Zonal Wind Components Research Articles

MF radar observations of 6.5-day wave in the equatorial mesosphere and lower thermosphere

Wind measurements made during January 1995 through December 1997 using Partial Reflection Radar located at Tirunelveli (8.7°N, 77.8°E), a tropical station in India, have been used to monitor the long-period planetary wave activity in mesosphere and lower thermosphere. Lomb-scargle periodogram analysis and wavelet techniques are used to study the temporal development of these oscillations and their vertical structure. The observed amplitude and power spectrum show a broad peak near 5–7 days, which corresponds to 6.5-day wave reported earlier. The 6.5-day wave is observed to be an equinoctial phenomenon when background wind is westward, being strong during April/May and September/October months. This shows possible effect of semi annual oscillation on this wave. The 6.5-day oscillation is particularly strong in the zonal wind component, and seems to be present almost continuously in the 84– 95 km height region. The wave activity is found to maximize during the weak-quasi biennial oscillation year. However, this observed feature need to be examined with a much longer dataset and we have initiated some efforts towards this direction. Mean climatology of these variations observed during three years (1995–1997) has been presented and found that there exists a strong interannual variability. The observed wave activity and its possible source mechanisms are compared and discussed with the available literature.

Variations in lower thermosphere dynamics at midlatitudes during intense geomagnetic storms

High‐resolution observations of the dynamics of the ionospheric E region (100–150 km) at midlatitudes have been made with the incoherent scatter radar at Millstone Hill (42.6°N, 71.5°W) during several intense geomagnetic storms (Kp ∼ 8–9). These included the storms of 25 September 1998, 15 July 2000, and 31 March 2001. The observations reveal electric fields of up to 100 mV/m during these events and indicate E region zonal plasma drifts of 300–1000 m/s, westward in the evening sector and eastward in the morning sector, with smaller meridional drifts. The tidal pattern of neutral winds in the lower thermosphere is heavily disrupted. The zonal component of the neutral wind follows convection‐driven ion flow, while the meridional component shows a more complex response to geomagnetic forcing.

Greenland cod (Gadus morhua): modeling recruitment variation during the second half of the 20th century

AbstractTwo approaches were used to qualify observed variability in Greenland cod (Gadus morhua) recruitment. In the first analysis, we used the linear trend of the Greenland cod recruitment time series and climatic variables, such as air temperatures from the Denmark Strait and wind conditions off East Greenland and Southwest Greenland, to explain the interannual variation in cod recruitment off Greenland. The model resulting from this ‘trend/environmental approach’, explained 79% of the interannual variation in cod recruitment off Greenland. In the second, analytical approach, the ‘regime approach’, multiple linear regression models were used, with the input data being the time series of cod recruitment and spawning stock biomass (SSB) from Iceland and Greenland, sea surface and air temperatures around Greenland, and zonal wind components between Iceland and Greenland. Model results indicated that, during the decades between 1950 and 1990, there were three different cause–effect regimes which significantly influenced the variability of cod recruitment. The three regimes included: (a) the 1950s and 1960s, a regime with favorable sea surface temperatures and a self‐sustaining cod stock off Greenland with high SSB that produced a series of above‐average, strong year classes; (b) the 1970s and 1980s, a regime of declining SSB and recruitment, with recruitment dependent on advection from Iceland; and (c) the 1990s, when the advective potential for recruitment from the Icelandic cod stock was the only available source for replenishment of the Greenland cod stocks, because cod recruitment in Greenland waters was negligible. The three models explained 76–77% of the observed interannual variation in cod recruitment off Greenland. Both approaches suggested that advective factors were the dominant influences for cod recruitment in the ‘Iceland–Greenland System’.

Coupled modes of large-scale climatic variables and regional precipitation in the western Mediterranean in autumn

