Wind Stress Curl Anomalies Research Articles

The Initiation and Developing Mechanisms of Central Pacific El Niños

Abstract The initiation and developing mechanisms of four major central Pacific (CP) El Niño events in 1994, 2002, 2004, and 2009 were investigated by analyzing oceanic and atmospheric reanalysis data. A mixed layer heat budget analysis was conducted and the result shows that the initiation mechanism of the 1994 CP El Niño is very different from other CP El Niños in 2000s, while the developing mechanisms are similar among these events. The initial sea surface temperature (SST) warming of the 1994 El Niño was caused by enhanced solar radiation, which was related to atmospheric meridional overturning circulation in association with positive SST anomaly forcing in the subtropical Pacific. The subtropical SST anomalies also induced anticyclonic surface wind stress curl anomalies, which caused the formation of subsurface warmer waters in the off-equatorial regions. The off-equatorial subsurface warmer waters were transported farther equatorward by the mean subsurface ocean currents, leading to the subsurface warming in the central equatorial Pacific. The deepened thermocline anomaly at the equator further promoted a positive advective and thermocline feedback so that the SST anomaly grew. During the initiation phase of the 2000s El Niños, ocean dynamics played a dominant role, while the effect of surface heat flux anomalies was minor. Preexisting subsurface warmer waters appeared in the equatorial region during their initiation phases. Such subsurface anomalies can cause the SST warming in the central Pacific through induced anomalous eastward zonal currents that advect high mean SST eastward. This positive zonal advective feedback, along with a positive thermocline feedback, continued to warm the local SST throughout the developing phase of the 2000s El Niño events.

Meridional volume transport in the South Pacific: Mean and SAM-related variability

The large increase in upper-ocean sampling during the past decade enables improved estimation of the mean meridional volume transport in the midlatitude South Pacific, and hence of the climatically important Meridional Overturning Circulation. Transport is computed using Argo float profile data for geostrophic shear and trajectory data for reference velocities at 1000 m. For the period 2004-2012, the mean geostrophic transport across 32S is 20.6 ± 6.0 Sv in the top 2000 m of the ocean. From west to east, this includes the southward East Australian Current (23.3 ± 2.9 Sv), its northward recirculation (16.3 ± 3.6 Sv), the broad interior northward flow (18.4 ± 4.1 Sv), and the net northward flow (9.2 ± 2.2 Sv) in opposing currents in the eastern Pacific. The basin-integrated geostrophic transport includes 7.3 ± 0.9 Sv of surface and thermocline waters, 4.9 ± 1.0 Sv of Subantarctic Mode Water, and 4.9 ± 1.4 Sv of Antarctic Intermediate Water. Interannual variability in volume transport across 32S in the South Pacific shows a Southern Annual Mode signature characterized by an increase during the positive phase of the Southern Annular Mode and a decrease during the negative phase. Maximum amplitudes in geostrophic transport anomalies, seen in the East Australian Current and East Australian Current recirculation, are consistent with wind stress curl anomalies near the western boundary. Key Points Improved meridional volume transport in the South Pacific using Argo EAC transport variability tied to the SAM © Published 2014. This article is a U.S. Government work and is in the public domain in the USA.

Wind-driven variability in sea surface temperature front distribution in the California Current System

Simultaneous satellite-derived observations from 2002 to 2009 are used to quantify the relation between sea surface temperature (SST) fronts and ocean winds in the California Current System (CCS). An edge-detection algorithm is applied to SST observations to generate monthly maps of frontal probabilities. Empirical orthogonal decompositions reveal that the seasonal evolution of fronts in the CCS is strongly related to the seasonal evolution of coastal alongshore wind stress. The seasonal development of SST fronts is remarkably different to the north and to the south of Cape Mendocino, however. While fronts to the north of the cape extend for hundreds of kilometers from the coast peaking during summer and fall, when upwelling winds are stronger off northern California and Oregon, the region to the south of Cape Mendocino is characterized by high frontal activity during spring in a much narrower band close to the coast. Throughout the region, anomalies in the intensity of upwelling-favorable wind stress are followed by anomalies in frontal activity. The width and speed of the widening of the region of high frontal activity are also related to coastal alongshore wind stress. Interannual variability in the timing of the widening of the region of high frontal activity in the lee of Cape Blanco compared to the timing of the spring transition to upwelling-favorable winds may be related to the wind stress curl distribution in the lee of the cape. Stronger upwelling-favorable wind stress curl anomalies lead to early widening of the region of high frontal activity.

