Sensitivity of Sverdrup transport to surface wind products over the tropical North Pacific Ocean
This study investigates the sensitivity of the Sverdrup transport to the NCEP/NCAR, ERA-Interim, CCMP, and QSCAT wind products over the tropical North Pacific Ocean during the period of 2000–2008. Our analyses show that all of the S-E transports in the reanalysis/analysis winds are northward in the band of 7–9° N except the QSCAT wind, which produces more realistic southward transport as shown in the meridional geostrophic transports (MGTs) calculated from WOA13 and Argo data. At 24° N, high-resolution wind products can better estimate the meridional transport in this region. At 8° N, although the CCMP has the same high resolution as the QSCAT, it fails to produce more realistic ocean circulation just as the coarse resolution wind products do there. The unrealistically large wind stress curl and small zonal wind stress in winter in reanalysis/analysis wind products account for this discrepancy with the QSCAT. This discrepancy also suggests that the model physics used in these reanalysis wind products is deficient in depicting the ocean circulation in this region, where the ocean fronts and eddy activity are both active.
KeywordsWind stress curl Wind stress Sverdrup balance El Niño ENSO Ocean fronts Meso-scale eddy
Traditionally, our understanding of the mean meridional transports of the ocean is based on Sverdrup theory (Sverdrup 1947), which obtains the meridional transport of the wind-driven ocean circulation by integrating the wind stress curl (WSC) without detailed information of the oceanic baroclinicity under a linear dynamic assumption. It describes a simple yet powerful balance between the WSC and the depth-integrated meridional transport in the interior ocean. It has been prominent in diagnoses of the tropical circulation in the Pacific Ocean, where the ocean state and its interaction with the atmosphere play an important role in the magnitude and time evolution of El Niño Southern Oscillation (ENSO) events and global climate system (Jin and Neelin 1993; Cane 1998; Meehl et al. 2001; Hu et al. 2015; Zhou et al. 2018). However, it has been difficult to provide strong observational evidence for its validity due to a lack of long-term ocean observations.
Early studies (Spillane and Niiler 1975; Wyrtki 1975; Meyers 1980) have shown that the Sverdrup transport calculated from 2 to 5° latitude grid wind stress data does not account for the Pacific North Equatorial Countercurrent (NECC). They attribute the discrepancy to strong vorticity advection and lateral mixing near the current boundaries, where the meridional shear is large. Recent studies also suggest that Sverdrup transports are found to underestimate the mean gyre circulation of the low-latitude northwestern Pacific Ocean significantly based on comparisons with Argo data collected in the past 10 years or so (Zhang et al. 2013; Yuan et al. 2014; Yang and Yuan 2016), and the difference has been attributed to oceanic nonlinearity. However, the wind stress data used in above studies are all coarse resolution (zonal resolution > 2°), which cannot resolve small structures of wind stress and wind stress curl in this region. Landsteiner et al. (1990) compared three different wind products in estimation of the Sverdrup transport in the tropical Pacific Ocean, and they concluded that detailed and accurate simulation of the general circulation in the tropical Pacific is limited more by the uncertainties in estimates of the surface wind stresses than by deviations from Sverdrup balance. Kessler et al. (2003) argued that the WSC structure in equatorial eastern Pacific is crucial in realistic estimation of ocean circulation there.
So, reliable knowledge of the wind stress fields and WSC at the sea surface is very important for understanding the ocean circulation in tropical North Pacific. Errors in wind stress forcing affect not only the interior ocean but the western boundary as well, since systematic errors in the wind field accumulate toward the west. This could lead to considerable uncertainty in model studies of higher-order processes such as meridional heat transport, which depend on accurate simulation of both interior and western boundary currents. However, considerable uncertainties exist in estimating the WSC due to inadequate temporal and spatial sampling of the observations, which causes substantial discrepancies in different wind stress analyses and thus in ocean meridional transport estimates and model simulations (Harrison 1989; Landsteiner et al. 1990; Chen 2003; Kessler et al. 2003).
The commonly used sea surface wind products can be divided into two types, one is observational data and the other is reanalysis/analysis data. For the observational wind products, the in situ observations have been constrained by limited sampling in both space and time. Remote sensing of sea surface winds has been widely used, and these data provide high spatial and temporal resolution, near global coverage and extensive validation. In general, they agree reasonably well with in situ observations and surface analysis fields, but some biases are found especially in regions with strong sea surface currents (e.g., Bentamy et al. 1999; Ebuchi 1999; Kelly et al. 2001; Meissner et al. 2001; Ebuchi et al. 2002; Bourassa et al. 2003; Curry et al. 2004; Yuan 2004; Chelton and Freilich 2005). Moreover, the temporal record of remote sensing wind data is less than 30 years, which is not suitable for long-term climate study.
