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- Author or Editor: Fan Wang x
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Abstract
Passage of the Indonesian Throughflow (ITF) water through the Indian Ocean constitutes an essential section of the upper limb of the global ocean conveyor belt. Although existing studies have identified a major exit of the ITF water to the Atlantic Ocean through the Agulhas Current system, our knowledge regarding other possible destinations and primary pathways remains limited. This study applies the Connectivity Modeling System (CMS) particle tracking algorithm to seven model-based ocean current datasets. The results reveal a robust return path of the ITF water to the Pacific Ocean. The partition ratio between the Atlantic and Pacific routes is 1.60 ± 0.54 to 1, with the uncertainty representing interdataset spread. The average transit time across the Indian Ocean is 10–20 years to the Atlantic and 15–30 years to the Pacific. The “transit velocity” is devised to describe the three-dimensional pathways in a quantitative sense. Its distribution demonstrates that the recirculation structures in the southwestern subtropical Indian Ocean favor the exit to the Atlantic, while the Antarctic Circumpolar Current in the Southern Ocean serves as the primary corridor to the Pacific. Our analysis also suggests the vital impact of vertical motions. In idealized tracing experiments inhibiting vertical currents and turbulent mixing, more water tends to linger over the Indian Ocean or return to the Pacific. Turbulence mixing also contributes to vertical motions but only slightly affects the destinations and pathways of ITF water.
Abstract
Passage of the Indonesian Throughflow (ITF) water through the Indian Ocean constitutes an essential section of the upper limb of the global ocean conveyor belt. Although existing studies have identified a major exit of the ITF water to the Atlantic Ocean through the Agulhas Current system, our knowledge regarding other possible destinations and primary pathways remains limited. This study applies the Connectivity Modeling System (CMS) particle tracking algorithm to seven model-based ocean current datasets. The results reveal a robust return path of the ITF water to the Pacific Ocean. The partition ratio between the Atlantic and Pacific routes is 1.60 ± 0.54 to 1, with the uncertainty representing interdataset spread. The average transit time across the Indian Ocean is 10–20 years to the Atlantic and 15–30 years to the Pacific. The “transit velocity” is devised to describe the three-dimensional pathways in a quantitative sense. Its distribution demonstrates that the recirculation structures in the southwestern subtropical Indian Ocean favor the exit to the Atlantic, while the Antarctic Circumpolar Current in the Southern Ocean serves as the primary corridor to the Pacific. Our analysis also suggests the vital impact of vertical motions. In idealized tracing experiments inhibiting vertical currents and turbulent mixing, more water tends to linger over the Indian Ocean or return to the Pacific. Turbulence mixing also contributes to vertical motions but only slightly affects the destinations and pathways of ITF water.
Abstract
The summer Asian–Pacific oscillation (APO) is a dominant teleconnection pattern over the extratropical Northern Hemisphere that links the large-scale atmospheric circulation anomalies over the Asian–North Pacific Ocean sector. In this study, the direct Development of a European Multimodel Ensemble System for Seasonal-to-Interannual Prediction (DEMETER) model outputs from 1960 to 2001, which are limited in predicting the interannual variability of the summer Asian upper-tropospheric temperature and the decadal variations, are applied using the interannual increment approach to improve the predictions of the summer APO. By treating the year-to-year increment as the predictand, the interannual increment scheme is shown to significantly improve the predictive ability for the interannual variability of the summer Asian upper-tropospheric temperature and the decadal variations. The improvements for the interannual and interdecadal summer APO variability predictions in the interannual increment scheme relative to the original scheme are clear and significant. Compared with the DEMETER direct outputs, the statistical model with two predictors of APO and sea surface temperature anomaly over the Atlantic shows a significantly improved ability to predict the interannual variability of the summer rainfall over the middle and lower reaches of the Yangtze River valley (SRYR). This study therefore describes a more efficient approach for predicting the APO and the SRYR.
