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Yukio Kurihara

Abstract

Stripe noise is a common issue in sea surface temperatures (SSTs) retrieved from thermal infrared data obtained by satellite-based multidetector radiometers. We developed a bispectral filter (BSF) to reduce the stripe noise. The BSF is a Gaussian filter and an optimal estimation method for the differences between the data obtained at the split window. A kernel function based on the physical processes of radiative transfer has made it possible to reduce stripe and random noise in retrieved SSTs without degrading the spatial resolution or generating bias. The Second-Generation Global Imager (SGLI) is an optical sensor on board the Global Change Observation Mission–Climate (GCOM-C) satellite. We applied the BSF to SGLI data and validated the retrieved SSTs. The validation results demonstrate the effectiveness of BSF, which reduced stripe noise in the retrieved SGLI SSTs without blurring SST fronts. It also improved the accuracy of the SSTs by about 0.04 K (about 13%) in the robust standard deviation.

Significance Statement

This method reduces stripe noise and improves the accuracy of SST data with minimal compromise of spatial resolution. The method assumes the relationship between the brightness temperature and the brightness temperature difference in the split window based on the physical background of atmospheric radiative transfer. The physical background of the data provides an easy solution to a complex problem. Although destriping generally requires a complex algorithm, our approach is based on a simple Gaussian filter and is easy to implement.

Open access
Clemente Lopez-Bravo
,
Claire L. Vincent
,
Yi Huang
, and
Todd P. Lane

Abstract

A West Sumatra squall line occurred on 10 January 2016, with a clear offshore propagation of convection. Satellite-derived products from Himawari-8 Advanced Himawari Imager and the Geostationary Cloud Algorithm Testbed Geocat are used to investigate the westward propagation of cloudiness from Sumatra to the Indian Ocean with a lifetime of 1.5 days. A convective mask based on deep convective cell detection and a cell-tracking algorithm are used to estimate the propagation speed of the cloud system. Two distinct mesoscale convective responses are identified: 1) a rapid development in South Sumatra is influenced by the convective environment over the Indian Ocean. The propagation speed is estimated to be ∼5 m s−1 within the first 200 km from the coast. This speed is consistent with density currents. In contrast, 2) the coupling to the inertia–gravity wave is only evident for the northwest of Sumatra with speeds of ∼12 m s−1. The analysis of brightness temperature from the 10.4-μm spectral band and cloud-top temperature showed that the lifetime of the squall line is approximately 30 h with a propagating distance of ∼1000 km. Retrieved cloud properties and tracking of the offshore propagation indicated that the cloud structure consisted of multiple types of cells, propagating as envelopes of convection, and revealed the influence of large-scale variability of the Indian Ocean. Filtered OLR anomalies, satellite-derived rainfall, moisture flux convergence, and background winds flow around Sumatra are used to explore the effects of Kelvin wave activity that likely influenced the lifetime of the squall line.

Restricted access
S. Sharmila
,
H. Hendon
,
O. Alves
,
A. Weisheimer
, and
M. Balmaseda

Abstract

Despite the growing demand for long-range ENSO predictions beyond 1 year, quantifying the skill at these lead times remains limited. This is partly due to inadequate long records of seasonal reforecasts that make skill estimates of irregular ENSO events quite challenging. Here, we investigate ENSO predictability and the dependency of prediction skill on the ENSO cycle using 110 years of 24-month-long 10-member ensemble reforecasts from ECMWF’s coupled model (SEAS5-20C) initialized on 1 November and 1 May during 1901–2010. Results show that Niño-3.4 SST can be skillfully predicted up to ∼18 lead months when initialized on 1 November, but skill drops at ∼12 lead months for May starts that encounter the boreal spring predictability barrier in year 2. The skill beyond the first year is highly conditioned to the phase of ENSO: Forecasts initialized at peak El Niño are more skillful in year 2 than those initialized at peak La Niña, with the transition to La Niña being more predictable than to El Niño. This asymmetry is related to the subsurface initial conditions in the western equatorial Pacific: peak El Niño states evolving into La Niña are associated with strong upper-ocean heat discharge of the western Pacific, the memory of which stays beyond 1 year. In contrast, the western Pacific recharged state associated with La Niña is usually weaker and shorter-lived, being a weaker preconditioner for subsequent El Niño, the year after. High prediction skill of ENSO events beyond 1 year provides motivation for extending the lead time of operational seasonal forecasts up to 2 years.

