Why Adi Sankara used the name Narayana when referring to Paramatma as Jagat-karaka:

Even though this country is well known for "tatwa vichara", it is even more well known (good name and bad name both) for giving us a large number of Gods (deities). Instead of the Absolute being considered as a "dry" abstract principle, the deities which are no different from the Absolute, have form, and appear as the ' Absolute come to life'. The Acharya, in his bhashyas, could have dealt with the Paramatma as the prapancha karana sakthi, the Absolute, as a dry abstract principle. Instead of doing so, and in order that the treatment may be appealing to the community which is used to relating itself to God with Form, he had thought it necessary to refer to the Absolute at several places by a Name. The question arises about which Name to choose. 

Though there are many deities, these can be generally considered as falling under the broad classification of Saiva and Vaishnava. Between Siva and Vishnu, he may have thought about which to choose. The Acharya was himself an Avatara of Siva. So, he may have thought why use my own Name. Siva is very dear to Vishnu. And, similarly, Vishnu is very dear to Siva. Therefore, wherever Vedanta refers to Paramatma as Jagat-karana-sakthi, Prapancha-mahasakthi, Saguna-bhramman, Iswaran, with an implicit suggestion of attributed Form, where Murthy Rupa is indicated, the Acharya has used NARAYANA as the name.

Avatara Siva talking about Adhara Siva does not look proper and talking about Vishnu appears more graceful, balanced and dignified. Is that not so?

So it is, in the Bhashya books, the root (moola) of the causal principle of the Jagat is referred to by the name Narayana. The supreme central meaning of these books is the Oneness (unity) of Paramatma and Jivatma, that is Advaita. In this way, the use by Siva Avatara Acharya of Vishnu's name brings in the Hari Hara Advaita - that Siva and Vishnu are one and the same.

Mahavishnu has many names. In fact, there are a thousand names. Then it may be asked why the Narayana name is chosen. The supreme astakshari mantra of Mahavishnu has the Narayana name in it.

Ayana means path (marga). Ayana also means the end (goal) of the path. In both these senses, Narayana is the Ayana for the Nara (Jivatma). Bhrama Vidya Sastra gives the path to salvation. Narayana is Bhrama Vidya. When He appears as Krishna Paramatma, he himself says "Adhyatma Vidya Vidhyanam" in the Gita. The final goal of that Vidya is also He only. Therefore it is that when Bhrahma Vidya Guru Parampara is talked about, it starts with "Narayanam". So it is quite appropriate that in the Bhashya books, which are Bhrahma Vidya Sastras, the Narayana name is used.

The name Sankara joining with the name Narayana has given rise to a name Sankaranarayanan. In south Pandya country, there is a place where the worshipped image (murthy) is the image of Siva and Vishnu in one body showing their oneness. That image is known as Sankaranarayanan. The place is called Sankaranarayanan Koil. This is now commonly pronounced as Sankaranainar Koil in usage.

The Acharya himself has used the above name in an important context. He has bequeathed to us in a question answer form a text called "Prasnottara Ratna Malika". In the penultimate sloka of this work, the question is: Who is called Bhagwan and Maheswara. Our Acharya does not in reply give the names of Siva or Vishnu. Our Advaita Acharya gives the reply as the one Atma which is a communion of Siva and Vishnu. Even there he does not use Siva Vishnu or Hari Hara but has used Sankaranarayanan.

Q: Kascha Bhagwan Mahesah?
A: Sankara Narayanatmaikah

Therefore it is appropriate that this Sankara also gives a special niche for the name Narayana.

Thus it is clear that the Acharya has referred to the great causal principle of the Jagat by the name NARAYANA. So, when he intends that the namaskaras performed to us must be conveyed to the Jagat-Karana-Vastu, he instructs: Convey to Narayana. And to carry out the instruction, he has made a rule, which appears easy on the face of it.

Only that Narayana, who has created all this and has endowed all this with vital energy (sakthi), has the "right" to accept all the namaskaras. Namaskara to any deity goes to Kesava. We recite the sloka: Sarva Deva Namaskara Kesavam Pratigachhati. When namaskaras performed to the deities go only to HIM, how can namaskaras performed to ordinary people belong to them? All these namaskaras also go to HIM only. It is that we have been asked to always remember when namakaras are performed to us. In order that we do not "misappropriate" the namaskaras rightfully belonging to HIM only and make sure that the namaskara is duly redirected to HIM, the Acharya has most kindly defined a rule for us a rule, which as I said earlier, is seemingly easy. The rule is that when someone performs a namaskara to us, we should say "Narayana, Narayana".