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Transient-mean flow interactions over the northern tropical Americas
Published:
11 November 2017
by MDPI
in First International Electronic Conference on the Hydrological Cycle
session Low-Level Jets, Warm Pools, Monsoons
Abstract:
The Mid Summer Drought (MSD) over the Mesoamerican region constitutes a unique feature of its precipitation seasonal cycle. The MSD is a relative minimum in convective activity during July and August that coincides with an intensification of the Caribbean Low Level Jet (CLLJ) (mean flow at 925 hPa). There is not a unique theory on what maintains the CLLJ is, but the effect of the mean meridional convergence of easterly momentum related to tropical extratropical interactions over the Caribbean Sea, appars to play an important role. The barotropicaly unstable nature of the CLLJ shows that when this mean circulation is intense the amplification of high frequency transients (eg, easterly waves) is inhibited. Empirical observational evidence shows that as the CLLJ intensifies above a certain magnitude, transient activity decreases. Such transient activity is related to easterly waves, a key element in producing precipitation over the tropical Americas. Therefore, the CLLJ acts as a modulator of tropical convection in relation to the MSD, as the period of relatively minor tropical convection coincides with a decrease in high frequency Perturbation Kinetic Energy (PKE). Its role in the modulation of convective activity over the tropical Americas on interannual and even on interdecadal time scales makes it one of the key elements to understand climate variability over the tropical Americas.
Keywords: Caribbean low level jet, transient mean-flow
Comments on this paper
Raquel Nieto
16 November 2017
data
This an interesting work,
my question is about the data, Why the periods are different?
Is the horizontal resolution the same for NCEP and ERA data?
- the authors need to specify the data use in each figure in the caption.
best regards
my question is about the data, Why the periods are different?
Is the horizontal resolution the same for NCEP and ERA data?
- the authors need to specify the data use in each figure in the caption.
best regards
Eduardo Herrera
16 November 2017
Yes Raquel, that is right, thank you for observation.
Horizontal resolution for NCEP is 2.5°, and for ERA is 1.0°.
Period for each climatology was diferent because availability of data.
We used NCEP data for first two images, and ERA-Interim for the last one (Figure 3).
Horizontal resolution for NCEP is 2.5°, and for ERA is 1.0°.
Period for each climatology was diferent because availability of data.
We used NCEP data for first two images, and ERA-Interim for the last one (Figure 3).
Teddy Allen
17 November 2017
CLLJ and other patterns
Like you, I am fascinated by the role of the CLLJ in regional summer rainfall dynamics. However, I often feel that the CLLJ gets too much "credit". The climatological max speed of the CLLJ is roughly 12m/s. Is this really enough to provide ageostrophic circulation and patterns of convergence and divergence via jet dynamics? (I don't think you necessarily suggest this in your paper....) Any jet is simply the result of restoring thermal wind balance. So, where might this breakdown in thermal wind balance occur? If the CLLJ is a true jet, then the related horizontal temperature gradient must also be observed within the same region. Perhaps the differential heating along the meridian along the southern Caribbean region is related to heating along the hills (2000-3000ft) of the north coast of South America and the mid-summer SST cold tongue.This may explain the peak wind speeds at the relatively low 925hPa level. If this is the case, then the westward extension of the North Atlantic Subtropical High and its enhanced trades may explain the initial cooling the of SST cold tongue. Just a thought.....not yet tested, but begging for more attention.
As for your paper, I have just a few minor comments. A PKE hovmuller from May to October might be a valuable tool to explore PKE across the summer at multiple levels to help identify at which level the monthly mean PKE is strongest.
Figure 3 shows the PKE climatology for July. What do the unfiltered and low and high pass filtered plots look like? Do we observe transient waves at 500hPa in July at 24N, 75W? Could the 500hPa max in PKE in figure 3 just be the result of the general circulation and not transient waves? Comparing similar plots for different types of filters could help prove this. Therefore, I think additional analysis must be considered before making any conclusions regarding the impact of easterly waves.
And finally, I think we must be careful on how we describe rainfall during the mid-summer (MSD period). Is the MSD the result of reduced "convective activity" or is it just the result of reduced total rainfall. As you know, rainfall is not zero during the MSD period, but is just less than the surrounding wet season months. It may be interesting to analyze station rainfall data to compare rainfall frequency throughout the April-Sept period. It is possible that it may rain more often during the MSD, but with significantly less intensity? If so, this type of rain pattern could be more related to local convection. To my knowledge, there have not been any studies that compare precipitation types throughout the summer. This is a topic I am extremely interested in examining. Is the rainfall pattern associated with the late spring Caribbean rain-belt more stratiform compared to mid-summer local convection? GPM radar data may help address this question.
Thank you for presenting a novel method to help improve our understanding of MesoAmerican rainfall patterns. You have given me much to think about and I foresee fruitful collaboration in the future.