Open access
Antonios Dimitrelos
,
Rodrigo Caballero
, and
Annica M. L. Ekman

Abstract

The main energy input to the polar regions in winter is the advection of warm, moist air from lower latitudes. This makes the polar climate sensitive to the temperature and moisture of extrapolar air. Here, we study this sensitivity from an air-mass transformation perspective. We perform simulations of an idealized maritime air mass brought into contact with sea ice employing a three-dimensional large-eddy simulation model coupled to a one-dimensional multilayer sea ice model. We study the response of cloud dynamics and surface warming during the air-mass transformation process to varying initial temperature and humidity conditions of the air mass. We find in all cases that a mixed-phase cloud is formed, initially near the surface but rising continuously with time. Surface warming of the sea ice is driven by downward longwave surface fluxes, which are largely controlled by the temperature and optical depth of the cloud. Cloud temperature, in turn, is robustly constrained by the initial dewpoint temperature of the air mass. Since dewpoint only depends on moisture, the overall result is that surface warming depends almost exclusively on initial humidity and is largely independent of initial temperature. We discuss possible climate implications of this result—in particular, for polar amplification of surface warming and the role played by atmospheric energy transports.

Open access
Timothy H. Raupach
,
Joshua Soderholm
,
Alain Protat
, and
Steven C. Sherwood

Abstract

We evaluated the performance in Australia of proxies designed to identify atmospheric conditions prone to hail and severe storms. In a convection-resolving but short-duration simulation, proxies that use instability and wind shear thresholds overestimated the probability of hail occurring when compared to the estimated occurrence of surface graupel in the model, particularly in Australia’s tropical north. We used reanalysis data and the Australian Bureau of Meteorology severe storm archive to examine atmospheric conditions at times and locations when hailstorms, other storms, and no storms were reported between January 1979 and March 2021. In instability–shear space, the best discriminator between hail and no-storm times was found to vary predictably with melting-level height, allowing a new proxy to better represent latitudinal trends in atmospheric conditions. We found extra conditions that can be applied to the new proxy to efficiently reduce the number of false alarms. The new proxy outperforms the tested existing proxies for detection of hail-prone conditions in Australia.

Significance Statement

Hail proxies take a description of the atmosphere, such as its temperature, moisture content, and wind properties at various heights, and determine the likelihood of hail forming and hitting the ground. It is a difficult task prone to uncertainty, but in many locations there are no direct observations of hail, and in these places information from proxies is valuable. Existing proxies have a tendency to overestimate the probability of hail falling in the north of Australia. In this study we developed an updated proxy that uses information about the atmosphere’s melting-level height to refine its hail predictions. The new proxy outperforms other tested proxies for hail in Australia. Accurate hail proxies are important for assessment of past and future changes to hail hazard and risk.

Restricted access
Qihua Peng
,
Shang-Ping Xie
,
Rui Xin Huang
,
Weiqiang Wang
,
Tingting Zu
, and
Dongxiao Wang

Abstract

The Indonesian Throughflow (ITF) is projected to slow down under anthropogenic warming. Several mechanisms—some mutually conflicting—have been proposed but the detailed processes causing this slowdown remain unclear. By turning on/off buoyancy and wind forcings globally and in key regions, this study investigates the dynamical adjustments underlying the centennial ITF slowdown in the global oceans and climate models. Our results show that the projected weakened ITF transport in the top 1500 m is dominated by remote anomalous buoyancy forcing in the North Atlantic Ocean. Specifically, surface freshening and warming over the North Atlantic Ocean slow the Atlantic meridional overturning circulation (AMOC), and the resultant dynamic signals propagate through the coastal-equatorial waveguide into the southeastern Indian Ocean and western Pacific Ocean, causing the reduction of ITF transport over a deep layer. In contrast, the anomalous surface buoyancy flux in the Indo-Pacific affects the ocean temperature and salinity in a shallow upper layer, resulting in ITF changes in forms of high baroclinic mode structure with negligible impacts on the net ITF transport. A vertical partitioning index is proposed to distinguish the remote forcing via the AMOC and regional forcing in the Indo-Pacific Ocean, which could be useful for monitoring, attributing, and predicting the changing ITF transport under global warming.