Abstract
The summer Asian–Pacific oscillation (APO) is a dominant teleconnection pattern over the extratropical Northern Hemisphere that links the large-scale atmospheric circulation anomalies over the Asian–North Pacific Ocean sector. In this study, the direct Development of a European Multimodel Ensemble System for Seasonal-to-Interannual Prediction (DEMETER) model outputs from 1960 to 2001, which are limited in predicting the interannual variability of the summer Asian upper-tropospheric temperature and the decadal variations, are applied using the interannual increment approach to improve the predictions of the summer APO. By treating the year-to-year increment as the predictand, the interannual increment scheme is shown to significantly improve the predictive ability for the interannual variability of the summer Asian upper-tropospheric temperature and the decadal variations. The improvements for the interannual and interdecadal summer APO variability predictions in the interannual increment scheme relative to the original scheme are clear and significant. Compared with the DEMETER direct outputs, the statistical model with two predictors of APO and sea surface temperature anomaly over the Atlantic shows a significantly improved ability to predict the interannual variability of the summer rainfall over the middle and lower reaches of the Yangtze River valley (SRYR). This study therefore describes a more efficient approach for predicting the APO and the SRYR.
Abstract
In this study, the authors found that the summer precipitation over China experienced different decadal variation features from north to south after the late 1990s. In northeastern and North China and the lower–middle reaches of the Yangtze River, precipitation decreased after 1999, while precipitation experienced a significant reduction over South and southwestern China and a significant increase over the southern parts of Hetao region and Huaihe River valley after 2003. The authors next analyzed the associated decadal variation of the atmospheric circulation and attempted to identify the mechanisms causing the two decadal variations of precipitation. The wind anomalies for the former exhibit a barotropic meridional dipole pattern, with anticyclonic anomalies over Mongolia to northern China and cyclonic anomalies over the southeastern Chinese coast to the northwestern Pacific. For the latter, there is a southeast–northwest-oriented dipole pattern in the middle and lower troposphere, with cyclonic anomalies over the northern parts of the Tibetan Plateau and anticyclonic anomalies over the lower–middle reaches of the Yangtze River to southern Japan. An anomalous anticyclone dominates the upper troposphere over China south of 40°N. The authors further found that the summer sea surface temperature (SST) warming over the tropical Atlantic played an important role in the decadal variation around 2003 via inducing teleconnections over Eurasia. In contrast, the decadal variation around 1999 may be caused by the phase shift of the Pacific decadal oscillation (PDO), as has previously been indicated.
Abstract
In this study, the authors found that the summer precipitation over China experienced different decadal variation features from north to south after the late 1990s. In northeastern and North China and the lower–middle reaches of the Yangtze River, precipitation decreased after 1999, while precipitation experienced a significant reduction over South and southwestern China and a significant increase over the southern parts of Hetao region and Huaihe River valley after 2003. The authors next analyzed the associated decadal variation of the atmospheric circulation and attempted to identify the mechanisms causing the two decadal variations of precipitation. The wind anomalies for the former exhibit a barotropic meridional dipole pattern, with anticyclonic anomalies over Mongolia to northern China and cyclonic anomalies over the southeastern Chinese coast to the northwestern Pacific. For the latter, there is a southeast–northwest-oriented dipole pattern in the middle and lower troposphere, with cyclonic anomalies over the northern parts of the Tibetan Plateau and anticyclonic anomalies over the lower–middle reaches of the Yangtze River to southern Japan. An anomalous anticyclone dominates the upper troposphere over China south of 40°N. The authors further found that the summer sea surface temperature (SST) warming over the tropical Atlantic played an important role in the decadal variation around 2003 via inducing teleconnections over Eurasia. In contrast, the decadal variation around 1999 may be caused by the phase shift of the Pacific decadal oscillation (PDO), as has previously been indicated.
