[1] A recent study shows that the blockage of the southwest monsoon by the mountain range on the east coast of Indochina triggers a chain of ocean-atmospheric response, including a wind jet and cold filament in the South China Sea (SCS). We extend this climatological analysis by using higher temporal resolution (weekly) to study intraseasonal variability in summer. Our analysis shows that the development of the wind jet and cold filament is not a smooth seasonal process but consists of several intraseasonal events each year at about 45-day intervals. In a typical intraseasonal event, the wind jet intensifies to above 12 m/s, followed in a week by the development of a cold filament advected by an offshore jet east of South Vietnam on the boundary of a double gyre circulation in the ocean. The double gyre circulation itself also strengthens in response to the intraseasonal wind event via Rossby wave adjustment, reaching the maximum strength in 2 to 3 weeks. The intraseasonal cold filaments appear to influence the surface wind, reducing the local wind speed because of the increased static stability in the near-surface atmosphere. To first order, the above sequence of events may be viewed as the SCS response to atmospheric intraseasonal wind pulses, which are part of the planetary-scale boreal summer intraseasonal oscillation characterized by the northeastward propagation of atmospheric deep convection. The intraseasonal anomalies of sea surface temperature and precipitation are in phase over the SCS, suggesting an oceanic feedback onto the atmosphere. As wind variations are now being routinely monitored by satellite, the lags of 1-3 weeks in oceanic response offer useful predictability that may be exploited.
[1] Subseasonal variability in sea surface height (SSH) over the East Pacific warm pool off Central America is investigated using satellite observations and an eddy-resolving ocean general circulation model. SSH variability is organized into two southwest-tilted bands on the northwest flank of the Tehuantepec and Papagayo wind jets and collocated with the thermocline troughs. Eddy-like features of wavelength $600 km propagate southwestward along the high-variance bands at a speed of 9-13 cm/s. Wind fluctuations are important for eddy formation in the Gulf of Tehuantepec, with a recurring interval of 40-90 days. When forced by satellite wind observations, the model reproduces the two high-variance bands and the phase propagation of the Tehuantepec eddies. Our observational analysis and model simulation suggest the following evolution of the Tehuantepec eddies. On the subseasonal timescale, in response to the gap wind intensification, a coastal anticyclonic eddy forms on the northwest flank of the wind jet and strengthens as it propagates offshore in the following two to three weeks. An energetics analysis based on the model simulation indicates that besides wind work, barotropic and baroclinic instabilities of the mean flow are important for the eddy growth. Both observational and model results suggest a re-intensification of the anticyclonic eddy in response to the subsequent wind jet event. Off Papagayo, ocean eddy formation is not well correlated with local wind jet variability. In both the Gulfs of Tehuantepec and Papagayo, subseasonal SSH variability is preferentially excited on the northwest flank of the wind jet. Factors for this asymmetry about the wind jet axis as well as the origins of wind jet variability are discussed.
This study uses the method of self-organizing maps (SOMs) to categorize the June–August atmospheric teleconnections in the 500-hPa geopotential height field of the Southern Hemisphere (SH) extratropics. This approach yields 12 SOM patterns that provide a discretized representation of the continuum of SH teleconnection patterns from 1979 to 2012. These 12 patterns are large in spatial scale, exhibiting a mix of annular mode characteristics and wave trains of zonal wavenumber varying from 2 to 4. All patterns vary with intrinsic time scales of about 5–10 days, but some patterns exhibit quasi-oscillatory behavior over a period of 20–30 days, whereas still others exhibit statistically significant enhanced and suppressed frequencies up to about four weeks in association with the Madden–Julian oscillation. Two patterns are significantly influenced by El Niño–Southern Oscillation (ENSO) on interannual time scales. All 12 patterns have strong influences on surface air temperature and sea ice concentrations, with the sea ice response occurring over a time scale of about 2–4 weeks. The austral winter has featured a positive frequency trend in patterns that project onto the negative phase of the southern annular mode (SAM) and a negative frequency trend in positive SAM-like patterns. Such atmospheric circulation trends over 34 yr may arise through atmospheric internal variability alone, and, unlike other seasons in the SH, it is not necessary to invoke external forcing as a dominant source of circulation trends.
