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Abstract
Cloud radiative effects have long been known to play a key role in governing the mean climate. In recent years, it has become clear that they also contribute to climate variability in the tropics. Here we build on recent work and probe the role of cloud radiative effects in extratropical sea surface temperature (SST) variability. The impact of cloud radiative effects on climate variability is explored in “cloud-locking” simulations run on an Earth System Model. The method involves comparing the output from two climate simulations: one in which clouds are coupled to atmospheric dynamic and thermodynamic processes, and another in which clouds are prescribed and thus decoupled from them. The results reveal that cloud–climate coupling leads to widespread increases in the amplitudes of extratropical SST variability from monthly to decadal time scales. Notably, it leads to ∼40%–100% increases in the amplitude of monthly to decadal variability over both the North Atlantic and North Pacific Oceans. These increases are consistent with the “reddening” of cloud shortwave radiative effects that arises when clouds respond to the dynamic and thermodynamic state of the atmosphere. The results suggest that a notable fraction of observed Northern Hemisphere SST variability—including that associated with North Pacific and North Atlantic decadal variability—is due to cloud–climate coupling.
Abstract
Cloud radiative effects have long been known to play a key role in governing the mean climate. In recent years, it has become clear that they also contribute to climate variability in the tropics. Here we build on recent work and probe the role of cloud radiative effects in extratropical sea surface temperature (SST) variability. The impact of cloud radiative effects on climate variability is explored in “cloud-locking” simulations run on an Earth System Model. The method involves comparing the output from two climate simulations: one in which clouds are coupled to atmospheric dynamic and thermodynamic processes, and another in which clouds are prescribed and thus decoupled from them. The results reveal that cloud–climate coupling leads to widespread increases in the amplitudes of extratropical SST variability from monthly to decadal time scales. Notably, it leads to ∼40%–100% increases in the amplitude of monthly to decadal variability over both the North Atlantic and North Pacific Oceans. These increases are consistent with the “reddening” of cloud shortwave radiative effects that arises when clouds respond to the dynamic and thermodynamic state of the atmosphere. The results suggest that a notable fraction of observed Northern Hemisphere SST variability—including that associated with North Pacific and North Atlantic decadal variability—is due to cloud–climate coupling.
Abstract
Using the multimodel simulations from phase 6 of the Coupled Model Intercomparison Project (CMIP6), we investigate the aridity changes in China and the associated mechanisms during the three geological periods of the Last Interglacial (LIG), Last Glacial Maximum (LGM), and mid-Holocene (MH), as well as the three future scenarios of the shared socioeconomic pathways of SSP1-2.6, SSP2-4.5, and SSP5-8.5. The aridity index is used to measure terrestrial moisture, which combines the effects of both precipitation and potential evapotranspiration (PET), with the latter representing the amount of water consumed by the atmosphere. The results show that relative to the preindustrial period, the total dryland area in China varies by −15%, 6%, and −13% during the LIG, LGM, and MH, respectively, and slightly varies in the three future scenarios. Over China, LGM dryland expansion and future dryland contraction are mainly attributed to precipitation changes, MH dryland contraction is mainly caused by PET changes, and LIG dryland contraction is comparably caused by PET and precipitation changes. For the LGM and three future scenarios, temperature is the leading factor of PET changes, while during the MH and LIG, the change in relative humidity is the main factor. In comparison, the simulated aridity changes in China are generally consistent with the reconstructed moisture changes for the three past periods, although uncertainties exist in reconstructions during the LGM and MH.