Oceanic Rossby waves induced by the meridional shift of the ITCZ in association with ENSO events

This study investigated the eastern Pacific Intertropical Convergence Zone (ITCZ) as an atmospheric forcing to the ocean by using various observed and reanalysis data sets over 29 years. Climatologically, a zonal band of positive wind stress curl (WSC) with a 10° meridional width was exhibited along the ITCZ. A southward shift of the positive WSC band during the El Nino phase induced a negative (positive) WSC anomaly along the northern (southern) portion of the ITCZ, and vice versa during the La Nina phase. This meridional dipole accounted for more than 25 % of interannual variances of the WSC anomalies (WSCAs), based on analysis of the period 1993–2008. The negative (positive) WSCA in the northern portion of the ITCZ during the El Nino (La Nina) phase was collocated with a positive (negative) sea surface height anomaly (SSHA) that propagated westward as a Rossby wave all the way to the western North Pacific. This finding indicates that this off-equatorial Rossby wave is induced by the WSCA around the ITCZ. Our analysis of a 1.5-layer reduced gravity model revealed that the Rossby waves are mostly explained by wind stress forcing, rather than by reflection of an equatorial Kelvin wave on the eastern coastal boundary. The off-equatorial Rossby wave had the same SSHA polarity as the equatorial Kelvin wave, and generation of a phase-preserving Rossby wave without the Kelvin wave reflection was explained by meridional movement of the ITCZ. Thus, the ITCZ acts as an atmospheric bridge that connects the equatorial and off-equatorial oceanic waves.

Response of North Atlantic Ocean Circulation to Atmospheric Weather Regimes

Abstract A new framework is proposed for investigating the atmospheric forcing of North Atlantic Ocean circulation. Instead of using classical modes of variability, such as the North Atlantic Oscillation (NAO) or the east Atlantic pattern, the weather regimes paradigm was used. Using this framework helped avoid problems associated with the assumptions of orthogonality and symmetry that are particular to modal analysis and known to be unsuitable for the NAO. Using ocean-only historical and sensitivity experiments, the impacts of the four winter weather regimes on horizontal and overturning circulations were investigated. The results suggest that the Atlantic Ridge (AR), negative NAO (NAO−), and positive NAO (NAO+) regimes induce a fast (monthly-to-interannual time scales) adjustment of the gyres via topographic Sverdrup dynamics and of the meridional overturning circulation via anomalous Ekman transport. The wind anomalies associated with the Scandinavian blocking regime (SBL) are ineffective in driving a fast wind-driven oceanic adjustment. The response of both gyre and overturning circulations to persistent regime conditions was also estimated. AR causes a strong, wind-driven reduction in the strengths of the subtropical and subpolar gyres, while NAO+ causes a strengthening of the subtropical gyre via wind stress curl anomalies and of the subpolar gyre via heat flux anomalies. NAO− induces a southward shift of the gyres through the southward displacement of the wind stress curl. The SBL is found to impact the subpolar gyre only via anomalous heat fluxes. The overturning circulation is shown to spin up following persistent SBL and NAO+ and to spin down following persistent AR and NAO− conditions. These responses are driven by changes in deep water formation in the Labrador Sea.

Regional Sea Level Variation on Interannual Timescale in the East China Sea

Interannual sea level variation is investigated with the oceanic and atmospheric datasets in the East China Sea (ECS). Two modes are distinct on the interannual timescale, illustrated as the basin mode and the dipole mode. They account for 20% and 18% to the total interannual sea level variance respectively. The basin mode corresponds to the variability of the Kuroshio transport which is modulated by the PDO while the dipole mode is likely related to the local oceanic and atmospheric adjustment. Large-scale atmospheric circulation effect is dominant in influencing the interannual sea level in the ECS. ECS sea level responds barotropically to the basin-wide wind field, which illustrates negative correlation to the zonal-mean wind stress curl in the Pacific Ocean. Sea level variation exhibits the negative correlation at 8 years lag with the basin mean wind stress curl anomalies on the interannual timescale. The lagging years are consistent with the timescale that the baroclinic Rossby waves propagate westward in the North Pacific Ocean. Wind stress curl anomalies could also change the strength of the Kuroshio transport, and thus affect the local sea level through sea surface height adjustment. Local oceanic and atmospheric effect illustrates as another influence process. Steric effect contributes more than 20% to the interannual sea level gradually in a belt from the Fujian and Zhejiang coasts to the Korea/Tsushima strait. Especially in the northeast part, its contribution could be up to 60%. While for the local atmospheric process, zonal wind acts as a more important role on sea level than meridional component.