For this reason, over the past decade, reanalysis/analysis wind products have found widespread application in many areas of research ranging from studies of climatic trends and climate modeling (Bromwich and Fogt 2004; Decker et al. 2012). These products are generated by the assimilation of observational data over a given period of time, which is advantageous in climate studies because of their high resolution in space and time with long-term range. However, the accuracy of these fields remains dependent to a large extent on the model physics.
In this study, we first compare the WSCs calculated from observational and reanalysis/analysis wind products, and then we investigate the sensitivity of Sverdrup transport to these wind products over the tropical North Pacific Ocean. In the following section, we describe the method and sea surface wind datasets used in this study. In section 3, we compare the WSC among different wind products. The Sverdrup circulation and its sensitivity to different wind products are discussed in section 4. Summary and discussions are given in section 5.
2 Data and methods
2.1 Surface wind data
Parameters of the wind datasets used in this study during 2000–2008
Included data sources
Numerical weather prediction (NWP) and observations
1.9° × 1.875°
NWP and observations
0.7° × 0.7°
QuikSCAT, SSM/I, SSMIS, AMSR-E, WindSat, ECMWF analyses
0.25° × 0.25°
0.25° × 0.25°
Surface wind vectors in NCEP/NCAR Reanalysis Products-1 (NNRP-1) data were produced by the National Centers for Environmental prediction (NCEP) in collaboration with the National Centre for Atmospheric Research (NCAR). The data assimilation system uses a 3D-variational analysis scheme, with 28 sigma levels in the vertical and a triangular truncation of 62 waves which corresponds to a horizontal resolution of approximately 2.5° × 2.5° (Kalnay et al. 1996).
ERA-Interim surface wind vectors are a version of reanalysis data provided by the European Centre for Medium-Range Weather Forecasts (ECMWF). The version has a spatial resolution of 0.7° × 0.7° and uses 4D-variational analysis on a spectral grid with triangular truncation of 255 waves (corresponds to approximately 80 km) and a hybrid vertical coordinate system with 60 levels (Simmons et al. 2007).
The CCMP ocean surface wind analysis velocity product has a high resolution of 0.25° × 0.25°, they use analyses and reanalyses from the ECMWF as prior information, and combine with all the satellite data from Remote Sensing Systems and available conventional data using a variational analysis method (Atlas et al. 2011).
The QSCAT surface wind vectors are at 25-km resolution over a single 1600-km swath centered on the satellite ground track obtained from the SeaWinds instrument on board QuikSCAT satellite. This microwave instrument measures backscatter radiation in Ku-band (~ 14 GHz). Rain is a well-known problem affecting scatterometers, which tends to result in erroneous cross-track vectors and/or unrealistically high speeds. The present work uses the QuikScat Ku-2011 (v4) data, which use 4 satellite microwave radiometers (F13 SSMI, F14 SSMI, F15 SSMI, and TMI) to determine if rain is present at the location of the QuikScat observation (Ricciardulli and Wentz 2011; Ricciardulli and Wentz 2015).
The WSC is calculated at the center of the wind stress data cells using a centered finite difference scheme following Saunders (1976). We compare the WSC calculated from wind stress by Garratt (1977), Large and Pond (1981), and Foreman and Emeis (2010), which shows no significant difference for the present study.
2.2 Hydrographic data and climate indices
The absolute geostrophic current (AGC) data was calculated from both gridded Argo data (http://www.jamstec.go.jp/ARGO/argo_web/argo/?page_id=83&lang=en) in the upper 2000 m during 2001 to 2008 and the climatological hydrographic data of the WOA13 in the upper 5000 m north of 4° N in the North Pacific Ocean using the P-vector method (Chu 1995; Yuan et al. 2014; Zhou et al. 2018). The mean meridional geostrophic transport (MGT) in the North Pacific Ocean is calculated by integrating the meridional geostrophic velocities from the eastern boundary to the 5° off the coast in the western boundary in order to avoid the influence of the Mindanao Current (MC) and the Mindanao eddy (ME), which represents the meridional volume transport in interior Ocean.
The El Niño index Niño 3.4 (the SST anomaly averaged over the east-central equatorial Pacific region 5° S–5° N, 170°–120° W) is downloaded from (https://www.esrl.noaa.gov/psd/gcos_wgsp/Timeseries/Data/nino34.long.anom.data), which is calculated from HadISST1 datasets (Rayner et al. 2003).
2.3 Sverdrup balance
Both sides of Eq. (3) vary with x and y. The left side is called the meridional geostrophic transport (MGT), which is equivalent to the geostrophic streamfunction, and can be calculated from ocean hydrographic data. The right side is the Sverdrup minus the Ekman streamfunction (S-E in abbreviation), which can be computed from surface wind stress.