Abstract
Using the multimodel simulations from phase 6 of the Coupled Model Intercomparison Project (CMIP6), we investigate the aridity changes in China and the associated mechanisms during the three geological periods of the Last Interglacial (LIG), Last Glacial Maximum (LGM), and mid-Holocene (MH), as well as the three future scenarios of the shared socioeconomic pathways of SSP1-2.6, SSP2-4.5, and SSP5-8.5. The aridity index is used to measure terrestrial moisture, which combines the effects of both precipitation and potential evapotranspiration (PET), with the latter representing the amount of water consumed by the atmosphere. The results show that relative to the preindustrial period, the total dryland area in China varies by −15%, 6%, and −13% during the LIG, LGM, and MH, respectively, and slightly varies in the three future scenarios. Over China, LGM dryland expansion and future dryland contraction are mainly attributed to precipitation changes, MH dryland contraction is mainly caused by PET changes, and LIG dryland contraction is comparably caused by PET and precipitation changes. For the LGM and three future scenarios, temperature is the leading factor of PET changes, while during the MH and LIG, the change in relative humidity is the main factor. In comparison, the simulated aridity changes in China are generally consistent with the reconstructed moisture changes for the three past periods, although uncertainties exist in reconstructions during the LGM and MH.
Abstract
We estimate ocean heat content (OHC) change in the upper 2000 m in the Gulf of Mexico (GOM) from 1950 to 2020 to improve understanding of regional warming. Our estimates are based on 192 890 temperature profiles from the World Ocean Database. Warming occurs at all depths and in most regions except for a small region at northeastern GOM between 200 and 600 m. GOM OHC in the upper 2000 m increases at a rate of 0.38 ± 0.13 ZJ decade−1 between 1970 and 2020, which is equivalent to 1.21 ± 0.41 terawatts (TW). The GOM sea surface temperature (SST) increased ∼1.0° ± 0.25°C between 1970 and 2020, equivalent to a warming rate of 0.19° ± 0.05°C decade−1. Although SST in the GOM increases at a rate approximately twice that for the global ocean, the full-depth ocean heat storage rate in the GOM (0.86 ± 0.26 W m−2) applied to the entire GOM surface is comparable to that for the global ocean (0.82–1.11 W m−2). The upper-1000-m layer accounts for approximately 80%–90% of the total warming and variations in the upper 2000 m in the GOM. The Loop Current advective net heat flux is estimated to be 40.7 ± 6.3 TW through the GOM. A heat budget analysis shows the difference between the advective heat flux and the ocean heat storage rate (1.76 ± 1.36 TW, 1992–2017) can be roughly balanced with the annual net surface heat flux from ECCO (−37.9 TW).
Abstract
We estimate ocean heat content (OHC) change in the upper 2000 m in the Gulf of Mexico (GOM) from 1950 to 2020 to improve understanding of regional warming. Our estimates are based on 192 890 temperature profiles from the World Ocean Database. Warming occurs at all depths and in most regions except for a small region at northeastern GOM between 200 and 600 m. GOM OHC in the upper 2000 m increases at a rate of 0.38 ± 0.13 ZJ decade−1 between 1970 and 2020, which is equivalent to 1.21 ± 0.41 terawatts (TW). The GOM sea surface temperature (SST) increased ∼1.0° ± 0.25°C between 1970 and 2020, equivalent to a warming rate of 0.19° ± 0.05°C decade−1. Although SST in the GOM increases at a rate approximately twice that for the global ocean, the full-depth ocean heat storage rate in the GOM (0.86 ± 0.26 W m−2) applied to the entire GOM surface is comparable to that for the global ocean (0.82–1.11 W m−2). The upper-1000-m layer accounts for approximately 80%–90% of the total warming and variations in the upper 2000 m in the GOM. The Loop Current advective net heat flux is estimated to be 40.7 ± 6.3 TW through the GOM. A heat budget analysis shows the difference between the advective heat flux and the ocean heat storage rate (1.76 ± 1.36 TW, 1992–2017) can be roughly balanced with the annual net surface heat flux from ECCO (−37.9 TW).