Gracias,
teddy
As for your paper, I have just a few minor comments. A PKE hovmuller from May to October might be a valuable tool to explore PKE across the summer at multiple levels to help identify at which level the monthly mean PKE is strongest.
Figure 3 shows the PKE climatology for July. What do the unfiltered and low and high pass filtered plots look like? Do we observe transient waves at 500hPa in July at 24N, 75W? Could the 500hPa max in PKE in figure 3 just be the result of the general circulation and not transient waves? Comparing similar plots for different types of filters could help prove this. Therefore, I think additional analysis must be considered before making any conclusions regarding the impact of easterly waves.
And finally, I think we must be careful on how we describe rainfall during the mid-summer (MSD period). Is the MSD the result of reduced "convective activity" or is it just the result of reduced total rainfall. As you know, rainfall is not zero during the MSD period, but is just less than the surrounding wet season months. It may be interesting to analyze station rainfall data to compare rainfall frequency throughout the April-Sept period. It is possible that it may rain more often during the MSD, but with significantly less intensity? If so, this type of rain pattern could be more related to local convection. To my knowledge, there have not been any studies that compare precipitation types throughout the summer. This is a topic I am extremely interested in examining. Is the rainfall pattern associated with the late spring Caribbean rain-belt more stratiform compared to mid-summer local convection? GPM radar data may help address this question.
Thank you for presenting a novel method to help improve our understanding of MesoAmerican rainfall patterns. You have given me much to think about and I foresee fruitful collaboration in the future.
Gracias,
teddy
Teddy Allen
17 November 2017
Upon further thought....what would happen if you only used the zonal part of the KPE equation? Could this better capture easterly wave contribution?
Eduardo Herrera
17 November 2017
Thank you Teddy for your comments.
Yes, CLLJ is a key feature in explaining Climate dynamics of the tropical Americas.
During the summer of 2001 under the field campaign ECAC (Experimento climático en las Albercas de Agua Caliente, Climatic experiment on tropical Americas' warm pools) several soundings were done to capture the intensity of the CLLJ. The wind magnitude around 925 hPa was 15 to 17 m/s wich is pretty strong. We are working on determinig tha range of wind strenght that may help to amplify disturbances with the spatial characteristics of easterly waves. In other words, if the jet is to intense our preliminary theoretical results indicate that disturbances would not amplify. It is in a range between 10 to 15 m/s that the disturbances tend to amplify. This coincides with the observations and analysis presented in this study.
Numerous analysis on the MSD based on station data have been conducted. Most precipitation in the region is usually related to deep convection. The number of days with precipitation decreases and this is part of the reason why we observe an MSD. Given that easterly waves are an important mechanism of tropical convection, less easterly waves result in less days of precipitation.
As you said, the challenge is to find all sources of momentum. Related with thermal wind and differential heating, Ernesto Muñoz et al. (2008) tried to establish the contribution of thermal wind to the acceleration of CLLJ, explaining part of the wind anomalies variability.
In Figure 3, the 500 hPa max in PKE could be associated with general circulation and not easterly waves?
Yes, as one approaches to the mid-latitudes, the mean flow increases its magnitude and so do its anomalies.
Right now I don't have results for zonal PKE, but as soon as possible I will comment something about it.
Yes, CLLJ is a key feature in explaining Climate dynamics of the tropical Americas.
During the summer of 2001 under the field campaign ECAC (Experimento climático en las Albercas de Agua Caliente, Climatic experiment on tropical Americas' warm pools) several soundings were done to capture the intensity of the CLLJ. The wind magnitude around 925 hPa was 15 to 17 m/s wich is pretty strong. We are working on determinig tha range of wind strenght that may help to amplify disturbances with the spatial characteristics of easterly waves. In other words, if the jet is to intense our preliminary theoretical results indicate that disturbances would not amplify. It is in a range between 10 to 15 m/s that the disturbances tend to amplify. This coincides with the observations and analysis presented in this study.
Numerous analysis on the MSD based on station data have been conducted. Most precipitation in the region is usually related to deep convection. The number of days with precipitation decreases and this is part of the reason why we observe an MSD. Given that easterly waves are an important mechanism of tropical convection, less easterly waves result in less days of precipitation.
As you said, the challenge is to find all sources of momentum. Related with thermal wind and differential heating, Ernesto Muñoz et al. (2008) tried to establish the contribution of thermal wind to the acceleration of CLLJ, explaining part of the wind anomalies variability.
In Figure 3, the 500 hPa max in PKE could be associated with general circulation and not easterly waves?
Yes, as one approaches to the mid-latitudes, the mean flow increases its magnitude and so do its anomalies.
Right now I don't have results for zonal PKE, but as soon as possible I will comment something about it.
Eduardo Herrera
18 November 2017
About zonal part of PKE, for Figure 1 zonal PKE appears to represent the two main guides of easterly waves.
For Figure 3, zonal PKE has the same pattern of total PKE.
For Figure 3, zonal PKE has the same pattern of total PKE.
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