Imprint of the Pacific Decadal Oscillation on the South China Sea Throughflow Variability*

Abstract Analysis of the 62-yr hindcast outputs from an eddy-resolving ocean general circulation model reveals a prominent decadal variability in the upper-layer (0–745 m) Luzon Strait transport (LST), a key component of the South China Sea throughflow. This variability is in phase with the basin-scale wind stress anomalies associated with the Pacific decadal oscillation (PDO). A composite analysis shows that during the positive phase of the PDO, the Aleutian low and its related positive wind stress curl anomalies intrude southward, reducing the trade winds and enhancing the westerly wind anomalies in the tropical North Pacific. In response, the North Equatorial Current bifurcation shifts northward, resulting in a weaker Kuroshio east of Luzon and consequently a stronger South China Sea throughflow in the upper 745 m.

Nutrient variability in Subantarctic Mode Waters forced by the Southern Annular Mode and ENSO

As the primary source of nutrients to the global thermocline, Subantarctic Mode Waters (SAMWs) play a key role in primary production and climate. Here we use repeat hydrographic World Ocean Circulation Experiment/Climate Variability and Predictability data to quantify interannual SAMW nutrient variability and its forcing. Pacific sector SAMW nutrients were significantly correlated with the Southern Annular Mode (SAM) and wind stress curl anomalies associated with a faster meridional overturning circulation (MOC). A stronger MOC results in greater upwelling of nutrients at high latitudes, increased Ekman transport of nutrients equatorward, and subduction of higher preformed nutrient loads in SAMWs. Australian sector SAMWs were significantly correlated with El Niño–Southern Oscillation (ENSO), likely due to its modulation of transport in the East Australian Current extension. Interannual variability in SAMW nutrients impacts downstream tropical export production by as much as 5–12% of the annual mean.

Revisit the interannual variability of the North Equatorial Current transport with ECMWF ORA‐S3

AbstractThe interannual variability of the North Equatorial Current (NEC) transport in the tropical northwestern Pacific Ocean is investigated with the output from ECMWF Ocean Analysis/Reanalysis System 3 (ORA‐S3). The results show that the amplitude and root mean square (RMS) of interannual NEC transport anomalies increase from about 3.0–4.0 Sv and 2.0 Sv at 170°E to above 5.0 and 3.4 Sv at 135°E, respectively. The NEC transport variation agrees well with the variation of the sea surface height (SSH) anomaly difference between the southern and northern boundaries of the NEC region. Further analysis near the Philippine coast suggests that their good agreement mainly comes from the agreement of the NEC transport and SSH variations south of the gyre boundary. Around the bifurcation point off the Philippine coast, the southern branch of the NEC transport is highly related to El Niño–Southern Oscillation (ENSO) events. During El Niño/La Niña years, westerly/easterly wind anomalies and positive/negative wind stress curl anomalies develop in the tropical northwestern Pacific Ocean south of 20°N before the mature phase. The wind forcing center moves eastward with time and reaches the easternmost position around 170°E several months before the mature phase. This wind forcing generates upwelling/downwelling Rossby waves, which propagate westward to result in negative/positive SSH anomalies, hence inducing a cyclonic/anticyclonic gyre anomaly, which is responsible for the increase/decrease of the NEC transport. The northern branch of the NEC transport near the Philippine coast has no significant simultaneous relation with ENSO events.

