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Claudia K. Parise
,
Luciano P. Pezzi
,
Kevin I. Hodges
, and
Flavio Justino

Abstract

The study analyzes the sensitivity and memory of the Southern Hemisphere coupled climate system to increased Antarctic sea ice (ASI), taking into account the persistence of the sea ice maxima in the current climate. The mechanisms involved in restoring the climate balance under two sets of experiments, which differ in regard to their sea ice models, are discussed. The experiments are perturbed with extremes of ASI and integrated for 10 yr in a large 30-member ensemble. The results show that an ASI maximum is able to persist for ~4 yr in the current climate, followed by a negative sea ice phase. The sea ice insulating effect during the positive phase reduces heat fluxes south of 60°S, while at the same time these are intensified at the sea ice edge. The increased air stability over the sea ice field strengthens the polar cell while the baroclinicity increases at midlatitudes. The mean sea level pressure is reduced (increased) over high latitudes (midlatitudes), typical of the southern annular mode (SAM) positive phase. The Southern Ocean (SO) becomes colder and fresher as the sea ice melts mainly through sea ice lateral melting, the consequence of which is an increase in the ocean stability by buoyancy and mixing changes. The climate sensitivity is triggered by the sea ice insulating process and the resulting freshwater pulse (fast response), while the climate equilibrium is restored by the heat stored in the SO subsurface layers (long response). It is concluded that the time needed for the ASI anomaly to be dissipated and/or melted is shortened by the sea ice dynamical processes.

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Matthew D. K. Priestley
,
Duncan Ackerley
,
Jennifer L. Catto
, and
Kevin I. Hodges

Abstract

The Southern Hemisphere storm tracks are commonly simulated too far equatorward in climate models for the historical period. In the latest generation of climate models from phase 6 of the Coupled Model Intercomparison Project (CMIP6), the equatorward bias that was present in CMIP5 models still persists, although it is reduced considerably. A further reduction of the equatorward bias is found in atmosphere-only simulations. Using diagnostic large-scale fields, we propose that an increase in the midlatitude temperature gradients contributes to the reduced equatorward bias in CMIP6 and AMIP6 models, reducing the biases relative to ERA5. These changes increase baroclinicity in the atmosphere and are associated with a storm track that is situated farther poleward. In CMIP6 models, the poleward shift of the storm tracks is associated with an amelioration of cold midlatitude SST biases in CMIP5 and not through a reduction of the long-standing warm Southern Ocean SST bias. We propose that increases in midlatitude temperature gradients in the atmosphere and ocean are connected to changes in the cloud radiative effect. Persistent track density biases to the south of Australia are shown to be connected to an apparent standing-wave pattern originating in the tropics, which modifies the split jet structure near Australia and subsequently the paths of cyclones.

Free access
Matthew D. K. Priestley
,
Duncan Ackerley
,
Jennifer L. Catto
, and
Kevin I. Hodges

Abstract

The ability of climate models to represent extratropical storm tracks is vital to provide useful projections. In previous work, the representation of the extratropical storm tracks in the Northern Hemisphere was found to have improved from phase 5 to phase 6 of the Coupled Model Intercomparison Project (CMIP). Here we investigate the remaining and persistent biases in models from phase 6 of CMIP, by contrasting the atmosphere-only simulations (AMIP6) with the historical coupled simulations (CMIP6). The comparison of AMIP6 and CMIP6 simulations reveals that biases in sea surface temperatures (SSTs) in the coupled simulations across the North Pacific Ocean in winter modify the atmospheric temperature gradient, which is associated with an equatorward bias of the storm track. In the North Atlantic Ocean, cyclones do not propagate poleward enough in coupled simulations, which is partly driven by cold SSTs to the south of Greenland, decreasing the latent heat fluxes. In summer, excessive heating across central Asia and the Tibetan Plateau reduces the local baroclinicity, causing fewer cyclones to form and propagate from eastern China into the North Pacific in both the coupled and atmosphere-only simulations. Several of the biases described in the coupled models are reduced considerably in the atmosphere-only models when the SSTs are prescribed. For example, the equatorward bias of the North Pacific storm track is reduced significantly. However, other biases are apparent in both CMIP6 and AMIP6 (e.g., persistent reduction in track density and cyclogenesis over eastern Asia in summer), which are associated with other processes (e.g., land surface temperatures).