Abstract
The boreal summer intraseasonal oscillation (BSISO) is the most prominent tropical subseasonal signature. Due to the restriction of methodology used to extract BSISO, most of the previous studies ignored its asymmetry. This study reexamines the BSISO events over the western North Pacific (WNP) for 1985–2010 with a hierarchical cluster analysis. Two categories of BSISO events are classified, the long-period (20–60 day) and short-period (10–20 day) events. The long-period BSISO events manifest as a northward-propagating mode with a significant phase asymmetry characterized by a fast development, but a slow decay of the intraseasonal convection. The phase asymmetry is found to be determined by the BSISO-induced amplitude-asymmetric sea surface temperature (SST) anomalies, in which the suppressed convection-induced positive SST anomalies are stronger than the active convection-induced negative ones. Such amplitude-asymmetric SST anomalies result from the nonlinear relationship between convection and surface downward shortwave radiation flux anomalies caused by the cloud transmission effect. The stronger positive SST anomalies–induced turbulence flux anomalies act as a negative feedback onto the atmosphere, making the transition from the convection suppressed phase to the active phase earlier and faster. The fast-developing convection tends to cause a fast northeastward retreat of the preceding enhanced western North Pacific subtropical high. Accordingly, the middle and lower reaches of Yangtze River valley experience a rapid reversal from the increased precipitation to the decreased. The asymmetric BSISO events over WNP and their impacts revealed in this study would provide a new potential for subseasonal-to-seasonal forecast of the East Asian summer monsoon precipitation.
Abstract
The boreal summer intraseasonal oscillation (BSISO) is the most prominent tropical subseasonal signature. Due to the restriction of methodology used to extract BSISO, most of the previous studies ignored its asymmetry. This study reexamines the BSISO events over the western North Pacific (WNP) for 1985–2010 with a hierarchical cluster analysis. Two categories of BSISO events are classified, the long-period (20–60 day) and short-period (10–20 day) events. The long-period BSISO events manifest as a northward-propagating mode with a significant phase asymmetry characterized by a fast development, but a slow decay of the intraseasonal convection. The phase asymmetry is found to be determined by the BSISO-induced amplitude-asymmetric sea surface temperature (SST) anomalies, in which the suppressed convection-induced positive SST anomalies are stronger than the active convection-induced negative ones. Such amplitude-asymmetric SST anomalies result from the nonlinear relationship between convection and surface downward shortwave radiation flux anomalies caused by the cloud transmission effect. The stronger positive SST anomalies–induced turbulence flux anomalies act as a negative feedback onto the atmosphere, making the transition from the convection suppressed phase to the active phase earlier and faster. The fast-developing convection tends to cause a fast northeastward retreat of the preceding enhanced western North Pacific subtropical high. Accordingly, the middle and lower reaches of Yangtze River valley experience a rapid reversal from the increased precipitation to the decreased. The asymmetric BSISO events over WNP and their impacts revealed in this study would provide a new potential for subseasonal-to-seasonal forecast of the East Asian summer monsoon precipitation.
Abstract
Antarctic margin and Southern Ocean surface freshening has been observed in recent decades and is projected to continue over the twenty-first century. Surface freshening due to precipitation and sea ice changes is represented in coupled climate models; however, Antarctic ice sheet/shelf meltwater contributions are not. Because Antarctic melting is projected to accelerate over the twenty-first century, this constitutes a fundamental shortcoming in present-day projections of high-latitude climate. Southern Ocean surface freshening has been shown to cause surface cooling by reducing both ocean convection and the entrainment of warm subsurface waters to the surface. Over the twenty-first century, Antarctic meltwater is expected to alter the pattern of projected surface warming as well as having other climatic effects. However, there remains considerable uncertainty in projected Antarctic meltwater amounts, and previous findings could be model dependent. Here, we use the ACCESS-ESM1.5 coupled model to investigate global climate responses to low and high Antarctic meltwater additions over the twenty-first century under a high-emissions climate scenario. Our high-meltwater simulations produce anomalous surface cooling, increased Antarctic sea ice, subsurface ocean warming, and hemispheric differences in precipitation. Our low-meltwater simulations suggest that the magnitude of surface temperature and Antarctic sea ice responses is strongly dependent on the applied meltwater amount. Taken together, these findings highlight the importance of constraining projections of Antarctic ice sheet/shelf melt to better project global surface climate changes over the twenty-first century.
Significance Statement
Antarctic ice sheets and shelves are melting, adding meltwater to the Southern Ocean and changing the ocean circulation. Antarctic meltwater stratifies the upper ocean, resulting in cooling of the surface Southern Ocean but warming at depth that could accelerate ice shelf melting. Coupled climate models used to project twenty-first-century climate do not represent ice sheets or shelves, neglecting important climate impacts. Here we conduct meltwater simulations with a coupled climate model and find that the magnitude of climate responses is strongly dependent on the applied meltwater amount. This highlights 1) the importance of constraining Antarctic meltwater projections to better project global climate over the twenty-first century and 2) that it is important that Antarctic meltwater be represented in future-generation coupled climate models.
