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Abstract
The ocean currents in the Halmahera Sea are studied using a subsurface mooring deployed in the Jailolo Strait from November 2015 to October 2017. The subtidal currents of the mooring measurements are characterized by a two-layer system, with the current variability below about 200 m in opposite phases to that in the upper layer. The mean along-strait velocity (ASV) is toward the Indonesian seas in the whole water column, producing an estimated mean transport of 2.44 ± 0.42 Sv (1 Sv ≡ 106 m3 s−1). The errors of the transport calculation based on the single mooring measurements are estimated to be less than 15% using simulations of high-resolution ocean models. A weak current is observed to flow northward during 2017 at the bottom of the strait. The ASV variability is found to be dominated by an annual cycle both in the upper and lower layers. The total transport, however, is dominated by semiannual variability because of the cancelation of the annual transports in the upper and lower layers. The variability of the transport is suggested to be driven by the pressure difference between the Pacific Ocean and the Indonesian seas, as evidenced by the agreement between the satellite pressure gradient and the two-layer transports. The transport of the Jailolo Strait during the 2015/16 super El Niño is found to be nearly the same as that during the 2016 La Niña, suggesting that the interannual variability of the transport is much smaller than the seasonal cycle.
Abstract
The ocean currents in the Halmahera Sea are studied using a subsurface mooring deployed in the Jailolo Strait from November 2015 to October 2017. The subtidal currents of the mooring measurements are characterized by a two-layer system, with the current variability below about 200 m in opposite phases to that in the upper layer. The mean along-strait velocity (ASV) is toward the Indonesian seas in the whole water column, producing an estimated mean transport of 2.44 ± 0.42 Sv (1 Sv ≡ 106 m3 s−1). The errors of the transport calculation based on the single mooring measurements are estimated to be less than 15% using simulations of high-resolution ocean models. A weak current is observed to flow northward during 2017 at the bottom of the strait. The ASV variability is found to be dominated by an annual cycle both in the upper and lower layers. The total transport, however, is dominated by semiannual variability because of the cancelation of the annual transports in the upper and lower layers. The variability of the transport is suggested to be driven by the pressure difference between the Pacific Ocean and the Indonesian seas, as evidenced by the agreement between the satellite pressure gradient and the two-layer transports. The transport of the Jailolo Strait during the 2015/16 super El Niño is found to be nearly the same as that during the 2016 La Niña, suggesting that the interannual variability of the transport is much smaller than the seasonal cycle.
Abstract
The net surface energy flux is computed as a residual of the energy budget using top-of-atmosphere radiation combined with the divergence of the column-integrated atmospheric energy transports, and then used with the vertically integrated ocean heat content tendencies to compute the ocean meridional heat transports (MHTs). The mean annual cycles and 12-month running mean MHTs as a function of latitude are presented for 2000–16. Effects of the Indonesian Throughflow (ITF), associated with a net volume flow around Australia accompanied by a heat transport, are fully included. Because the ITF-related flow necessitates a return current northward in the Tasman Sea that relaxes during El Niño, the reduced ITF during El Niño may contribute to warming in the south Tasman Sea by allowing the East Australian Current to push farther south even as it gains volume from the tropical waters not flowing through the ITF. Although evident in 2015/16, when a major marine heat wave occurred, these effects can be overwhelmed by changes in the atmospheric circulation. Large interannual MHT variability in the Pacific is 4 times that of the Atlantic. Strong relationships reveal influences from the southern subtropics on ENSO for this period. At the equator, northward ocean MHT arises mainly in the Atlantic (0.75 PW), offset by the Pacific (−0.33 PW) and Indian Oceans (−0.20 PW) while the atmosphere transports energy southward (−0.35 PW). The net equatorial MHT southward (−0.18 PW) is enhanced by −0.1 PW that contributes to the greater warming of the southern (vs northern) oceans.
