Recent studies suggest that eddy Properties are significantly influenced by the mean current shear associated with the western Sargasso Sea subtropical frontal zone (SFZ). Between 19° and 34°N, the mean density structure is characterized by three layers separated by a climatologically permanent upper (“seasonal”) thermocline that shoals, and a lower (main) thermocline that deepens, toward the north; the thermoclines are separated by the wedge-shaped southern part of the subtropical mode water pool. The SFZ is evident as a zonal band between about 26° and 32°N where subtropical frontogenesis between the westerlies and trades enhances the slope of the mean seasonal thermocline. Classical linear as well as more recent nonlinear stability theories predict that the mean SFZ flow should be unstable. The linear eigenvalue problem suggests that the most unstable perturbations have wavelengths between 150 and 200 km. Analysis of a channel version of the Miami isopycnic-coordinate primitive equation numerical model verified these predictions and also characterized the further nonlinear evolution of the eddy field. As nonlinear effects become increasingly important, the eddies with wavelengths of 150-200 km predicted by linear theory that initially dominate the model fields continue to grow as the wavenumber spectrum becomes saturated. Following this, the eddies stop growing and energy shifts to longer wavelengths as predicted by geostrophic turbulence theory and observed in Geosat altimeter data. Zonal bands of mean flow also appear after the onset of the nonlinear energy cascade to larger scales as predicted by theory. Model results suggest baroclinic energy conversion and atmospheric forcing contribute roughly equally to eddy variability within the SFZ. Over three-fourths of the available potential energy released by the instability is extracted from the model seasonal thermocline. This agrees with the strong dependence of the strength of the instability on seasonal thermocline slope predicted by linear theory, and also agrees with the concentration of eddy potential energy within the seasonal thermocline revealed by analysis of historical XBT data. This may be one reason why clear evidence of baroclinic instability in the Sargasso Sea SFZ was not obtained from earlier moored measurements in this region (e.g., MODE); measurements in the main thermocline were emphasized.

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