Abstract
Antarctic margin and Southern Ocean surface freshening has been observed in recent decades and is projected to continue over the twenty-first century. Surface freshening due to precipitation and sea ice changes is represented in coupled climate models; however, Antarctic ice sheet/shelf meltwater contributions are not. Because Antarctic melting is projected to accelerate over the twenty-first century, this constitutes a fundamental shortcoming in present-day projections of high-latitude climate. Southern Ocean surface freshening has been shown to cause surface cooling by reducing both ocean convection and the entrainment of warm subsurface waters to the surface. Over the twenty-first century, Antarctic meltwater is expected to alter the pattern of projected surface warming as well as having other climatic effects. However, there remains considerable uncertainty in projected Antarctic meltwater amounts, and previous findings could be model dependent. Here, we use the ACCESS-ESM1.5 coupled model to investigate global climate responses to low and high Antarctic meltwater additions over the twenty-first century under a high-emissions climate scenario. Our high-meltwater simulations produce anomalous surface cooling, increased Antarctic sea ice, subsurface ocean warming, and hemispheric differences in precipitation. Our low-meltwater simulations suggest that the magnitude of surface temperature and Antarctic sea ice responses is strongly dependent on the applied meltwater amount. Taken together, these findings highlight the importance of constraining projections of Antarctic ice sheet/shelf melt to better project global surface climate changes over the twenty-first century.
Significance Statement
Antarctic ice sheets and shelves are melting, adding meltwater to the Southern Ocean and changing the ocean circulation. Antarctic meltwater stratifies the upper ocean, resulting in cooling of the surface Southern Ocean but warming at depth that could accelerate ice shelf melting. Coupled climate models used to project twenty-first-century climate do not represent ice sheets or shelves, neglecting important climate impacts. Here we conduct meltwater simulations with a coupled climate model and find that the magnitude of climate responses is strongly dependent on the applied meltwater amount. This highlights 1) the importance of constraining Antarctic meltwater projections to better project global climate over the twenty-first century and 2) that it is important that Antarctic meltwater be represented in future-generation coupled climate models.
Abstract
An information theory–based framework is developed to assess the predictability and quantify the forecast uncertainty of ENSO complexity, which includes different types of ENSO events with diverse characteristics. With the assistance of a recently developed multiscale stochastic conceptual model that successfully captures both the large-scale dynamics and many crucial statistical properties of the observed ENSO complexity, it is shown that different ENSO events possess distinct predictability limits. Beyond the ensemble mean value, the spread of the ensemble members also contains valuable information about predictability. First, La Niña events are most predictable at long lead times, especially as a subsequent transition after eastern Pacific (EP) El Niño events or during multiyear La Niña phases. Second, EP El Niños tend to be more predictable than the central Pacific (CP) El Niño events up to one year ahead due to a more favorable signal-to-noise ratio, even though their onset remains hard to predict. Third, 4 out of 6 CP El Niño events seem to be predictable up to 24 months ahead, where such strong predictability is often converted to skillful forecast. Fourth, strengthening/weakening the Walker circulation intensity increases/decreases CP predictability at long leads. Fifth, accounting for intraseasonal wind events in the initial condition strongly contributes to EP predictability at lead times of less than one year. Finally, it is shown that a Gaussian approximation of the information gain computation is accurate, making the information theory approach tractable for studying the predictability of more sophisticated models.