Abstract
The net surface energy flux is computed as a residual of the energy budget using top-of-atmosphere radiation combined with the divergence of the column-integrated atmospheric energy transports, and then used with the vertically integrated ocean heat content tendencies to compute the ocean meridional heat transports (MHTs). The mean annual cycles and 12-month running mean MHTs as a function of latitude are presented for 2000–16. Effects of the Indonesian Throughflow (ITF), associated with a net volume flow around Australia accompanied by a heat transport, are fully included. Because the ITF-related flow necessitates a return current northward in the Tasman Sea that relaxes during El Niño, the reduced ITF during El Niño may contribute to warming in the south Tasman Sea by allowing the East Australian Current to push farther south even as it gains volume from the tropical waters not flowing through the ITF. Although evident in 2015/16, when a major marine heat wave occurred, these effects can be overwhelmed by changes in the atmospheric circulation. Large interannual MHT variability in the Pacific is 4 times that of the Atlantic. Strong relationships reveal influences from the southern subtropics on ENSO for this period. At the equator, northward ocean MHT arises mainly in the Atlantic (0.75 PW), offset by the Pacific (−0.33 PW) and Indian Oceans (−0.20 PW) while the atmosphere transports energy southward (−0.35 PW). The net equatorial MHT southward (−0.18 PW) is enhanced by −0.1 PW that contributes to the greater warming of the southern (vs northern) oceans.
Abstract
Convective processes in the atmosphere over the Maritime Continent and their diurnal cycles have important repercussions for the circulations in the tropics and beyond. In this work, we present a new dataset of precipitable water vapor (PWV) obtained from the Sumatran GPS Array (SuGAr), a dense network of GPS stations principally for examining seismic and tectonic activity along the western coast of Sumatra and several offshore islands. The data provide an opportunity to examine the characteristics of convection over the area in greater detail than before. In particular, our results show that the diurnal cycle of PWV on Sumatra has a single late afternoon peak, while that offshore has both a midday and a nocturnal peak. The SuGAr data are in good agreement with GPS radio occultation data from the Constellation Observing System for Meteorology, Ionosphere, and Climate (COSMIC) mission, as well as with imaging spectrometer data from the Ozone Measuring Instrument (OMI). A comparison between SuGAr and the NASA Water Vapor Project (NVAP), however, shows significant differences, most likely due to discrepancies in the temporal and spatial resolutions. To further understand the diurnal cycle contained in the SuGAr data, we explore the impact of the Madden–Julian oscillation (MJO) on the diurnal cycle with the aid of the Weather Research and Forecasting (WRF) Model. Results show that the daily mean and the amplitude of the diurnal cycle appear smaller during the suppressed phase relative to the developing/active MJO phase. Furthermore, the evening/nighttime peaks of PWV offshore appear later during the suppressed phase of the MJO compared to the active phase.
Abstract
Convective processes in the atmosphere over the Maritime Continent and their diurnal cycles have important repercussions for the circulations in the tropics and beyond. In this work, we present a new dataset of precipitable water vapor (PWV) obtained from the Sumatran GPS Array (SuGAr), a dense network of GPS stations principally for examining seismic and tectonic activity along the western coast of Sumatra and several offshore islands. The data provide an opportunity to examine the characteristics of convection over the area in greater detail than before. In particular, our results show that the diurnal cycle of PWV on Sumatra has a single late afternoon peak, while that offshore has both a midday and a nocturnal peak. The SuGAr data are in good agreement with GPS radio occultation data from the Constellation Observing System for Meteorology, Ionosphere, and Climate (COSMIC) mission, as well as with imaging spectrometer data from the Ozone Measuring Instrument (OMI). A comparison between SuGAr and the NASA Water Vapor Project (NVAP), however, shows significant differences, most likely due to discrepancies in the temporal and spatial resolutions. To further understand the diurnal cycle contained in the SuGAr data, we explore the impact of the Madden–Julian oscillation (MJO) on the diurnal cycle with the aid of the Weather Research and Forecasting (WRF) Model. Results show that the daily mean and the amplitude of the diurnal cycle appear smaller during the suppressed phase relative to the developing/active MJO phase. Furthermore, the evening/nighttime peaks of PWV offshore appear later during the suppressed phase of the MJO compared to the active phase.