The variability of autumn precipitation in the western Mediterranean and its relationship to the large-scale atmospheric variability during the period 1948 to 1989 is assessed. A singular value decomposition analysis is used to establish modes of coupled variability between regional precipitation and geopotential height (Z300), zonal (U-wind) and meridional (V-wind) wind components at the 300 hPa level. The Z300/precipitation coupling, which accounts for 52% of the total squared covariance, is strong during the autumn. The first Z300 coupled mode, in its positive phase, is characterised by a dipole structure with negative anomalies over Scandinavia and positive anomalies over the Iberian Peninsula in the Z300 and negative precipitation anomalies in the western Mediterranean. In its negative phase, a coupled pattern is found showing a high-over-low block and positive precipitation anomalies over the Mediterranean area. The coupling depicted by the second mode is weaker than that found in the first mode. The second coupled mode (21% of the total squared covariance) is characterised by negative anomalies in the eastern North Atlantic and positive ones over North Africa and the central Mediterranean in the Z300 and negative anomalies in the regional precipitation. Consistent with the results of the two first modes mentioned, the coupled patterns of either U-wind/precipitation or V-wind/precipitation are found to be coherent with those for Z300/precipitation. Composite maps were obtained to give a representation of the average circulation associated with coherent precipitation variability in the western Mediterranean. The regional impacts of both modes are investigated and the large-scale dynamic patterns presented are important modes of variability. Taking into account data for the whole of the twentieth century, results show that the first singular mode is responsible for the decadal trends and long term changes in precipitation. The late 1970s and 1980s is shown as the drier period and the late 1950s and early 1960s as the wettest years of the century.

Systematic errors of IMD Operational NWP model

The systematic errors of IMD (India Meteorological Department) limited area model over Indian region using 48 hours forecast fields for winter and summer seasons are examined in this paper. The study reveals that the core of the sub tropical westerly jet and tropical easterly jet are well simulated in the zonal component of wind. In case of meridional component of wind significant difference is noticed between forecast and analysis. In the upper tropospheric levels (above 100 hPa) northerly biases are noticed. Thermal structure of the model exhibits cool bias in middle and lower tropospheric levels over the northern latitudes. The nature of model biases of the geopotential height changes with the season. Model drying over the tropical belt and moistening towards the northern latitudes are found in all the seasons studied.

Statistical Downscaling Estimates of High Temperature in Summer in Japan

The development of a quantitative method to predict regional extreme high temperature due to global warming is necessary. In this study, a statistical downscaling model for estimating monthly mean daily maximum temperature in August in Japan was proposed and examined. The model related the local variable with the principal components of large-scale climate variables over the region using stepwise multiple regression analysis. The principal components of air temperature and zonal wind at 850hPa level, sea level pressure, and geopotential height at 500hPa level were selected as eligible predictors. The statistical model was evaluated by the cross validation procedure. The correlation coefficients between the observation and the regression estimates were significant at most stations, and the model estimated the observation fairly well. Therefore it was confirmed that the method is applicable to the estimation of high temperature in the region. The method was then applied to the output of NCAR-CSM. The 1×CO2 climate downscaled from the global model output was generally cooler than the observation due to the underestimate of 850hPa air temperature to the north of Japan showing that the downscaling model reflects deviations in the global model.

Variational data assimilation in the tropics: The impact of a background‐error constraint

AbstractGeostrophic adjustment theory predicts that wind information is a primary information source in the tropics. We ask whether this is true when variational data assimilation including a background‐error constraint is applied. The question is investigated by carrying out three‐ and four‐dimensional variational (4D‐Var) data assimilation experiments with nonlinear shallow‐water equations and idealized waves. The contribution to the analysis from mass and wind field observations is contrasted to that of spectral characteristics and multivariate relationships of equatorial waves built into a background‐error covariance matrix. A background‐error term for the analysis is built by using the tropical eigenmodes and the observed variability is used to determine the relative weights for different modes.Single observation sensitivity experiments illustrate an important impact of Kelvin and mixed Rossby–gravity waves in reducing the coupling between the mass and wind fields, found when only equatorial Rossby waves are utilized in the background‐error constraint.The assimilation experiments demonstrate two main features. First, the wind field information is of greater value than the mass field information, in spite of the known error statistics. A second intriguing feature is that the assimilation of wind observations better resolves smaller‐scale features of the height field than the assimilation of height data. When height data are used, a somewhat different analysis response is obtained for the zonal and meridional wind components, due to the strong projection onto the Kelvin wave structure for height observations. Finally, a comparison of different time windows for 4D‐Var illuminates the sensitivity of height field assimilation to the length of the assimilation window. Copyright © 2004 Royal Meteorological Society

The influence of wind and topography on precipitation distribution in Sweden: statistical analysis and modelling