Abstract
An information theory–based framework is developed to assess the predictability and quantify the forecast uncertainty of ENSO complexity, which includes different types of ENSO events with diverse characteristics. With the assistance of a recently developed multiscale stochastic conceptual model that successfully captures both the large-scale dynamics and many crucial statistical properties of the observed ENSO complexity, it is shown that different ENSO events possess distinct predictability limits. Beyond the ensemble mean value, the spread of the ensemble members also contains valuable information about predictability. First, La Niña events are most predictable at long lead times, especially as a subsequent transition after eastern Pacific (EP) El Niño events or during multiyear La Niña phases. Second, EP El Niños tend to be more predictable than the central Pacific (CP) El Niño events up to one year ahead due to a more favorable signal-to-noise ratio, even though their onset remains hard to predict. Third, 4 out of 6 CP El Niño events seem to be predictable up to 24 months ahead, where such strong predictability is often converted to skillful forecast. Fourth, strengthening/weakening the Walker circulation intensity increases/decreases CP predictability at long leads. Fifth, accounting for intraseasonal wind events in the initial condition strongly contributes to EP predictability at lead times of less than one year. Finally, it is shown that a Gaussian approximation of the information gain computation is accurate, making the information theory approach tractable for studying the predictability of more sophisticated models.
Abstract
Ocean heat uptake is asymmetric with respect to the sign of radiative forcing. It is already known that surface cooling anomalies penetrate into the ocean faster than surface warming anomalies. Because of this asymmetry, the time-variable component of radiative forcing can induce a long-term, rectified cooling trend in ocean heat content, which is this work’s primary focus. Here, we explore this asymmetry and rectification on global and interannual scales, its implications, and its possible dependence on model parameters. We do so using a full-complexity global ocean–sea ice general circulation model and an idealized one-dimensional vertical mixing model, both forced with idealized abrupt and oscillatory surface forcing anomalies. In both models, the ocean heat uptake response to an abrupt cooling perturbation is larger than an equal-magnitude warming perturbation. This asymmetry is shown to be larger when the background vertical diffusivity is smaller, and is therefore likely model dependent. Sinusoidal oscillatory forcing with zero time mean induces a rectified cooling trend in both models, whose magnitude depends on both the diffusivity and the frequency of the interannual oscillatory forcing. The net rectified cooling can reach a rate of approximately −0.11 W m−2, which is substantial relative to the estimated anthropogenic warming rate of 0.47 W m−2 (von Schuckmann et al.). We discuss this rectification effect in the context of volcanic forcing in climate models, whose time-variable component may cause model-dependent ocean cooling in CMIP6 historical simulations, reaching 5%–30% the size of the total impact of volcanic forcing in our one-dimensional model. Correcting for this cooling may help reduce uncertainties in modeled ocean heat content evolution.
Abstract
Ocean heat uptake is asymmetric with respect to the sign of radiative forcing. It is already known that surface cooling anomalies penetrate into the ocean faster than surface warming anomalies. Because of this asymmetry, the time-variable component of radiative forcing can induce a long-term, rectified cooling trend in ocean heat content, which is this work’s primary focus. Here, we explore this asymmetry and rectification on global and interannual scales, its implications, and its possible dependence on model parameters. We do so using a full-complexity global ocean–sea ice general circulation model and an idealized one-dimensional vertical mixing model, both forced with idealized abrupt and oscillatory surface forcing anomalies. In both models, the ocean heat uptake response to an abrupt cooling perturbation is larger than an equal-magnitude warming perturbation. This asymmetry is shown to be larger when the background vertical diffusivity is smaller, and is therefore likely model dependent. Sinusoidal oscillatory forcing with zero time mean induces a rectified cooling trend in both models, whose magnitude depends on both the diffusivity and the frequency of the interannual oscillatory forcing. The net rectified cooling can reach a rate of approximately −0.11 W m−2, which is substantial relative to the estimated anthropogenic warming rate of 0.47 W m−2 (von Schuckmann et al.). We discuss this rectification effect in the context of volcanic forcing in climate models, whose time-variable component may cause model-dependent ocean cooling in CMIP6 historical simulations, reaching 5%–30% the size of the total impact of volcanic forcing in our one-dimensional model. Correcting for this cooling may help reduce uncertainties in modeled ocean heat content evolution.