ABSTRACT
We studied the scale interactions of the convectively coupled Kelvin waves (KWs) over the South China Sea (SCS) and Maritime Continent (MC) during December 2016. Three KWs were observed near the equator in this month while the Madden–Julian oscillation (MJO) was inactive. The impacts of these KWs on the rainfall variability of various time scales are diagnosed, including synoptic disturbances, diurnal cycle (DC), and the onset of the Australian monsoon. Four interaction events between the KWs and the westward-propagating waves over the off-equatorial regions were examined; two events led to KW enhancements and the other two contributed to the formation of a tropical depression/tropical cyclone. Over the KW convectively active region of the MC, the DC of precipitation was enhanced in major islands and neighboring oceans. Over the land, the DC hot spots were modulated depending on the background winds and the terrain effects. Over the ocean, the “coastal regime” of the DC appeared at specific coastal areas. Last, the Australian summer monsoon onset occurred with the passage of a KW, which provided favorable conditions of low-level westerlies and initial convection over southern MC and the Arafura Sea. This effect may be helped by the warm sea surface temperature anomalies associated with the La Niña condition of this month. The current results showcase that KWs and their associated scale interactions can provide useful references for weather monitoring and forecast of this region when the MJO is absent.
ABSTRACT
We studied the scale interactions of the convectively coupled Kelvin waves (KWs) over the South China Sea (SCS) and Maritime Continent (MC) during December 2016. Three KWs were observed near the equator in this month while the Madden–Julian oscillation (MJO) was inactive. The impacts of these KWs on the rainfall variability of various time scales are diagnosed, including synoptic disturbances, diurnal cycle (DC), and the onset of the Australian monsoon. Four interaction events between the KWs and the westward-propagating waves over the off-equatorial regions were examined; two events led to KW enhancements and the other two contributed to the formation of a tropical depression/tropical cyclone. Over the KW convectively active region of the MC, the DC of precipitation was enhanced in major islands and neighboring oceans. Over the land, the DC hot spots were modulated depending on the background winds and the terrain effects. Over the ocean, the “coastal regime” of the DC appeared at specific coastal areas. Last, the Australian summer monsoon onset occurred with the passage of a KW, which provided favorable conditions of low-level westerlies and initial convection over southern MC and the Arafura Sea. This effect may be helped by the warm sea surface temperature anomalies associated with the La Niña condition of this month. The current results showcase that KWs and their associated scale interactions can provide useful references for weather monitoring and forecast of this region when the MJO is absent.
Abstract
Variability of oceanic salinity, an indicator of the global hydrological cycle, plays an important role in the basin-scale ocean circulation. In this study, interannual to decadal variability of salinity in the upper layer of the Indian Ocean is investigated using Argo observations since 2004 and data assimilating model outputs (1992–2015). The southeastern Indian Ocean shows the strongest interannual to decadal variability of upper-ocean salinity in the Indian Ocean. Westward propagation of salinity anomalies along isopycnal surfaces is detected in the southern Indian Ocean and attributed to zonal salinity advection anomalies associated with the Indonesian Throughflow and the South Equatorial Current. Composite and salinity budget analyses show that horizontal advection is a major contributor to the interannual to decadal salinity variability of the southern Indian Ocean, and the local air–sea freshwater flux plays a secondary role. The Pacific decadal oscillation (PDO) and El Niño–Southern Oscillation (ENSO) modulate the salinity variability in the southeastern Indian Ocean, with low salinity anomalies occurring during the negative phases of the PDO and ENSO and high salinity anomalies during their positive phases. The Indonesian Throughflow plays an essential role in transmitting the PDO- and ENSO-related salinity signals into the Indian Ocean. A statistical model is proposed based on the PDO index, which successfully predicts the southeastern Indian Ocean salinity variability with a lead time of 10 months.
Abstract
Variability of oceanic salinity, an indicator of the global hydrological cycle, plays an important role in the basin-scale ocean circulation. In this study, interannual to decadal variability of salinity in the upper layer of the Indian Ocean is investigated using Argo observations since 2004 and data assimilating model outputs (1992–2015). The southeastern Indian Ocean shows the strongest interannual to decadal variability of upper-ocean salinity in the Indian Ocean. Westward propagation of salinity anomalies along isopycnal surfaces is detected in the southern Indian Ocean and attributed to zonal salinity advection anomalies associated with the Indonesian Throughflow and the South Equatorial Current. Composite and salinity budget analyses show that horizontal advection is a major contributor to the interannual to decadal salinity variability of the southern Indian Ocean, and the local air–sea freshwater flux plays a secondary role. The Pacific decadal oscillation (PDO) and El Niño–Southern Oscillation (ENSO) modulate the salinity variability in the southeastern Indian Ocean, with low salinity anomalies occurring during the negative phases of the PDO and ENSO and high salinity anomalies during their positive phases. The Indonesian Throughflow plays an essential role in transmitting the PDO- and ENSO-related salinity signals into the Indian Ocean. A statistical model is proposed based on the PDO index, which successfully predicts the southeastern Indian Ocean salinity variability with a lead time of 10 months.