Abstract
Ningaloo Niño—the interannually occurring warming episode in the southeast Indian Ocean (SEIO)—has strong signatures in ocean temperature and circulation and exerts profound impacts on regional climate and marine biosystems. Analysis of observational data and eddy-resolving regional ocean model simulations reveals that the Ningaloo Niño/Niña can also induce pronounced variability in ocean salinity, causing large-scale sea surface salinity (SSS) freshening of 0.15–0.20 psu in the SEIO during its warm phase. Model experiments are performed to understand the underlying processes. This SSS freshening is mutually caused by the increased local precipitation (~68%) and enhanced freshwater transport of the Indonesian Throughflow (ITF; ~28%) during Ningaloo Niño events. The effects of other processes, such as local winds and evaporation, are secondary (~18%). The ITF enhances the southward freshwater advection near the eastern boundary, which is critical in causing the strong freshening (>0.20 psu) near the Western Australian coast. Owing to the strong modulation effect of the ITF, SSS near the coast bears a higher correlation with El Niño–Southern Oscillation (0.57, 0.77, and 0.70 with the Niño-3, Niño-4, and Niño-3.4 indices, respectively) than sea surface temperature (−0.27, −0.42, and −0.35) during 1993–2016. Yet, an idealized model experiment with artificial damping for salinity anomaly indicates that ocean salinity has limited impact on ocean near-surface stratification and thus minimal feedback effect on the warming of Ningaloo Niño.
Abstract
Ningaloo Niño—the interannually occurring warming episode in the southeast Indian Ocean (SEIO)—has strong signatures in ocean temperature and circulation and exerts profound impacts on regional climate and marine biosystems. Analysis of observational data and eddy-resolving regional ocean model simulations reveals that the Ningaloo Niño/Niña can also induce pronounced variability in ocean salinity, causing large-scale sea surface salinity (SSS) freshening of 0.15–0.20 psu in the SEIO during its warm phase. Model experiments are performed to understand the underlying processes. This SSS freshening is mutually caused by the increased local precipitation (~68%) and enhanced freshwater transport of the Indonesian Throughflow (ITF; ~28%) during Ningaloo Niño events. The effects of other processes, such as local winds and evaporation, are secondary (~18%). The ITF enhances the southward freshwater advection near the eastern boundary, which is critical in causing the strong freshening (>0.20 psu) near the Western Australian coast. Owing to the strong modulation effect of the ITF, SSS near the coast bears a higher correlation with El Niño–Southern Oscillation (0.57, 0.77, and 0.70 with the Niño-3, Niño-4, and Niño-3.4 indices, respectively) than sea surface temperature (−0.27, −0.42, and −0.35) during 1993–2016. Yet, an idealized model experiment with artificial damping for salinity anomaly indicates that ocean salinity has limited impact on ocean near-surface stratification and thus minimal feedback effect on the warming of Ningaloo Niño.
Abstract
Interannual variabilities of sea level and upper-ocean gyre circulation of the western tropical Pacific Ocean (WTPO) have been predominantly attributed to El Niño–Southern Oscillation (ENSO). The results of the present study put forward important modulation effects by the Indian Ocean dipole (IOD) mode. The observed sea level in the WTPO shows significant instantaneous and lagged correlations (around −0.60 and 0.40, respectively) with the IOD mode index (DMI). A composite of 14 “independent” IOD events for 1958–2017 shows negative sea level anomalies (SLAs) of 4–7 cm in the WTPO during positive IOD events and positive SLAs of 6–8 cm in the following year that are opposite in sign to the El Niño effect. The IOD impacts are reproduced by large-ensemble simulations of a climate model that generate respectively 430 and 519 positive and negative independent IOD events. A positive IOD induces westerly winds over the western and central tropical Pacific and causes negative SLAs through Ekman upwelling, and it facilitates the establishment of a La Niña condition in the following year that involves enhanced Pacific trade winds and causes positive SLAs in the WTPO. Ocean model experiments confirm that the IOD affects the WTPO sea level mainly through modulating the tropical Pacific winds. Variability of the Indonesian Throughflow (ITF) induced by IOD winds has a relatively weak effect on the WTPO. The IOD’s impacts on the major upper-ocean currents are also considerable, causing anomalies of 1–4 Sv (1 Sv ≡ 106 m3 s−1) in the South Equatorial Current (SEC) and North Equatorial Countercurrent (NECC) volume transports.