AbstractTo estimate daily catchment precipitation from point observations there is a need to understand the spatial pattern, particularly in mountainous regions. One of the most important processes occurring there is orographic enhancement, which is affected by, among other things, wind speed and wind direction. The objective of this paper was to investigate whether the relationship between precipitation, airflow and topography could be described by statistical relationships using data easily available in an operational environment. The purpose was to establish a statistical model to describe basic patterns of precipitation distribution. This model, if successful, can be used to account for the topographical influence in precipitation interpolation schemes. A statistical analysis was carried out to define the most relevant variables, and, based on that analysis, a regression model was established through stepwise regression. Some 15 years of precipitation data from 370 stations in Sweden were used for the analysis. The geostrophic wind, computed from pressure observations, was assumed to represent the airflow at the relevant altitude. Precipitation data for each station were divided into 48 classes representing different wind directions and wind speeds. Among the variables selected, the single most important one was found to be the location of a station with respect to a mountain range. On the upwind side, precipitation increased with increasing wind speed. On the leeward side there was less variation in precipitation, and wind speed did not affect the precipitation amounts to the same degree. For ascending air, slope multiplied by wind speed was another important factor. The effect of slope was enhanced close to the coast, and reduced for mountain valleys with upwind barriers. The stepwise procedure led to a regression model that also included the meridional and zonal wind components. Their inclusion might indicate the importance of air mass characteristics not explicitly accounted for. Copyright © 2003 Royal Meteorological Society

The 6–9 day wave and rainfall modulation in northern Africa during summer 1981

Zonal and meridional wind components and geopotential height from European Centre for Medium‐Range Weather Forecasts model analyses and daily rainfall data from the Institut Français de Recherche Scientifique pour le Développement en Coopération (ORSTOM) are used to study westward propagating 6–9 day waves and rainfall modulation in northern Africa during summer 1981. The 6–9 day wave structure is determined using a composite method. In this structure, there are two vortices of opposite circulation on either side of the latitude 12.5°N. The rainfall maxima are associated with cyclonic vortices and the rainfall minima with anticyclonic vortices, coinciding with the minima and the maxima of geopotential height anomalies, respectively. The composite variability shows that the 6–9 day wave is associated with positive rainfall anomalies in West Africa in the band of latitude 7.5°–17.5°N, in the western part of the area around Senegal and Guinea and in the center toward Lake Chad. The rainfall anomalies are linked to the zonal wind anomalies, and the increase in rainfall is associated with large modulation of the African Easterly Jet zonal wind component, mainly in the cyclonic circulation. The main zones of decreasing rainfall appear north of 17.5°–20°N, toward Sudan, and south of 8°N, near Ivory Coast.

Simulation of marine boundary layer characteristics using a 1-D PBL model over the Bay of Bengal during BOBMEX-99

The characteristic features of the marine boundary layer (MBL) over the Bay of Bengal during the southwest monsoon and the factors influencing it are investigated. The Bay of Bengal and Monsoon Experiment (BOBMEX) carried out during July–August 1999 is the first observational experiment under the Indian Climate Research Programme (ICRP). A very high-resolution data in the vertical was obtained during this experiment, which was used to study the MBL characteristics off the east coast of India in the north and south Bay of Bengal. Spells of active and suppressed convection over the Bay were observed, of which, three representative convective episodes were considered for the study. For this purpose a one-dimensional multi-level PBL model with a TKE-ε closure scheme was used. The soundings, viz., the vertical profiles of temperature, humidity, zonal and meridional component of wind, obtained onboard ORV Sagar Kanya and from coastal stations along the east coast are used for the study. The temporal evolution of turbulent kinetic energy, marine boundary layer height (MBLH), sensible and latent heat fluxes and drag coefficient of momentum are simulated for different epochs of monsoon and monsoon depressions during BOBMEX-99.The model also generates the vertical profiles of potential temperature, specific humidity, zonal and meridional wind. These simulated values compared reasonably well with the observations available from BOBMEX.