ABSTRACT
The simulated Madden–Julian oscillation (MJO) events in 27 general circulation models (GCMs) are identified using an MJO tracking method. The results suggest that the occurrence frequencies of simulated MJO events can represent a model’s ability to simulate several characteristics of the MJO to a certain extent during boreal winter, such as propagation range, strength, and termination longitude. All tracked MJO events are classified into those that propagate through the Maritime Continent (MC) (MJO-C) and those that do not (MJO-B), and the weakening and blocking effects on MJO propagation by the MC in GCMs were quantified. In general, if a GCM shows a stronger weakening effect on MJO strength over the MC, it tends to produce a stronger blocking effect on MJO propagation over the MC during boreal winter. The barrier effect of the MC on MJO propagation is exaggerated in most GCMs, while it can be underestimated in some GCMs, especially the coupled GCMs. Strong lower-tropospheric premoistening is identified ahead of the MJO convection center when it is over the central MC for MJO-C but not for MJO-B in most GCMs. Such strong premoistening is mainly attributed to the zonal gradient of lower-tropospheric easterly anomalies within the front Walker cell, which could be a precursor leading to the eastward propagation of MJO convection. In contrast to the observation, the role of the background sea surface temperature and land–sea precipitation contrast in the barrier effect on MJO propagation by the MC is not well captured by most GCMs.
ABSTRACT
The simulated Madden–Julian oscillation (MJO) events in 27 general circulation models (GCMs) are identified using an MJO tracking method. The results suggest that the occurrence frequencies of simulated MJO events can represent a model’s ability to simulate several characteristics of the MJO to a certain extent during boreal winter, such as propagation range, strength, and termination longitude. All tracked MJO events are classified into those that propagate through the Maritime Continent (MC) (MJO-C) and those that do not (MJO-B), and the weakening and blocking effects on MJO propagation by the MC in GCMs were quantified. In general, if a GCM shows a stronger weakening effect on MJO strength over the MC, it tends to produce a stronger blocking effect on MJO propagation over the MC during boreal winter. The barrier effect of the MC on MJO propagation is exaggerated in most GCMs, while it can be underestimated in some GCMs, especially the coupled GCMs. Strong lower-tropospheric premoistening is identified ahead of the MJO convection center when it is over the central MC for MJO-C but not for MJO-B in most GCMs. Such strong premoistening is mainly attributed to the zonal gradient of lower-tropospheric easterly anomalies within the front Walker cell, which could be a precursor leading to the eastward propagation of MJO convection. In contrast to the observation, the role of the background sea surface temperature and land–sea precipitation contrast in the barrier effect on MJO propagation by the MC is not well captured by most GCMs.
Abstract
An unfiltered zonal Hovmöller depiction of rainfall in the Maritime Continent (MC) reveals remarkable spatiotemporal continuity of zonally propagating disturbances with a diurnal period, which endure over multiple days and propagate faster than the individual convective storms they coupled with. This phenomenon and its sensitivity to the Madden–Julian oscillation (MJO) during the 2011/12 Dynamics of the MJO (DYNAMO) field campaign is examined here through a well-validated, convection-permitting model simulation conducted on a large domain. We find that these disturbances are zonally propagating diurnal gravity waves excited by vigorous nocturnal mesoscale convective systems over Sumatra and Borneo. These gravity waves are diurnally phase locked: their wavelength very closely matches the distance between these two islands (~1500 km), while their particular zonal phase speed (~±17 m s−1) allows them to propagate this distance in one diurnal cycle. We therefore hypothesize that these waves are amplified by resonant interaction due to diurnal phase locking. While these zonal gravity waves decouple from convection once beyond the MC, their divergent flow signature endures well across the Indian Ocean, provoking the notion that they may influence rainfall at far remote locations. The exact controls over this zonal phase speed remain uncertain; we note, however, that it is roughly consistent with diurnal offshore-propagating modes documented previously. Further study is required to tie this down, and more generally, to understand the sensitivity of these modes to background flow strength and the geography of the MC.