Abstract
Interannual variabilities of sea level and upper-ocean gyre circulation of the western tropical Pacific Ocean (WTPO) have been predominantly attributed to El Niño–Southern Oscillation (ENSO). The results of the present study put forward important modulation effects by the Indian Ocean dipole (IOD) mode. The observed sea level in the WTPO shows significant instantaneous and lagged correlations (around −0.60 and 0.40, respectively) with the IOD mode index (DMI). A composite of 14 “independent” IOD events for 1958–2017 shows negative sea level anomalies (SLAs) of 4–7 cm in the WTPO during positive IOD events and positive SLAs of 6–8 cm in the following year that are opposite in sign to the El Niño effect. The IOD impacts are reproduced by large-ensemble simulations of a climate model that generate respectively 430 and 519 positive and negative independent IOD events. A positive IOD induces westerly winds over the western and central tropical Pacific and causes negative SLAs through Ekman upwelling, and it facilitates the establishment of a La Niña condition in the following year that involves enhanced Pacific trade winds and causes positive SLAs in the WTPO. Ocean model experiments confirm that the IOD affects the WTPO sea level mainly through modulating the tropical Pacific winds. Variability of the Indonesian Throughflow (ITF) induced by IOD winds has a relatively weak effect on the WTPO. The IOD’s impacts on the major upper-ocean currents are also considerable, causing anomalies of 1–4 Sv (1 Sv ≡ 106 m3 s−1) in the South Equatorial Current (SEC) and North Equatorial Countercurrent (NECC) volume transports.
Abstract
The southeast Indian Ocean (SEIO) exhibits decadal variability in sea surface temperature (SST) with amplitudes of ~0.2–0.3 K and covaries with the central Pacific (r = −0.63 with Niño-4 index for 1975–2010). In this study, the generation mechanisms of decadal SST variability are explored using an ocean general circulation model (OGCM), and its impact on atmosphere is evaluated using an atmospheric general circulation model (AGCM). OGCM experiments reveal that Pacific forcing through the Indonesian Throughflow explains <20% of the total SST variability, and the contribution of local wind stress is also small. These wind-forced anomalies mainly occur near the Western Australian coast. The majority of SST variability is attributed to surface heat fluxes. The reduced upward turbulent heat flux (Q T ; latent plus sensible heat flux), owing to decreased wind speed and anomalous warm, moist air advection, is essential for the growth of warm SST anomalies (SSTAs). The warming causes reduction of low cloud cover that increases surface shortwave radiation (SWR) and further promotes the warming. However, the resultant high SST, along with the increased wind speed in the offshore area, enhances the upward Q T and begins to cool the ocean. Warm SSTAs co-occur with cyclonic low-level wind anomalies in the SEIO and enhanced rainfall over Indonesia and northwest Australia. AGCM experiments suggest that although the tropical Pacific SST has strong effects on the SEIO region through atmospheric teleconnection, the cyclonic winds and increased rainfall are mainly caused by the SEIO warming through local air–sea interactions.
Abstract
The southeast Indian Ocean (SEIO) exhibits decadal variability in sea surface temperature (SST) with amplitudes of ~0.2–0.3 K and covaries with the central Pacific (r = −0.63 with Niño-4 index for 1975–2010). In this study, the generation mechanisms of decadal SST variability are explored using an ocean general circulation model (OGCM), and its impact on atmosphere is evaluated using an atmospheric general circulation model (AGCM). OGCM experiments reveal that Pacific forcing through the Indonesian Throughflow explains <20% of the total SST variability, and the contribution of local wind stress is also small. These wind-forced anomalies mainly occur near the Western Australian coast. The majority of SST variability is attributed to surface heat fluxes. The reduced upward turbulent heat flux (Q T ; latent plus sensible heat flux), owing to decreased wind speed and anomalous warm, moist air advection, is essential for the growth of warm SST anomalies (SSTAs). The warming causes reduction of low cloud cover that increases surface shortwave radiation (SWR) and further promotes the warming. However, the resultant high SST, along with the increased wind speed in the offshore area, enhances the upward Q T and begins to cool the ocean. Warm SSTAs co-occur with cyclonic low-level wind anomalies in the SEIO and enhanced rainfall over Indonesia and northwest Australia. AGCM experiments suggest that although the tropical Pacific SST has strong effects on the SEIO region through atmospheric teleconnection, the cyclonic winds and increased rainfall are mainly caused by the SEIO warming through local air–sea interactions.