Mesoscale Correlation Length Scales from NSCAT and Minimet Surface Wind Retrievals in the Labrador Sea

Observations of the surface wind speed and direction in the Labrador Sea for the period October 1996‐May 1997 were obtained by the NASA scatterometer (NSCAT), and by 21 newly developed Minimet drifting buoys. Minimet wind speeds are inferred, hourly, from observations of acoustic pressure in the Wind-Speed Observation Through Ambient Noise (WOTAN) technology. Wind directions are inferred from a direction histogram, also accumulated hourly, as determined by the orientation of a wind vane attached to the surface floatation. Effective temporal averaging of acoustic pressure (20 min), and the interval over which the direction histogram is accumulated (160 s), are shown to be consistent with low-pass filtering to preserve mesoscale time- and spacescale signals in the surface wind. Minimet wind speed and direction retrievals in the Labrador Sea were calibrated with collocated NSCAT data. The NSCAT calibrations extend over the full field lifetimes of each Minimet (90 days on average). Wind speed variabilities of O(5 m s21) and wind direction variabilities of O(408) are evident on timescales of one to several hours in Minimet time series. Wind speed and direction rms differences versus spatial separation comparisons (from 0 to 400 km) for the NSCAT and Minimet records demonstrate similar rms differences in wind speed as a function of spatial separation, but O(208) larger rms differences in Minimet direction. These differences are consistent with spatial smoothing effects in the median filter step for wind direction retrievals within the NSCAT swath. Zonal and meridional surface wind components are constructed from the calibrated Minimet wind speed and direction dataset. Rms differences versus spatial separation for these components are used to estimate mesoscale spatial correlation length scales of 250 and 290 km in the zonal and meridional directions, respectively.

Intradiurnal wind variations in the midlatitude and high‐latitude mesosphere and lower thermosphere

MF radar observations of neutral zonal and meridional wind components in the mesosphere and lower thermosphere region (78–98 km) made at Adelaide (35°S, 138°E) and Davis (68°S, 78°E) from January 1994 to December 1999 are analyzed with an emphasis on intradiurnal wind variations common to both stations. Periodograms were used to investigate the spectral content of the wave motions from 6 hours to 20 days and reveal the presence of a rich spectrum of waves. Motions due to tides and intradiurnal oscillations (6–15 hour periods) are particularly evident. Cross‐spectral analyses were used to assess the global coherence of the intradiurnal oscillations. They are found to be short‐lived (∼2–4 days) and have wave amplitudes in the range of 10–20 ms−1. The cross‐spectral analyses revealed discrete intervals during which intradiurnal oscillations showed a high degree of coherence between the two stations. The cross‐spectral phase differences suggest that oscillations with periods between 8 and 14 hours are westward propagating with zonal wave number 1. These oscillations are identified as the atmospheric manifestation of gravitational normal modes or Lamb waves.

Simulation of the Madden–Julian Oscillation in a Coupled General Circulation Model. Part II: The Role of the Basic State

Abstract In Part I of this study it was shown that air–sea coupling had a positive impact on some aspects of the simulation of the Madden–Julian oscillation (MJO) by a GCM. However, errors in the basic-state climate of that GCM appeared to be preventing the MJO-related convection from propagating into the west Pacific. In this paper, the actual impact of these errors will be addressed. An integration of a flux-adjusted version of the coupled model has been performed, which has reduced basic-state errors in the west Pacific. In this version of the coupled GCM the MJO does propagate into the west Pacific. The simulation of the MJO by a coupled model with the same atmospheric component but a different ocean GCM is also analyzed. This coupled GCM has similar systematic errors in low-level zonal wind and precipitation to the model studied in Part I, but with warmer SSTs. Results from this experiment, together with the other available evidence, suggest that it is the errors in the low-level zonal wind component...

Resonant nonlinear interactions between atmospheric waves in the polar summer mesopause region

Data obtained from the mobile SOUSY VHF radar at Andoya/Norway in summer 1987 have been used to study the nonlinear interactions between planetary waves, tides and gravity waves in the polar mesosphere, and the instability of background atmosphere above the mesopause. It is observed that 35-h planetary wave, diurnal, semidiurnal and terdiurnal tides are the prominent perturbations in the Lomb-Scargle spectra of the zonal wind component. By inspecting the frequency combinations, several triads are identified. By bispectral analysis it is shown that most bispectral peaks stand for quadratic coupling between tidal harmonics or between tide and planetary or gravity wave, and the height dependence of bispectral peaks reflects the variation of wave-wave interactions. Above the mesopause, the occurrence heights of the maximum L-S power spectral peaks corresponding to the prominent wave components ten to increase with their frequencies. This may result from the process in which two low frequency waves interact to generate a high frequency wave. Intensities of the planetary wave and tides increase gradually, arrive at their maxima, and then decay quickly in turn with increasing height. This kind of scene correlates with a “chain” of wave-wave resonant interactions that shifts with height from lower frequency segment to higher frequency segment. By instability analysis, it is observed that above the mesopause, theRichardson number becomes smaller and smaller with height, implying that the turbulent motion grows stronger and stronger and accordingly the background atmosphere more and more instable. It is suggested that the wave-wave sum resonant interaction and the wave dissipation due to instability are two dominant dynamical processes that occur in the mesopause region. The former invokes the energy transfer from lower frequency waves to higher frequency waves. The latter results in the heating of the atmosphere and accelerating of the background flow.