Free access
Ju Liang
,
Jennifer L. Catto
,
Matthew Hawcroft
,
Kevin I. Hodges
,
Mou Leong Tan
, and
James M. Haywood

Abstract

Borneo vortices (BVs) are intense precipitating winter storms that develop over the equatorial South China Sea and strongly affect the weather and climate over the western Maritime Continent because of their association with deep convection and heavy rainfall. In this study, the ability of the Hadley Centre Global Environment Model 3–Global Coupled, version 3.1 (HadGEM3-GC3.1), global climate model to simulate the climatology of BVs at different horizontal resolutions is examined using an objective feature-tracking algorithm. The HadGEM3-GC3.1 at the N512 (25 km) horizontal resolution simulates BVs with well-represented characteristics, including their frequency, spatial distribution, and lower-tropospheric structures when compared with BVs identified in a climate reanalysis, whereas the BVs in the N96 (~135 km) and N216 (~65 km) simulations are much weaker and less frequent. Also, the N512 simulation better captures the contribution of BVs to the winter precipitation in Borneo and the Malay Peninsula when compared with precipitation from a reanalysis data and from observations, whereas the N96 and N216 simulations underestimate this contribution because of the overly weak low-level convergence of the simulated BVs. The N512 simulation also exhibits an improved ability to reproduce the modulation of BV activity by the occurrence of northeasterly cold surges and active phases of the Madden–Julian oscillation in the region, including increased BV track densities, intensities, and lifetimes. A sufficiently high model resolution is thus found to be important to realistically simulate the present-climate precipitation extremes associated with BVs and to study their possible changes in a warmer climate.

Open access
Elliott M. Sainsbury
,
Reinhard K. H. Schiemann
,
Kevin I. Hodges
,
Alexander J. Baker
,
Len C. Shaffrey
, and
Kieran T. Bhatia

Abstract

Recurving tropical cyclones (TCs) can cause extensive damage along the U.S. East Coast and later in their life cycle over Europe as post-tropical cyclones. While the existing literature attempts to understand the drivers of basinwide and regional TC variability, less work has been undertaken looking at recurving TCs. The roles played by the interannual variabilities of TC frequency and the steering flow in governing recurving TC interannual variability are investigated in this study. Using a track-matching algorithm, we identify observed TC tracks from the NHC “best track” hurricane database, version 2 (HURDAT2) in the ERA5 and MERRA2 reanalyses. This allows for detailed analysis of the post-tropical stages of the tracks in the observational TC record, enabling robust identification and separation of TCs that recurve. We show that over 75% of the interannual variance in annual recurving TC frequency can be explained by just two predictors—the frequency of TCs forming in the subtropical Atlantic, and hurricanes (TCs with wind speeds > 33 m s−1) forming in the main development region (MDR). An index describing the seasonal mean meridional steering flow shows a weak, nonsignificant relationship with recurving TC frequency, supported by composite analysis. These results show that the interannual variability in recurving TC frequency is primarily driven by the seasonal TC activity of the MDR and the subtropical Atlantic, with seasonal anomalies in the steering flow playing a much smaller, secondary role. These results help to quantify the extent to which skillful seasonal forecasts of Atlantic hurricane activity benefit regions vulnerable to recurving TCs.

Significance Statement

Recurving tropical cyclones (TCs) can cause extensive damage to the U.S. East Coast, eastern Canada, and Europe. It is, therefore, crucial to understand why some years have a higher frequency of recurving TCs than other years. In this study, we show that the frequency of recurving TCs is very strongly linked to the frequency that hurricanes (TCs with wind speeds > 33 m s−1) form in the main development region, and the frequency that TCs form in the subtropical Atlantic. This result suggests that skillful seasonal prediction of hurricane activity could be used to give enhanced seasonal warning to the regions often impacted by recurving TCs.