Southern Ocean organic carbon export plays an important role in the global carbon cycle, yet its basin-scale climatology and variability are uncertain due to limited coverage of in situ observations. In this study, a neural network approach based on the self-organizing map (SOM) is adopted to construct weekly gridded (1° × 1°) maps of organic carbon export for the Southern Ocean from 1998 to 2009. The SOM is trained with in situ measurements of O2 / Ar-derived net community production (NCP) that are tightly linked to the carbon export in the mixed layer on timescales of 1–2 weeks, and six potential NCP predictors: photosynthetically available radiation (PAR), particulate organic carbon (POC), chlorophyll (Chl), sea surface temperature (SST), sea surface height (SSH), and mixed layer depth (MLD). This non-parametric approach is based entirely on the observed statistical relationships between NCP and the predictors, and therefore is strongly constrained by observations.
A thorough cross-validation yields three retained NCP predictors, Chl, PAR, and MLD. Our constructed NCP is further validated by good agreement with previously published independent in situ derived NCP of weekly or longer temporal resolution through real-time and climatological comparisons at various sampling sites. The resulting November–March NCP climatology reveals a pronounced zonal band of high NCP roughly following the subtropical front in the Atlantic, Indian and western Pacific sectors, and turns southeastward shortly after the dateline. Other regions of elevated NCP include the upwelling zones off Chile and Namibia, Patagonian Shelf, Antarctic coast, and areas surrounding the Islands of Kerguelen, South Georgia, and Crozet. This basin-scale NCP climatology closely resembles that of the satellite POC field and observed air-sea CO2 flux. The long-term mean area-integrated NCP south of 50° S from our dataset, 14 mmol C m–2 d–1, falls within the range of 8.3–24 mmol C m–2 d–1 from other model estimates. A broad agreement is found in the basin-wide NCP climatology among various models but with significant spatial variations, particularly in the Patagonian Shelf. Our approach provides a comprehensive view of the Southern Ocean NCP climatology and a potential opportunity to further investigate interannual and intraseasonal variability
During the modern satellite era since 1979, the Pacific Walker circulation (PWC) experienced an intensification and a westward shift, which has broad impacts on the global climate variability. While the strengthening of the PWC has been shown to be driven by both the regional Pacific sea surface temperature (SST) and the remote forcing from other basins, its westward shift is primarily attributed to the phase change of the Atlantic Multidecadal variability. In this study, we investigate the potential effect of the remote SST forcing from the Atlantic and the Indian Oceans on the westward shift of the PWC, through statistical analysis and numerical experiments using atmospheric and coupled models. Results show that the tropical Atlantic warming plays a key (decisive) role in driving the PWC westward shift by triggering a Gill–Matsuno-type circulation anomaly in the tropics. This circulation response drives anomalous surface westerlies over the eastern Pacific and subsidence over the central Pacific that weakens the eastern part of the PWC, meanwhile generating easterly wind anomalies over the central-western Pacific and anomalous atmospheric convection over the western Pacific that intensifies the western part of the PWC. This direct forcing contributes ~ 32% of the observed PWC movement, while the Atlantic-induced inter-basin SST changes contribute another ~ 36% of its westward shift according to coupled model simulation results. Our results reinforce the importance of the inter-basin interactions in adjusting the tropical climate variabilities, and have broad implication for projecting the global climate.