Analysis of recent circulation and thermal advection change in the northern Antarctic Peninsula

AbstractChanges in circulation at 500 and 850 hPa and horizontal thermal advection between these two levels are examined at Bellingshausen station in the northern Antarctic Peninsula between 1969 and 2000. A synthesis of radiosonde observations and reanalysis data is employed, the latter being ‘corrected’ to remove any bias and trend versus the observations. These data only reveal statistically significant increases in the annual zonal wind component; although the mean direction of the thermal advection is 339° there is no indication of increasing northerlies coincident with the remarkable regional surface warming. Seasonally, the zonal wind is most highly correlated with surface temperature in autumn and winter, the seasons showing the greatest temperature rise. This suggests that the increasing westerlies have played a role in the recent warming, and several authors have postulated that this results from a trend towards the positive phase of the Southern Hemisphere annular mode (SAM). However, a comparison between a regional zonal circulation index derived independently from observations and the reanalysis data used in past studies of the SAM indicates long‐term increases in the latter may be an artefact caused by errors in the early reanalysis data. While observations do indicate an increase in the zonal wind since the late 1970s, coincident with a decline in the strength of the semi‐annual oscillation, there is no clear evidence of circulation changes driving the long‐term Antarctic Peninsula surface warming. Copyright © 2002 Royal Meteorological Society.

Momentum transport of wave zero during March: A possible predictor for the Indian summer monsoon

Analysis of monthly momentum transport of zonal waves at 850 hPa for the period 1979 to 1993, between ‡S and ‡N for January to April, using zonal (u) and meridional (v) components of wind taken from the ECMWF reanalysis field, shows a positive correlation (.1% level of significance) between the Indian summer monsoon rainfall (June through September) and the momentum transport of wave zero TM(0) over latitudinal belt between 25‡S and 5‡N (LB) during March. Northward (Southward) TM(0) observed in March over LB subsequently leads to a good (drought) monsoon season over India which is found to be true even when the year is marked with the El-Nino event. Similarly a strong westerly zone in the Indian Ocean during March, indicates a good monsoon season for the country, even if the year is marked with El-Nino. The study thus suggests two predictors, TM(0) over LB and the strength of westerly zone in the Indian Ocean during March.

Observations of Variability on Synoptic Timescales in the East Pacific ITCZ*

Abstract Data obtained in the eastern Pacific intertropical convergence zone (ITCZ) during the Tropical Eastern Pacific Process Study (TEPPS) show a 3–6-day variability. The NOAA ship Ronald H. Brown collected surface meteorological observations, C-band Doppler radar volumes, atmospheric soundings, and rainfall data while on station at 7.8°N, 125°W from 8–23 August 1997. The 3–6-day variability was a prominent timescale in the meridional wind and humidity data. The maxima of the surface-to-700-mb meridional wind anomalies were 5–12 m s–1. Maxima of the moisture anomalies at the same levels were ∼1–3 g kg–1, in phase with the low-level southerlies, while upper-level moisture lagged the southerlies by less than a day. The radar indicated a close connection between precipitation in the vicinity of the ship and the meridional wind, with the precipitation occurring when the surface southerlies were strongest. The meridional and zonal wind components had a maximum of 850–450-mb wind shear during the southerly e...