Open access
Matthew D. K. Priestley
,
Duncan Ackerley
,
Jennifer L. Catto
,
Kevin I. Hodges
,
Ruth E. McDonald
, and
Robert W. Lee

Abstract

The representation of the winter and summer extratropical storm tracks in both hemispheres is evaluated in detail for the available models in phase 6 of the Coupled Model intercomparison Project (CMIP6). The state of the storm tracks from 1979 to 2014 is compared to that in ERA5 using a Lagrangian objective cyclone tracking algorithm. It is found that the main biases present in the previous generation of models (CMIP5) still persist, albeit to a lesser extent. The equatorward bias around the SH is much reduced and there appears to be some improvement in mean biases with the higher-resolution models, such as the zonal tilt of the North Atlantic storm track. Low-resolution models have a tendency to underestimate the frequency of high-intensity cyclones with all models simulating a peak intensity that is too low for cyclones in the SH. Explosively developing cyclones are underestimated across all ocean basins and in both hemispheres. In particular the models struggle to capture the rapid deepening required for these cyclones. For all measures, the CMIP6 models exhibit an overall improvement compared to the previous generation of CMIP5 models. In the NH most improvements can be attributed to increased horizontal resolution, whereas in the SH the impact of resolution is less apparent and any improvements are likely a result of improved model physics.

Open access
Wataru Yanase
,
Hiroshi Niino
,
Shun-ichi I. Watanabe
,
Kevin Hodges
,
Matthias Zahn
,
Thomas Spengler
, and
Irina A. Gurvich

Abstract

Polar lows are intense meso-α-scale cyclones that develop over the oceans poleward of the main baroclinic zone. A number of previous studies have reported polar low formation over the Sea of Japan within the East Asian winter monsoon. To understand the climatology of polar lows over the Sea of Japan, a tracking algorithm for polar lows is applied to the recent JRA-55 reanalysis. The polar low tracking is applied to 36 cold seasons (October–March) from October 1979 to March 2015. The polar lows over the Sea of Japan reach their maximum intensity on the southeastern side of the midline between the Japanese islands and the Asian continent. Consistent with previous case studies, composite analysis demonstrates that the polar low development is associated with the enhanced northerly flow on the western side of a synoptic-scale extratropical cyclone, with the cold trough in the midtroposphere and with increased heat fluxes from the sea surface. Furthermore, the present climatological study has revealed two dominant directions of motion of the polar lows: southward and eastward. Southward-moving polar lows are steered by a strong northerly flow in the lower troposphere, which is enhanced on the western side of synoptic-scale extratropical cyclones, while the eastward-moving polar lows occur within a planetary-scale westerly flow in the midlatitudes. Thus, the direction of polar low motion reflects the difference in planetary- and synoptic-scale conditions.

Full access
Julia V. Manganello
,
Kevin I. Hodges
,
Benjamin A. Cash
,
James L. Kinter III
,
Eric L. Altshuler
,
Michael J. Fennessy
,
Frederic Vitart
,
Franco Molteni
, and
Peter Towers

Abstract

Seasonal forecast skill of the basinwide and regional tropical cyclone (TC) activity in an experimental coupled prediction system based on the ECMWF System 4 is assessed. As part of a collaboration between the Center for Ocean–Land–Atmosphere Studies (COLA) and the ECMWF called Project Minerva, the system is integrated at the atmospheric horizontal spectral resolutions of T319, T639, and T1279. Seven-month hindcasts starting from 1 May for the years 1980–2011 are produced at all three resolutions with at least 15 ensemble members. The Minerva system demonstrates statistically significant skill for retrospective forecasts of TC frequency and accumulated cyclone energy (ACE) in the North Atlantic (NA), eastern North Pacific (EP), and western North Pacific. While the highest scores overall are achieved in the North Pacific, the skill in the NA appears to be limited by an overly strong influence of the tropical Pacific variability. Higher model resolution improves skill scores for the ACE and, to a lesser extent, the TC frequency, even though the influence of large-scale climate variations on these TC activity measures is largely independent of resolution changes. The biggest gain occurs in transition from T319 to T639. Significant skill in regional TC forecasts is achieved over broad areas of the Northern Hemisphere. The highest-resolution hindcasts exhibit additional locations with skill in the NA and EP, including land-adjacent areas. The feasibility of regional intensity forecasts is assessed. In the presence of the coupled model biases, the benefits of high resolution for seasonal TC forecasting may be underestimated.