Abstract
Central Pacific (CP) El Niño (i.e., CP El Niño) events have occurred more frequently during recent decades. Wind stress patterns are argued to have significant effects on the generation and evolution of CP El Niño. However, the winds differ in different CP El Niño events, making it hard in previous studies to avoid overgeneralizing the timing and location of the winds that indeed matter. In this study, the theoretically favorable wind perturbations (FWPs) that may warm the Niño-4 region, in terms of their directions, horizontal structures, and bounds, in each month before the peak month (December) of CP El Niños are determined, using an adjoint sensitivity method. The mechanisms of the FWPs are interpreted. Primarily, zonal temperature advection via the equatorial wave–associated velocity anomalies is responsible. In particular, easterly FWPs over the central equatorial Pacific with off-equatorial westerly FWPs (constituting a wind structure with a strong north–south gradient) during the first half year can play a positive role in warming the Niño-4 region and so can the westerly FWPs over the western tropical Pacific, while westerly FWPs in the western-central tropical Pacific in the second half year show higher efficiency. Meanwhile, the particular wind structure of the first half year (i.e., the easterly anomaly over the central equatorial Pacific with strong wind stress curl off the equator) has also been verified to be able to produce a CP-type warming in an intermediate coupled model (ICM); similar wind stress anomalies had been observed in some CP El Niño events. Thus, the FWPs provide helpful guidance in analyzing the generation and evolving processes of the wind-driven CP El Niño.
Abstract
Central Pacific (CP) El Niño (i.e., CP El Niño) events have occurred more frequently during recent decades. Wind stress patterns are argued to have significant effects on the generation and evolution of CP El Niño. However, the winds differ in different CP El Niño events, making it hard in previous studies to avoid overgeneralizing the timing and location of the winds that indeed matter. In this study, the theoretically favorable wind perturbations (FWPs) that may warm the Niño-4 region, in terms of their directions, horizontal structures, and bounds, in each month before the peak month (December) of CP El Niños are determined, using an adjoint sensitivity method. The mechanisms of the FWPs are interpreted. Primarily, zonal temperature advection via the equatorial wave–associated velocity anomalies is responsible. In particular, easterly FWPs over the central equatorial Pacific with off-equatorial westerly FWPs (constituting a wind structure with a strong north–south gradient) during the first half year can play a positive role in warming the Niño-4 region and so can the westerly FWPs over the western tropical Pacific, while westerly FWPs in the western-central tropical Pacific in the second half year show higher efficiency. Meanwhile, the particular wind structure of the first half year (i.e., the easterly anomaly over the central equatorial Pacific with strong wind stress curl off the equator) has also been verified to be able to produce a CP-type warming in an intermediate coupled model (ICM); similar wind stress anomalies had been observed in some CP El Niño events. Thus, the FWPs provide helpful guidance in analyzing the generation and evolving processes of the wind-driven CP El Niño.
Abstract
Tropical cyclones (TCs) can pump heat downward into the ocean through inducing intense vertical mixing. Many efforts have been made to estimate the TC-induced ocean heat uptake (OHU), but spatiotemporal variability of TC-induced OHU remains unclear. This study estimates the TC-induced OHU, which takes into account the heat loss at the air–sea interface during TC passage compared to previous studies and investigates the spatiotemporal variability of TC-induced OHU and its potential impacts on ocean heat content (OHC) during the period 1985–2018. It is found that the spatial distribution of OHU is inhomogeneous, with the largest OHU occurring in the northwest Pacific, and category 3–5 TCs contribute approximately 51% of the total global OHU per year. The annually accumulated TC-induced OHUs in the regional basins exhibit pronounced interannual variability, which is closely related to the TC power dissipation index (PDI). By decomposing PDI into TC intensity, frequency, and duration, we find that the TC characteristics influencing OHU variability vary by basin. Correlation analyses suggest that the interannual variations of OHUs are linked to El Niño–Southern Oscillation (ENSO). In addition, the OHU might have the potential to influence OHC variability, especially in the equatorial eastern Pacific where there are significant positive correlations between the OHU and OHC with lags of 2–6 months. This has an important implication that TC-induced OHU might have potential effects on ENSO evolution.
