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CLLJ and WHWP heat content as a constrain to North American Monsoon activation moisture supply
Ana Durán-Quesada * 1 , Rodrigo Castillo 1 , Marie Hundsdoerfer 2 , Luis Gimeno 3
1  Atmospheric, Oceanic and Planetary Physics Department, School of Physics, University of Costa Rica, Costa Rica.
2  University of Kiel, Germany
3  Environmental Physics Laboratory, University of Vigo, Spain


With a well defined long term basis analysis of moisture supply to the North American Monsoon (NAM) domain based upon FLEXPART Lagrangian trajectories, the role of the Caribbean Sea and the Gulf of Mexico as the primary moisture source for the monsoon onset is analyzed. Regardless the NAM area requires the input from other sources, it is the eastern source which provides the required supply to activate the land moisture processes. Here we study how the warm SSTs of the WHWP enclosed region increase the moisture content, modulate the depth of the boundary layer and to which extent the CLLJ is able to advect the moist air towards the eastern Sierra Madre region. The analysis focuses on how different these processes are for warm and cold ENSO events and the relevance of this variability mode as a control of the meridional rainfall distribution across tropical north America under ENSO forcing. For this analysis, a different approach for the WHWP characterization is implemented as a new volume heat content definition is used for the WHWP instead of the traditional area defined index.

Keywords: CLLJ, moisture transport, WHWP
Comments on this paper
Paulina Ordoñez
monsoon onset dates
Interesting work. However, it is not clearly pointed out how the monsoon onset dates are established.
Ana Durán-Quesada
You are right, that was not explicitly mentioned, since different from other monsoon systems the NAM does not properly fulfill the wind reversal criteria, the onset was determined based on precipitation from CHIRPS using the Zeng and Liu criteria for consecutive days of rainfall for the golden ratio threshold(Zeng, X. and Lu, E., 2004. Globally unified monsoon onset and retreat indexes. Journal of Climate, 17(11), pp.2241-2248.)

Victor .Magaña
scales of quasi-stationary circulations
Is it possible that the role of the Gulf of California moisture flow be modulated by a larger scale flow forced by ENSO conditions, considering for instance a forced planetary scale circulation as in the PNA pattern?
Ana Durán-Quesada
I think it sure does, still the moisture export signature contains a lot of noise which makes hard to underpin only the Gulf evaporation contribution, looking for such relationship has been normally conducted by performing EOF analysis or projecting the pattern onto the SST fields, in those cases some correlations can be found but the signal is not that good. This changes when you repeat the analysis using other variables that have a direct physic link to evaporation such as the heat release (we aim to shown this result in the full manuscript of this contribution for WATER).

You can think of a bunch of nice experiments to check on how large the forcing could be by imposing the pattern as a temperature gradient for example in an aquaplanet simulation.

Victor .Magaña
Hello Ana
There are some suggestion on the mechanisms that connect the Intra Americas Seas (moisture fluxes) to the NAM region. For instance, Adams and Stensrud (2007) suggested that Tropical Easterly Waves may be playing an important role in forcing convective activity over the NAM region. Have you included how TEW activity changes might result in forcing moisture fluxes and the onset of the NAM over northwestern Mexico? My point is that we should consider a dynamical mechanism to connect to distant regions

Teddy Allen
other earlier moisture sources? Panama Low?
Hi Ana! I know you focus here on the NAM during the summer, but did you do any moisture analysis for the late spring period? I have noticed that the development of the "Panama Low" (closed 850hPa height contours near the SW Caribbean) can provide the circulation pattern to advect moisture from the tropics into the central Caribbean. Are you familiar with the Panama Low pattern? Research on this topic is scant.
Ana Durán-Quesada
Hi Ted!
Not directly for this work but we have checked on the low system as a possible precursor of instabilities during the first rainy season in southern Central America and the intensification of rainfall in the Caribbean slope (our question there was how that low pressure could be related with the CLLJ and its associated transport). My feeling on that is that it might result from the influence of the exit region of the CLLJ and the subsidence associated with the descending branch ITCZ that is 'pivoted' in northern South America. If that suggestion is true you should at least observe a marked intensification and a zonal displacement of the low during warm ENSO compared to cold ENSO and neutral years. We did some compositing and the relationship was certainly there but we think we are missing to incorporate in the picture a few thing namely the SST gradients, the boundary layer height which should be tilted when the low intensifies and also filter out the days with heavy rain for which deep convection was influencing the area (but we have not done that yet...).