Abstract
An unfiltered zonal Hovmöller depiction of rainfall in the Maritime Continent (MC) reveals remarkable spatiotemporal continuity of zonally propagating disturbances with a diurnal period, which endure over multiple days and propagate faster than the individual convective storms they coupled with. This phenomenon and its sensitivity to the Madden–Julian oscillation (MJO) during the 2011/12 Dynamics of the MJO (DYNAMO) field campaign is examined here through a well-validated, convection-permitting model simulation conducted on a large domain. We find that these disturbances are zonally propagating diurnal gravity waves excited by vigorous nocturnal mesoscale convective systems over Sumatra and Borneo. These gravity waves are diurnally phase locked: their wavelength very closely matches the distance between these two islands (~1500 km), while their particular zonal phase speed (~±17 m s−1) allows them to propagate this distance in one diurnal cycle. We therefore hypothesize that these waves are amplified by resonant interaction due to diurnal phase locking. While these zonal gravity waves decouple from convection once beyond the MC, their divergent flow signature endures well across the Indian Ocean, provoking the notion that they may influence rainfall at far remote locations. The exact controls over this zonal phase speed remain uncertain; we note, however, that it is roughly consistent with diurnal offshore-propagating modes documented previously. Further study is required to tie this down, and more generally, to understand the sensitivity of these modes to background flow strength and the geography of the MC.
Abstract
The diurnal cycle over tropical coastal waters is characterized by offshore migration of precipitation area during nighttime. This study analyzes in situ observational data collected during the YMC-Sumatra 2017 field campaign around the western coast of Sumatra Island, Indonesia, to examine the offshore migration phenomenon during 5–31 December 2017, when the Research Vessel Mirai was deployed about 90 km off the coast to perform observation. The offshore migration is observed in only less than a half of the 27 days. A comparison of radiosonde data at the vessel between days with and without the offshore migration reveals that vertical wind shear in the lower troposphere is a key environmental condition. In late afternoon of the days with the offshore migration, offshore (northeasterly) wind shear with height with considerable magnitude is observed, which is due to weaker daily mean southwesterly wind in the lower free troposphere, stronger southwesterly wind in the boundary layer, and sea breeze. As this condition is considered favorable for regeneration of convective cells to the offshore side of old ones, these results support an idea that the regeneration process is critical for the offshore migration. The Madden–Julian oscillation and cold surges play some roles in the weakening of the free-tropospheric wind. The migration speed is estimated at 2–3 m s−1, which is lower than that observed in another field campaign conducted in 2015 (Pre-YMC 2015). This difference is partly due to the difference in the environmental wind in the lower to midtroposphere.
Abstract
The diurnal cycle over tropical coastal waters is characterized by offshore migration of precipitation area during nighttime. This study analyzes in situ observational data collected during the YMC-Sumatra 2017 field campaign around the western coast of Sumatra Island, Indonesia, to examine the offshore migration phenomenon during 5–31 December 2017, when the Research Vessel Mirai was deployed about 90 km off the coast to perform observation. The offshore migration is observed in only less than a half of the 27 days. A comparison of radiosonde data at the vessel between days with and without the offshore migration reveals that vertical wind shear in the lower troposphere is a key environmental condition. In late afternoon of the days with the offshore migration, offshore (northeasterly) wind shear with height with considerable magnitude is observed, which is due to weaker daily mean southwesterly wind in the lower free troposphere, stronger southwesterly wind in the boundary layer, and sea breeze. As this condition is considered favorable for regeneration of convective cells to the offshore side of old ones, these results support an idea that the regeneration process is critical for the offshore migration. The Madden–Julian oscillation and cold surges play some roles in the weakening of the free-tropospheric wind. The migration speed is estimated at 2–3 m s−1, which is lower than that observed in another field campaign conducted in 2015 (Pre-YMC 2015). This difference is partly due to the difference in the environmental wind in the lower to midtroposphere.