Abstract
The Pacific Meridional Modes (PMMs) are the leading ocean-atmosphere coupled modes in the subtropical northeastern (NPMM) and southeastern (SPMM) Pacific, respectively, and have been suggested to be key precursors to equatorial Pacific variability. Previous studies pointed out that both PMMs-related sea surface temperature (SST) anomalies are primarily driven by net surface heat flux variations during their equatorward evolution. However, whether ocean heat advective processes would play a role during the evolution remains unclear. To address this issue, we perform an ocean mixed-layer heat budget analysis based on observations and three ocean reanalysis datasets, and then reveal the effect of ocean advections on the evolution by comparing a fully coupled dynamic ocean model (DOM) to a slab ocean model (SOM). Our results suggest that for the NPMM evolution, ocean advections—primarily by anomalous meridional Ekman heat advections driven by mean and anomalous zonal wind stresses—play a damping role in the south of the NPMM, resulting in the NPMM shifts poleward but still freely propagates westward from preceding boreal winter to the following summer. This finding challenges the traditional view that the NPMM propagates equatorward through the wind-evaporation-SST feedback. For the SPMM evolution, ocean advections play a damping role in the center of the SPMM from boreal spring to summer, as well as an intensification role in the southwest Pacific during summer. However, the effect of the intensification on the SPMM evolution is hard to be revealed due to the strong simulation bias of the SPMM evolution in the DOM.
Abstract
The Pacific Meridional Modes (PMMs) are the leading ocean-atmosphere coupled modes in the subtropical northeastern (NPMM) and southeastern (SPMM) Pacific, respectively, and have been suggested to be key precursors to equatorial Pacific variability. Previous studies pointed out that both PMMs-related sea surface temperature (SST) anomalies are primarily driven by net surface heat flux variations during their equatorward evolution. However, whether ocean heat advective processes would play a role during the evolution remains unclear. To address this issue, we perform an ocean mixed-layer heat budget analysis based on observations and three ocean reanalysis datasets, and then reveal the effect of ocean advections on the evolution by comparing a fully coupled dynamic ocean model (DOM) to a slab ocean model (SOM). Our results suggest that for the NPMM evolution, ocean advections—primarily by anomalous meridional Ekman heat advections driven by mean and anomalous zonal wind stresses—play a damping role in the south of the NPMM, resulting in the NPMM shifts poleward but still freely propagates westward from preceding boreal winter to the following summer. This finding challenges the traditional view that the NPMM propagates equatorward through the wind-evaporation-SST feedback. For the SPMM evolution, ocean advections play a damping role in the center of the SPMM from boreal spring to summer, as well as an intensification role in the southwest Pacific during summer. However, the effect of the intensification on the SPMM evolution is hard to be revealed due to the strong simulation bias of the SPMM evolution in the DOM.
Abstract
This study reported that the intensification of tropical cyclones (TCs) to major TCs (MTCs) in the Western North Pacific (WNP) region exhibited strong difference between boreal autumn (SON) and summer (JJA) since the early 2000s; the ratio of MTCs to the total number of TCs (MTC ratio) has continuously increased in SON but not in JJA. Due to this difference, more MTCs form and pass through the western flank of the WNP region in SON.
The increase of the MTC-ratio in SON was associated with interdecadal changes in TC activity and 30–60-day intraseasonal oscillations (ISOs). The mean genesis location of TCs and ISOs accompanied by a negative outgoing longwave radiation anomaly shrunk and shifted westward simultaneously in SON since the early 2000s due to the westward extension of the WNP subtropical high. However, this change was not observed in JJA. This westward shift of ISO substantially modulated large-scale thermodynamic and dynamic conditions, which in turn enhanced the TC–ISO interaction and accelerated energy conversion between TC and ISO. The kinetic energy budget along the MTC track was further analyzed to understand the TC–ISO interaction. Both the lower-level barotropic energy conversion (CK) and upper-level baroclinic energy conversion (CE) contributed to the intensification of TCs. CK mainly resulted from the scale interaction between TCs and ISO, whereas CE resulted from TC-related perturbations.
Abstract
This study reported that the intensification of tropical cyclones (TCs) to major TCs (MTCs) in the Western North Pacific (WNP) region exhibited strong difference between boreal autumn (SON) and summer (JJA) since the early 2000s; the ratio of MTCs to the total number of TCs (MTC ratio) has continuously increased in SON but not in JJA. Due to this difference, more MTCs form and pass through the western flank of the WNP region in SON.