2.4 The standard error of the mean
2.5 The empirical orthogonal function analysis
3 Analysis of the WSC fields
In this section, we compare the annual mean, seasonal, and interannual variability of the sea surface WSC fields over the tropical North Pacific Ocean among NCEP/NCAR, ERA-Interim, CCMP, and QSCAT datasets.
3.1 Annual mean
The dipole WSC structure near the Hawaii land mass in response to orographic effects of the islands is quite strong in all wind products (Xie et al. 2001). High resolution clearly has a more consistent structure of this dipole with the QSCAT. Although the NCEP/NCAR has a coarse resolution, its amplitude is much larger than the other winds, and it has an unrealistic WSC dipole centering along the zero WSC line around 165° E (Fig. 1a). Previous studies have suggested the tropical bias in the NCEP/NCAR wind stresses, which tend to be systematically low when compared with observations by ships and scatterometer (Stricherz et al. 1997; Milliff et al. 1999; Smith et al. 1999). However, the WSC calculated from the present version of NCEP/NCAR products during 2000–2008 shows systematically high amplitude compared with other three wind products in the tropical Pacific Ocean. The zero line of WSC between tropical and subtropical gyre locates around 12–15° N with a little northward inclination in the western boundary, which is consistent with the mean surface bifurcation latitude of North Equatorial Current observed from altimetry data (Wang and Hu 2006; Qiu and Chen 2010). Large differences are identified near the western and eastern boundary region among the three reanalysis/analysis wind products, which may be caused by different resolutions.
3.2 Seasonal variability
3.3 Interannual variability
The time series of the first principal component (PC1) from the four wind products show very high simultaneous correlations with the Niño3.4 index with the lowest value of 0.78 for NCEP/NCAR and the highest value of 0.91 for the Interim (Fig. 3e–h). The high correlations suggest that the major interannual modes of the surface WSC are dominated by ENSO. Since ENSO behavior is highly sensitive to changes in the wind forcing (Neelin 1990; Kirtman 1997; An and Wang 2000; Cassou and Perigaud 2000), it is very important to investigate the differences of the interannual variations in the WSC in tropical North Pacific Ocean among the four wind products.
4 The Sverdrup transport in tropical North Pacific Ocean
The Sverdrup theory provides an efficient way to estimate the meridional transport of the wind-driven ocean circulation by integrating the WSC. This provides a simple way to assess the large scale differences in these four WSC fields by comparing the predicted Sverdrup transport distributions with the observed MGT calculated from long-term mean hydrographic data.
4.1 Mean field over the tropical North Pacific Ocean
The MGT is integrated from the 1500 m depth to the sea surface and from the eastern boundary of the North Pacific Ocean to the grids 5° away from the western boundary, and the S-E transport is also zonally integrated the same way as that for the MGT. The mean MGT from WOA13 is mostly southward in the tropical North Pacific Ocean, which mainly represents the interior subtropical-tropical gyre circulation (Fig. 5a), with the maximum transport of 40 Sv (1 Sv = 106 m3 s−1) locating east of Taiwan Island. This is consistent with the mean MGT calculated from 57-year long pentadal hydrographic data by Zhou et al. (2018). The MGT calculated from 8-year long Argo profiling float data shows basically consistent patterns only the maximum transport exceeds 45 Sv. The consistency between the MGT from WOA13 and Argo data suggests that the tropical Pacific Ocean is in a steady state for the 2000–2008 period’s mean.
In comparison, the S-E transports show quite similar spatial pattern in the four wind products in the subtropical gyre, which are qualitatively consistent with that of the MGT in the subtropical gyre and near the eastern boundary in the North Pacific Ocean (Fig. 5b-e), suggesting the dominance of wind-driven circulation in these areas. However, the maximum southward S-E transports in all wind products are not co-located with those of the MGT, with the S-E maximum shifted a little southward toward the Luzon Strait. Another obvious difference is that all of the S-E transports are northward in the band of 7°–9° N except that of the QSCAT, which produces more realistic southward transport as shown in the MGT.
The amplitudes of the differences between the MGT and the S-E transports in the four datasets are shown in Figs. 5g–j. The maximum positive difference (10~15 Sv) roughly locates east of the Luzon Island (15°–20° N), where the maximum S-E transport locates. This large positive value indicates the large differences of the zero line of WSC in tropical-subtropical gyre in the western Pacific Ocean, which is consistent with the distribution of WSC shown in Fig. 1and previous studies (Yuan et al. 2014; Zhou et al. 2018). There is also a maximum negative difference (− 15~− 20 Sv) in the three reanalysis/analysis wind products east of the Mindanao Island (Fig. 5g–i), which is absent in the QSCAT (Fig. 5j). Generally, the differences in Fig. 5j are the smallest, suggesting that the QSCAT wind predicts a more realistic meridional transport compared with the other three reanalysis/analysis wind products in the tropical Pacific Ocean.