Abstract
The two-way interaction between Madden–Julian oscillation (MJO) and higher-frequency waves (HFW) over the Maritime Continent (MC) during boreal winter of 1984–2005 is investigated. It is noted from observational analysis that strengthened (weakened) HFW activity appears to the west (east) of and under MJO convection during the MJO active phase and the opposite is seen during the MJO suppressed phase. Sensitivity model experiments indicate that the control of HFW activity by MJO is through change of the background vertical wind shear and specific humidity. The upscale feedbacks from HFW to MJO through nonlinear rectification of condensational heating and eddy momentum transport are also investigated with observational data. A significantly large amount (25%–40%) of positive heating anomaly (
Abstract
The two-way interaction between Madden–Julian oscillation (MJO) and higher-frequency waves (HFW) over the Maritime Continent (MC) during boreal winter of 1984–2005 is investigated. It is noted from observational analysis that strengthened (weakened) HFW activity appears to the west (east) of and under MJO convection during the MJO active phase and the opposite is seen during the MJO suppressed phase. Sensitivity model experiments indicate that the control of HFW activity by MJO is through change of the background vertical wind shear and specific humidity. The upscale feedbacks from HFW to MJO through nonlinear rectification of condensational heating and eddy momentum transport are also investigated with observational data. A significantly large amount (25%–40%) of positive heating anomaly (
Abstract
The present study shows that winter cold events over eastern China can be induced by Madden–Julian oscillation (MJO)-associated anomalous convection over the Maritime Continent. We conduct composite analysis separately for identified intraseasonal cold events over eastern China that occur following anomalous convection over the Maritime Continent and the tropical Indian Ocean. For cold events related to anomalous convection over the Maritime Continent, the southward intrusion of cold air into eastern China takes an eastward path in association with an eastward location of an anomalous Siberian high compared to cold events related to anomalous convection over the tropical Indian Ocean. The Maritime Continent convection-related cold events tend to occur with a negative Arctic Oscillation (AO), whereas the relationship between the tropical Indian Ocean convection-related cold events and the AO is weak. Anomalous convective heating over the Maritime Continent triggers a poleward Rossby wave train, which, together with an AO-related southward wave train from northern Eurasia, contributes to the deepening of the East Asian trough. The poleward wave energy dispersion is similarly triggered by anomalous convective heating over the tropical Indian Ocean. In both types of cold events, anomalous tropical heating induces a meridional vertical circulation, with large-scale airmass convergence in the upper midtroposphere and descending of air on the northern branch of the vertical cell over Siberia. The upper-level mass convergence and the radiative cooling over Siberia work together for the enhancement and southeastward expansion of the Siberian high and the southward intrusion of cold anomalies to eastern China.
Abstract
The present study shows that winter cold events over eastern China can be induced by Madden–Julian oscillation (MJO)-associated anomalous convection over the Maritime Continent. We conduct composite analysis separately for identified intraseasonal cold events over eastern China that occur following anomalous convection over the Maritime Continent and the tropical Indian Ocean. For cold events related to anomalous convection over the Maritime Continent, the southward intrusion of cold air into eastern China takes an eastward path in association with an eastward location of an anomalous Siberian high compared to cold events related to anomalous convection over the tropical Indian Ocean. The Maritime Continent convection-related cold events tend to occur with a negative Arctic Oscillation (AO), whereas the relationship between the tropical Indian Ocean convection-related cold events and the AO is weak. Anomalous convective heating over the Maritime Continent triggers a poleward Rossby wave train, which, together with an AO-related southward wave train from northern Eurasia, contributes to the deepening of the East Asian trough. The poleward wave energy dispersion is similarly triggered by anomalous convective heating over the tropical Indian Ocean. In both types of cold events, anomalous tropical heating induces a meridional vertical circulation, with large-scale airmass convergence in the upper midtroposphere and descending of air on the northern branch of the vertical cell over Siberia. The upper-level mass convergence and the radiative cooling over Siberia work together for the enhancement and southeastward expansion of the Siberian high and the southward intrusion of cold anomalies to eastern China.