Restricted access
Ori Adam
,
Alexander Farnsworth
, and
Daniel J. Lunt

Abstract

The tropical rain belt varies between unimodal and bimodal meridional precipitation distributions, both regionally and on seasonal to geological time scales. Here we show that this variation is largely driven by equatorial precipitation inhibition, and quantify it using an equatorial modality index (EMI) that varies continuously between 1 and 2 for purely unimodal and bimodal distributions. We show that tropical modality is a fundamental characteristic of tropical climate, which we define as annual-mean EMI. We examine large-scale aspects of tropical modality across 73 climate models from phases 5 and 6 of the Coupled Model Intercomparison Project, 45 paleo simulations (∼300 million years ago to present), and observations. We find increased tropical modality to be strongly related to increased width of the tropical rain belt, wider and weaker meridional overturning circulation, colder equatorial cold tongues, and more severe double intertropical convergence zone bias in modern climate models. Tropical sectors (or global zonal means) with low tropical modality are characterized by monsoonal seasonal variations (i.e., seasonal migrations of rainbands following the sun). In sectors with high tropical modality we identify three important seasonal modes: (i) migration of the precipitation distribution toward the warmer hemisphere, (ii) variation in the latitudinal separation between hemispheric rainbands, and (iii) seesaw variation in the intensity of the hemispheric rainbands. In high tropical modality sectors, due to contrasting shifts of the migration and separation modes, counter to general wisdom, seasonal migrations of tropical rainbands cannot be generally assumed to follow the sun.

Significance Statement

The tropical rain belt is a band of intense precipitation that encircles the tropics. Important tropical phenomena such as monsoons and seasonal shifts of marine rainbands are driven by seasonal migrations of the tropical rain belt, which therefore govern key socioeconomic aspects of tropical populations. This work examines how changes in the north–south profile of tropical precipitation affect large-scale aspects of tropical climate, on seasonal to geological time scales. Specifically, we examine the tendency of the profile of the tropical rain belt to vary from having one to two peaks (i.e., from being unimodal to bimodal). We define an objective quantitative measure of this modality variation, which varies between 1 and 2 for unimodal and bimodal profiles. We then show that the annual mean of this measure is an important general characteristic of tropical climate, which we define as tropical modality. We also show that in tropical regions where tropical modality is low (close to 1), rainbands follow the sun in their seasonal migrations, and conform to the canonical model of the tropical overturning circulation, known as the Hadley circulation, which goes along with monsoonal seasonal variations. However, in regions with high tropical modality (i.e., close to 2), the common theoretical expectation that rainbands follow the sun (or migrate toward the warming hemisphere) is not generally justified. Instead, we identify three important independent seasonal modes of variation: (i) migration of the precipitation distribution toward the warmer hemisphere, (ii) variation in the latitudinal separation between hemispheric rainbands (or width of the precipitation profile), and (iii) seesaw variation in the intensity of the hemispheric rainbands.

Restricted access
D. L. Suhas
and
William R. Boos

Abstract

Transient, synoptic-scale vortices produce a large fraction of total rainfall in most monsoon regions and are often associated with extreme precipitation. However, the mechanism of their amplification remains a topic of active research. For monsoon depressions, which are the most prominent synoptic-scale vortex in the Asian–Australian monsoon, recent work has suggested that meridional gradients in zonal wind in the vortex environment may produce growth through barotropic instability, while meridional gradients in environmental humidity have also been proposed to cause amplification through coupling with precipitating convection. Here, a two-dimensional shallow water model on a sphere with parameterized precipitation is used to examine the relative role played by these two environmental gradients. By systematically varying the meridional moisture gradient and meridional wind shear for both weak, quasi-linear waves and finite-amplitude isolated vortices, we show that rotational winds in the initial vortex are amplified most strongly by meridional shear of the environmental zonal wind, while vortex precipitation rates are most sensitive to environmental moisture gradients. The growth rate in the presence of both gradients is less than the sum of growth rates in the presence of isolated gradients, as the phase relation between moisture and vorticity anomalies becomes distorted with increasing shear. These results suggest that background meridional gradients in both zonal wind and environmental humidity can contribute to the amplification of vortices to monsoon depression strength, but with some degree of decoupling of the dry rotational flow and the moist convection.

