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  • Author or Editor: Christopher S. Meinen x
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Christopher S. Meinen

Abstract

Altimetric observations of sea surface height anomaly (SSHA) from the TOPEX/Poseidon and ERS satellites, hydrography, and the ECMWF and Florida State University wind products are used to track warm water (≥20°C) as it is exchanged between the equatorial Pacific Ocean and the higher latitudes during 1993–2003. The large El Niño event of 1997–98 resulted in a significant discharge of warm water toward the higher latitudes within the interior of the Pacific Ocean. The exchange of anomalous warm water volume with the Northern Hemisphere appears to be blocked under the intertropical convergence zone, consistent with most current ideas on the time-mean tropical–subtropical exchange. Little of the warm water discharged northward across 5° and 8°N during the 1997–98 El Niño event could be traced as far as 10°N. To the south, however, these anomalous volumes of warm water were visible at least as far as 20°S, primarily in the longitudes around 130°–160°W. In both hemispheres most of the warm water appeared to flow westward before returning to the Tropics during the recharge phase of the El Niño–La Niña cycle. The buildup of warm water in the Tropics before the 1997–98 El Niño is shown to be fed primarily by warm water drawn from the region in the western Pacific within 5°S–15°N. The exchange cycle between the equatorial band and the higher latitudes north of the equator leads the cycle in the south by 6–8 months. These results are found in all three datasets used herein, hydrography, altimetric observations of SSHA, and Sverdrup transports calculated from multiple wind products, which demonstrates the robustness of the results.

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Christopher S. Meinen
and
Michael J. McPhaden

Abstract

Previous studies have indicated that the volume of warm water (WWV) in the equatorial Pacific is intrinsically related to the dynamics of the El Niño–Southern Oscillation (ENSO) cycle. A gridded subsurface temperature dataset, which incorporates temperature measurements from expendable bathythermograph profiles as well as measurements from the moorings in the Tropical Atmosphere and Ocean Array, was used along with historical hydrography to quantify the variability of the WWV (with temperatures greater than 20°C) within the region 8°S–8°N, 156°E–95°W during 1993–99. These data were also used to estimate geostrophic transports of warm water relative to a level of no motion at 1000 dbar (1 dbar = 104 Pa). Ekman transports were estimated using wind data from gridded observations, assimilating model output, and satellite scatterometer measurements. The results indicate that the buildup of WWV between the weak 1994–95 El Niño event and the onset of the strong 1997–98 El Niño event resulted primarily from an anomalous decrease in the net southward transport (geostrophic plus Ekman) across 8°S in the western and central Pacific combined with an anomalous increase in eastward flow across 156°E. Afterward, beginning in April 1997, WWV in the equatorial region was reduced by about 26% coincident with the 1997–98 El Niño. Approximately half of the warm water lost during this period is accounted for by poleward transport across a wide range of longitudes, while the remaining half is accounted for by vertical water mass transformations. The implications of the results for understanding and modeling the ENSO cycle are discussed.

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Christopher S. Meinen
,
Michael J. McPhaden
, and
Gregory C. Johnson

Abstract

Geostrophic and Ekman transports calculated from observations of subsurface thermal structure and surface winds are used to determine vertical velocities and transports as a function of time, depth, and longitude in the equatorial Pacific within 5°S–5°N, 165°E–95°W during 1993–99 via a box volume balance. The vertical transports are determined in boxes of 10° of latitude by generally 15° in longitude. The corresponding vertical velocities represent a spatial average over these regions. Both the total vertical velocity and the cross-isopycnal component of the vertical velocity (approximated by the cross-isothermal component) are calculated on seasonal and interannual timescales. For the eastern equatorial Pacific (5°S–5°N, 155°–95°W) the mean vertical transport across 50 m is (24 ± 3) × 106 m3 s−1. Variability in the vertical velocity is large relative to the mean. On interannual timescales this variability is well correlated with the local winds in the western portion of the study area, while the correspondence is weaker in the east where wind variability is much smaller. At seasonal timescales there is good correspondence between the vertical velocity and the local winds throughout the study region. The cross-isothermal vertical velocity is significantly smaller than the total vertical velocity, and the means of both compare well with the few historical estimates available.

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Ricardo M. Domingues
,
William E. Johns
, and
Christopher S. Meinen

Abstract

In this study, mechanisms causing year-to-year changes in the Florida Current seasonality are investigated using controlled realistic numerical experiments designed to isolate the western boundary responses to westward-propagating open ocean signals. The experiments reveal two distinct processes by which westward-propagating signals can modulate the phase of the Florida Current variability, which we refer to as the “direct” and “indirect” response mechanisms. The direct response mechanism involves a two-stage response to open ocean anticyclonic eddies characterized by the direct influence of Rossby wave barotropic anomalies and baroclinic wall jets that propagate through Northwest Providence Channel. In the indirect response mechanism, open ocean signals act as small perturbations to the stochastic Gulf Stream variability downstream, which are then transmitted upstream to the Florida Straits through baroclinic coastally trapped signals that can rapidly travel along the U.S. East Coast. Experiments indicate that westward-propagating eddies play a key role in modulating the phase of the Florida Current variability, but not the amplitude, which is determined by its intrinsic variability in our simulations. Results from this study further suggest that the Antilles Current may act as a semipermeable barrier to incoming signals, favoring the interaction through the indirect response mechanism. The mechanisms reported here can be potentially linked to year-to-year changes in the seasonality of the Atlantic meridional overturning circulation and may also be present in other western boundary current systems.

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Pedro N. DiNezio
,
Lewis J. Gramer
,
William E. Johns
,
Christopher S. Meinen
, and
Molly O. Baringer

Abstract

The role of wind stress curl (WSC) forcing in the observed interannual variability of the Florida Current (FC) transport is investigated. Evidence is provided for baroclinic adjustment as a physical mechanism linking interannual changes in WSC forcing and changes in the circulation of the North Atlantic subtropical gyre. A continuous monthly time series of FC transport is constructed using daily transports estimated from undersea telephone cables near 27°N in the Straits of Florida. This 25-yr-long time series is linearly regressed against interannual WSC variability derived from the NCEP–NCAR reanalysis. The results indicate that a substantial fraction of the FC transport variability at 3–12-yr periods is explained by low-frequency WSC variations. A lagged regression analysis is performed to explore hypothetical adjustment times of the wind-driven circulation. The estimated lag times are at least 2 times faster than those predicted by linear beta-plane planetary wave theory. Possible reasons for this discrepancy are discussed within the context of recent observational and theoretical developments. The results are then linked with earlier findings of a low-frequency anticorrelation between FC transport and the North Atlantic Oscillation (NAO) index, showing that this relationship could result from the positive (negative) WSC anomalies that develop between 20° and 30°N in the western North Atlantic during high (low) NAO phases. Ultimately, the observed role of wind forcing on the interannual variability of the FC could represent a benchmark for current efforts to monitor and predict the North Atlantic circulation.

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