4.2 Along 24° N and 8° N
At 24° N, the S-E transports and the MGT show consistent monotone increase of southward transport ranging from 30 to 42 Sv, which is consistent with historical calculation (Godfrey 1989; Trenberth et al. 1990; Qiu and Joyce 1992; Hautala et al. 1994). But the amplitudes of S-E transport are about 7–15 Sv smaller than that of the MGT with the Interim being the smallest and the QSCAT being the largest. At this latitude, the S-E transports calculated from high-resolution CCMP and QSCAT wind products seem to have more realistic value suggesting the importance of the horizontal resolution of wind products in calculating the meridional transport in this region.
At 8° N, all of the S-E transports show a net northward transport across the Pacific Ocean except the QSCAT wind, which shows consistent increasing southward transport from 160° W to the western boundary region. These results are consistent with that in Fig. 5a, e. Although the CCMP assimilates the QSCAT and has the same high resolution as the QSCAT, it fails to produce more realistic ocean circulation just as the coarse resolution wind products do here. Both the Sverdrup and Ekman transports in all wind products show consistent northward transport at 8° N (Fig. 7c, d), but the amplitudes differ from each other significantly. The reanalysis/analysis winds seem to overestimate the Sverdrup transport and underestimate the Ekman transport in this region.
5 Discussions and summary
The region between 7° and 9° N in Pacific Ocean is also the boundary between the NEC and NECC, where active activities of meso-scale eddies and fronts associated with velocity shear of the two currents and the meanders of the NECC locate. Many studies have shown that ocean fronts and eddies can influence the wind stress and WSC structures (Kelly et al. 2001; Cornillon and Park 2001; Chelton et al. 2004; Xie 2004). Numerical weather prediction models tend to underestimate the small scale structures in WSC and divergence (Milliff and Morzel 2001; Milliff et al. 2004), overestimate the latent and sensible hear fluxes in these frontal or eddy region (Rouault et al. 2003), which results in unrealistic wind stress and wind stress curl over oceanic fronts and meso-scale eddies.
The ME and the Mindanao Dome also locate in the band of 7°–9° N in western Pacific, where the thermocline is very shallow due to strong upwelling associated with the ME and Ekman pumping induced by local wind forcing (Masumoto and Yamagata 1991; Tozuka et al. 2002; Zhou et al. 2010). Kutsuwada et al. (2018) suggest that simulated ocean currents and oceanic internal structure using NCEP winds exhibit significant differences from that of QSCAT and observations along 10° N in the western tropical Pacific Ocean by comparing outputs from ocean general circulation models driven by NCEP and QSCAT wind datasets. They argue that the shallower thermocline depth is more sensitive to unrealistically strong wind stress curl, which results in substantial bias in the oceanic field in the ocean interior. Further analysis about the dynamics of this discrepancy is of course very valuable, which will be investigated in our future study.
This paper investigates the sensitivity of the Sverdrup transport to the NCEP/NCAR, ERA-Interim, CCMP, and QSCAT wind products over the tropical North Pacific Ocean during the period of 2000–2008. The major results are summarized as follows: All of the S-E transports in the reanalysis/analysis winds are northward in the band of 7°–9° N except the QSCAT wind, which produces more realistic southward transport as shown in the MGTs calculated from WOA13 and Argo data. At 24° N, high-resolution wind products can better estimate the meridional transport in this region. At 8° N, although the CCMP has the same high resolution as the QSCAT, it fails to produce more realistic ocean circulation just as the coarse resolution wind products do there. This suggests the deficiency of the model physics used in these reanalysis/analysis wind products in this region, which tend to overestimate the latent and sensible hear fluxes in ocean frontal or eddy region, results in unrealistic wind stress and wind stress curl over oceanic fronts and meso-scale eddies.
Given the fact that most climate models are coarse resolution and basically follow the Sverdrup dynamics, the accurate and detailed simulation of the general ocean circulation will rely on better wind estimates. Although winds are internally generated in ocean-atmosphere coupled models, accurate wind forcing is very important for model initialization especially when these models are used for prediction. Errors in wind stress forcing affect not only the interior ocean but the western boundary as well, since systematic errors in the wind field accumulate toward the west. The failure of the reanalysis/analysis wind products in depicting the front-induced positive strip of WSC in the equatorial eastern Pacific and the southward S-E transport in the tropical Pacific Ocean between 7° and 9° N could lead to considerable uncertainty in model studies of higher-order processes such as meridional heat transport, which depend on accurate simulation of both interior and western boundary currents. Further studies are highly needed to investigate the dynamical mechanisms in these discrepancies in this region.
We thank two anonymous reviewers for their valuable comments.
The work is supported by the grants NSFC (41876009), QMSNL (2016ASKJ12), and NSFC (41421005, 41720104008).
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