The Pacific Walker circulation (WC) is a major component of the global climate system. It connects the Pacific sea surface temperature (SST) variability to the climate variabilities from the other ocean basins to the mid- and high latitudes. Previous studies indicated that the ENSO-related atmospheric feedback, in particular, the surface wind response is largely underestimated in AMIP and CMIP models. In this study, we further investigate the responses in the WC stream function and the sea level pressure (SLP) to the ENSO-related SST variability by comparing the responses in 45 AMIP and 63 CMIP models and six reanalysis datasets. We reveal a diversity in the performances of simulated SLP and WC between different models. While the SLP responses to the El Niño-related SST variability are well simulated in most of the atmospheric and coupled models, the WC stream function responses are largely underestimated in most of these models. The WC responses in the AMIP5/6 models capture ~ 75% of those in the reanalysis, whereas the CMIP5/6 models capture ~ 58% of the responses. Further analysis indicates that these underestimated circulation responses could be partially attributed to the biases in the precipitation scheme in both the atmospheric and coupled models, as well as the biases in the simulated ENSO-related SST patterns in the coupled models. One should pay special attention to these biases when studying the WC or the tropical atmosphere–ocean interactions using numerical models.
Abstract. Southern Ocean organic carbon export plays an important role in the global carbon cycle, yet its basin-scale climatology and variability are uncertain due to limited coverage of in situ observations. In this study, a neural network approach based on the self-organizing map (SOM) is adopted to construct weekly gridded (1 • × 1 • ) maps of organic carbon export for the Southern Ocean from 1998 to 2009. The SOM is trained with in situ measurements of O 2 / Ar-derived net community production (NCP) that are tightly linked to the carbon export in the mixed layer on timescales of one to two weeks and with six potential NCP predictors: photosynthetically available radiation (PAR), particulate organic carbon (POC), chlorophyll (Chl), sea surface temperature (SST), sea surface height (SSH), and mixed layer depth (MLD). This nonparametric approach is based entirely on the observed statistical relationships between NCP and the predictors and, therefore, is strongly constrained by observations.A thorough cross-validation yields three retained NCP predictors, Chl, PAR, and MLD. Our constructed NCP is further validated by good agreement with previously published, independent in situ derived NCP of weekly or longer temporal resolution through real-time and climatological comparisons at various sampling sites. The resulting NovemberMarch NCP climatology reveals a pronounced zonal band of high NCP roughly following the Subtropical Front in the Atlantic, Indian, and western Pacific sectors, and turns southeastward shortly after the dateline. Other regions of elevated NCP include the upwelling zones off Chile and Namibia, the Patagonian Shelf, the Antarctic coast, and areas surrounding the Islands of Kerguelen, South Georgia, and Crozet. This basin-scale NCP climatology closely resembles that of the satellite POC field and observed air-sea CO 2 flux. The long-term mean area-integrated NCP south of 50 • S from our dataset, 17.9 mmol C m −2 d −1 , falls within the range of 8.3 to 24 mmol C m −2 d −1 from other model estimates. A broad agreement is found in the basin-wide NCP climatology among various models but with significant spatial variations, particularly in the Patagonian Shelf. Our approach provides a comprehensive view of the Southern Ocean NCP climatology and a potential opportunity to further investigate interannual and intraseasonal variability.
The Arctic has experienced dramatic climate changes, characterized by rapid surface warming and sea-ice loss over the past four decades, with broad implications for climate variability over remote regions. Some studies report that Arctic warming may simultaneously induce a widespread cooling over Eurasia and frequent cold events over North America, especially during boreal winter. In contrast, other studies suggest a seesaw pattern of extreme temperature events with cold weather over East Asia accompanied by warm weather in North America on sub-seasonal time scales. It is unclear whether a systematic linkage in surface air temperature exists between the two continents, let alone their interaction with Arctic sea ice. Here, we reveal a dipole pattern of surface air temperature in boreal winter featuring a cooling (warming) in the Eurasian continent accompanied by a warming (cooling) in the North American continent, which is induced by an anomalous Barents-Kara sea-ice decline (increase). The dipole operates on interannual and multidecadal time scales. We find that an anomalous sea-ice loss over the Barents-Kara Seas triggers a wavenumber one atmospheric circulation pattern over the high-latitude Northern Hemisphere, with an anomalous high-pressure center over Siberia and an anomalous low-pressure center over high-latitude North America. The circulation adjustment generates the dipole temperature pattern through thermal advection. Our finding has important implications for Northern Hemisphere climate variability, extreme weather events, and their prediction and projection.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.