The increase of the MTC-ratio in SON was associated with interdecadal changes in TC activity and 30–60-day intraseasonal oscillations (ISOs). The mean genesis location of TCs and ISOs accompanied by a negative outgoing longwave radiation anomaly shrunk and shifted westward simultaneously in SON since the early 2000s due to the westward extension of the WNP subtropical high. However, this change was not observed in JJA. This westward shift of ISO substantially modulated large-scale thermodynamic and dynamic conditions, which in turn enhanced the TC–ISO interaction and accelerated energy conversion between TC and ISO. The kinetic energy budget along the MTC track was further analyzed to understand the TC–ISO interaction. Both the lower-level barotropic energy conversion (CK) and upper-level baroclinic energy conversion (CE) contributed to the intensification of TCs. CK mainly resulted from the scale interaction between TCs and ISO, whereas CE resulted from TC-related perturbations.
Abstract
The impact of tropical Atlantic Ocean variability modes in the variability of the upper-ocean circulation has been investigated. For this purpose, we use three oceanic reanalyses, an interannual forced-ocean simulation, and satellite data for the period 1982–2018. We have explored the changes in the main surface and subsurface ocean currents during the emergence of Atlantic meridional mode (AMM), Atlantic zonal mode (AZM), and AMM–AZM connection. The developing phase of the AMM is associated with a boreal spring intensification of North Equatorial Countercurrent (NECC) and a reinforced summer Eastern Equatorial Undercurrent (EEUC) and north South Equatorial Current (nSEC). During the decaying phase, the reduction of the wind forcing and zonal sea surface height gradient produces a weakening of surface circulation. For the connected AMM–AZM, in addition to the intensified NECC, EEUC, and nSEC in spring, an anomalous north-equatorial wind curl excites an oceanic Rossby wave (RW) that is boundary-reflected into an equatorial Kelvin wave (KW). The KW reverses the thermocline slope, weakening the nSEC and EUC in boreal summer and autumn, respectively. During the developing spring phase of the AZM, the nSEC is considerably reduced with no consistent impact at subsurface levels. During the autumn decaying phase, the upwelling RW-reflected mechanism is activated, modifying the zonal pressure gradient that intensifies the nSEC. The NECC is reduced in boreal spring–summer. Our results reveal a robust alteration of the upper-ocean circulation during AMM, AZM, and AMM–AZM, highlighting the decisive role of ocean waves in connecting the tropical and equatorial ocean transport.
Abstract
The impact of tropical Atlantic Ocean variability modes in the variability of the upper-ocean circulation has been investigated. For this purpose, we use three oceanic reanalyses, an interannual forced-ocean simulation, and satellite data for the period 1982–2018. We have explored the changes in the main surface and subsurface ocean currents during the emergence of Atlantic meridional mode (AMM), Atlantic zonal mode (AZM), and AMM–AZM connection. The developing phase of the AMM is associated with a boreal spring intensification of North Equatorial Countercurrent (NECC) and a reinforced summer Eastern Equatorial Undercurrent (EEUC) and north South Equatorial Current (nSEC). During the decaying phase, the reduction of the wind forcing and zonal sea surface height gradient produces a weakening of surface circulation. For the connected AMM–AZM, in addition to the intensified NECC, EEUC, and nSEC in spring, an anomalous north-equatorial wind curl excites an oceanic Rossby wave (RW) that is boundary-reflected into an equatorial Kelvin wave (KW). The KW reverses the thermocline slope, weakening the nSEC and EUC in boreal summer and autumn, respectively. During the developing spring phase of the AZM, the nSEC is considerably reduced with no consistent impact at subsurface levels. During the autumn decaying phase, the upwelling RW-reflected mechanism is activated, modifying the zonal pressure gradient that intensifies the nSEC. The NECC is reduced in boreal spring–summer. Our results reveal a robust alteration of the upper-ocean circulation during AMM, AZM, and AMM–AZM, highlighting the decisive role of ocean waves in connecting the tropical and equatorial ocean transport.