Sea Surface Temperature and Wind Stress Curl Variability near a Cape

Abstract The coupling between sea surface temperature (SST), SST gradients, and wind stress curl variability near a cape off Brazil is investigated using satellite observations and several different SST high-resolution analyses. The cape is characterized by strong SST fronts year-round, associated with upwelling and advection of warm water offshore in a western boundary current. Observations reveal a strong coupling between crosswind SST gradients and wind stress curl variability, with the predominantly negative crosswind gradients leading to negative, upwelling favorable wind stress curl anomalies. The spatial correlation between empirical orthogonal functions (EOF) of those variables is ~0.6, while the correlation between the EOF amplitude time series of the wind stress curl and crosswind SST gradients is larger than 0.7. The coupling occurs during summer and winter and is strongly modulated by variations in the wind stress directional steadiness. The intensity of the coupling is weaker than around capes on the California Current system, presumably because of higher variability in wind direction off Brazil. During periods of high wind stress directional steadiness off Cape Frio, the coupling is intensified by up to 40%–75%. Wind stress curl is also correlated with SST itself, especially in the vicinity of the cape, although not as strongly as with crosswind SST gradients. The analyses suggest that the observed wind stress curl anomalies can lead to surface cooling of as much as 1°C. If the enhanced upwelling leads to further strengthening of the upwelling front, negative wind stress curl anomalies may be intensified in a positive feedback mechanism.

A window for carbon uptake in the southern subtropical Indian Ocean

Atmospheric CO2 sinks in the southern Indian Ocean was examined for their seasonal, interannual and interdecadal variability. Two distinct zones of CO2 uptake are identified; located in the 15°S–35°S band is the northern box where CO2 uptake is dominated by the solubility pump, and in the latitudinal range of 35°S–50°S is the southern box where both the solubility and biological pump are equal players. The anthropogenic CO2 inventories appear to be a result of deepening subduction of CO2 and subsequent invasion into the northern domain. The seasonal and interannual variability of CO2 sinks to the north are rather surface trapped, while the deep CO2 variability is found to be coherent with the atmospheric forcing, consistent with decadal wind stress curl anomalies. This is a step towards separating the secular trends of deep ocean CO2 from its natural variability in the analysis region.

Interannual Variations of Subsurface Spiciness in the Philippine Sea: Observations and Mechanism

Abstract The Philippine Sea (PS) is a key region connecting North Pacific subtropics to the equator via western boundary currents. Using available measurements from Argo profiling floats, satellite altimeters, and research surveys, the authors investigate the characteristics and mechanism of subsurface spiciness variability (represented by salinity changes between 23.5 and 24.5 σθ) in the PS. During the past decade, low-frequency salinity variability was dominated by interannual signals characterized by out-of-phase changes between the southern and northern PS with peak-to-peak amplitudes exceeding 0.1 psu. These salinity anomalies are mainly generated locally by anomalous cross-front geostrophic advections. In 2003, an anomalous cyclonic circulation developed in the PS, which transported greater (less) than normal high-salinity North Pacific Tropical Water to the northern (southern) PS and produced positive (negative) salinity anomalies there. In 2009, an anomalous anticyclone emerged, which produced negative (positive) salinity anomalies in the northern (southern) PS. These year-to-year variations are closely associated with ENSO cycle. During strong El Niño (La Niña) episodes, positive (negative) wind stress curl anomalies between 8° and 18°N evoke westward-propagating upwelling (downwelling) Rossby waves in the central Pacific and positive (negative) anomalous Ekman pumping in the western Pacific, resulting in the observed current and salinity changes in the PS. Further analysis suggests that these locally generated spiciness anomalies disperse quickly while propagating to the equatorial Pacific in the Mindanao Current (MC). In the meantime, anomalies advected from higher latitudes are nearly diminished upon reaching the PS. The western boundary of the North Pacific seems quite efficient in damping extratropical signals.

Effect of different types of El Niño on primary productivity in the South China Sea

More than ten years of satellite-derived net primary production (NPP) data were used to investigate the interannual variations of NPP associated with two different types of El Niño in the South China Sea (SCS). Results of the empirical orthogonal function analysis (EOF) showed that the first two modes had significant interannual variations and were sensitive to two types of El Niño. The first mode is highly correlated to the canonical El Niño. This mode is characterized by basin-scale decreased NPP during the canonical El Niño years, predominantly diminished off the east coast of Vietnam and the northwest coast of Luzon. The second mode is well correlated to the El Niño Modoki, showing increased NPP in the central and eastern SCS during the El Niño Modoki years. The interannual NPP variability is linked to anomalous atmospheric and oceanic conditions associated with these two types of El Niño. During the developing phase of El Niño Modoki, enhanced western North Pacific summer monsoon and moderate northeasterly wind divergence gave rise to the positive wind stress curl anomalies in the central and eastern SCS, inducing strong Ekman upwelling and increased upward nutrient supply. The moderate Ekman upwelling in the mature phase of El Niño Modoki also sustained the ocean primary productivity. Basin-scale decreased NPP in the SCS was attributed to anomalous weakened wind and abnormal warm sea surface temperature throughout the period of canonical El Niño.