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Elliott M. Sainsbury
,
Reinhard K. H. Schiemann
,
Kevin I. Hodges
,
Alexander J. Baker
,
Len C. Shaffrey
, and
Kieran T. Bhatia

Abstract

Post-tropical cyclones (PTCs) can bring high winds and extreme precipitation to Europe. Although the structure and intensity of observed Europe-impacting PTCs has been investigated in previous studies, a quantitative understanding of the factors important for PTCs to reach Europe has not been established. By tracking and identifying the full life cycle of tropical cyclones (TCs) in the ERA5 reanalysis, we investigate why some PTCs impact Europe and why others do not, using a composite analysis. We show that PTCs that impact Europe are typically ∼4–6 m s−1 stronger at their lifetime maximum intensity and throughout the extratropical transition process. They are also twice as likely to reintensify in the midlatitudes. During ET, the Europe-impacting PTCs interact more strongly with an upstream upper-level trough in a significantly more baroclinic environment. The Europe-impacting PTCs are steered on a more poleward trajectory across a midlatitude jet streak. It is during the crossing of the jet that these cyclones often undergo their reintensification. Using contingency table analysis, TC lifetime maximum intensity, and whether post-ET reintensification occurs are shown to be significantly associated with the odds that a PTC reaches Europe. This supports our composite analysis and further indicates that TC intensity and reintensification both modulate the likelihood that a PTC will impact Europe.

Significance Statement

Some post-tropical cyclones (PTCs) reach Europe, often associated with extreme precipitation and high winds. It is currently unclear what factors allow this to occur. In this study, we track cyclones in two reanalyses using a feature tracking scheme and identify the PTCs by matching (in space and time) reanalysis tracks with observed tracks. Using a composite analysis, we show that 1) tropical cyclones (TCs) that are more intense, and 2) TCs that reintensify after extratropical transition, are more likely to reach Europe. TCs that reintensify interact strongly with an upper-level upstream trough and cross a midlatitude jet streak. Reintensification occurs as the cyclones cross this jet streak.

Open access
Matthew J. Widlansky
,
H. Annamalai
,
Stephen B. Gingerich
,
Curt D. Storlazzi
,
John J. Marra
,
Kevin I. Hodges
,
Barry Choy
, and
Akio Kitoh

Abstract

Potential changing climate threats in the tropical and subtropical North Pacific Ocean were assessed, using coupled ocean–atmosphere and atmosphere-only general circulation models, to explore their response to projected increasing greenhouse gas emissions. Tropical cyclone occurrence, described by frequency and intensity, near islands housing major U.S. defense installations was the primary focus. Four island regions—Guam and Kwajalein Atoll in the tropical northwestern Pacific, Okinawa in the subtropical northwestern Pacific, and Oahu in the tropical north-central Pacific—were considered, as they provide unique climate and geographical characteristics that either enhance or reduce the tropical cyclone risk. Guam experiences the most frequent and severe tropical cyclones, which often originate as weak systems close to the equator near Kwajalein and sometimes track far enough north to affect Okinawa, whereas intense storms are the least frequent around Oahu. From assessments of models that simulate well the tropical Pacific climate, it was determined that, with a projected warming climate, the number of tropical cyclones is likely to decrease for Guam and Kwajalein but remain about the same near Okinawa and Oahu; however, the maximum intensity of the strongest storms may increase in most regions. The likelihood of fewer but stronger storms will necessitate new localized assessments of the risk and vulnerabilities to tropical cyclones in the North Pacific.

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