Restricted access
Laura E. Queen
,
Sam Dean
,
Dáithí Stone
,
Roddy Henderson
, and
James Renwick

Abstract

Anthropogenic climate change is affecting rivers worldwide, threatening water availability and altering the risk of natural hazards. Understanding the pattern of regional streamflow trends can help to inform region-specific policies to mitigate and adapt to any negative impacts on society and the environment. We present a benchmark dataset of long, near-natural streamflow records across Aotearoa New Zealand (NZ) and the first nationwide analysis of observed spatiotemporal streamflow trends. Individual records rarely have significant trends, but when aggregated within homogenous hydrologic regions (determined through cluster analyses), significant regional trends emerge. A multitemporal approach that uses all available data for each region and considers trend significance over time reveals the influence of decadal variability in some seasons and regions, and consistent trends in others. Over the last 50+ years, winter streamflow has significantly increased in the west South Island and has significantly decreased in the north North Island; summer streamflow has significantly decreased for most of the North Island; autumn streamflow has generally dried nationwide; and spring streamflow has increased along the west coast and decreased along the east coast. Correlations between streamflow and dynamic and thermodynamic climate indices reveal the dominant drivers of hydrologic behavior across NZ. Consistencies between the observed near-natural streamflow trends and observed changes in circulation and thermodynamic processes suggest possible climate change impacts on NZ hydrology.

Open access
Mengmeng Lu
,
Song Yang
,
Congwen Zhu
,
Junbin Wang
,
Shuheng Lin
,
Wei Wei
, and
Hanjie Fan

Abstract

While it is commonly accepted that the thermal effect of the Tibetan Plateau (TP) strengthens the Asian summer monsoon, a recent analysis based mainly on idealized model experiments revealed that the TP effect weakened the Southeast Asian summer monsoon (SEASM). Based on both observational analyses and model experiments, the current study further deciphers the physical mechanism for the TP’s thermal impact on the SEASM and the modulation of this impact by the sea surface temperature (SST) in the tropical Atlantic. When diabatic heating is enhanced over the southern TP, the South Asian high (SAH) intensifies and extends eastward, leading to convergence over the southeastern flank of the anomalous upper-level anticyclone and sinking motion that cause downward advection of negative vorticity. Accompanied by this anomalous anticyclonic pattern, the western Pacific subtropical high (WPSH) extends westward and the monsoon over Southeast Asia is weakened. The TP–SEASM relationship is enhanced when SST and convection increase over the tropical Atlantic, which cause an anomalous barotropic wave train propagating southeastward from eastern North America to East Asia, leading to an eastward extension of the SAH and a westward extension of the WPSH. The anomalous heating over the tropical Atlantic also modulates the Walker circulation through two anomalous vertical cells, with ascending motions over the Maritime Continent and the eastern tropical Indian Ocean, inducing a lower-level anticyclone over Southeast Asia as a Gill-type response. Thus, a warming tropical Atlantic can intensify the TP’s thermal forcing, weaken the SEASM, and then modulate the TP–SEASM relationship through both the extratropical wave train and the tropical zonal circulation.

Significance Statement

The Southeast Asian summer monsoon (SEASM) exhibits significant interannual variability. This study is aimed at better understanding the thermal impact of the Tibetan Plateau (TP) on the interannual variability of SEASM intensity and the possible modulating effect of the tropical Atlantic on the TP–SEASM relationship. We find that enhanced heating over the southern TP weakens the SEASM circulation and its associated precipitation by extending the South Asian high eastward and expanding the western Pacific subtropical high westward. Tropical Atlantic warming enhances this TP–SEASM relationship through both the extratropical wave train and the tropical zonal circulation. Thus, seasonal prediction of the SEASM can be improved by further considering the synergistic impact of the TP’s thermal forcing and the tropical Atlantic surface temperature.

Open access