Marine biogeochemical responses to the North Atlantic Oscillation in a coupled climate model

In this study a coupled ocean‐atmosphere model containing interactive marine biogeochemistry is used to analyze interannual, lagged, and decadal marine biogeochemical responses to the North Atlantic Oscillation (NAO), the dominant mode of North Atlantic atmospheric variability. The coupled model adequately reproduces present‐day climatologies and NAO atmospheric variability. It is shown that marine biogeochemical responses to the NAO are governed by different mechanisms according to the time scale considered. On interannual time scales, local changes in vertical mixing, caused by modifications in air‐sea heat, freshwater, and momentum fluxes, are most relevant in influencing phytoplankton growth through light and nutrient limitation mechanisms. At subpolar latitudes, deeper mixing occurring during positive NAO winters causes a slight decrease in late winter chlorophyll concentration due to light limitation and a 10%–20% increase in spring chlorophyll concentration due to higher nutrient availability. The lagged response of physical and biogeochemical properties to a high NAO winter shows some memory in the following 2 years. In particular, subsurface nutrient anomalies generated by local changes in mixing near the American coast are advected along the North Atlantic Current, where they are suggested to affect downstream chlorophyll concentration with 1 year lag. On decadal time scales, local and remote mechanisms act contemporaneously in shaping the decadal biogeochemical response to the NAO. The slow circulation adjustment, in response to NAO wind stress curl anomalies, causes a basin redistribution of heat, freshwater, and biogeochemical properties which, in turn, modifies the spatial structure of the subpolar chlorophyll bloom.

Observations of the Kuroshio's barotropic and baroclinic responses to basin-wide wind forcing

[1] Observations show that the Kuroshio in the East China Sea (ECS-Kuroshio) responds to the large-scale wind stress curl field at two time scales. It is argued that these two responses are related to barotropic and baroclinic modes that reach the ECS via different waveguides. Variability in the ECS-Kuroshio is assessed by comparing satellite altimetry, historical hydrography, and the Pacific Decadal Oscillation (PDO) index with the latter used as a proxy for the large-scale wind stress curl forcing. Sea level difference across the ECS-Kuroshio is positively correlated with PDO at zero lag and negatively correlated at 7 year lag. In contrast, pycnocline steepness and PDO are uncorrelated at zero lag and negatively correlated at 7 year lag. These signals in the ECS-Kuroshio, considered together with wind stress curl anomalies in the open ocean, are consistent with a barotropic response to the wind at zero lag. The barotropic response is likely forced in the central North Pacific by wind stress curl anomalies of opposite sign, one of which is centered at ECS latitudes (∼27°N) while the other sits further north. This suggests that, in general, the absolute transport at a given latitude is not simply that predicted by the Sverdrup balance along the latitude. This is a consequence of waveguides that can steer the barotropic mode across latitude lines. In contrast, the signals that lag PDO by 7 years are consistent with a baroclinic mode, which represents the ocean's time-integrated response to the wind stress curl along a single latitude band between 24°N and 27°N.

Analysis of Decadal Climate Variability in the Tropical Pacific by Coupled GCM

This study presents the spatial and temporal structures of the decadal variability of the Pacific from an extended control run of a coupled global climate model (GCM). The GCM used was version g2.0 of the Flexible Global Ocean Atmosphere Land System (FGOALS-g2.0) developed at LASG/IAP. The GCM FGOALS-g2.0 reproduces similar spatial-temporal structures of sea surface temperature (SST) as observed in the Pacific decadal oscillation (PDO) with a significant period of approximately 14 years. Correspondingly, the PDO signals were closely related to the decadal change both in the upper-ocean temperature anomalies and in the atmospheric circulation. The present results suggest that warm SST anomalies along the equator relax the trade winds, causing the SSTs to warm even more in the eastern equatorial Pacific, which is a positive feedback. Meanwhile, warm SST anomalies along the equator force characteristic off-equatorial wind stress curl anomalies, inducing much more poleward transport of heat, which is a negative feedback. The upper-ocean meridional heat transport, which is associated with the PDO phase transition, links the equatorial to the off-equatorial Pacific Ocean, acting as a major mechanism responsible for the tropical Pacific decadal variations. Therefore, the positive and negative feedbacks working together eventually result in the decadal oscillation in the Pacific.