Equatorial temperature and wind anomaly (ETWA)—a review

We review the observational evidence for describing the characteristics of the equatorial temperature and zonal wind anomaly (ETWA) in the low-latitude thermosphere in solar maximum and minimum periods. In spite of some new results on ETWA in the last decade, including its discovery, there is no satisfactory explanation in our understanding of the phenomenon. The two suggested mechanisms for heating at the crests of the equatorial ionization anomaly (EIA) to form the equatorial temperature anomaly (ETA) are due to (1) the ion-drag on the zonal winds resulting in the transfer of kinetic energy into heat energy and (2) the exothermic chemical reactions involved in the dominant O +ion re-combinations. To verify which of the two suggested mechanisms is the most effective in causing ETA, it is necessary to measure simultaneously a few parameters in situ by the satellite-borne instruments. They are (1) the electron density ( N e) and temperature ( T e), (2) the molecular and atomic ion densities ( N ij ) and ion temperatures ( T i), (3) the gas temperatures ( T) and densities of the gas constituents, (4) the vector winds or at least the zonal ( Z) and vertical ( V) wind components and (5) the drift velocities of the ionization. These together with the simultaneous ground-based measurements, will resolve identifying not only the dominant mechanism(s) for ETWA, but also the processes responsible for the enigmatic phenomena, such as the equatorial spread-F (ESF), the midnight temperature maximum (MTM) and the possible role of the EIA in their occurrences.

Comparison of Na lidar and meteor radar wind measurements at Starfire Optical Range, NM, USA

Simultaneous wind measurements in the mesopause region at Starfire Optical Range near Albuquerque, NM with Na wind/temperature lidar and meteor radar have been performed and compared. 20 nights of hourly data recorded with these two instruments at two layers around 86 and 93 km altitude are compared for both zonal and meridional wind components. The mean values are found to have no statistically significant differences. The correlation coefficients vary between 0.63 and 0.70, indicating that the two sets of measurements are broadly consistent. When comparing the averaged variations over the night, the two measurements are highly correlated, with correlation coefficients varying from 0.84 to 0.95. It indicates that the strong tidal variation is well captured by both instruments. Differences are however significant at individual hours, which are believed to be mainly due to the fact that the meteor radar wind is an average over the entire sky while the lidar measures wind within a volume about 100 m in diameter.

The Nondeterministic Nature of Kuroshio Penetration and Eddy Shedding in the South China Sea*

A ⅛°, 6-layer Pacific version of the Naval Research Laboratory Layered Ocean Model is used to investigate the nondeterministic nature of Kuroshio intrusion and eddy shedding into the South China Sea (SCS) on annual and interannual timescales. Four simulations, which only differ in the initial state, are forced with 1979–93 European Centre for Medium-Range Weather Forecasts reanalysis 1000 hectopascal (hPa) winds and then continued in 1994–97 with ECMWF operational 1000-hPa winds. The model shows differing amounts of Kuroshio penetration across all four simulations for the yearly means, indicating a large degree of nondeterminism at this timescale. This nondeterminism is quantified by a technique that separates the variability of a model variable into deterministic (caused by direct atmospheric forcing) and nondeterministic (caused by mesoscale flow instabilities) components. Analysis indicates substantial nondeterministic sea surface height and upper-layer velocity variability in the vicinity of Luzon Strait. A quantitative measure of Kuroshio intrusion into the SCS is presented that allows interexperiment comparisons and investigation of interannual variability, and attempts are made to positively correlate it with the oceanic and atmospheric environment. Yearly mean Kuroshio intrusion is not strongly linked to Luzon Strait transports or to changes in the North Equatorial Current bifurcation latitude (which is related to the northward Kuroshio transport east of Luzon). Likewise, no relationship could be found that linked interannual variability of yearly mean Kuroshio intrusion or monsoon season mean Luzon Strait transport with the corresponding zonal or meridional wind components, wind stress magnitude, or wind stress curl. However, there was a close relationship between the mean seasonal cycles of the Luzon Strait transport and the northeast–southwest monsoon. Eddy shedding and deep Kuroshio intrusion are rare events during the period of ECMWF reanalysis forcing, but are persistent features during the ECMWF operational time frame. While the wind stress is consistent across the reanalysis/operational time boundary, large differences exist in the wind stress curl pattern over the Luzon Strait and interior of the SCS basin. For contemporaneous years, the ECMWF operational winds produce higher curl extrema (by a factor of 2) and a much sharper north–south gradient in Luzon Strait. The net effect is to produce more Ekman pumping, a deepening of the thermocline, and a more deeply penetrating Kuroshio during the 1994–97 ECMWF operational forcing time frame. Thus, while normal interannual variations of the wind curl did not produce a deterministic response of simulated Kuroshio intrusion, the marked differences in curl between the two atmospheric products did have a substantial impact.