Abstract
Tropical cyclones (TCs) can pump heat downward into the ocean through inducing intense vertical mixing. Many efforts have been made to estimate the TC-induced ocean heat uptake (OHU), but spatiotemporal variability of TC-induced OHU remains unclear. This study estimates the TC-induced OHU, which takes into account the heat loss at the air–sea interface during TC passage compared to previous studies and investigates the spatiotemporal variability of TC-induced OHU and its potential impacts on ocean heat content (OHC) during the period 1985–2018. It is found that the spatial distribution of OHU is inhomogeneous, with the largest OHU occurring in the northwest Pacific, and category 3–5 TCs contribute approximately 51% of the total global OHU per year. The annually accumulated TC-induced OHUs in the regional basins exhibit pronounced interannual variability, which is closely related to the TC power dissipation index (PDI). By decomposing PDI into TC intensity, frequency, and duration, we find that the TC characteristics influencing OHU variability vary by basin. Correlation analyses suggest that the interannual variations of OHUs are linked to El Niño–Southern Oscillation (ENSO). In addition, the OHU might have the potential to influence OHC variability, especially in the equatorial eastern Pacific where there are significant positive correlations between the OHU and OHC with lags of 2–6 months. This has an important implication that TC-induced OHU might have potential effects on ENSO evolution.
Abstract
The Pacific meridional mode (PMM) can modulate El Niño–Southern Oscillation (ENSO) and is also affected by ENSO-related tropical Pacific sea surface temperature anomalies (SSTAs). Two tropical feedbacks on the PMM have been proposed: a positive one of central tropical Pacific SSTAs and a negative one of eastern tropical Pacific (ETP) SSTAs, the latter of which is suggested to be active only during strong eastern Pacific (EP) El Niño events like those in 1982/83 and 1997/98. However, we find that no strong, negative PMM-like SSTAs appeared, although the PMM indices (PMMIs) were strongly negative in spring of 1983 and 1998. Observation and model experiments show that tropical warming in 1983 and 1998 not only occurred in the ETP but also extended to the date line, thus inducing wind anomalies unfavorable for establishing the wind–evaporation–SST feedback for a negative PMM in the subtropics. To understand the discrepancy between the large negative PMMIs and weak PMM-related subtropical cooling during strong EP El Niño events, we isolate the relative contributions of subtropical and tropical SSTAs to the PMMIs by calculating their spatial projections on the PMM. Analysis combined using observation and CMIP6 models shows that despite the large contribution from subtropical SSTAs, the large tropical SSTAs, especially the extreme ETP warming, could cause large negative PMMIs during strong EP El Niño events even without strong, negative subtropical SSTAs. Our study clarifies the impact of ETP warming in causing a negative PMM and indicates the overstatement of negative PMMIs by tropical SSTAs during strong EP El Niño events.
Significance Statement
This paper aims to reevaluate the previously proposed effect of strong eastern Pacific El Niño events, like those in 1982/83 and 1997/98, on exciting a negative Pacific meridional mode (PMM). We find that although the PMM indices were strongly negative during the decay of strong eastern Pacific El Niño events, the large negative PMM sea surface temperature anomalies (SSTAs) could not be observed in the subtropical Pacific. Further diagnosis indicates that the PMM index can be large if strong SSTAs occur in eastern tropical Pacific even without subtropical SSTAs, implying that one should be careful when using the PMM index.