Dynamic mechanism of interannual sea surface height variability in the North Pacific Subtropical Gyre

In this study, the dynamic mechanisms of interannual sea surface height (SSH) variability are investigated based on the first-mode baroclinic Rossby wave model, with a focus on the effects of different levels of wind stress curl (WSC). Maximum covariance analysis (MCA) of WSC and SSH anomalies displays a mode with significant WSC anomalies located primarily in the mid-latitude eastern North Pacific and central tropical Pacific with corresponding SSH anomalies located to the west. This leading mode can be attributed to Ekman pumping induced by local wind stress and the westward-propagating Rossby wave driven by large-scale wind stress. It is further found that in the middle latitudes, the SSH anomalies are largely determined by WSC variations associated with the North Pacific Gyre Oscillation (NPGO), rather than the Pacific Decadal Oscillation (PDO). The sensitivity of the predictive skill of the linear first-mode baroclinic model to different wind products is also examined.

Interannual heat content variability in the South China Sea and its response to ENSO

The interannual variability of upper ocean heat content (OHC) in the South China Sea (SCS) for the period of 1987–2006 and its response to ENSO events are investigated. It is found that the variability has a good correspondence with ENSO events, but with opposite phase. Negative OHC anomalies appear during ENSO warm phases, while positive OHC anomalies occur during ENSO cool phases. In addition, negative (positive) OHC anomalies propagate westward obviously during ENSO warm (cool) phases in the northern SCS. In contrast, OHC anomalies in the southern SCS do not exhibit distinct westward propagation during ENSO events. To explore why the OHC anomalies cannot propagate westward in the southern SCS, the interannual variability of oceanic and atmospheric anomaly fields including wind stress curl (WSC), horizontal wind stress, latent heat flux (LHF) and sea level pressure (SLP) is investigated. The results show that after a mature phase of ENSO warm (cool) event, negative (positive) OHC anomalies first appear in the northern SCS, which comes from the western Pacific through Luzon Strait. Then cyclonic (anticyclonic) wind stress anomalies occur in the northern SCS, which leads to positive (negative) WSC anomalies. Meanwhile, positive (negative) LHF anomalies which correspond to oceanic heat loss (gain) occur in this region. The effects of WSC and LHF, combined with the westward propagating negative (positive) OHC anomalies from the western Pacific, may contribute to rapid growth and propagation of the OHC anomalies in the northern SCS. On the contrary, the negative (positive) WSC and LHF anomalies associated with positive (negative) SLP in the southern SCS seem to be the important processes responsible for the weakening and non-propagation of the OHC anomalies in the southern SCS after a mature phase of ENSO warm (cool) event.

Interannual spring bloom variability and Ekman pumping in the coastal Gulf of Alaska

A lower trophic level ecosystem model is coupled to a three‐dimensional coastal ocean circulation model for the northwestern coastal Gulf of Alaska (CGOA) to investigate regional ecosystem dynamics on seasonal and interannual timescales. By including explicit growth limitation by light, nitrate, ammonium, silicic acid, and iron, the ecosystem model provides a comprehensive framework to investigate the combined role of macronutrients and micronutrients in shaping phytoplankton community structure. On the basis of comparisons with available in situ and remotely sensed observations for 1998 through 2002, the model reproduces the dominant modes of variability associated with the northwestern CGOA ecosystem dynamics. An empirical orthogonal function (EOF) analysis of biological and surface forcing fields suggests that, for the 5‐year period, interannual variability in peak chlorophyll concentrations during the spring bloom is related to interannual variability in the wind stress curl over the northern CGOA during the previous winter. Positive wind stress curl anomalies during winter result in enhanced Ekman pumping and, thus, nutrient upwelling on the shelf and at the shelfbreak. Since phytoplankton growth is severely light‐limited when Ekman pumping occurs, nutrient upwelling associated with the wind stress curl acts as a priming mechanism for the CGOA shelf, leading to higher peak chlorophyll concentrations during the following spring bloom. Enhanced wind stress curl and Ekman pumping are associated with the presence of a low‐pressure system in the northern CGOA, and potentially connected to the large‐scale variability of the Aleutian Low in the North Pacific.

Observed Interannual Variability of the Florida Current: Wind Forcing and the North Atlantic Oscillation

AbstractThe 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.