Abstract
The Pacific meridional mode (PMM) can modulate El Niño–Southern Oscillation (ENSO) and is also affected by ENSO-related tropical Pacific sea surface temperature anomalies (SSTAs). Two tropical feedbacks on the PMM have been proposed: a positive one of central tropical Pacific SSTAs and a negative one of eastern tropical Pacific (ETP) SSTAs, the latter of which is suggested to be active only during strong eastern Pacific (EP) El Niño events like those in 1982/83 and 1997/98. However, we find that no strong, negative PMM-like SSTAs appeared, although the PMM indices (PMMIs) were strongly negative in spring of 1983 and 1998. Observation and model experiments show that tropical warming in 1983 and 1998 not only occurred in the ETP but also extended to the date line, thus inducing wind anomalies unfavorable for establishing the wind–evaporation–SST feedback for a negative PMM in the subtropics. To understand the discrepancy between the large negative PMMIs and weak PMM-related subtropical cooling during strong EP El Niño events, we isolate the relative contributions of subtropical and tropical SSTAs to the PMMIs by calculating their spatial projections on the PMM. Analysis combined using observation and CMIP6 models shows that despite the large contribution from subtropical SSTAs, the large tropical SSTAs, especially the extreme ETP warming, could cause large negative PMMIs during strong EP El Niño events even without strong, negative subtropical SSTAs. Our study clarifies the impact of ETP warming in causing a negative PMM and indicates the overstatement of negative PMMIs by tropical SSTAs during strong EP El Niño events.
Significance Statement
This paper aims to reevaluate the previously proposed effect of strong eastern Pacific El Niño events, like those in 1982/83 and 1997/98, on exciting a negative Pacific meridional mode (PMM). We find that although the PMM indices were strongly negative during the decay of strong eastern Pacific El Niño events, the large negative PMM sea surface temperature anomalies (SSTAs) could not be observed in the subtropical Pacific. Further diagnosis indicates that the PMM index can be large if strong SSTAs occur in eastern tropical Pacific even without subtropical SSTAs, implying that one should be careful when using the PMM index.
Abstract
This study identifies several modes of coevolution of various types of El Niño–Southern Oscillation (ENSO) and Indian Ocean dipole (IOD) by performing rotated season-reliant empirical orthogonal function (S-EOF) analysis with consideration of ENSO asymmetry. The first two modes reveal that early-onset ENSO is associated with subsequent strong IOD development, whereas late-onset ENSO forces an obscure IOD pattern with marginal SST anomalies in the western Indian Ocean. Further studies show that El Niño starting before early summer can more easily force an IOD event than that starting in late summer or fall, even when they are of equivalent magnitudes. This is because the atmospheric responses over the Indian Ocean to the eastern Pacific warming are in sharp contrast between early and late summer. Early-onset (late onset) El Niño can (cannot) cause favorable atmospheric circulation conditions over the Indian Ocean for inducing the western Indian Ocean warming, which facilitates the subsequent IOD development. In addition, the different propagations of ocean dynamic Rossby waves during the early- or late-onset types of ENSO are also accountable for the different IOD development. For the higher-order modes, the rotated S-EOF of “Niño only” cases shows a coevolution between a negative IOD mode and a date line Pacific El Niño, with warm sea surface temperature anomalies originating from the northern Pacific meridional mode.
Abstract
This study identifies several modes of coevolution of various types of El Niño–Southern Oscillation (ENSO) and Indian Ocean dipole (IOD) by performing rotated season-reliant empirical orthogonal function (S-EOF) analysis with consideration of ENSO asymmetry. The first two modes reveal that early-onset ENSO is associated with subsequent strong IOD development, whereas late-onset ENSO forces an obscure IOD pattern with marginal SST anomalies in the western Indian Ocean. Further studies show that El Niño starting before early summer can more easily force an IOD event than that starting in late summer or fall, even when they are of equivalent magnitudes. This is because the atmospheric responses over the Indian Ocean to the eastern Pacific warming are in sharp contrast between early and late summer. Early-onset (late onset) El Niño can (cannot) cause favorable atmospheric circulation conditions over the Indian Ocean for inducing the western Indian Ocean warming, which facilitates the subsequent IOD development. In addition, the different propagations of ocean dynamic Rossby waves during the early- or late-onset types of ENSO are also accountable for the different IOD development. For the higher-order modes, the rotated S-EOF of “Niño only” cases shows a coevolution between a negative IOD mode and a date line Pacific El Niño, with warm sea surface temperature anomalies originating from the northern Pacific meridional mode.
